Earthquake Monitoring and Early Warning Systems P 1...Seismology Bibliography Glossary Active fault...

Earthquake Monitoring andEarly Warning Systems

William H K Lee1 and Yih-Min Wu21US Geological Survey Menlo Park CA USA2Department of Geosciences National TaiwanUniversity Taipei Taiwan

Article Outline

GlossaryDefinition of the SubjectIntroductionEarthquake Monitoring InstrumentationHistorical DevelopmentsTechnical ConsiderationsEarthquake Monitoring in the Digital EraEarthquake Monitoring Regional and Local

NetworksA Brief HistorySome Recent AdvancesRecording Damaging Ground ShakingSeismograms and Derived ProductsEarthquake LocationEarthquake MagnitudeQuantification of the Earthquake SourceLimitations of Earthquake CatalogsEarthquake Early Warning (EEW) SystemsPhysical Basis and Limitations of EEW SystemsDesign Considerations for EEW SystemsRegional Warning Versus Onsite WarningSome Recent EEWAdvancesFuture DirectionsAppendix A Progress Report on Rotational

SeismologyBibliography

Glossary

Active fault A fault (qv) that has moved inhistoric (eg past 10000 years) or recent geo-logical time (eg past 500000 years)

Body waves Waves which propagate through theinterior of a body For the Earth there are twotypes of seismic bodywaves (1) compressionalor longitudinal (P wave) and (2) shear ortransverse (S wave)

Coda waves Waves which are recorded on aseismogram (qv) after the passage of bodywaves (qv) and surface waves (qv) Theyare thought to be back-scattered waves due tothe Earthrsquos inhomogeneities

Earthquake early warning system (EEWS)An earthquake monitoring system that is capa-ble of issuing warning message after an earth-quake occurred and before strong groundshaking begins

Earthquake precursor Anomalous phenome-non preceding an earthquake

Earthquake prediction A statement in advanceof the event of the time location and magni-tude (qv) of a future earthquake

Epicenter The point on the Earthrsquos surface ver-tically above the hypocenter (qv)

Far-field Observations made at large distancesfrom the hypocenter (qv) compared to thewave-length andor the source dimension

Fault slip The relative displacement of points onopposite sides of a fault (qv) measured on thefault surface

Fault A fracture or fracture zone in the Earthalong which the two sides have been displacedrelative to one another parallel to the fracture

Focal mechanism A description of the orienta-tion and sense of slip on the causative faultplane derived from analysis of seismic waves(qv)

Hypocenter Point in the Earth where the ruptureof the rocks originates during an earthquakeand seismic waves (qv) begin to radiate Itsposition is usually determined from arrivaltimes of seismic waves (qv) recorded by seis-mographs (qv)

Intensity earthquake Rating of the effects ofearthquake vibrations at a specific place Inten-sity can be estimated from instrumental mea-surements however it is formally a rating

copy Springer Science Business Media New York (outside the USA) 2019R A Meyers (ed) Encyclopedia of Complexity and Systems Sciencehttpsdoiorg101007978-3-642-27737-5_152-2

1

assigned by an observer of these effects using adescriptive scale Intensity grades are com-monly given in Roman numerals (in the caseof the Modified Mercalli Intensity Scale fromI for ldquonot perceptiblerdquo to XII for ldquototaldestructionrdquo)

Magnitude earthquake Quantity intended tomeasure the size of earthquake at its sourceindependent of the place of observation Rich-ter magnitude (ML) was originally defined in1935 as the logarithm of the maximum ampli-tude of seismic waves in a seismogram writtenby a Wood-Anderson seismograph (correctedto) a distance of 100 km from the epicenterMany types of magnitudes exist such as body-wave magnitude (mb) surface-wave magni-tude (MS) and moment magnitude (MW)

Moment tensor A symmetric second-order ten-sor that characterizes an internal seismic pointsource completely For a finite source it repre-sents a point source approximation and can bedetermined from the analysis of seismic waves(qv) whose wavelengths are much greaterthan the source dimensions

Near-field A term for the area near the causativerupture of an earthquake often taken asextending a distance from the rupture equal toits length It is also used to specify a distance toa seismic source comparable or shorter than thewavelength concerned In engineering applica-tions near-field is often defined as the areawithin 25 km of the fault rupture

Plate tectonics A theory of global tectonics(qv) in which the Earthrsquos lithosphere isdivided into a number of essentially rigidplates These plates are in relative motioncausing earthquakes and deformation alongthe plate boundaries and adjacent regions

Probabilistic seismic hazard analysis Availableinformation on earthquake sources in a givenregion is combined with theoretical and empir-ical relations among earthquake magnitude(qv) distance from the source and local siteconditions to evaluate the exceedance probabil-ity of a certain ground motion parameter suchas the peak ground acceleration at a given siteduring a prescribed time period

Seismic hazard Any physical phenomena asso-ciated with an earthquake (eg ground motionground failure liquefaction and tsunami) andtheir effects on land use man-made structureand socioeconomic systems that have thepotential to produce a loss

Seismic hazard analysis The calculation of theseismic hazard (qv) expressed in probabilis-tic terms (See probabilistic seismic hazardanalysis qv) The result is usually displayedin a seismic hazard map (qv)

Seismic hazardmap Amap showing contours ofa specified ground-motion parameter or responsespectrum ordinate for a given probabilistic seis-mic hazard analysis (qv) or return period

Seismic moment The magnitude of the compo-nent couple of the double couple that is the pointforce system equivalent to a fault slip (qv) in anisotropic elastic body It is equal to rigidity timesthe fault slip integrated over the fault plane Itcan be estimated from the far-field seismic spec-trum at wave lengths much longer than thesource size It can also be estimated from thenear-field seismic geologic and geodetic dataAlso called ldquoscalar seismic momentrdquo to distin-guish it from moment tensor (qv)

Seismic risk The risk to life and property fromearthquakes

Seismic wave A general term for waves gener-ated by earthquakes or explosions There aremany types of seismic waves The principleones are body waves (qv) surface waves(qv) and coda waves (qv)

Seismogram Record of ground motions madeby a seismograph (qv)

Seismograph Instrument which detects andrecords ground motion (and especially vibra-tions due to earthquakes) along with timinginformation It consists of a seismometer(qv) a precise timing device and a recordingunit (often including telemetry)

Seismometer Inertial sensor which responds toground motions and produces a signal that canbe recorded

Source parameters of an earthquake Theparameters specified for an earthquake sourcedepends on the assumed earthquake model

2 Earthquake Monitoring and Early Warning Systems

They are origin time hypocenter (qv) mag-nitude (qv) focal mechanism (qv) andmoment tensor (qv) for a point sourcemodel They include fault geometry rupturevelocity stress drop slip distribution etc fora finite fault model

Surface waves Waves which propagate alongthe surface of a body or along a subsurfaceinterface For the Earth there are two commontypes of seismic surface waves Rayleighwaves and Love waves (both named aftertheir discoverers)

Tectonics Branch of Earth science which dealswith the structure evolution and relativemotion of the outer part of the Earth the lith-osphere The lithosphere includes the Earthrsquoscrust and part of the Earthrsquos upper mantle andaverages about 100 km thick See plate tecton-ics (qv)

Teleseism An earthquake at an epicentral dis-tance greater than about 20 or 2000 km fromthe place of observation

Definition of the Subject

When a sudden rupture occurs in the Earth elastic(seismic) waves are generated When these wavesreach the Earthrsquos surface we may feel them as aseries of vibrations which we call an earthquakeSeismology is derived from the Greek wordseιsmo0B (seismos or earthquake) and lo0goB(logos or discourse) thus it is the science of earth-quakes and related phenomena Seismic waves canbe generated naturally by earthquakes or artificiallyby explosions or other means We define earth-quake monitoring as a branch of seismologywhich systematically observes earthquakes withinstruments over a long period of time

Instrumental recordings of earthquakes havebeen made since the later part of the nineteenthcentury by seismographic stations and networksof various sizes from local to global scales Theobserved data have been used for example (1) tocompute the source parameters of earthquakes(2) to determine the physical properties of theEarthrsquos interior (3) to test the theory of platetectonics (4) to map active faults (5) to infer the

nature of damaging ground shaking and (6) tocarry out seismic hazard analyses Constructinga satisfactory theory of the complex earthquakeprocess has not yet been achieved within the con-text of physical laws eg realistic equations formodeling earthquakes do not exist at presentGood progress however has been made in build-ing a physical foundation for the earthquakesource process (Kanamori and Brodsky 2000)partly as a result of research directed toward earth-quake prediction

Earthquakes release large amounts of energythat potentially can cause significant damage andhuman deaths During an earthquake potentialenergy (mainly elastic strain energy and some grav-itational energy) that has accumulated in the hypo-central region over decades to centuries or longer isreleased suddenly (Kanamori and Rivera 2006)This energy is partitioned into (1) radiated energyin the form of propagating seismic waves(2) energy consumed in overcoming fault friction(3) the energy which expands the rupture surfacearea or changes its properties (eg by pulverizingrock) and (4) heat The radiated seismic energy is asmall fraction (about 7) of the total energy bud-get and it can be estimated using the recordedseismograms Take for example the 1971 SanFernando earthquake (MW = 66) in southern Cal-ifornia Its radiated energywas about 5 1021 ergsor about 120 kilotons of TNT explosives or theenergy released by six atomic bombs of the sizeused in World War II The largest earthquakerecorded instrumentally (so far) is the 1960Chileanearthquake (MW = 95) Its radiated energy wasabout 11 1026 ergs an equivalent of about 2600megatons of TNT explosives the energy releasedby about 130000 atomic bombs It is therefore nosurprise that an earthquake can cause up to hun-dreds of thousands of human deaths and produceeconomic losses of up to hundreds of billions ofdollars

Monitoring earthquakes is essential for provid-ing scientific data to investigate complex earth-quake phenomena and to mitigate seismichazards The present article is a brief overviewof earthquake monitoring and early warning sys-tems it is intended for a general scientific audi-ence and technical details can be found in the

Earthquake Monitoring and Early Warning Systems 3

cited references Earthquakes are complex naturalphenomena and their monitoring requires an inter-disciplinary approach including using tools fromcomputer science electrical and electronic engi-neering mathematics physics and others Earth-quake early warning systems (which are based onearthquake monitoring) offer practical informa-tion for reducing seismic hazards in earthquake-prone regions

After the ldquoIntroductionrdquo we will present asummary of earthquake monitoring a descriptionof the products derived from the analysis ofseismograms and a discussion of the limitationsof these products Earthquake early warning sys-tems are then presented briefly and we concludewith a section on future directions including aprogress report on rotational seismology(Appendix) We present overviews of most topicsin earthquake monitoring and an extensive bibli-ography is provided for additional reading andtechnical details

Introduction

Earthquakes both directly and indirectly havecaused much suffering to mankind During thetwentieth century alone about two million peoplewere killed as a result of earthquakes A list ofdeadly earthquakes (death tolls 25) of the worldduring the past five centuries was compiled byUtsu (2002a) It shows that earthquakes of mag-nitude6 (~150 per year worldwide) can be dam-aging and deadly if they occur in populated areasand if their focal depths are shallow (lt50 km)Seismic risk can be illustrated by plotting the mostdeadly earthquakes of the past five centuries(1500ndash2000) over a map of current populationdensity This approach was used by Utsu(2002a) and his result is shown in Fig 1 Mostof these deadly earthquakes are concentrated(1) along the coasts of Central America the Carib-bean western South America and Indonesia and(2) along a belt that extends from southernEurope the Middle East Iran Pakistan andIndia to China and Japan

Table 1 lists the most deadly earthquakes (deathtoll gt20 000) of the past 110 years based onofficial estimates (often underestimated for politi-cal reasons or lack of accurate census data in manyareas of the world) In the first 5 years of thetwenty-first century four disastrous earthquakesoccurred in India Indonesia Iran and PakistanIn the twentieth century the average death tollcaused by earthquakes (and tsunamis they trig-gered) was about 16000 per year For the past7 years the yearly death toll was about 60000 ndashfour times higher than the average in the previouscentury In Fig 2 we extracted a portion of Fig 1 toillustrate the relationship between past earthquakesand population in India Pakistan northern Indo-nesia and adjoining regions We numbered thefour most recent disastrous earthquakes in Fig 2It is obvious that the large populations in IndiaIndonesia Iran Pakistan and their adjoiningregions (over 15 billion people) has been andwill continue to be adversely affected by earth-quakes Fatalities depend largely on resistance ofbuilding construction to shaking in addition topopulation density and earthquake occurrence

In recent decades population increases accel-erated urbanization and population concentrationalong coastal areas prone to earthquakes suggestthat many more earthquake-related fatalities willoccur unless effective steps are taken to minimizeearthquake and tsunami hazards

Earthquake MonitoringInstrumentation

Besides geodetic data (Feigl 2002) the primaryinstrumental data for the quantitative study ofearthquakes are seismograms records of groundmotion caused by the passage of seismic wavesSeismograms are written by seismographs instru-ments which detect and record ground motionalong with timing information A seismographconsists of three basic components (1) a seis-mometer which responds to ground motion andproduces a signal proportional to accelerationvelocity or displacement over a range of ampli-tudes and frequencies (2) a timing device

(3) either a local recording unit which writesseismograms on paper film or electronic storagemedia or more recently(4) a telemetry system fordelivering the seismograms to a central laboratoryfor recording Technical discussions of seismom-etry may be found for example in Wielandt(2002) and of seismic instruments in Havskovand Alguacil (2004) An overview of challengesin observational earthquake seismology is givenby Lee (2002) and a useful manual of seismolog-ical observatory practice is provided byBormann (2002)

