Earthquakes: seismology & measurementSeismologyAs we have seen, most earthquakes are essentially the product of tectonic stresses which are generated at the boundaries of the Earth's tectonic plates. The released energy travels as shock waves, called seismic waves, which may be felt and measured. The study of seismic waves is known as seismology, a word derived from a Greek word meaning "to shake."Seismographs are the instruments which record earthquakes. Scientists use these instruments as their principal tool to study seismic waves. They are very sensitive instruments that can detect, measure and record ground vibrations and their intensities during an earthquake.A seismograph is a simple pendulum. As the ground shakes the base and frame move with the vibrations but inertia keeps the pendulum bob in place. It moves relative to vibrating ground. The pendulum displacement records the changes over time the tracing out the record of the event is called a seismogram.

Image: A seismographThe image above shows the seismograph in Sunset Crater Volcano National Monument's visitor centre. A pen at the top of the device records a zig-zag line on the moving, paper-covered cylinder whenever an earthquake is detected. The seismogram (paper record) is removed each day and replaced. The earthquake data from the seismogram is then analyzed by scientists. Seismographs can help us determine the time,epicentre, focus, and the type of faulting which produced an earthquake as well as estimate how much energy was released.Magnitude and other units of measurementThe severity of an earthquake can vary from events which are barely detectable even using the most sophisticated devices, to devastating events which can level cities and trigger Tsunamis and sometimes even volcanic activity. The severity of an earthquake is called its magnitude. Various scales were proposed to measure the magnitude of earthquakes until 1935, when the Richter Scale was developed by a seismologist named Conrad Richter to measure the intensity of the seismic waves.The amplitude (height) of the largest recorded wave of an earthquake at a specific distance is called the Richter magnitude. Under the Richter scale, each order of magnitude is 10 times more intensive than the last one, which means that a two is 10 times more intense than a one and a three is one hundred times greater. But it is to

point out that, while it is correct to say that for each increase in 1 in the Richter magnitude there is a tenfold increase in amplitude of the wave, it is incorrect to say that each increase of 1 in Richter magnitude represents a tenfold increase in the size of the earthquake (as is commonly incorrectly stated by the press). A better measure of the size of an earthquake is the amount of energy released by the earthquake, which is related to the Richter Scale by the following equation:

log E = 11.8 + 1.5 Mwhere Log refers to the logarithm to the base 10, E is the energy released in ergs and M the Richter magnitude.

Image: Earthquake Severity - Richter ScaleTectonic earthquakes can range in size from magnitudes less than zero, resulting from fault slippage of a few centimetres, to the largest events (magnitude greater than 9), where fault displacements are on the order of many metres. Earthquake size is determined not only by amount of displacement but also area of the ruptured fault plane. Hence the larger the rupture area, the larger is the earthquake. A magnitude 7 earthquake ruptures a fault area of about 1000 km2 or about 50 km long and 20 km wide.Also depth is an important factor influencing earthquake severity. We know that earthquakes can originate at various depths within the Earth’s solid core. The deeper the earthquake, the more powerful it is, but it is also far less likely to reach the surface. That’s why shallow earthquakes are more common and more dangerous, because the shallower an earthquake, the more damage to surface structures it can cause.There is no limit to the possible magnitude of an earthquake but historically just over magnitude 9 is the record. The earthquake of most recent history to reach 9 on the

Richter scale was the Japan quake of March 2011. This was also the largest recorded Japanese earthquake of all time.

Image: Earthquake in Japan, March 2011

Monitoring InstrumentsThe USGS maintains a variety of fault and volcano monitoring instruments around the western United States, including Hawaii. The data from all instruments are transmitted in real time to the USGS in Menlo Park using satellite, radio and telephone telemetry.

CreepmetersA creepmeter measures fault slip by recording the displacement between 2 piers or monuments located on opposite sides of the fault, spaced 30 meters apart. Typically, an invar wire (or a graphic rod) is anchored to one pier and is stretched across the fault. Its displacement relative to the second pier is measured electronically and checked periodically with a mechanical measurement. Using the angle of the wire from the strike of the fault, the change in distance between the two piers is directly proportionally to fault slip.

Because the piers are anchored to about 2 meters depth, they are subject to the influence of seasonal (winter) rainfall. Many of the creepmeters show an annual cycle due to the wetting and drying of the near-surface materials within the fault zone. In addition, creep is influenced by large rainfall events and nearby earthquakes.

