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    Why earthquakes happen?Pe rformance of low-rise be aring-wall house s in ea rthqua kesReferences

    Why earthquakes happen?

    Plate tectonicsA shaking of the ground caused by sudden movements in the earth's crust is termed as an earthquake.

    The crust is the ou termost layer of the e arth and is com pose d from brittle so lid rock fo rmations. The crust vary inthickness and can be only 5 to 10 km a t the m id ocean ridges and as much as a 100 k m unde r the Himalayas.Lithosphere is called the solid part of the earth, including the crust and the uppermost mantle. The mantle being theportion of the e arth underneath the crust. The lithosphe re is about 100 k m thick, although its thickne ss is a gede p ende nt i.e. older lithosph ere is thicker. Asthenosph ere is called the d uctile part of the earth just below thelithosphere, including the lower mantle. The asthenosphere is about 180 km thick and is relatively soft.

    According to the com mo nly accepted plate tectonics theo ry, the ea rth's lithosphere is m ade up of 2 1 tectonic solidplates(Figure 1), of various area, that are constantly moving. The plates move as rigid bodies on top of theasthenosphere, causing their edges to either slide past each other, slide under each other, push into each other or pullapart.

    Figure 1- Tectonic plates on the ea rth's surface (Im age courtesy of U. S. Geological Survey)

    The caus e for the plate motions is believed to be the energy generated from temperature and pressure differencesbetween the surface and the core of the earth.It should be no ted however, that the process of pla te mo tions or cree p is rather slow. For instance the rate of spread ingalong the m id Atlantic ridge (i.e. betwee n North and Sou th American p lates a nd Eurasia n and African p lates respe ctively)

    is estimated to be about 2.5 cm per year.Therefore the plates become gradually into contact causing gradual increase in deformation. Thus the deformed rockformations s lowly accumu late stresse s which after certain time can approach the strength o f the m aterial. A fault (Figure2) is a fracture a long which the rock fo rmations of crust on either side ha ve mo ved relative to one anothe r parallel to thefracture. Th e su dden fracture o f rock creates a n imp act and that cause s bod y waves in the crust to propaga te and theea rth's surface beg in to vibrate. Th e location in the crust where a seism ic rupture begins is called a fo cus. The e picenterof an e arthquake is the point on the e arth's surface vertically above the fo cus (Figure 2).Earthquakes can also happen as a result of volcanic eruptions and human activities such as explosions as well as largeexcavations in m ines.

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    Figure 2- A fault slip causing an earthquake (Image courtesy of Kian H. Chong, Univ. of California, Davis)

    Most of the e arthquake s occur at the plates b ounda ries. However there are isolated e vents of ea rthquak es within themiddle of the tectonic plates- such as ea rthquakes in the m iddle of the Indian peninsula. Depending on the m otions a tthe interface, the plates bounda ries a re classified a s follows (Figure 3):transform boundaries, divergent boundaries, convergent boundaries, and plate boundary zones.

    Figure 3- The ma in types of tectonic plate bounda ries (I ma ge courtesy of U. S. Geological Survey, This dyna mic Earth,by Robert Tilling and Jacquelyne Kious)

    Transform boundaries are the ones where movement is parallel to the fault line. An example of such boundary is theSan Andreas fault in Califrornia.Divergent bou ndaries o ccur when two pla tes in contact are m oving ap art and ne w crust is be ing crea ted by ho t ma gmapushing up from the mantle (see Figure 3). The mid Atlantic ridge is an example of divergent plates.Convergent bo undaries o ccur when two plates are m oving towards ea ch other and collide. There a re several types of convergent bou ndaries a nd they are pictured be low. On Figure 4 is pictured convergence be twee n an oceanic plate a nd alargely continental plate. An example of such convergence can be found off the Peru-Chile coast in South America. Theoceanic Nazca pla te is pushing into the South Ame rican plate an d as a result the ocea nic tectonic plate is be ing forcedunder the continental plate. Ma ny of the e arth's a ctive volcano es a re also lo cated at ocean ic-continental convergenceboundaries.

    Figure 4 (Ima ge courtesy of U. S. Geological Survey, This dyna mic Earth, by Robe rt Tilling and Ja cquelyne Kious)

    On Figure 5 is pictured convergence betwee n two ocea nic plates. Whe n ocean ic tectonic plates collide on e o f the plates isusua lly subducted un der the other. The process of su bduction results in the formation of isla nds in the o cean. Suchislands are typically grouped in arcs, called island a rcs.