An accelerograph is a seismograph designedto record on scale the acceleration time history ofstrong ground motions Measuring acceleration isimportant for studying response of buildings tostrong ground motions close to earthquakesMany modern sensitive seismographs arevelocigraphs recording the time history of groundvelocity They are designed to measure seismicwaves of small amplitudes (because seismic

waves attenuate quickly from their sources) eitherfrom small earthquakes nearby or from largeearthquakes that are far away

A seismic network (or an ldquoarrayrdquo) is a group ofseismographs ldquolinkedrdquo to a central headquartersNowadays the link is by various methods oftelemetry but in early days the links were bymail or telegrams or simply by manual collectionof the records When we speak of a seismic sta-tion we may mean an observatory with multipleinstruments in special vaults or a small instrumentpackage at a remote site

Seismographs were first developed in the latenineteenth century and individual seismographicobservatories (often a part of astronomical or mete-orological observatories) began earthquake moni-toring by issuing earthquake information in theirstation bulletins and other publications Howeverin order to accurately locate an earthquake datafrom several seismographic stations are necessaryIt was then natural for many governments to assume

Earthquake Monitoring and Early Warning Systems Fig 1 Location of deadly earthquakes around the world1500ndash2000 Population density is shown by the background colors See Utsu (2002a) for details

responsibility for monitoring earthquakes withintheir territories However because seismic wavesfrom earthquakes do not recognize national bound-aries the need for international cooperation became

clear In the following subsections we present anoverview of the history and results of earthquakemonitoring

Earthquake Monitoring and Early Warning Systems Table 1 Deadly Earthquakes Tsunamis from 1896ndash2005(Utsu (2002a) and recent sources)

Origin TimeYear MMDD HrMin (UTCexcept L = local)

Hypocenter Magnitude Location

Deaths(Approximate)

Lat(deg)

Lon(deg)

Depth(km)

2005 1008 350 34432 73573 10 76 PakistanKashmir

80361+

2004 1226 058 3298 95778 7 92 IndonesiaSumatra

283106+

2003 1226 156 29004 58337 15 66 Iran Bam 26000

2001 0126 316 23420 70230 16 77 IndiaGujaratBhuj

20000+

1990 0620 2100 37008 49213 18 74 Iran western 40000

1988 1207 741 40919 44119 7 68 ArmeniaSpitak

40000

1976 0727 1942 39605 117889 17 76 ChinaTangshan

242000

1976 0204 901 15298 89145 13 75 Guatemala 23000

1970 0531 2023 9248 78842 73 75 Peru 67000

1948 1005 2012 37500 58000 0 72 USSRAshgabat

65000

1939 1226 2357 39770 39533 35 77 TurkeyErzincan

33000

1939 0125 332 36200 72200 0 77 ChileChillian

28000

1935 0530 2132 28894 66176 35 81 PakistanQuetta

60000

1932 1225 204 39771 96690 25 76 ChinaGansu

70000

1927 0522 2232 37386 102311 25 77 ChinaTsinghai

100000

1923 0901 258 35405 139084 35 79 Japan Kanto 143000

1920 1216 1205 36601 105317 25 86 ChinaGansu

240000

1915 0113 652 42000 13500 0 69 ItalyAvezzano

33000

1908 1228 420 38000 15500 0 70 ItalyMessina

82000

1906 0817 040 33000 72000 0 82 ChileValparaiso

20000

1905 0404 050 33000 76000 0 81 IndiaKangra

20000

1896 0615 1932 L 39500 144000 0 82 JapanSanriku-oki

22000

ldquordquo denotes large uncertainties because a range of deaths had been reported ldquo+rdquo denotes a minimum value

Historical Developments

In 1897 John Milne designed the first inexpensiveseismograph which was capable of recording verylarge earthquakes anywhere in the world With asmall grant from the British Association for theAdvancement of Science (BAAS) a few otherdonations and his own money Milne managed todeploy about 30 of his instruments around theworld forming the first worldwide seismographicnetwork At the same time seismogram readingswere reported voluntarily to Milnersquos observatory atShide on the Isle of Wight England A globalearthquake summary with these seismogram read-ings was issued byMilne beginning in 1899 These

summaries are now known as the ldquoShide Circu-larsrdquo Milne also published progress and results inthe ldquoReports of the BAAS Seismological Commit-teerdquo from 1895 to 1913 A review of Milnersquos workand a reproduction of his publications as computerreadable files were given by Schweitzer and Lee(2003) and its attached CD-ROM After Milnersquosdeath in 1913 Herbert H Turner continuedMilnersquosefforts and in 1918 established publication of theInternational Seismological Summary (ISS)

The shortcomings of the Milne seismograph(low magnification no damping and poor timeresolution) were soon recognized Several improvedseismographs (notably the Omori Bosch-OmoriWiechert Galitzin and Milne-Shaw) were

Earthquake Monitoringand Early WarningSystems Fig 2 Locationof the four most deadlyearthquakes of the twenty-first century (up to the endof 2007) on a map showingthe location of the deadlyearthquakes from sixteenthto twentieth centuries(After Utsu (2002a) andTable 1)

developed and deployed in the first three decades ofthe twentieth century Figure 3 shows several ofthese classical seismographs (see Schweitzer andLee (2003) for further explanation) Although theISS provided an authoritative compilation arrival-time data of seismic waves and determinations ofearthquake hypocenters beginning in 1918 its short-comings were also evident These include

difficulties in collecting the available arrival-timedata around the world (which were submitted on avoluntary basis) and in the processing and analysisof data from many different types of seismographsRevolutions and wars during the first half of thetwentieth century frequently disrupted progress par-ticularly impacting collection and distribution earth-quake information

Earthquake Monitoring and Early Warning Systems Fig 3 Some classical seismographs a Milne b Bosch-Omori c Wiechert and d Galitzin (After Schweitzer and Lee (2003))

In the late 1950s attempts to negotiate a com-prehensive nuclear test ban treaty failed in partbecause of perceptions that seismic methods wereinadequate for monitoring underground nucleartests (Richards 2002) The influential Berknerreport of 1959 therefore advocated major supportfor seismology (Kisslinger and Howell 2003) As aresult the Worldwide Standardized SeismographNetwork (WWSSN) was created in the early 1960swith about 120 continuously recording stationslocated across much of the world except Chinaand the USSR (Oliver and Murphy 1971) Each

WWSSN station was equipped with identical setsof short-period and long-period three-componentseismographs and accurate chronometers Figure 4shows some of the equipment at aWWSSN stationincluding three-components of long-period seis-mometers long-period recording and test instru-ments and the time and power console A similarset of three-component short-period seismometersand recording and test instruments nearly identicalin appearance was also deployed at each stationSeismograms from the WWSSN were sent to theUnited States to be photographed on 70 mm film

Earthquake Monitoring and Early Warning SystemsFig 4 Some WWSSN station equipment a Three-component long-period seismometers installed on a seis-mic pier b Long-period recording and test instruments

and c Time and power console A similar set of three-component short-period seismometers and recordingtestinstruments is not shown

chips for distribution (about US$ 1 per chip as thensold to any interested person)

The WWSSN network is credited with makingpossible rapid progress in global seismology andwith helping to spark the plate tectonics revolu-tion of the late 1960s (Uyeda 2002) At about thesame time the Unified System of Seismic

Observations (ESSN) of the former USSR andits allied countries was established consisting ofalmost 100 stations equipped with Kirnos short-period 1ndash20 s displacement sensors and long-period seismographs

Samples of seismograms recorded on smokedpaper and photographic paper or film by analog

EarthquakeMonitoring and EarlyWarning Systems Fig 5 Some sample analog seismograms recorded on smokedpaper

seismographs are shown in Figs 5 and 6 Twoefforts to preserve and make such records availableonline are now underway the SeismoArchives(wwwiriseduseismo (Lee and Benson 2008))and Sismos (sismosrmingvit (Michelini et al2005))

With the establishment of the WWSSN theUnited States also assumed the task of monitoringearthquakes on a global scale beginning in theearly 1960s The mission of the US NationalEarthquake Information Center (NEIC now partof the USA Geological Survey) is ldquoto determine

Earthquake Monitoring and Early Warning Systems Fig 6 Some sample analog seismograms recorded onphotographic paper or film

rapidly the location and size of all destructiveearthquakes worldwide and to immediately dis-seminate this information to concerned nationaland international agencies scientists and the gen-eral publicrdquo (httpearthquakeusgsgovregionalneic)

In 1964 the ISS was reorganized as the Inter-national Seismological Centre (ISC) Since thenthe ISC (httpwwwiscacuk) has issued annualglobal earthquake catalogs with a time lag ofabout 2 years (Willemann and Storchak 2001)

Technical Considerations

To record seismic waves we must consider both theavailable technology for designing seismographs

and the nature of the Earthrsquos background noise(Webb 2002) The Earth is constantly in motionThis ldquobackgroundrdquo noise is usually classified aseither (1) microseisms which typically have fre-quencies below about 1 Hz are often the largestbackground signals and are usually caused by nat-ural disturbances (largely caused by ocean wavesnear shorelines) or (2) microtremors which havefrequencies higher than about 1 Hz and are due tohuman activities (such as traffic and machinery) andlocal natural sources (such as wind and movingvegetation) Ground motions from earthquakesvarymore than ten orders ofmagnitude in amplitudeand six orders ofmagnitude in frequency dependingon the size of the earthquake and the distance atwhich it is recorded Figure 7 illustrates the relativedynamic range of some common seismometers for

Earthquake Monitoring and Early Warning SystemsFig 7 Relative dynamic range of some common seis-mometers for global earthquake monitoring (Modified

from Fig 1 in Hutt et al (2002)) The Y-axis is marked indecibel (dB) where dB = 20 log (AA0) A is the signalamplitude and A0 is the reference signal amplitude

global earthquake monitoring A ldquolow Earth noiserdquomodel (Berger et al 2004 Peterson 1993) is thelower limit of Earthrsquos natural noise in its quietestlocations ndash it is desirable to have instruments that aresensitive enough to detect this minimal backgroundEarth signal In the analog instrument era (ie priorto about 1980) short-period and long-period seis-mometers were designed separately to avoid micro-seisms which have predominant periods of about6 s Short-period seismometers were designed todetect tiny ground motions from smaller nearbyearthquakes while long-period instruments weredesigned to recover the motions of distant largerearthquakes (ldquoteleseismsrdquo) Additionally strong-motion accelerometers generally recording directlyonto 70 mm-wide film strips were used to measurelarge motions from nearby earthquakes In todayrsquosmuch more capable digital instrumentation twomajor types of instruments are deployed (1) ldquobroad-bandrdquo seismometers which replace and improveupon both short-period and long-period seismome-ters and (2) strong-motion accelerometers for high-amplitude high-frequency seismic waves fromlocal earthquakes which often drive broadband seis-mometers off scale While rare examples of the oldanalog instruments are still in use the vast majorityof instruments presently operating are digital

In addition to having large variations in ampli-tudes and frequencies seismic waves from earth-quakes also attenuate rapidly with distance thatis they lose energy as they travel particularly athigher frequencies We must consider theseeffects in order to monitor seismic waveseffectively

In 1935 CF Richter introduced the concept ofmagnitude to classify local earthquakes by theirldquosizerdquo effectively the amount of energy radiatedat the actual rupture surface within the Earth Seethe entry by Bormann and Saul ldquoEarthquakeMagnituderdquo for a discussion of the various mag-nitude scales in use While every effort is made tomake these different scales overlap cleanly eachhas strengths and weaknesses that make one oranother preferable in a given situation Probablythe most general and robust of these methods iscalled a ldquomoment magnituderdquo symbolized asMWExisting instruments and environments are suchthat the smallest natural earthquakes we routinely

observe close by are about magnitude =1 Thelargest earthquake so far recorded by instrumen-tals is the MW = 95 Chilean earthquake in 1960In 1941 B Gutenberg and CF Richter discov-ered that over large geographic regions the rate ofearthquake occurrence is empirically related totheir magnitudes by

logN frac14 a b M (1)

where N is the number of earthquakes of magni-tude M or greater and a and b are numericalconstants It turns out that b is usually about 1which implies that M = 6 earthquakes are aboutten times more frequent than M = 7 earthquakesEngdahl and Villasenor (2002) show that there hasbeen an average of about 15 major (M⩾ 7) earth-quakes per year over the past 100 years and about150 large (M 6) earthquakes per year during thissame time interval Strong ground motions (above01 g in acceleration) over sizeable areas are gen-erated byM 6 earthquakes these are potentiallydamaging levels of ground shaking

Earthquakes are classified by magnitude (M) asmajor if M 7 as moderate to large if M rangesfrom 5 to 7 as small if M ranges from 3 to 5 asmicro if M lt 3 and as nano if M lt 0 Anearthquake with M 7 34 is often called greatand if M 9 mega

EarthquakeMonitoring in the Digital Era

Figure 8 shows the expected amplitudes of seis-mic waves by earthquake magnitude The topframe is a plot of the equivalent peak groundacceleration versus frequency The two heavycurves denote the ldquominimum Earth noiserdquo andthe ldquomaximum Earth noiserdquo (ie for seismo-graphic station located in the continental interiorversus near the coast)

The two domains of the WWSSN equipmentshort-period long-period seismometers are shownas gray shading The domains for two other instru-ments SRO (Seismic Research ObservatoriesSeismograph) and IDA (International Deploy-ment of Accelerometers) are also shown thesewere the early models of the current instruments

Earthquake Monitoring and Early Warning Systems Fig 8 Expected amplitudes of seismic waves by earthquakemagnitude See text for explanations

now in operation in the Global SeismographicNetwork (GSN) The bottom two frames indicateexpected amplitudes of seismic waves from earth-quakes of a range of magnitudes (we use themoment magnitudeMW) For simplicity we con-sider two cases (bottom left) global earthquakesrecorded at a large distance with a seismographicnetwork spaced at intervals of about 1000 km and(bottom right) local earthquakes recorded at shortdistances with a seismic array spaced at intervalsof about 50 km In the bottom left plot the global-scale network the expected amplitudes of P-waveand surface wave at 3000 km from the earthquakesource are shown for the bottom right plot a localseismic array the expected amplitudes of S-waveat 10 km and 100 km from the earthquake sourceare shown Seismologists use this and similarfigures in planning seismographic networksLocal noise surveys are usually conducted aswell when designing specific seismographicnetworks