Pore Pressure MonitorsThese instruments record fluid pressure changes in deep boreholes that may be driven by fault activity. Measurements can be made to better than 0.1 millibar.

Strainmeters

Strainmeters for continuous crustal strain monitoring are highly sensitive instruments with precision of less than 1 part per billion (i.e. less than 1 inch in 16,000 miles). They are usually installed in boreholes where surface noise is greatly reduced. These instruments monitor the change in crustal strain near active faults and volcanoes associated with fault slip, earthquakes, and volcanic activity. Currently, numerous instruments have been installed by the USGS along the San Andreas fault, in the Long Valley Caldera, and by other institutions near active faults and volcanoes in the US, Japan, China, Iceland, Italy, and Taiwan.

The Sacks-Evertson dilational borehole strainmeter consists of a stainless-steel cylinder with an annulus filled with silicone oil. Changes in volumetric strain in the ground are detected by small movements of the walls of the borehole and are measured relative to the borehole diameter. This is translated into displacement and voltage by an expansion bellows attached to a linear voltage displacement transducer. The instrument is cemented into the ground at a depth of about 200 meters.The Sacks-Evertson tensor strainmeter is similar in principle but the annulus is divided into three independent segments, 120 degrees apart. Strain is determined in these three directions.Borehole tensor strain is also measured with a GTSM strainmeter built byGTSM Technologies in Queensland, Australia. These instruments measure strain in three directions, 120 degrees apart (a fourth redundant component is also included at 90 degrees) with a differential capacitance displacement transducer.

Networks of dilational and tensor strainmeters were initially installed in San Juan Bautista, Parkfield, Southern California and Long Valley in the early 1980's. These networks were later supplemented as increased hazard was identified in Parkfield in the mid-1980's, Long Valley in 1989, and the San Francisco Bay area in the late 1990's. A network of six dilational strainmeters and two tensor strainmeters was initially installed along the Hayward fault in the San Francisco Bay area in 1992 with an additional five DTM tensor strainmeters added through the Bay Area in 2001.

The figure shows a borehole strainmeter being installed to a depth of about 200 meters near the Hayward fault, San Francisco Bay, California. A commercial drill rig is used to drill and case 6" to 8" diameter holes and to core the bottom of these holes until a section of about 10 feet of unfractured rock is obtained. The strainmeter is then installed into a bath of expansive grout within the cored section of the hole. After the grout has set, the instrument detects deformation of the rock. This deformation is converted to ground strain through calibrations obtained from comparing the measured earth tide with the predicted tides in the solid earth corrected for ocean tide loading.

Tiltmeters

Tiltmeters are highly sensitive instruments used to measure ground tilt (rotation) near faults and volcanoes caused by fault slip and volcanic uplift. The precision to which tilt can be measured is less

than 1 part per billion (i.e. less than 1 inch in 16,000 miles). For crustal monitoring applications, these instruments are mostly installed in boreholes to avoid spurious ground tilts produced by differential thermal expansion in near-surface materials, rainfall and pumping effects.

Tilt detections systems vary depending on the particular instrument design types used. These design types include simple pendulums (boreholes), liquid level systems (vaults or observatories) or the position of a bubble under concave quartz (similar to a carpenters level). Differential capacitance transducers (DCT's) are usually used to detect the position of pendulums hanging between two capacitor plates within a tube in a borehole. Linear voltage differential transformers (LVDT's) and DCT's are used to determine apparent height changes between the two ends of a liquid level tiltmeter such as the 10 meter mercury-liquid level tiltmeters installed at the Presidio in San Francisco and at Berkeley in the 1960's and more recently, the 500 meter long baseline tiltmeters installed in Long Valley and the Pacific Northwest by R. Bilham of the University of Colorado. A resistance bridge that locates the position of a bubble under a concave quartz lens is used for both shallow and deep borehole tiltmeters. This type of detection system was used in five of the seven tiltmeters installed in the San Francisco Bay area from 1992 to 2001.

The figure shows borehole strainmeters and tiltmeters being installed at a depth of about 200 meters near the Hayward fault, San Francisco Bay, California. A water-well drill rig is usually used to drill and case these boreholes and to core the bottom of these holes. Tiltmeters are usually cemented well within the casing to avoid tilting from movement on localized cracks and fractures. Tiltmeters are calibrated using theoretical earth tides in the solid earth with ocean loading corrections.