With advances in digital technology earthquakemonitoring entered the digital era in the 1980sOlder analog equipment was gradually phased outas modern digital equipment replaced it (Hutt et al2002) The WWSSN was replaced by the GlobalSeismographic Network (GSN) a collaboration ofseveral institutions under the IRIS consortium(httpwwwirisedu) The goal of the GSN(httpwwwiriseduaboutGSNindexhtm) is ldquotodeploy over 128 permanent seismic recording sta-tions uniformly over the Earthrsquos surfacerdquo The GSNproject provides funding for two network opera-tors (1) the IRISASL Network Operations Centerin Albuquerque NewMexico (operated by the USGeological Survey) and (2) the IRISIDANetworkOperations Center in La Jolla California (operatedby personnel from the Scripps Institution of Ocean-ography) Components of a modern IRIS GSNseismograph system which include broadbandseismometers accelerometers and recordingequipment are shown in Fig 9

Figure 10 shows the station map of the GlobalSeismographic Network as of 2007 IRIS GSN sta-tions continuously record seismic data from verybroad band seismometers at 20 samples per second(sps) and also include high-frequency (40 sps) andstrong-motion (1 and 100 sps) sensors where

scientifically warranted It is the goal of the GSNproject to provide real-time access to its data viaInternet or satellite Since 1991 the IRIS DataMan-agement Center has been providing easy access tocomprehensive seismic data from the GSN andelsewhere (Ahern 2003)

Earthquake Monitoring Regional andLocal Networks

A major development in earthquake monitoringwas the establishment of seismographic networksoptimized to record the many frequent but smallerregional and local earthquakes occurring in manylocations To observe as many of these nearbyearthquakes as possible inexpensive seismo-graphs with high magnifications and lowdynamic-range telemetry are used to record thesmallest earthquakes feasible with current tech-nology and local background noise As a resultthe recorded amplitudes often overdrive theinstruments for earthquakes with M ≳ 3 withinabout 50 km of such seismographs This is not aserious defect since the emphasis for these net-works is to obtain as many first arrival times aspossible and to detect and to locate the maximumnumber of earthquakes Because seismic wavesfrom small earthquakes are quickly attenuatedwith increasing distance it is also necessary todeploy many instruments at small station spacing(generally from a few to a few tens of kilometers)and to cover as large a territory as possible in orderto record at least a few earthquakes every weekSince funding often is limited these local andregional seismic networks are commonly opti-mized for the largest number of stations ratherthan for the highest quality data

A Brief History

In the 1910s the Carnegie Institution ofWashington DC (CIW) was spending a greatdeal of money building the worldrsquos then largesttelescope (100 inch) at Mount Wilson Observa-tory southern California (Goodstein 1991) Sinceastronomers were concerned about earthquakes

that might disturb their telescopes Harry OWoodwas able to persuade CIW to support earthquakeinvestigations and as a result a regional networkof about a dozen Wood-Anderson seismographswas established in southern California in the1920s See Goodstein (1991) for the early historyleading to the establishment of the CaliforniaInstitute of Technology (Caltech) and its Seismo-logical Laboratory Astronomers played importantroles in getting seismic monitoring established invarious other regions of the world as well

Regional networks using different types ofseismographs were established in many countriesabout this time such as in Japan New Zealandand the USSR and its allies In the 1960s high-gain short-period telemetered networks weredeveloped to study microearthquakes To supportdetailed studies of local earthquakes and

especially for the purpose of earthquake predic-tion over 100 microearthquake networks wereestablished in various parts of the world by theend of the 1970s (Lee and Stewart 1981) Thesemicroearthquake networks comprised from tens tohundreds of short-period seismometers generallywith their signals telemetered into central record-ing sites for processing and analysis High mag-nification was achieved through electronicamplification permitting recording of very smallearthquakes (down toM= 0) though this came atthe expense of saturated records for earthquakesof M ≳ 3 within about 50 km Unfortunately thehope of discovering some sort of earthquake pre-cursor from the data obtained by these microearth-quake networks did not work out For a review ofthe earthquake prediction efforts please readKanamori (2003)

Earthquake Monitoring and Early Warning Systems Fig 9 Components of the IRIS-2 GSN System broadbandseismometers accelerometers and recording equipment

Some Recent Advances

Because of recent advances in electronics com-munications and microcomputers it is now pos-sible to deploy sophisticated digital seismographstations at global national regional and localscales for real-time seismology (Kanamori et al1997) Many such networks including temporaryportable networks have been implemented inmany countries In particular various real-timeand near real-time seismic systems began opera-tion in the 1990s for example in Mexico(Espinosa-Aranda et al 1995) California (Geeet al 2003 Hauksson et al 2003) and Taiwan(Teng et al 1997) The Real-Time Data (RTD)system operated by the Central Weather Bureau(CWB) of Taiwan is based on a network of tele-metered digital accelerographs (Shin et al 2003)since 1995 this system has used pagers e-mail

and other techniques to automatically and rapidlydisseminate information about the hypocentermagnitude and shaking amplitude of felt earth-quakes (M ≳ 4) in the Taiwan region The disas-trous Chi-Chi earthquake (MW = 76) of20 September 1999 caused 2471 deaths and totaleconomic losses of US$ 115 billion For thisearthquake sequence the RTD system deliveredaccurate information to government officials102 s after the origin time of the main shock(about 50 s for most aftershocks) and proved tobe useful in the emergency response of the Taiwangovernment (Goltz et al 2001 Wu et al 2000)

Recording Damaging Ground Shaking

Observing teleseisms on a global scale with sta-tion spacing of several hundreds of kilometers

Earthquake Monitoring and Early Warning Systems Fig 10 Station map of the Global Seismographic Network(GSN) as of 2007

does not yield critical information about near-source strong ground shaking required for earth-quake structural engineering purposes and seis-mic hazard reduction Broadband seismometerswhich are optimized to record earthquakes at greatdistances do not perform well in the near-field ofa major earthquake For example during the 1999Chi-Chi earthquake the nearest broadband stationin Taiwan (epicentral distance of about 20 km)was badly overdriven recorded no useful databeyond the arrival time of the initial P-wave andfinally failed about 1 min into the shock

A regional seismic network with station spacingof a few tens of kilometers cannot do the job eitherthe station spacing is still too large and the recordsare typically overdriven for earthquakes of M ≳ 3(any large earthquake would certainly overdrivethese sensitive instruments in the entire network)In his account of early earthquake engineeringHousner (2002) credited John R Freeman an emi-nent engineer with persuading the then US Secre-tary of Commerce to authorize a strong-motionprogram and in 1930 the design of anaccelerograph for engineering purposes In a letterto RR Martel Housnerrsquos professor at CaltechFreeman wrote

I stated that the data which had been given tostructural engineers on acceleration and limits ofmotion in earthquakes as a basis for their designswere all based on guesswork that there had neveryet been a precise measurement of accelerationmade That of the five seismographs around SanFrancisco Bay which tried to record the earthquakeof 1906 not one was able to tell the truth

Strong-motion recordings useful to engineersmust be on-scale for damaging earthquakes andin particular from instruments located on or nearbuilt structures in densely urbanized environ-ments within about 25 km of the earthquake-rupture zone for sites on rock or within about100 km for sites on soft soils Recordings ofmotions sufficient to cause damage at sites atgreater distances are also of interest for earth-quake engineering in areas likely to be affectedby major subduction-zone earthquakes and inareas with exceptionally low attenuation rates(Borcherdt 1997) In addition densely-spacednetworks of strong-motion recorders are needed

to study the large variations in these motions overshort distances (Evans et al 2005 Field andHough 1997)

Although several interesting accelerogramswere recorded in southern California in the 1930sand 1940s most seismologists did not pursuestrong-motion monitoring until much later The1971 San Fernando earthquake emphatically dem-onstrated the need for more strong-motion data(Anderson 2003) Two important programsemerged in the United States ndash the NationalStrong-Motion Program (httpnsmpwrusgsgov)and the California Strong Motion InstrumentationProgram (httpdocinet3consrvcagovcsmip)However the budgets for these programs wereand continue to be small in comparison to otherearthquake programs High levels of funding forstrong-motion monitoring comparable to that ofthe GSN and the regional seismic networksbecame available in Taiwan in the early 1990sand in Japan in the mid-1990s The Consortium ofOrganizations for Strong-Motion ObservationSystems (httpwwwcosmos-eqorg) wasestablished recently to promote the acquisitionand application of strong-motion data

Seismograms and Derived Products

Even before instruments were developed to recordseismic waves from earthquakes many scholarscompiled catalogs of earthquake events noted inhistorical and other documents Robert Mallet in1852ndash1854 published the first extensive earthquakecatalog of theworld (1606BCndashAD 1842) totaling6831 events (Mallet 1858) Based on this compila-tion Mallet prepared the first significant seismicitymap of the Earth in 1858 Malletrsquos map is remark-able in that it correctly identifies the major earth-quake zones of the Earth excepting for parts of theoceans Although Malletrsquos earthquake catalog andsimilar compilations contain awealth of informationabout earthquakes they were made without the aidof instruments and thus were subject to the biases ofthe observers as well as to population distributionsThese non-instrumental earthquake catalogs alsocontain errors because the source materials werecommonly incomplete and inconsistent regarding

date time place names and reported damageAmbraseys et al (2002) discusses these difficultiesfor a regional case and Guidoboni (2002) addressesthe matter in general

Today seismograms are the fundamental dataproduced by earthquake monitoring An analystrsquosfirst task is to find out when and where the earth-quakes occurred its size and other characteris-tics The accuracy of determining earthquakeparameters as well as the number of parametersused to characterize an earthquake has progressedalong with the availability of seismograms andcomputers as well as advances in seismology Inthe analog era earthquake parameters were pri-marily the origin time geographical location(epicenter) focal depth and magnitude A list ofthese parameters for earthquakes occurring oversome time interval is called an earthquake cata-log A useful and common illustration of suchresults is a map showing the locations of earth-quakes by magnitude (a seismicity map)Figure 11 is such a seismicity map for1900ndash1999 as prepared by Engdahl andVillasensor (2002) The map shows that moderateand large earthquakes are concentrated in tectonicactive areas while most areas of the Earth areaseismic

Earthquake Location

Several methods have been developed to locateearthquakes (ie determine origin time latitudeand longitude of the epicenter and focal depth)Common to most of these methods is the use ofarrivals times of initial P- and S-waves In partic-ular Geiger (1912) applied the Gauss-Newtonmethod to solve for earthquake location whichis a nonlinear problem by formulating it as aninverse problem However since Geigerrsquos methodis computational intensive it was not practical toapply it for the routine determinations of earth-quake hypocenters until the advance of moderncomputers in the early 1960s

Before computers became widely availablestarting in the 1960s earthquakes were usuallylocated by a manual graphical method In any

location method we assume that an earthquakeis a point source and its sole parameters are origintime (time of occurrence to) and hypocenter posi-tion (xo yo zo) If both P- and S-arrival times areavailable one may use the time intervals betweenP- and S-waves at each station (S-P times) andestimates of seismic wave velocities in the Earthto obtain a rough estimate of the epicentral dis-tance D from that station

D frac14 V PV S= V P V Seth THORNfrac12 TS TPeth THORN (2)

where VP is the P-wave velocity VS the S-wavevelocity TS the S-wave arrival time and TP theP-wave arrival time For a typical crustal P-wavevelocity of 6 kms and VPVS 18 the distanceD in kilometers is about 75 times the S-P intervalmeasured in seconds If three or more epicentraldistances are available the epicenter may beplaced at the intersection of circles with the sta-tions as centers and the appropriateD as radii Theintersection will seldom be a point and its arealextent gives a rough estimate of the uncertainty ofthe epicenter and hypocentral (focal) depth In theearly days the focal depth was usually assumed oroccasionally determined using a ldquodepth phaserdquo(generally a ray that travels upward from thehypocenter and reflects back from the Earthrsquossurface then arcs through the Earth to reach adistant seismograph)

Although Geiger (1912) presented a method fordetermining the origin time and epicenter themethod can be extended easily to include focaldepth To locate an earthquake using a set of arrivaltimes tk from stations at positions (xk yk zk)k= 1 2 m wemust assume amodel of seismicvelocities from which theoretical travel times T kfor a trial hypocenter at (x y z) to the stationscan be computed Let us consider a given trialorigin time and hypocenter as the trial vector w

in a four-dimensional Euclidean space

w frac14 txyzeth THORNT (3)

where the superscript T(T) denotes the vectortranspose Theoretical arrival time tk from w tothe k-th station is the theoretical travel time Tk

plus the trial origin time t We now define thearrival time residual at the k-th station rk as thedifference between the observed and the theoret-ical arrival times We may consider this set ofstation residuals as a vector in an m-dimensionalEuclidean space and write

r frac14 r1 weth THORN r2 weth THORN rm weth THORNeth THORNT (4)

We now apply the least squares method toobtain a set of linear equations solving for anadjustment vector dw

ATAdx frac14 ATr (5)

where A is the Jacobian matrix consisting of par-tial derivatives of travel time with respect to t x yand z A detailed derivation of the Geiger methodis given by Lee and Stewart (see pp 132ndash134 inLee and Stewart (1981)) There are many practicaldifficulties in implementing Geigerrsquos method forlocating earthquakes as discussed by Lee andStewart (see pp 134ndash139 in Lee and Stewart(1981)) Although standard errors for these earth-quake locations can be computed they are often

not meaningful because errors in the measurementof arrival times usually do not obey a Gaussianprobability distribution In recent years manyauthors applied various nonlinear methods tolocate earthquakes a review of these methods isgiven by Lomax et al ldquoEarthquake LocationDirect Global-Search Methodsrdquo

Earthquake Magnitude

After an earthquake is located the next question ishow big was it The Richter magnitude scale wasoriginally devised to measure the ldquosizerdquo of an earth-quake in southern California Richter (1935) definedthe local (earthquake) magnitude ML of an earth-quake observed at any particular station to be

ML frac14 logA logA0 Deth THORN (6)

where A is the maximum amplitude in millimetersas recorded by aWood-Anderson seismograph foran earthquake at epicentral distance of D km Thecorrection factor logA0(D) is the maximumamplitude at D km for a ldquostandardrdquo earthquake

Earthquake Monitoring and Early Warning Systems Fig 11 Seismicity of the Earth 1900ndash1999 (see Engdahl andVillasenor (2002) for details)

Thus three arbitrary choices enter into the defini-tion of local magnitude (1) the use of the Wood-Anderson seismographs (2) the use of the com-mon logarithm (ie the logarithm to the base 10)and (3) the selection of the standard earthquakewhose amplitudes as a function of distance D arerepresented by A0(D)

In the 1940s B Gutenberg and CF Richterextended the local magnitude scale to includemore distant earthquakes Gutenberg (1945)defined the surface-wave magnitude MS as

MS frac14 log A=Teth THORN logA0 Doeth THORN (7)

where A is the maximum combined horizontalground displacement in micrometers (mm) for sur-face waves with a period of 20 s and logA0 istabulated as a function of epicentral distance D indegrees in a similar manner to that for the localmagnitudersquos A0 (D) Specifically surface-wavemagnitude is calculated from

MS frac14 logAthorn 1656 logDthorn 1818 (8)

using the prominent 20 s period surface wavescommonly observed on the two horizontal-component seismograms from earthquakes ofshallow focal depth

Both magnitude scales were derived empiri-cally and have scale-saturation problems egfor very large earthquakes above a certain sizethe computed magnitudes of a particular type areall about the same After the pioneering work ofCharles F Richter and Beno Gutenberg numer-ous authors have developed alternative magnitudescales as reviewed recently by Utsu (2002b) andby Bormann and Saul ldquoEarthquake Magni-tuderdquo A current magnitude scale widely acceptedas ldquobestrdquo (as having the least saturation problemand being a close match to an earthquakersquos totalrelease of stress and strain) is the ldquomoment mag-nituderdquo MW computed from an earthquakersquosldquomoment tensorrdquo

Quantification of the Earthquake Source

As pointed out by Kanamori (1978) it is not asimple matter to find a single measure of theldquosizerdquo of an earthquake simply because earth-quakes result from complex physical processesThe elastic rebound theory of Harry F Reid sug-gests that earthquakes originate from spontaneousslippage on active faults after a long period ofelastic strain accumulation (Reid 1910) Faultsmay be considered the slip surfaces across whichdiscontinuous displacement occurs in the Earthwhile the faulting process may be modeled math-ematically as a shear dislocation in an elasticmedium (see Savage (1978) for a review)A shear dislocation (or slip) is equivalent to adouble-couple body force (Burridge and Knopoff1964 Maruyama 1963) The scaling parameter ofeach component couple of a double-couple bodyforce is its moment Using the equivalencebetween slip and body forces Aki (1966) intro-duced the seismic moment M0 as

M 0 frac14 methD Aeth THORNdA frac14 msA (9)

where m is the shear modulus of the medium A isthe area of the slipped surface or source area ands is the slip D(A) averaged over the area A If anearthquake produces surface faulting we mayestimate its rupture length L and its averageslip s from measurement of that faulting Thearea A may be approximated by Lh where h isthe focal depth (it is often but not always foundthat the hypocenter is near the bottom of therupture surface) A reasonable estimate for m is3 1011 dynescm2 With these quantities wecan estimate the seismic moment from Eq (9)

Seismic moment also can be estimated inde-pendently from seismograms From dislocationtheory the seismic moment can be related to thefar-field seismic displacement recorded by seis-mographs For example Hanks and Wyss (1972)showed that

M 0 frac14 O0=cyf

4ppRu3 (10)

where O0 is the long-period limit of the displace-ment spectrum of either P or S waves cy f is afunction accounting for the body-wave radiationpattern r is the density of the medium R is afunction accounting for the geometric spreadingof body waves and v is the body-wave velocitySimilarly seismic moment can be determinedfrom surface waves or coda waves (Aki 19661969)

In 1977 Hiroo Kanamori recognized that anew magnitude scale can be developed using seis-mic moment (M0) by comparing the earthquakeenergy and seismic moment relation

ES frac14 Ds=2meth THORNM 0 (11)

where Ds is the stress drop and m is the shearmodulus with the surface-wave magnitude andenergy relation (Gutenberg and Richter 1956)

logES frac14 15MS thorn 118 (12)

where ES and M0 are expressed in ergs and dyne-cm respectively The average value of (Ds2m) isapproximately equal to 10 104 If we use thisvalue in Eq (11) we obtain

logM 0 frac14 15MS thorn 161 (13)

It is known that MS values saturate for greatearthquakes (M0 about 10

29 dyne cm or more)and therefore that Eqs (12) and (13) do not holdfor such great earthquakes If a new moment-magnitude scale using the notation MW is definedby

logM 0 frac14 15MW thorn 161 (14)

thenMW is equivalent toMS below saturation andprovides a reasonable estimate for great earth-quakes without the saturation problem(Kanamori 1977) The subscript letter W standsfor the work at an earthquake fault but soon MW

became known as the moment magnitude Deter-mining earthquake magnitude using seismic

moment is clearly a better approach because ithas a physical basis

The concept of seismic moment led to thedevelopment of moment tensor solutions forquantifying the earthquake source including itsfocal mechanism (Gilbert 1971 Gilbert andDziewonski 1975) the seismic moment is justthe scalar value of the moment tensor Since the1980s Centroid-Moment-Tensor (CMT) solu-tions have been produced routinely for eventswith moment magnitudes (MW) greater thanabout 55 The CMT methodology is describedby Dziewonski et al (1981) and Dziewonski andWoodhouse (1983a) a comprehensive review isgiven in Dziewonski and Woodhouse (1983b)These CMT solutions are published yearly in thejournal Physics of the Earth and Planetary Inte-riors and the entire database is accessible onlineThis useful service is now performed by theGlobal CMT Project (httpwwwglobalcmtorg)and more than 25000 moment tensors have beendetermined for large earthquakes from 1976 to2007 In the most recent decade Quick CMTsolutions (Ekstroumlm 1994) determined in near-realtime have been added and are distributed widelyvia e-mail (httpwwwseismologyharvardeduprojectsCMTQuickCMTs)

Limitations of Earthquake Catalogs

In addition to international efforts to catalog earth-quakes on a global scale observatories and gov-ernment agencies issue more-detailed earthquakecatalogs at local regional and national scalesHowever earthquake catalogs from local to globalscales vary greatly in spatial and temporal coverageand in quality with respect to completeness andaccuracy because of the ongoing evolution ofinstrumentation data processing procedures andagency staff An earthquake catalog to be usedfor research should have at least the followingsource parameters origin time epicenter (latitudeand longitude) focal depth and magnitude

The International Seismological Summary andits predecessors provided compilations of arrivaltimes and locations of earthquakes determinedmanually from about 1900 to 1963 Despite their

limitations (notably the lack of magnitude esti-mates) these materials remain valuable The firstglobal earthquake catalog that contains both loca-tions and magnitudes was published by Guten-berg and Richter in 1949 and was followed by asecond edition in 1954 (Gutenberg and Richter1954) This catalog contains over 4000 earth-quakes from 1904 to 1951 Unfortunately its tem-poral and spatial coverage is uneven as a result ofrapid changes in seismic instrumentation and ofthe interference of both World Wars Neverthe-less the procedures used for earthquake locationand magnitude estimation were the same through-out using the arrival-time and amplitude dataavailable to Gutenberg and Richter during the1940s and early 1950s

Since 1964 the International SeismologicalCentre has performed systematic cataloging ofearthquakes worldwide by using computers andmore modern seismograph networks The spatialcoverage of this catalog is not complete for someareas of the Earth (especially the oceans) becauseof the paucity of seismographic stations in suchareas By plotting the cumulative numbers ofearthquakes above a certain magnitude versusmagnitude and using Eq (1) the lower limit ofcompleteness of an earthquake catalog may beestimated ndash it is the magnitude below which thedata deviate below a linear fit to Eq (1)

A Centennial Earthquake Catalog coveringISS- and ISC-reported global earthquakes from1900ndash1999 was generated using an improvedEarth model that takes into account regional var-iations in seismic wave velocities in the Earthrsquoscrust and upper mantle (Engdahl and Villasenor2002 Villasenor and Engdahl 2007) Engdahl andVillasenor (2002) also compiled existing magni-tude data from various authors and suggestedpreferred values However these ldquopreferred mag-nitudesrdquo were not determined by the same pro-cedures At present the Global CMT Project(httpwwwglobalcmtorg) provides the mostcomplete online source parameters for globalearthquakes (withMWgt 55) including Centroid-Moment-Tensor solutions Although the CMTcatalog starts in 1976 the improved global

coverage of modern broadband digital seismo-graphs began only in about 1990

In summary earthquake catalogs have been usedextensively for earthquake prediction research andseismic hazard assessment since the first such cata-log was produced Reservations have beenexpressed about the reliability of the results andinterpretations from these studies because the cata-logs cover too little time and have limitations incompleteness and accuracy (both random and sys-tematic) Nevertheless advances have been made inusing earthquake catalogs to (1) study the nature ofseismicity (eg ldquoSeismicity Critical States ofFromModels to Practical Seismic Hazard EstimatesSpacerdquo) (2) investigate earthquake statistics (eg ldquoEarthquake Occurrence and Mechanisms Sto-chastic Models forrdquo) (3) forecast earthquakes (eg ldquoEarthquake Forecasting and Verificationrdquo)(4) predict earthquakes (eg ldquoGeo-complexityand Earthquake Predictionrdquo) and (5) assess seismichazards and risk and so forth

Earthquake Early Warning (EEW)Systems

With increasing urbanization worldwide earth-quake hazards pose ever greater threats to livesproperty and livelihoods in populated areas nearmajor active faults on land or near offshore sub-duction zones Earthquake early-warning systemscan be useful tools for reducing the impact ofearthquakes provided that cities are favorablylocated with respect to earthquake sources andtheir citizens are properly trained to respond tothe warning messages Recent reviews of earth-quake early warning systems may be found in Leeand Espinosa-Aranda (2003) Kanamori (2005)and Allen (2007) as well as a monograph on thesubject by Gasparini et al (2007)

Under favorable conditions an EEW systemcan forewarn an urban area of impending strongshaking with lead times that range from a fewseconds to a few tens of seconds A lead time isthe time interval between issuing a warning andthe arrival of the S-waves which are the most

destructive seismic waves Even a few seconds ofadvanced warning is useful for preprogrammedemergency measures at various critical facilitiessuch as the deceleration of rapid-transit vehiclesand high-speed trains the orderly shutoff ofgas pipelines the controlled shutdown of somehigh-technological manufacturing operations thesafe-guarding of computer facilities (eg disk-head parking) and bringing elevators to a stop atthe nearest floor

Physical Basis and Limitations of EEWSystems

The physical basis for earthquake early warning issimple damaging strong ground shaking iscaused primarily by shear (S) and subsequentsurface waves both of which travel more slowlythat the primary (P) waves and seismic wavestravel much more slowly than electromagneticsignals transmitted by telephone or radio How-ever certain physical limitations must be consid-ered as shown by Fig 12

Figure 12 is a plot of the travel time for theP-wave and S-wave as a function of distance froman earthquake We make the following assump-tions about a typical destructive earthquake(1) focal depth at ~20 km (2) P-wave velocity~8 kms and (3) S-wave velocity ~45 kms If

an earthquake is located 100 km from a city theP-wave arrives at the city after about 13 s and theS-waves in about 22 s (Fig 12) If we deploy adense seismic network near the earthquake sourcearea (capable of locating and determining the sizeof the event in about 10 s) we will have about 3 sto issue the warning before the P-wave arrives andabout 12 s before the more destructive S-wavesand surface waves arrive at the city We haveassumed that it takes negligible time to send asignal from the seismic network to the city viaelectromagnetic waves which travel at about one-third the velocity of light or faster (between about100000 and 300000 kms depending on themethod of transmission)

From Fig 12 it is clear that this strategy maywork for earthquakes located at least about 60 kmfrom the urban area For earthquakes at shorterdistances (~20 to ~60 km) we must reduce thetime needed to detect the event and issue a warn-ing to well under 10 s This requirement impliesthat we must deploy a very dense seismic networkvery close to the fault and estimate the necessaryparameters very fast However such dense net-works are not economical to deploy using existingseismic instruments

For earthquakes within 20 km of a city there islittle one can do other than installing motion-sensitive automatic shut-off devices at criticalfacilities (natural gas for example) and hope thatthey are either very quick when responding toS-waves or are triggered by the onset of theP-wave Normally an earthquake rupture morethan ~100 km from an urban area does not com-monly pose a large threat (seismic waves wouldbe attenuated and spread out farther) There areexceptions caused either by unusual local siteconditions such as Mexico City or by earth-quakes with large rupture zones which thereforeradiate efficiently to greater distances

Design Considerations for EEW Systems

In the above discussion we have assumed that oneimplements an earthquake early warning systemwith a traditional seismic network Such EEWsystems have limitation as illustrated by Fig 13