Seismometers are instruments that measure motion of the ground, including those ofseismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.

The word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, measure and was coined by David Milne-Homein 1841, to describe an instrument designed by Scottish physicist James David Forbes.[1]

Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this distinguishes them

from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.[2]

The concerning technical discipline is called seismometry,[3] a branch of seismology

Basic principles[edit]

A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the earthʼs surface. As the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. If a recording system is installed, such as a rotating drum attached to the frame, and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce a history of ground motion, called a seismogram.

Any movement of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the movement between the frame and the mass, the motion of the ground can be determined.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the weight is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones used in seismic surveys for oil and gas.

Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and the vertical. If only one axis can be measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves.

The foundation of a seismic station is critical.[4] A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally, European seismographs

were placed in a particular area after a destructive earthquake. Today, they are spread to provide appropriate coverage (in the case of weak-motion seismology) or concentrated in high-risk regions (strong-motion seismology).[5]

History[edit]

Ancient era[edit]

Replica of Zhang Heng's seismoscope Houfeng Didong Yi

See also: List of Chinese inventions

In AD 132, Zhang Heng of China's Han dynasty invented the first seismoscope (by the definition above), which was called Houfeng Didong Yi (translated as, "instrument for measuring the seasonal winds and the movements of the Earth"). The description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter; at eight points around the top were dragon's heads holding bronze balls. When there was an earthquake, one of the mouths would open and drop its ball into a bronze toad at the base, making a sound and supposedly showing the direction of the earthquake. On at least one occasion, probably at the time of a large earthquake in Gansu in AD 143, the seismoscope indicated an earthquake even though one was not felt. The available text says that inside the vessel was a central column that could move along eight tracks; this is thought to refer to a pendulum, though it is not known exactly how this was linked to a mechanism that would open only one dragon's mouth. The first ever earthquake recorded by this seismoscope was supposedly somewhere in the east.Days later, a rider from the east reported this earthquake.[6][7]

Modern designs[edit]

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with astylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus

scratches a pattern corresponding with the Earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the pressure waves and thus help find the epicenter of a local earthquake – such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further re-analysis was performed in the 1980s using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south.

Milne horizontal pendulum seismometer. One of the Important Cultural Properties of Japan. Exhibit in

the National Museum of Nature and Science, Tokyo, Japan.

After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked in Japan from 1880 to 1895.[5] These seismometers used damped horizontal pendulums. After World War II, these were adapted into the widely used Press-Ewing seismometer.

Later, professional suites of instruments for the world-wide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge.

The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical," that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents.

Zollner described torsionally suspended horizontal pendulums as early as 1869, but developed them for gravimetry rather than seismometry.

Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors called a photomultiplier. The voltage generated in the photomultiplier was used to drive galvanometers which had a small mirror mounted on the axis. The moving reflected light beam would strike the surface of the turning drum, which was covered with photo-sensitive paper. The expense of developing photo sensitive paper caused many seismic observatories to switch to ink or thermal-sensitive paper.

Modern instruments[edit]

CMG-40T triaxial broadband seismometer

Modern instruments use electronic sensors, amplifiers, and recording devices. Most are broadband covering a wide range of frequencies. Some seismometers can measure motions with frequencies from 500 Hz to 0.00118 Hz (1/500 = 0.002 seconds per cycle, to 1/0.00118 = 850 seconds per cycle). The mechanical suspension for horizontal instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force suspension, such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring to provide a long period (high sensitivity).[8][9] Some modern instruments use a "triaxial" design, in which three identical motion sensors are set at the same angle to the vertical but 120 degrees apart on the horizontal. Vertical and horizontal motions can be computed from the outputs of the three sensors.

Seismometers unavoidably introduce some distortion into the signals they measure, but professionally designed systems have carefully characterized frequency transforms.

Modern sensitivities come in three broad ranges: geophones, 50 to 750 V/m; local geologic seismographs, about 1,500 V/m; and teleseismographs, used for world survey, about 20,000 V/m. Instruments come in three main varieties: short period, long period and broadband. The short and long period measure velocity and are very sensitive, however they 'clip' the signal or go off-scale for

ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly one part per million.

Delivered seismometers come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. The output of a digital seismograph can be simply input to a computer. It presents the data in a standard digital format (often "SE2" over Ethernet).