Earthquake Monitoring and Early Warning SystemsFig 12 Travel time of P-waves and of S-waves versusdistance for a typical earthquake

which shows the expected early warning times fora repeat of the 1999 Chi-Chi earthquake HoweverNakamura and his colleagues have been successfulin applying a single-station approach (Nakamura1988 Saita and Nakamura 2003) where seismicsignals are recorded and processed locally by theseismograph and an earthquake warning is issuedwhenever ground motions there exceed some trig-ger threshold We will next discuss these two basicapproaches regional versus on-site in designing anearthquake early warning system

Earthquake early warning capability can beimplemented through a rapid reporting system(RRS) from a traditional network assuming real-time telemetry into the networkrsquos central labora-tory This type of system provides to populatedareas and other sensitive locations primary eventinformation (hypocenter magnitude groundshaking intensities and potential damage) about

1 min after the earthquake begins The RRS trans-mits this critical information electronically toemergency response agencies and other interestedorganizations and to individuals Each recipientcan then take action (some of which may be pre-programmed) shortly after the earthquake beginsResponse measures can include the timely dis-patch of rescue equipment and emergency sup-plies to the likely areas of damage

Californiarsquos ShakeMap (Wald et al 1999a b)Taiwanrsquos CWB and Japanrsquos JMA systems are typ-ical examples of RSS In the case of the TaiwanRRS the CWB has since 1995 provided intensitymaps hypocenters and magnitudes within 1 minof the occurrence ofMgt 4 earthquakes (Teng et al1997 Wu et al 1997) This systemrsquos reliabilitydocumented by electronic messages to governmentagencies and scientists has been close to perfectparticularly for large damaging earthquakesFigure 14 shows a block diagram of the TaiwanRRS and details may be found in Wu et al (1997)

Using a set of empirical relationships derivedfrom the large data set collected during the 1999Chi-Chi earthquake CWB now releases within afewminutes of an event the estimated distributionsof PGA and PGV refined magnitudes and damageestimates (Wu et al 2007a) This near-real-timedamage assessment is useful for rapid post-disasteremergency response and rescue missions

Regional Warning Versus OnsiteWarning

Two approaches have been adopted for earthquakeearly warning systems (1) regional warning and(2) on-site warning The first approach relies ontraditional seismological methods in which datafrom a seismic network are used to locate an earth-quake determine the magnitude and estimate theground motion in the region involved In the sec-ond approach the initial ground motions (mainlyP wave) observed at a site are used to predict theensuing ground motions (mainly S and surfacewaves) at the same site

The regional approach is more comprehensivebut takes a longer time to issue an earthquakewarning An advantage of this approach is that

Earthquake Monitoring and Early Warning SystemsFig 13 Expected EWS early warning times (indicated bycircles) in Taiwan with respect to the occurrence of anevent similar to the Chi-Chi earthquake of 20 September1999 Triangles are locations of elementary schools whichcan be regarded as a good indicator for the populationdensity of Taiwan

EarthquakeMonitoring and Early Warning Systems Fig 14 A block diagram showing the hardware of the TaiwanEarthquake Rapid Reporting System

estimates of the timing of expected strong motionsthroughout the affected region can be predictedmore reliably The early warning system in Taiwanis a typical example and it uses a regional warningsystem called virtual subnetwork approach (VSN)that requires an average of 22 s to determine earth-quake parameters with magnitude uncertainties of025 It provides a warning for areas beyondabout 70 km from the epicenter (Fig 13) Thissystem has been in operation since 2002 withalmost no false alarms (Wu et al 2007a) Withthe advancement of new methodology and moredense seismic networks regional systems arebeginning to be able to provide early warnings toareas closer to the earthquake epicenter

The regional approach has also been used inother areas The method used in Mexico(Espinosa-Aranda et al 1995) is slightly differentfrom the traditional seismological method It is aspecial case of EEW system due to the relativelylarge distance (about 300 km in this case) betweenthe earthquake source region (west coast of Cen-tral America) and the warning site (Mexico City)However the warning is conceptually ldquoregionalrdquo

In Japan various EEW techniques have beendeveloped and deployed by the National ResearchInstitute for Earth Science and Disaster Preven-tion (NIED) and Japan Meteorological Agency(JMA) since 2000 (Horiuchi et al 2005Kamigaichi 2004 Odaka et al 2003) ldquoTsu-nami Forecasting and Warningrdquo In particularJMA has started sending early warning messagesto potential users responsible for emergencyresponses (Hoshiba et al 2008) The potentialusers include railway systems construction com-panies and others and they are familiar with theimplications of early warning messages as well asthe technical limitations of EEW (Kamigaichi2004)

Some Recent EEW Advances

Allen and Kanamori (2003) proposed the Earth-quake Alarm System (ElarmS) to issue an earth-quake warning based on information determinedfrom the P-wave arrival only Kanamori (2005)extended the method of Nakamura (1988) and

Allen and Kanamori (2003) to determine a periodparameter tc from the initial 3 s of the P wave tcis defined as

tc frac14 2p=ffiffir

p(15)

where

r frac14ETH t00 _u2 teth THORN dtETH t00 u2 teth THORN dt

(16)

u(t) is the ground-motion displacement t0 is theduration of record used (usually 3 s) and tcwhich represents the size of an earthquake canbe computed from the incoming data sequentially

The tc method was used for earthquake earlywarning in southern California Taiwan andJapan by Wu and Kanamori (2005a b 2008)and Wu et al (2007b) At a given site the mag-nitude of an event is estimated from tc and thepeak ground-motion velocity (PGV) from Pd (thepeak amplitude of displacement in the first 3 safter the arrival of the P wave) The incomingthree-component signals are recursivelyconverted to ground acceleration velocity anddisplacement The displacements are recursivelyfiltered using an accusal Butterworth high-passfilter with a cutoff frequency of 0075 Hz and aP-wave threshold trigger is constantly moni-tored When a trigger occurs tc and Pd are com-puted The relationships between tc andmagnitude (M) and Pd and peak ground velocity(PGV) for southern California Taiwan andJapan were investigated Figure 15 shows agood correlation between tc and MW from theK-NET records in Japan and Fig 16 shows thePd versus PGV plot for southern California Tai-wan and Japan These relationships may be usedto detect the occurrence of a large earthquake andprovide onsite warning in the area immediatelyaround the station where the onset of strongground motion is expected within a few secondsafter the arrival of the P-wave When the stationdensity is high the onsite warning methods maybe applied to data from multiple stations toincrease the robustness of an onsite early warn-ing and to complement the regional warning

approach In an ideal situation such warningswould be available within 10 s of the origintime of a large earthquake whose subsequentground motion may last for tens of seconds

Wu and Zhao (2006) investigated the attenu-ation of Pd with the hypocentral distance R insouthern California as a function of magnitudeM and obtained the following relationships

MPd frac14 4748thorn 1371 log Pdeth THORN thorn 1883 log Reth THORN (17)

and

log Pdeth THORN frac14 3463thorn 0729M 1374 log Reth THORN (18)

For the regional warning approach when anearthquake location is determined by the P-wavearrival times at stations close to the epicenter thisrelationship can be used to estimate the earth-quake magnitude Their result shows that forearthquakes in southern California the Pd

Earthquake Monitoringand Early WarningSystems Fig 15 tcestimates from 20 eventsusing the nearest sixstations of the K-NETSmall open circles showsingle-record results andlarge circles show event-average values with onestandard deviation barsSolid line shows the leastsquares fit to the event-average values and the twodashed lines show the rangeof one standard deviation

Earthquake Monitoring and Early Warning SystemsFig 16 Relationship between peak initial displacementamplitude (Pd) measurements and peak ground velocity(PGV) for the records with epicentral distances less than30 km from the epicenter in Southern California (red solidcircles) Taiwan (blue diamonds) and Japan (black solidtriangles) Solid line shows the least squares fit and the twodashed lines show the range of one standard deviation

magnitudes agree with the catalog magnitudeswith a standard deviation of 018 for events lessthan magnitude 65 They concluded that Pd is arobust measurement for estimating the magni-tudes of earthquakes for regional early warningpurposes in southern California This method hasalso applied to Italian region by Zollo et al (2006)with a very good performance

Because the on-site approach is faster than theregional approach it can provide useful earlywarning to sites at short distances from the earth-quake epicenter where early warning is mostneeded Onsite early warning can be generatedby either a single station or by a dense array Fora single station operation signals from P-wavesare used for magnitude and hypocenter determi-nation to predict strong ground shakingNakamura (1984) first proposed this conceptdeveloped the Urgent Earthquake Detection andAlarm System or UrEDAS (Nakamura and Saita2007) and introduced a simple strong-motionindex for onsite EEW (Nakamura 2004) How-ever the reliability of on-site earthquake informa-tion is generally less than that obtained with theregional warning system There currently is atrade-off between warning time and the reliabilityof the earthquake information Generally aninformation updating procedure is necessary forany EEW system On-site warning methods canbe especially useful in regions where a denseseismic network is deployed

The Japan Meteorological Agency (JMA)began distribution of earthquake early warninginformation to the public in October 1 2007through several means such as TV and radio(Hoshiba et al 2008) (httpwwwjmagojpjmaenActivitieseewhtml) The JMA system was suc-cessfully activated during the recent Noto Hantoand Niigata Chuetsu-Oki earthquakes in 2007 andprovided accurate information of hypocenter mag-nitude and intensity about 38 s after the arrival ofP-waves at nearby stations The warning messagereached sites further than about 30 km from theepicenter as an early warning alert (ie informationarrived before shaking started at the site) This is aremarkable performance of the system for damag-ing earthquakes and gives promise of an early

warning system as a practical means for earthquakedamage mitigation Although warning alert is mostneeded within 30 km of the epicenter it is notfeasible with the current density and configurationof the JMA network

Lawrence and Cochran (2007) proposed a col-laborative project for rapid earthquake responseand early warning by using the accelerometersthat are already installed inside many laptop com-puters Their Quake-Catcher Network (QCN) willemploy existing laptops which have accelerome-ters already installed and desktops outfitted withinexpensive (under $ 50) USB accelerometers toform the worldrsquos largest high-density distributedcomputing seismic network for monitoring strongground motions (httpqcnstanfordedu) Byfreely distributing the necessary software anyonehaving a computer with an Internet connectioncan join the project as a collaborative memberThe Quake-Catcher Network also has the poten-tial to provide better understanding of earth-quakes and the client-based software is alsointended to be educational with instructive mate-rial displaying the current seismic signal andorrecent earthquakes in the region It is an effectiveway to bring earthquake awareness to studentsand the general public

Future Directions

To be successful monitoring earthquakes requireslarge stable funding over a long period of timeThe most direct argument for governments tosupport long-term earthquake monitoring is tocollect scientific data for hazard mitigation Inthe past two decades about half a million ofhuman lives have been lost due to earthquakesand economic losses from earthquake damagetotal about $ 200 billion Future losses will beeven greater as rapid urbanization is taking placeworldwide For example the recent Japanese Fun-damental Seismic Survey and Observation Plan(costing several hundred million US dollars) is adirect response to the economic losses of about$ 100 billion due to the 1995 Kobe earthquake Inaddition to scientific and technological challenges

in monitoring earthquakes seismologists mustpay attention to achieve (1) stable long-termfunding (2) effective management and executionand (3) delivery of useful products to the users

Seismologists benefit greatly from scientificand technological advances in other fields Forexample Global Positioning Systems (GPS)open a new window for monitoring crustal defor-mation which is important to understand the driv-ing forces that generate earthquakes ( ldquoGPSApplications in Crustal Deformation MonitoringCrustal Deformation During the Seismic CycleInterpreting Geodetic Observations ofrdquo) Underthe US Earth Scope Program (httpwwwearthscopeorg) the Plate Boundary Observatory(PBO) is covering the western Northern Americaand Alaska with a network of high precision GPSand strain-meter stations in order to measuredeformation across the active boundary betweenthe Pacific and North America plates (httpwwwearthscopeorgobservatoriespbo) As the sam-pling rate of GPS data increases they can providetime histories of displacement during an earth-quake Monitoring earthquakes with multipletypes of instruments and sensors is now increas-ingly popular and ldquointegratedrdquo or ldquosuperrdquo stations

are increasingly common Figure 17 shows anexample of an integrated station (HGSD) in east-ern Taiwan Instruments deployed at the HGSDstation in eastern Taiwan include a broadband

seismometer a continuous GPS instrument astrain-meter and a six-channel accelerograph(Model K2 byKinemetrics) with an internal accel-erometer and a rotational sensor (Model R-1 byeentec) A digital seismogram recorded at theHGSD station from an earthquake (MW = 51)of July 23 2007 at a distance of 34 km is shown inFig 18 The importance of rotational seismologyand its current status are given in the Appendix

A radically different design of seismographicnetworks (and earthquake early warning system inparticular) is now possible using the ldquoSensor Net-workrdquo developed by Intel Research Intel is work-ing with the academic community and industrycollaborators to actively explore the potential ofwireless sensor networks This research is alreadydemonstrating the potential for this new technol-ogy to enhance public safety reduce the cost ofdoing business and bring a host of other benefitsto business and society (httpwwwintelcomresearchexploratorywireless_sensorshtm)

Earthquake Monitoring and Early Warning SystemsFig 17 Instruments deployed at the HGSD station ineastern Taiwan Clockwise from the top (1) A broadbandseismometer (Model CMG-3TB) installed at a depth of100 m) (2) A continuous GPS instrument (3) A strain-

meter installed at a depth of 210 m) (4) A Model Q3306-channel recorder with an accelerometer (ModelEpiSensor) and a short-period seismometer (Model L2)and (5) A Model K2 6-channel accelerograph with aninternal accelerometer and a rotational sensor (Model R-1)

It has been very difficult historically to obtainadequate and stable funding for long-term earth-quake monitoring largely because disastrousearthquakes occur infrequently Since there aremany pressing problems facing modern societiesalmost all governments react to earthquake (and

tsunami) disasters only after the fact and eventhen for relatively short periods of time Toadvance earthquake prediction research and todevelop effective earthquake warning systemswill require continuous earthquake monitoringwith extensive instrumentations in the near-field

Earthquake Monitoring and Early Warning SystemsFig 18 A digital seismogram recorded at the HGSDstation from an earthquake (MW = 51) of July23 2007 at a distance of 34 km Top frame three-

component translational accelerations Bottom framethree-component rotation velocity motions N = North-South V = Vertical and E = East-West

for decades and even centuries Therefore inno-vative approaches must be developed and perse-verance is needed

Appendix A Progress Report onRotational Seismology

Seismology is based primarily on the observationand modeling of three orthogonal components oftranslational ground motions Although effects ofrotational motions due to earthquakes have longbeen observed (eg Mallet 1862) Richter (seep 213 in Richter (1958)) stated that

Perfectly general motion would also involve rota-tions about three perpendicular axes and threemore instruments for these Theory indicates andobservation confirms that such rotations arenegligible

However Richter provided no references forthis claim and the available instruments at thattime did not have the sensitivity to measure thevery small rotation motions that the classical elas-ticity theory predicts

Some theoretical seismologists (e g Aki andRichards 1980 2002) and earthquake engineershave argued for decades that the rotational part ofground motions should also be recorded It is wellknown that standard seismometers and accelerom-eters are profoundly sensitive to rotations particu-larly tilt and therefore subject to rotation-inducederrors (see eg Graizer 1991 2005 2006 Pilletand Virieux 2007) The paucity of instrumentalobservations of rotational ground motions ismainly the result of the fact that until recentlythe rotational sensors did not have sufficient reso-lution to measure small rotational motions due toearthquakes

Measurement of rotational motions has implica-tions for (1) recovering the complete ground-displacement history from seismometer recordings(2) further constraining earthquake rupture proper-ties (3) extracting information about subsurfaceproperties and (4) providing additional groundmotion information to engineers for seismic design

In this Appendix we will first briefly reviewelastic wave propagation that is based on the

linear elasticity theory of simple homogeneousmaterials under infinitesimal strain This theorywas developed mostly in the early nineteenth cen-tury the differential equations of the linear elastictheory were first derived by Louis Navier in 1821and Augustin Cauchy gave his formulation in1822 that remains virtually unchanged to the pre-sent day (Sokolnikoff 1956) From this theorySimeon Poisson demonstrated in 1828 the exis-tence of longitudinal and transverse elastic wavesand in 1885 Lord Rayleigh confirmed the exis-tence of elastic surface waves George Green putthis theory on a physical basis by introducing theconcept of strain energy and in 1837 derived thebasic equations of elasticity from the principle ofenergy conservation In 1897 Richard Oldhamfirst identified these three types of waves inseismograms and linear elasticity theory hasbeen embedded in seismology ever since

In the following we summarize recent pro-gress in rotational seismology and the need toinclude measurements of rotational groundmotions in earthquake monitoring The mono-graph by Teisseyre et al (2006) provides a usefulsummary of rotational seismology

Elastic Wave PropagationThe equations of motion for a homogeneous iso-tropic and initially unstressed elastic bodymay beobtained using the conservation principles of con-tinuum mechanics (eg Fung 1965) as

r2uit2

frac14 lthorn meth THORN yxi

thorn mnabla2ui i frac14 123 (19)

and

y frac14X

juj=xj (20)

where y is the dilatation r is the density ui is theith component of the displacement vector u

t is

the time and l and m are the elastic constants ofthe media Eq (19) may be rewritten in vectorform as

r 2 u=t2

frac14 lthorn meth THORNnabla nablabull u

thorn mnabla2 u

(21)

If we differentiate both sides of Eq (19) withrespect to xi sum over the three components andbring r to the right-hand side we obtain

2y=t2 frac14 lthorn 2meth THORN=rfrac12 nabla2y (22)

If we apply the curl operator (nabla) to both sidesof Eq (21) and note that

nablabull nabla u

frac14 0 (23)

we obtain

2 nabla u

=t2 frac14 m=reth THORNnabla2 nabla u

(24)

Now Eqs (22) and (24) are in the form of theclassical wave equation

2C=t2 frac14 v2nabla2C (25)

where C is the wave potential and v is the wave-propagation velocity (a pseudovector wave slow-ness is a proper vector) Thus a dilatational distur-bance y (or a compressional wave) may betransmitted through a homogenous elastic bodywith a velocity VP where

V P frac14ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffilthorn 2meth THORN=rfrac12

p(26)

according to Eq (22) and a rotational disturbancenabla u

(or a shear wave) may be transmitted with a

wave velocity VS where

VS frac14ffiffiffiffiffiffiffiffim=r

p(27)

according to Eq (24) In seismology and forhistorical reasons these two types of waves arecalled the primary (P) and the secondary (S)waves respectively

For a heterogeneous isotropic and elasticmedium the equation of motion is more complex

than Eq (21) and is given by Karal and Keller(1959) as

r 2 u=t2

thorn mnabla2 u

thornnablal nablabull u

thorn nablam nabla u

thorn 2 nablambullnablaeth THORN u

(28)

Furthermore the compressional wave motionis no longer purely longitudinal and the shearwave motion is no longer purely transverseA review of seismic wave propagation and imag-ing in complex media may be found in the entry byIgel et al ldquoSeismic Wave Propagation in Mediawith Complex Geometries Simulation ofrdquo

A significant portion of seismological researchis based on the solution of the elastic wave equa-tions with the appropriate initial and boundaryconditions However explicit and unique solu-tions are rare except for a few simple problemsOne approach is to transform the wave equation tothe eikonal equation and seek solutions in terms ofwave fronts and rays that are valid at high fre-quencies Another approach is to develop throughspecific boundary conditions a solution in terms ofnormal modes (Lognonne and Clevede 2002)Although ray theory is only an approximation(Chapman 2002) the classic work of Jeffreysand Bullen and Gutenberg used it to determineEarth structure and locate earthquakes thatoccurred in the first half of the twentieth centuryIt remains a principal tool used by seismologistseven today Impressive developments in normalmode and surface wave studies (in both theoryand observation) started in the second half of thetwentieth century leading to realistic quantifica-tion of earthquakes using moment tensor method-ology (Dziewonski and Woodhouse 1983b)

Rotational Ground MotionsRotations in ground motion and in structuralresponses have been deduced indirectly fromaccelerometer arrays but such estimates arevalid only for long wavelengths compared to thedistances between sensors (eg Castellani andBoffi 1986 Ghayamghamian and Nouri 2007Huang 2003 Niazi 1987 Oliveira and Bolt

1989 Spudich et al 1995) The rotational com-ponents of ground motion have also been esti-mated theoretically using kinematic sourcemodels and linear elastodynamic theory of wavepropagation in elastic solids (Bouchon and Aki1982 Lee and Trifunac 1985 1987 Trifunac1982)

In the past decade rotational motions from tele-seismic and small local earthquakes were also suc-cessfully recorded by sensitive rotational sensorsin Japan Poland Germany New Zealand andTaiwan (eg Huang et al 2006 Igel et al 20052007 Suryanto et al 2006 Takeo 1998 Takeo andIto 1997 Teisseyre et al 2003) The observationsin Japan and Taiwan show that the amplitudes ofrotations can be one to two orders of magnitudegreater than expected from the classical linear the-ory Theoretical work has also suggested that ingranular materials or cracked continuaasymmetries of the stress and strain fields cancreate rotations in addition to those predicted bythe classical elastodynamic theory for a perfectcontinuum ( ldquoEarthquake Source Asymmetryand Rotation Effectsrdquo)

Because of lack of instrumentation rotationalmotions have not yet been recorded in the near-field (within ~25 km of fault ruptures) of strongearthquakes (magnitudegt65) where the discrep-ancy between observations and theoretical predic-tions may be the largest Recording such groundmotions will require extensive seismic instrumen-tation along some well-chosen active faults andluck To this end several seismologists have beenadvocating such measurements and a currentdeployment in southwestern Taiwan by its CentralWeather Bureau is designed to ldquocapturerdquo a repeatof the 1906 Meishan earthquake (magnitude 71)with both translational and rotational instruments

Rotations in structural response and the con-tributions to the response from the rotational com-ponents of the ground motion have also been ofinterest for many decades (eg Luco 1976Newmark 1969 Rutenberg and Heidebrecht1985) Recent reviews on rotational motions inseismology and on the effects of the rotationalcomponents of ground motion on structures canbe found for examples in Cochard et al (2006)

and Pillet and Virieux (2007) and Trifunac(2006) respectively

Growing Interest ndash The IWGoRSVarious factors have led to spontaneous organiza-tion within the scientific and engineering commu-nities interested in rotational motions Suchfactors include the growing number of successfuldirect measurements of rotational ground motions(eg by ring laser gyros fiber optic gyros andsensors based on electrochemical technology)increasing awareness about the usefulness of theinformation they provide (eg in constraining theearthquake rupture properties extracting informa-tion about subsurface properties and about defor-mation of structures during seismic and otherexcitation) and a greater appreciation for the lim-itations on information that can be extracted fromthe translational sensors due to their sensitivity torotational motions eg computation of perma-nent displacements from accelerograms (egBoroschek and Legrand 2006 Graizer 19912005 2006 Pillet and Virieux 2007 Trifunacand Todorovska 2001)

A small workshop on Rotational Seismologywas organized by WHK Lee K Hudnut andJR Evans of the USGS on 16 February 2006 inresponse to grassroots interest It was held at theUSGS offices in Menlo Park and in PasadenaCalifornia with about 30 participants from abouta dozen institutions participating via teleconfer-encing and telephone (Evans et al 2007) Thisevent led to the formation of the InternationalWorking Group on Rotational Seismology in2006 inaugurated at a luncheon during the AGU2006 Fall Meeting in San Francisco

The International Working Group on Rota-tional Seismology (IWGoRS) aims to promoteinvestigations of rotational motions and theirimplications and the sharing of experience datasoftware and results in an open web-based envi-ronment (httpwwwrotational-seismologyorg)It consists of volunteers and has no official statusH Igel and WHK Lee currently serve as ldquoco-organizersrdquo Its charter is accessible on theIWGoRS web site The Working Group has anumber of active members leading task groupsthat focus on the organization of workshops and

scientific projects including testing and verifyingrotational sensors broadband observations withring laser systems and developing a field labora-tory for rotational motions The IWGoRS web sitealso contains the presentations and posters fromrelated meetings and eventually will provideaccess to rotational data from many sources

The IWGoRS organized a special session onRotational Motions in Seismology convened byH Igel WHK Lee and M Todorovska duringthe 2006 AGU Fall Meeting (Lee et al 2007b)The goal of that session was to discuss rotationalsensors observations modeling theoreticalaspects and potential applications of rotationalground motions A total of 21 papers were sub-mitted for this session and over 100 individualsattended the oral session

The large attendance at this session reflectedcommon interests in rotational motions from awide range of geophysical disciplines includingstrong-motion seismology exploration geophys-ics broadband seismology earthquake engineer-ing earthquake physics seismic instrumentationseismic hazards geodesy and astrophysics thusconfirming the timeliness of IWGoRS It becameapparent that to establish an effective internationalcollaboration within the IWGoRS a larger work-shop was needed to allow sufficient time to dis-cuss the many issues of interest and to draftresearch plans for rotational seismology and engi-neering applications

First International WorkshopThe First International Workshop on RotationalSeismology and Engineering Applications washeld in Menlo Park California on 18ndash19September 2007 This workshop was hosted bythe US Geological Survey (USGS) which recog-nized this topic as a new research frontier forenabling a better understanding of the earthquakeprocess and for the reduction of seismic hazardsThe technical program consisted of three presen-tation sessions plenary (4 papers) and oral(6 papers) held during the first day and poster(30 papers) held during the morning of the secondday A post-workshop session was held on themorning of September 20 in which scientists ofthe Laser Interferometer Gravitational-waveObservatory (LIGO) presented their work on

seismic isolation of their ultra-high precisionfacility which requires very accurate recordingof translational and rotational components ofground motions (3 papers) Proceedings of thisWorkshop were released in Lee et al (2007a)with a DVD disc that contains all the presentationfiles and supplementary information

One afternoon of the workshop was devoted toin-depth discussions on the key outstanding issuesand future directions The participants could joinone of five panels on the following topics (1) the-oretical studies of rotational motions (chaired byL Knopoff) (2) measuring far-field rotationalmotions (chaired by H Igel) (3) measuringnear-field rotational motions (chaired byTL Teng) (4) engineering applications of rota-tional motions (chaired by MD Trifunac) and(5) instrument design and testing (chaired byJR Evans) The panel reports on key issues andunsolved problems and on research strategies andplans can be found in Appendices 21 through 25in Lee et al (2007a) Following the in-depthgroup discussions the panel chairs reported onthe group discussions in a common session withfurther discussions among all the participants

DiscussionsSince rotational ground motions may play a sig-nificant role in the near-field of earthquakes rota-tional seismology has emerged as a new frontierof research During the Workshop discussionsL Knopoff asked Is there a quadratic rotation-energy relation in the spirit of Greenrsquos strain-energy relation coupled to it or independent ofit Can we write a rotation-torque formula analo-gous to Hookersquos law for linear elasticity in theform

Lij frac14 dijklokl (29)

where okl is the rotation

okl frac14 1

2ukl ulk

(30)

Lij is the torque density and dijkl are the coef-ficients of rotational elasticity How are the drsquosrelated to the usual crsquos of elasticity If we definethe rotation vector as

Ofrac14 1

2nabla u

(31)

we obtain

V 2snabla nabla O

frac14 2O

=t2

1

2r1 nabla f

(32)

where the torque density is nabla f fis the body

force density and r is density of the medium Thisshows that rotational waves propagate withS-wave velocity and that it may be possible tostore torques Eq (15) is essentially an extensionusing the classical elasticity theory