Teleseismometers[edit]

A low-frequency 3-direction ocean-bottom seismometer (cover removed). Two masses for x- and y-direction

can be seen, the third one for z-direction is below. This model is a CMG-40TOBS, manufactured by Güralp

Systems Ltd and is part of the Monterey Accelerated Research System.

The modern broadband seismograph can record a very broad range of frequencies. It consists of a small "proof mass", confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.

In most designs the electronics holds a mass motionless relative to the frame. This device is called a "force balance accelerometer". It measures acceleration instead of velocity of ground movement. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Some instruments use a linear variable differential capacitor.

That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker, except that the coil is attached to the mass, and the magnet is mounted on the frame. The result is that the mass stays nearly motionless.

Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. The current to the drive coil provides a sensitive, accurate measurement of the force

between the mass and frame, thus measuring directly the ground's acceleration (using f=ma where f=force, m=mass, a=acceleration).

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of the air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckeisen model has a thick glass base that must be glued to its pier without bubbles in the glue.

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a material that interacts minimally with magnetic fields. A seismograph is also sensitive to changes in temperature so many instruments are constructed from low expansion materials such as nonmagnetic invar.

The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has been improved. The most successful public domain designs use thin foil hinges in a clamp.

Another issue is that the transfer function of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most instruments are characterized on a variable frequency shaking table.

Strong-motion seismometers[edit]

Another type of seismometer is a digital strong-motion seismometer, or accelerograph. The data from such an instrument is essential to understand how an earthquake affects manmade structures.

A strong-motion seismometer measures acceleration. This can be mathematically integrated later to give velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking.

Other forms[edit]

A Kinemetrics seismograph, formerly used by the United States Department of the Interior.

Accelerographs and geophones are often heavy cylindrical magnets with a spring-mounted coil inside. As case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to 1 Hz. Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs.

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones.

Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through an electret sponge or a conductive fluid through amagnetic field.

Interconnected seismometers[edit]

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves to propagate away from the hypocenter, the initiating point of fault rupture (See also Earthquake location). Interconnected seismometers are also used to detect underground nuclear test explosions. These seismometer are often used as part of a large scale, multimillion-dollar governmental or scientific project, but some organizations, such as the Quake-Catcher Network, can use residential size detectors built into computers to detect earthquakes as well.

In reflection seismology, an array of seismometers image sub-surface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars.

A world-wide array of seismometers can actually image the interior of the Earth in wave-speed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a "public seismograph network."[10]

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline ofgeophones unrolled behind a truck. Now most short-range systems use "thumpers" that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an 18" (0.5 m) plastic field case with a computer, display and printer in the cover.

Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.

Recording[edit]

Viewing of a Develocorder film

Further information: Seismogram

Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection; for amateurs, a PC with a sound card and associated software is adequate. Most systems record continuously, but some record only when a signal is detected, as shown by a short-term increase in the variation of the signal, compared to its long-term average (which can vary slowly because of changes in seismic noise).[citation needed]

Prior to the availability of digital processing of seismic data in the late 1970s, the records were done in a few different forms on different types of media. A "Helicorder" drum was a device used to record data into photographic paper or in the form of paper and ink. A "Develocorder" was a machine that record data from up to 20 channels into a 16-mm film. The recorded film can be viewed by a machine. The reading and measuring from these types of media can be done by hand. After the digital processing has been used, the archives of the seismic data were recorded in magnetic tapes. Due to the deterioration of older magnetic tape medias, large number of waveforms from the archives are not recoverable.

 In 1880 John Milne, an English seismologist and geologist, is credited for the development of the first modern seismograph in 1880.  Milne called his seismograph the “Horizontal seismograph” (“Inventors”).  Milne’s seismograph consists of three parts: the inertia member, the transducer, and a recorder. 

  (Drawing by Professor Stephen A. Nelson)  The inertia member is a weight suspended by a wire or a spring.  It is similar to a pendulum; however, it can only swing in one direction.  The transducer is a device that detected the motion between the mass and the ground.  This motion is then converted into a form that can be recorded.  The transducer can be a mechanical lever or an electrodynamic system.  In an electrodynamic system, a coil of wire moved back and forth in a magnetic field.  This movement created an electric current that passed through a galvanometer then recorded on a sheet of paper (Kauffman and Judson 182). Sir James Alfred Ewing, Thomas Grey and John Milne founded the Seismological Society of Japan.  The society funded the invention of seismographs (“Inventors”). 