Lakes (1995) pointed out that the behavior ofsolids can be represented by a variety of contin-uum theories In particular the elasticity theory ofthe Cosserat brothers (Cosserat and Cosserat1909) incorporates (1) a local rotation of pointsas well as the translation motion assumed in theclassical theory and (2) a couple stress (a torqueper unit area) as well as the force stress (force perunit area) In the constitutive equation for theclassical elasticity theory there are two indepen-dent elastic constants whereas for the Cosseratelastic theory there are six Lakes (personal com-munication 2007) advocates that there is substan-tial potential for using generalized continuumtheories in geo-mechanics and any theory musthave a strong link with experiment (to determinethe constants in the constitutive equation) andwith physical reality

Indeed some steps towards better understand-ings of rotational motions have taken place Forexample Twiss et al (1993) argued that brittledeformation of the Earthrsquos crust ( ldquoBrittle Tec-tonics A Non-linear Dynamical Systemrdquo) involv-ing block rotations is comparable to thedeformation of a granular material with faultblocks acting like the grains They realized theinadequacy of classical continuum mechanicsand applied the Cosserat or micropolar continuumtheory to take into account two separate scales ofmotions macro-motion (large-scale average

motion composed of macrostrain rate and macro-spin) and micro-motion (local motion composedof microspin) A theoretical link is thenestablished between the kinematics of crustaldeformation involving block rotations and theeffects on the seismic moment tensor and focalmechanism solutions

Recognizing that rotational seismology is anemerging field the Bulletin of Seismological Soci-ety of Americawill be publishing in 2009 a specialissue under the guest editorship of WHK LeeM Ccedilelebi MI Todorovska and H Igel

Bibliography

Primary LiteratureAhern TK (2003) The FDSN and IRIS Data Management

System providing easy access to terabytes of informa-tion In Lee WHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earth-quake and engineering seismology part B AcademicAmsterdam pp 1645ndash1655

Aki K (1966) Generation and propagation of G waves fromthe Niigata Earthquake of June 16 1964 part 1 Astatistical analysis Bull Earthq Res Inst 4423ndash72Part 2 Estimation of earthquake moment releasedenergy and stress-strain drop from the G wave spec-trum Bull Earthq Res Inst 4473ndash88

Aki K (1969) Analysis of the seismic coda of local earth-quakes as scattered waves J Geophys Res746215ndash6231

Aki K Richards PG (1980) Quantitative seismologyWH Freeman San Francisco

Aki K Richards PG (2002) Quantitative seismology the-ory and methods 2nd edn University Science BooksSausalito

Allen RM (2007) Earthquake hazard mitigation new direc-tions and opportunities In Kanamori H (ed) Earthquakeseismology Treatise on geophysics vol 4 ElsevierAmsterdam pp 607ndash648

Allen RM Kanamori H (2003) The potential for earth-quake early warning in Southern California Science300786ndash789

Ambraseys NN Jackson JAMelville CP (2002) Historicalseismicity and tectonics the case of the Eastern Med-iterranean and the Middle East In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seis-mology part A Academic Amsterdam pp 747ndash763

Anderson JG (2003) Strong-motion seismology In LeeWHK Kanamori H Jennings PC Kisslinger C (eds)International handbook of earthquake and engineeringseismology part B Academic Amsterdam pp 937ndash965

Berger J Davis P EkstroumlmG (2004) Ambient earth noise asurvey of the Global Seismographic NetworkJ Geophys Res 109B11307

Borcherdt RD(ed) (1997) Vision for the future of the USNational Strong-Motion Program The committee forthe future of the US National Strong Motion ProgramUS Geol Surv Open-File Rept B97530

Bormann P (ed) (2002) New Manual of SeismologicalObservatory Practice GeoForschungsZentrum Pots-dam httpwwwgfz-potsdamdebibnmsop_formularhtml

Boroschek R Legrand D (2006) Tilt motion effects on thedouble-time integration of linear accelerometers anexperimental approach Bull Seism Soc Am962072ndash2089

Bouchon M Aki K (1982) Strain tilt and rotation associ-ated with strong ground motion in the vicinity of earth-quake faults Bull Seism Soc Am 721717ndash1738

Burridge R Knopoff L (1964) Body force equivalents forseismic dislocations Bull Seism Soc Am541875ndash1888

Castellani A Boffi G (1986) Rotational components of thesurface ground motion during an earthquake EarthqEng Struct Dyn 14751ndash767

Chapman CH (2002) Seismic ray theory and finite fre-quency extensions In Lee WHK Kanamori HJennings PC Kisslinger C (eds) International hand-book of earthquake and engineering seismology partA Academic Amsterdam pp 103ndash123

Cochard A Igel H Schuberth B Suryanto WVelikoseltsev A Schreiber U Wassermann JScherbaum F Vollmer D (2006) Rotational motions inseismology theory observation simulation InTeisseyre R Takeo M Majewski M (eds) Earthquakesource asymmetry structural media and rotationeffects Springer Heidelberg pp 391ndash411

Cosserat E Cosserat F (1909) Theorie des Corps Deform-ables Hermann Paris

Dziewonski AM Woodhouse JH (1983a) An experimentin the systematic study of global seismicity centroid-moment tensor solutions for 201 moderate and largeearthquakes of 1981 J Geophys Res 883247ndash3271

Dziewonski AM Woodhouse JH (1983b) Studies of theseismic source using normal-mode theory InKanamori H Boschi B (eds) Earthquakes observationtheory and interpretation North-Holland Amsterdampp 45ndash137

Dziewonski AM Chou TA Woodhouse JH (1981) Deter-mination of earthquake source parameters from wave-form data for studies of global and regional seismicityJ Geophys Res 862825ndash2852

Ekstroumlm G (1994) Rapid earthquake analysis utilizes theInternet Comput Phys 8632ndash638

Engdahl ER Villasenor A (2002) Global seismicity 1900-1999 In Lee WHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earth-quake and engineering seismology part A AcademicAmsterdam pp 665ndash690

Espinosa-Aranda JM Jimenez A Ibarrola G Alcantar FAguilar A Inostroza M Maldonado S (1995) MexicoCity seismic alert system Seism Res Lett 66(6)42ndash53

Evans JR Hamstra RH Kuumlndig C Camina P Rogers JA(2005) TREMOR a wireless MEMS accelerograph fordense arrays Earthq Spectr 21(1)91ndash124

Evans JR Cochard A Graizer V Huang B-S Hudnut KWHutt CR Igel H Lee WHK Liu C-C Majewski ENigbor R Safak E Savage WU Schreiber UTeisseyre R Trifunac M Wassermann J Wu C-F(2007) Report of a workshop on rotational groundmotion US Geol Surv Open File Rep 202007ndash1145httppubsusgsgovof20071145

Feigl KL (2002) Estimating earthquake source parametersfrom geodetic measurements In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seismol-ogy part A Academic Amsterdam pp 607ndash620

Field EH Hough SE (1997) The variability of PSVresponse spectra across a dense array deployed duringthe Northridge aftershock sequence Earthq Spectr13243ndash257

Fung YC (1965) Foundations of solid mechanics Prentice-Hall Englewood Cliffs

Gasparini P Manfredi G Zschau J (eds) (2007) Seismicearly warning systems Springer Berlin

Gee L Neuhauser D Dreger D Pasyanos MUhrhammer R Romanowicz B (2003) The rapid earth-quake data integration project In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seis-mology part B Academic Amsterdam pp 1261ndash1273

Geiger LC (1912) Probability method for the determinationof earthquake epicenters from the arrival time onlyBull St Louis Univ 860ndash71

Ghayamghamian MR Nouri GR (2007) On the character-istics of ground motion rotational components usingChiba dense array data Earthq Eng Struct Dyn36(10)1407ndash1429

Gilbert F (1971) Excitation of the normal modes of theEarth by earthquake sources Geophys J R Astron Soc22223ndash226

Gilbert F Dziewonski AM (1975) Application of normalmode theory to the retrieval of structural parametersand source mechanisms from seismic spectra PhilTrans Roy Soc Lond A 278187ndash269

Goltz JD Flores PJ Chang SE Atsumi T (2001) Emer-gency response and early recovery In 1999 Chi-ChiTaiwan earthquake reconnaissance report EarthqSpectra Suppl A 17173ndash183

Goodstein JR (1991) Millikanrsquos school a history of theCalifornia Institute of Technology Norton New York

Graizer VM (1991) Inertial seismometry methods IzvEarth Phys Akad Nauk SSSR 27(1)51ndash61

Graizer VM (2005) Effect of tilt on strong motion dataprocessing Soil Dyn Earthq Eng 25197ndash204

Graizer VM (2006) Tilts in strong groundmotion Bull SeisSoc Am 962090ndash2106

Guidoboni E (2002) Historical seismology the long mem-ory of the inhabited world In Lee WHK Kanamori HJennings PC Kisslinger C (eds) International hand-book of earthquake and engineering seismology partA Academic Amsterdam pp 775ndash790

Gutenberg B (1945) Amplitudes of surface waves andmagnitudes of shallow earthquakes Bull Seism SocAm 353ndash12

Gutenberg B Richter CF (1954) Seismicity of the earth2nd edn Princeton University Press Princeton

Gutenberg B Richter CF (1956) Magnitude and energy ofearthquakes Ann Geofis 91ndash15

Hanks TCWyssM (1972) The use of body wave spectra inthe determination of seismic source parameters BullSeism Soc Am 62561ndash589

Hauksson E Jones LM Shakal AF (2003) TriNet a mod-ern ground motion seismic network In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seis-mology part B Academic Amsterdam pp 1275ndash1284

Havskov J Alguacil G (2004) Instrumentation in earth-quake seismology Springer Berlin

Horiuchi S Negishi H Abe K Kamimura A FujinawaY (2005) An automatic processing system for broad-casting earthquake alarms Bull Seism Soc Am95708ndash718

Hoshiba M Kamigaichi O Saito M Tsukada S HamadaN (2008) Earthquake early warning starts nationwide inJapan EOS Trans Am Geophys Un 89(8)73ndash74

Housner GW (2002) Historical view of earthquake engi-neering In Lee WHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earth-quake and engineering seismology part A AcademicAmsterdam pp 13ndash18

Huang BS (2003) Ground rotational motions of the 1991Chi-Chi Taiwan earthquake as inferred from dense arrayobservations Geophys Res Lett 30(6)1307ndash1310

Huang BS Liu CC Lin CR Wu CF Lee WHK(2006)Measuring mid- and near-field rotational groundmotions in Taiwan Poster presented at 2006 Fall AGUMeeting San Francisco

Hutt CR Bolton HF Holcomb LG (2002) US contributionto digital global seismograph networks In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seis-mology part A Academic Amsterdam pp 319ndash322

Igel H Schreiber U Flaws A Schuberth BVelikoseltsev A Cochard A (2005) Rotational motionsinduced by the M81 Tokachi-oki earthquakeSeptember 25 2003 Geophys Res Lett 32L08309httpsdoiorg1010292004GL022336

Igel H Cochard A Wassermann J Schreiber UVelikoseltsev A Dinh NP (2007) Broadband observa-tions of rotational ground motions Geophys J Int168(1)182ndash197

Kamigaichi O (2004) JMA earthquake early warningJ Japan Assoc Earthq Eng 4134ndash137

Kanamori H (1977) The energy release in great earth-quakes J Geophys Res 822921ndash2987

Kanamori H (1978) Quantification of earthquakes Nature271411ndash414

Kanamori H (2003) Earthquake prediction an overviewIn Lee WHK Kanamori H Jennings PC KisslingerC (eds) International handbook of earthquake and engi-neering seismology part B Academic Amsterdampp 1205ndash1216

Kanamori H (2005) Real-time seismology and earthquakedamage mitigation Annu Rev Earth Planet Sci33195ndash214

Kanamori H Brodsky EE (2000) The physics of earth-quakes Phys Today 54(6)34ndash40

Kanamori H Rivera L (2006) Energy partitioning duringan earthquake In Abercrombie R McGarr AKanamori H Di Toro G (eds) Earthquakes radiatedenergy and the physics of faulting Geophysical mono-graph vol 170 Am Geophys Union Washington DCpp 3ndash13

Kanamori H Hauksson E Heaton T (1997) Real-timeseismology and earthquake hazard mitigation Nature390461ndash464

Karal FC Keller JB (1959) Elastic wave propagation inhomogeneous and inhomogeneous media J AcoustSoc Am 31694ndash705

Kisslinger C Howell BF (2003) Seismology and physicsof the Earthrsquos interior in the USA 1900ndash1960 In LeeWHK Kanamori H Jennings PC Kisslinger C (eds)International handbook of earthquake and engineeringseismology part B Academic Amsterdam p 1453

Lakes RS (1995) Experimental methods for study ofCosserat elastic solids and other generalized continuaIn Muumlhlhaus H (ed) Continuum models for materialswith micro-structure Wiley New York pp 1ndash22

Lawrence JF Cochran ES (2007) The Quake Catcher Net-work cyberinfrastructure bringing seismology intoschools and homes American Geophysical UnionFall Meeting 2007 abstract ED11C-0633

Lee WHK (2002) Challenges in observational seismologyIn Lee WHK Kanamori H Jennings PC KisslingerC (eds) International handbook of earthquake and engi-neering seismology part A Academic Amsterdampp 269ndash281

Lee WHK Benson RB (2008) Making non-digitally-recorded seismograms accessible online for studyingearthquakes In Freacutechet J Meghraoui M StucchiM (eds) Modern approach in historical seismologyinterdisciplinary studies of past and recent earthquakesSpringer Berlin pp 403ndash427

Lee WHK Espinosa-Aranda JM (2003) Earthquake earlywarning systems current status and perspectives InZschau J Kuppers AN (eds) Early warning systems fornatural disaster reduction Springer Berlin pp 409ndash423

Lee WHK Stewart SW (1981) Principles and applicationsof microearthquake networks Academic New York

Lee VW Trifunac MD (1985) Torsional accelerograms IntJ Soil Dyn Earthq Eng 4(3)132ndash139

Lee VW Trifunac MD (1987) Rocking strong earthquakeaccelerations Int J Soil Dyn Earthq Eng 6(2)75ndash89

LeeWHK Celebi M TodorovskaMI Diggles MF (2007a)Rotational seismology and engineering applicationsproceedings for the first international workshopMenlo Park California USA 18ndash19 September USGeol Surv Open File Rep 2007ndash1144 httppubsusgsgovof20071144

Lee WHK Igel H Todorovska MI Evans JR (2007b)Rotational seismology AGU session working groupand website US Geol Surv Open File Rep 2007ndash1263httppubsusgsgovof20071263

Lognonne P Clevede E (2002) Normal modes of the Earthand planets In Lee WHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earth-quake and engineering seismology part A AcademicAmsterdam pp 125ndash147

Luco JE (1976) Torsional response of structures toobliquely incident seismic SH waves Earthq EngStruct Dyn 4207ndash219

Mallet R (1858) Fourth report on the facts of earthquakephenomena Ann Rep Brit Assn Adv Sci 281ndash136

Mallet R (1862) Great Neapolitan earthquake of 1857vol I II Chapman and Hall London

Maruyama T (1963) On the force equivalents of dynamicalelastic dislocations with reference to the earthquakemechanism Bull Earthq Res Inst 41467ndash486

Michelini A De Simoni B Amato A Boschi E (2005)Collecting digitizing and distributing historical seis-mological data EOS 86(28)261ndash266

Minson SE Lee WHK (2014) Bayesian historical earth-quake relocation an example from the 1909Taipeiearthquake Geophysical Journal International 1981419ndash1430

Nakamura Y (1984) Development of the earthquake early-warning system for the Shinkansen some recent earth-quake engineering research and practical in Japan TheJapanese National Committee of the InternationalAssociation for Earthquake Engineering pp 224ndash238

Nakamura Y (1988) On the urgent earthquake detectionand alarm system UrEDAS Proceedings of the ninthworld conference earthquake engineering 7673ndash678

Nakamura Y (2004) On a rational strong motion indexcompared with other various indices 13th world con-ference earthquake engineering paper no 910

Nakamura Y Saita J (2007) UrEDAS The earthquakewarning system today and tomorrow In Gasparini PManfredi G Zschau J (eds) Earthquake early warningsystems Springer Berlin pp 249ndash281

Newmark NM (1969) Torsion in symmetrical buildingsProceedings of the fourth world conference on earth-quake engineering vol II pp A319-A332

Niazi M (1987) Inferred displacements velocities androtations of a long rigid foundation located atEl-Centro differential array site during the 1979 Impe-rial Valley California earthquake Earthq Eng StructDyn 14531ndash542

Odaka T Ashiya K Tsukada S Sato S Ohtake K NozakaD (2003) A new method of quickly estimating epicen-tral distance and magnitude from a single seismicrecord Bull Seism Soc Am 93526ndash532

Oliveira CS Bolt BA (1989) Rotational components ofsurface strong ground motion Earthq Eng Struct Dyn18517ndash526

Oliver J Murphy L (1971) WWNSS seismologyrsquos globalnetwork of observing stations Science 174254ndash261

Peterson J (1993) Observations and modeling of seismicbackground noise US Geol Surv Open File Rep93ndash322 p 95

Pillet R Virieux J (2007) The effects of seismic rotationson inertial sensors Geophys J Int httpsdoiorg101111j1365-246X200703617x

Reid HF (1910) The California earthquake of 18 April1906 vol 2 The mechanics of the earthquake CarnegieInst Washington DC

Richards PG (2002) Seismological methods of monitoringcompliance with the Comprehensive Nuclear Test-BanTreaty In Lee WHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earthquakeand engineering seismology part A Academic Amster-dam pp 369ndash382

Richter CF (1935) An instrumental earthquake magnitudescale Bull Seis Soc Am 251ndash32

Richter CF (1958) Elementary seismology Freeman SanFrancisco

Rutenberg A Heidebrecht AC (1985) Rotational groundmotion and seismic codes Can J Civ Eng12(3)583ndash592

Saita J Nakamura Y (2003) UrEDAS the early warningsystem for mitigation of disasters caused by earth-quakes and tsunamis In Zschau J Kuppers AN (eds)Early warning systems for natural disaster reductionSpringer Berlin pp 453ndash460

Savage JC (1978) Dislocation in seismology In NabarroFRN (ed) Dislocation in solids North-HollandAmsterdam pp 251ndash339

Schweitzer J Lee WHK (2003) Old seismic bulletins acollective heritage from early seismologists InWHK L Kanamori H Jennings PC KisslingerC (eds) International handbook of earthquake and engi-neering seismology part B Academic Amsterdampp 1665ndash1717 (with CD-ROM)

Shin TC Tsai YB Yeh YT Liu CC Wu YM (2003) Strong-motion instrumentation programs in Taiwan In LeeWHK Kanamori H Jennings PC Kisslinger C (eds)International handbook of earthquake and engineeringseismology part B Academic Amsterdam pp1057ndash1062

Sokolnikoff IS (1956) Mathematical theory of elasticity2nd edn McGraw-Hill New York

Spudich P Steck LK Hellweg M Fletcher JB Baker LM(1995) Transient stresses at Park-field California pro-duced by the m 74 Landers earthquake of 28 June1992 observations from the UPSAR dense seismo-graph array J Geophys Res 100675ndash690

Suryanto W Igel H Wassermann J Cochard ASchubert B Vollmer D Scherbaum F (2006) Compar-ison of seismic array-derived rotational motions withdirect ring laser measurements Bull Seism Soc Am96(6)2059ndash2071

Takeo M (1998) Ground rotational motions recorded innear-source region Geophys Res Lett 25(6)789ndash792

Takeo M Ito HM (1997) What can be learned from rota-tional motions excited by earthquakes Geophys J Int129319ndash329

Teisseyre R Suchcicki J Teisseyre KP Wiszniowski JPalangio P (2003) Seismic rotation waves basic ele-ments of theory and recording Ann Geofis 46671ndash685

TeisseyreR TakeoMMajewski E (eds) (2006) Earthquakesource asymmetry structural media and rotationeffects Springer Berlin

Teng TL Wu L Shin TC Tsai YB Lee WHK (1997) Oneminute after strongmotion map effective epicenter andeffective magnitude Bull Seism Soc Am 871209ndash1219

Trifunac MD (1982) A note on rotational components ofearthquake motions on ground surface for incidentbody waves Soil Dyn Earthq Eng 111ndash19

Trifunac MD (2006) Effects of torsional and rocking exci-tations on the response of structures In Teisseyre RTakeo M Majewski M (eds) Earthquake source asym-metry structural media and rotation effects SpringerHeidelberg pp 569ndash582

TrifunacMD TodorovskaMI (2001) A note on the useabledynamic range of accelerographs recording translationSoil Dyn Earthq Eng 21(4)275ndash286

Twiss R Souter B Unruh J (1993) The effect of blockrotations on the global seismic moment tensor and thepatterns of seismic P and T axes J Geophys Res98(B1)645ndash674

Utsu T (2002a) A list of deadly earthquakes in the world(1500-2000) In LeeWHK Kanamori H Jennings PCKisslinger C (eds) International handbook of earth-quake and engineering seismology part A AcademicAmsterdam pp 691ndash717

Utsu T (2002b) Relationships between magnitude scalesIn Lee WHK Kanamori H Jennings PC KisslingerC (eds) International handbook of earthquake and engi-neering seismology part A Academic Amsterdampp 733ndash746

Uyeda S (2002) Continental drift sea-floor spreading andplateplume tectonics In Lee WHK Kanamori HJennings PC Kisslinger C (eds) International hand-book of earthquake and engineering seismology partA Academic Amsterdam pp 51ndash67

Villasenor A Engdahl ER (2007) Systematic relocation ofearly instrumental seismicity Earthquakes in the Inter-national Seismological Summary for 1960ndash1963 BullSeismol Soc Am 971820ndash1832

Wald DJ Quitoriano V Heaton TH Kanamori H (1999a)Relationships between peak ground acceleration peakground velocity and modified Mercalli intensity inCalifornia Earthq Spectra 15557ndash564

Wald DJ Quitoriano V Heaton TH Kanamori H ScrivnerCW Worden CB (1999b) TriNet ldquoShakeMapsrdquo Rapidgeneration of peak ground motion and intensity mapsfor earthquakes in Southern California Earthq Spectr15537ndash555

Webb SC (2002) Seismic noise on land and on the seafloorIn Lee WHK Kanamori H Jennings PC KisslingerC (eds) International handbook of earthquake and engi-neering seismology part A Academic Amsterdampp 305ndash318

Wielandt E (2002) Seismometry In Lee WHKKanamori H Jennings PC Kisslinger C (eds) Interna-tional handbook of earthquake and engineering seis-mology part A Academic Amsterdam pp 283ndash304

Willemann RJ Storchak DA (2001) Data collection at theinternational seismological centre Seism Res Lett72440ndash453

Wu YM Kanamori H (2005a) Experiment on an onsiteearly warning method for the Taiwan early warningsystem Bull Seism Soc Am 95347ndash353

Wu YM Kanamori H (2005b) Rapid assessment of dam-aging potential of earthquakes in Taiwan from thebeginning of P Waves Bull Seism Soc Am951181ndash1185

Wu YM Kanamori H (2008) Exploring the feasibility ofon-site earthquake early warning using close-in recordsof the 2007 Noto Hanto earthquake Earth PlanetsSpace 60155ndash160

Wu YM Zhao L (2006) Magnitude estimation using thefirst three seconds P-wave amplitude in earthquakeearly warning Geophys Res Lett 33L16312 httpsdoiorg1010292006GL026871

Wu YM Chen CC Shin TC Tsai YB Lee WHK Teng TL(1997) Taiwan rapid earthquake information releasesystem Seismol Res Lett 68931ndash943

Wu YM LeeWHK Chen CC Shin TC Teng TL Tsai YB(2000) Performance of the Taiwan Rapid EarthquakeInformation Release System (RTD) during the 1999Chi-Chi (Taiwan) earthquake Seismol Res Lett71338ndash343

Wu YM Hsiao NC Lee WHK Teng TL Shin TC (2007a)State of the art and progresses of early warning systemin Taiwan In Gasparini P Manfredi G Zschau J (eds)Earthquake early warning systems Springer Berlinpp 283ndash306

Wu YM Kanamori H Allen R Hauksson E (2007b)Determination of earthquake early warning parameterstc and P d for southern California Geophys J Int169667ndash674

Zollo A LancieriM Nielsen S (2006) Earthquakemagnitudeestimation from peak amplitudes of very early seismicsignals on strong motion records Geophys Res Lett 33L23312 httpsdoiorg1010292006GL027795

Books and ReviewsAbercrombie R McGarr A Kanamori H Di Toro G (2006)

Earthquakes radiated energy and the physics offaulting Geophysical monograph vol 170 AmericanGeophysical Union Washington DC

Bolt BA (1993) Earthquakes WH Freeman New YorkChen YT Panza GF Wu ZL (2004) Earthquake hazard

risk and strong ground motion Seismological PressBeijing

Kanamori H (ed) (2007) Earthquake seismology treatiseon geophysics vol 4 Elsevier Amsterdam

Kanamori H Boschi E (1983) Earthquakes observationtheory and interpretation North-Holland Amsterdam

Keilis-Borok VI Soloviev AA (eds) (2003) Nonlineardynamics of the lithosphere and earthquake predictionSpringer Berlin

Pujol J (2003) Elastic wave propagation and generation inseismology Cambridge Univ Press Cambridge

WHK L Meyers H Shimazaki K (eds) (1988) Historicalseismograms and earthquakes of the world AcademicSan Diego

WHK L Kanamori H Jennings JC Kisslinger C (eds)(2002) International handbook of earthquake and engi-neering seismology part A Academic San Diegop 933 (and 1 CD-ROM)

WHK L Kanamori H Jennings JC Kisslinger C (eds)(2003) International handbook of earthquake and engi-neering seismology part B Academic San Diegop 1009 (and 2 CD-ROMs)

Zschau J Kuppers AN (eds) (2003) Early warning systemsfor natural disaster reduction Springer Berlin

Earthquake Monitoring and Early Warning Systems
- Glossary
- Definition of the Subject
- Introduction
- Earthquake Monitoring Instrumentation
- Historical Developments
- Technical Considerations
- Earthquake Monitoring in the Digital Era
- Earthquake Monitoring Regional and Local Networks
- A Brief History
- Some Recent Advances
- Recording Damaging Ground Shaking
- Seismograms and Derived Products
- Earthquake Location
- Earthquake Magnitude
- Quantification of the Earthquake Source
- Limitations of Earthquake Catalogs
- Earthquake Early Warning (EEW) Systems
- Physical Basis and Limitations of EEW Systems
- Design Considerations for EEW Systems
- Regional Warning Versus Onsite Warning
- Some Recent EEW Advances
- Future Directions
- Appendix A Progress Report on Rotational Seismology
- - Elastic Wave Propagation
  - Rotational Ground Motions
  - Growing Interest - The IWGoRS
  - First International Workshop
  - Discussions
  - - Bibliography
    - - Primary Literature
      - Books and Reviews

Page 2: Earthquake Monitoring and Early Warning Systems P 1...Seismology Bibliography Glossary Active fault A fault (q.v.) that has moved in historic (e.g., past 10,000 years) or recent geo-logical