            The Horizontal Pendulum seismograph was improved after World War II.  The new device is called the Press-Ewing seismograph.  The Press-Ewing seismograph is widely used throughout the United States to record long period waves.  The difference between the Press-Ewing and the Horizontal pendulum seismograph is that the Press-Ewing seismograph has a pivot that hold the pendulum was replaced with elastic wire to avoid friction (“Inventors”).  

The design of a seismograph is a weight freely suspended from a support that is attached to bedrock.  When the seismic waves reached the seismograph, the inertia of the weight kept the device stable while the ground and support shake.  The movement of the ground in relation to the movement of the weight it recorded onto a piece of paper that is wrapped around a rotating drum (Lutgens and Edwards 306).  To record motion in all directions, three seismographs are required.  One seismograph is needed to measure vertical motion, and two to

record horizontal motion.  The two seismographs recording horizontal directions, record in 90-degree angles (Kauffman and Judson 182).  

Seismographs record in a zigzag trace that shoes the varying amplitude of ground oscillations beneath the instrument. Depending on the sensitivity of the seismograph, earthquakes can be detected anywhere in the world.  Using data collected by the seismographs, the time, location, and the magnitude can be determined (Bellis).  The magnitude of an earthquake can be determined by a mathematical formula called the Richter Magnitude Scale.    

Charles F. Richter developed the Richter Magnitude Scale in 1934.  Richter defined the scale as “The logarithm to base 10 of the maximum seismic wave amplitude recorded on a standard seismograph at a distance of 100 kilometers from the earthquake epicenter.”  The seismic wave used in the calculation is not specified (Bolt 104).    Because it is not specified, S waves or P waves can be used.  The Richter scale was originally designed by Richter to differentiate between earthquakes with a low focus point in southern California.  The Richter scale is referred to ML, with “L” for local.  After many seismograph stations were established, it became clear the formula was only valid for a certain frequency and distance ranges (“Measuring Earthquakes”).  The original scale was modified to measure earthquakes at any distance, focal depth and compensate for geological variations from place to place (Lutgens and Edwards).  Adjustments were also included into the formula to compensate for the variation of the distance between different seismographs (“Severity”).  

Reading the Richter scale is a difficult task to accomplish.  Each whole number increase in the scale releases 32 times more energy than the preceding number (U.S. Geological Survey).  Although this process seems simple, it is actually very complex.  Seismologists are given seismograms then had to determine the magnitude at its source.  Earthquakes with a magnitude of 2.0 or less are called microearthquakes (Bellis).  The Richter scale has no upper limit.  Although the largest earthquake is recorded to be an 8.9, it is possible the earth can withstand the stress to produce an earthquake larger than 9.0 (McConnel).  According to the chart, large earthquakes will be felt by everyone

and even cause serious damage.  However, people may not feel some large-magnitude earthquakes beneath the ocean at all (Bellis).

How Geoscience Australia monitors earthquakesGeoscience Australia monitors seismic data from more than 60 stations on the Australian National Seismograph Network and in excess of 300 stations worldwide in near real-time, 24 hours a day, seven days a week. Most of the 40 samples per second data are delivered within 30 seconds of being recorded at the seismometer to Geoscience Australia’s central processing facility in Canberra through various digital satellite and broadband communication systems.

Seismic data are also provided by overseas Governments which have national seismic networks. Geoscience Australia uses data provided by the Governments of New Zealand, Indonesia, Malaysia, Singapore and China and has access to data from global seismic networks provided by the USA, Japan, Germany and France. The Comprehensive Nuclear Test Ban Treaty Organisation’s    International

Monitoring System also provides seismic data for tsunami warning purposes.

The seismic data are collected and analysed automatically and immediately reviewed by Geoscience Australia’s Duty Seismologist.

As part of the Joint Australian Tsunami Warning Centre (JATWC), Duty Seismologists also are responsible for analysing and reporting within 10 minutes of the origin time, on earthquakes which have the potential to generate a tsunami. An earthquake alert is then sent to Geoscience Australia’s partner in the JATWC, the Australian Bureau of Meteorology, to determine tsunami advice and publish tsunami bulletins.

Teledyne Geotech Helicorder

used in the past to detect

earthquake activity.