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    Figure 5 (Ima ge courtesy of U. S. Geological Survey, This dyna mic Earth, by Robe rt Tilling and Ja cquelyne Kious)

    On Figure 6 is displayed collision be twee n two continental plates. Th e e arth's crust as e xplaine d ea rlier is much thicke r

    under the continents. I t also is constituted o f lighter rocks a nd therefore s ubduction does n't occur at leas t initially.Therefore the rock ma terial tends to pile u p or spread side ways. In such a way are forme d the Hima layas a nd theTibetan plateau between 40 and 50 m illion years ago.

    Figure 6 (Ima ge courtesy of U. S. Geological Survey, This dyna mic Earth, by Robe rt Tilling and Ja cquelyne Kious)

    India was a large island about 225 million years ago, situated not far from Australia. Research scientists have estimatedthat some 80 million years ago India was located about 6400 km south of Asia and moving northwards by 9m every 100years. India reached a nd collide with the continent of Asia a bout 50 m illion years ago. It is calculated that the Him alayasmo untains continue to rise with a rate of 1 cm per year. This could sum up to 10 km in one m illion years.

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    Figure 7- R econstruction of move me nt of the Indian plate a nd its collision with Eurasian plate (Ima ge courtesy of U. S.Geological Survey, This dyna mic Earth, by Robe rt Tilling and Ja cquelyne Kious)

    Plate bo unda ry zone s refer to zo nes where the bo unda ry betwee n the tectonic plates is no t well define d. Such zone isthe Med iterranean region which forms part of the boun dary between the African and Eurasia n plates .

    Types of seismic wavesThe slipping of rocks alon g a fault crea tes se ismic body waves in the crust. There a re two types of body waves: P -wavesand S-waves. P -waves are called also longitudinal waves because they propagate in longitudinal-forward waycompressing m aterial along circular surfaces. T he velocity of P-waves is faster than the other seism ic waves. T hey canalso p ropagate in a ir and o ther fluids as P-waves a re esse ntially sound waves. O n Figure 8 is illustrated P-wavepropagation causing vibration in the sam e direction. The se waves can also be referred to a s primary or compressivewaves.

    Figure 8 P-waves (Image courtesy of European Centre for Geo dynamics and Seismology)

    S-waves are transverse waves causing vibration a t right ang les to the direction that the wave is travelling. The se wavescan also be referred to a s seconda ry or shear waves. S-waves can no t propa gate in fluids like air because fluids do notpossess a shear strength. Depending on the material properties of the crust S-waves travel at about half the speed of P-waves. On Figure 9 is illustrated S-wave propagation causing vibration in a transverse direction.

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    Figure 9 S-waves (I ma ge courtesy of European Cen tre for Geo dynam ics and Seismo logy)

    The body waves g ene rated in the focus in the ea rth's crust travel through various layers of rock and s oil as the y reflectand refract during propag ation. Whe n these waves reach the e arth's surface they propagate in the form of surfacewaves. T here a re two types of su rface wave: R ayleigh waves and Love waves.

    Rale igh waves vibrate in a plan e pe rpendicular to the ea rth's surface (Figure 10). T he Love waves causes vibrations in aplane parallel to the e arth's surface an d pe rpendicular to the direction of wave propagation (Figure 11). Surface wavesrequire time and d istance to am plify. Their vibrations are sm all nea r the e picentre of a n ea rthquak e, while further theycan amplify or attenuate.

    Fig 10 Rayleigh waves (Image courtesy of European Centre for Geo dynamics and Seismology)

    Figure 11 Love waves (Image courtesy of European Centre for Geo dynamics and Seismology)

    The difference between magnitude and intensity of an earthquakeThe impact of an earthquake on built environment is closely related to the amount of energy released in the focus. Theme asure of relea sed e nergy is the m agnitude M, which was first defined by Richter in 1935, an d is therefore called -Richter's magnitude. According to Richter, the magnitude of an earthquake M is given by a logarithm of the maximumdisplacement amplitude A (in mm), recorded by a standardised instrument, located at exactly 100 km from theepicentre.

    Hence the magnitude, M measures the absolute "size" of the earthquake, irrespective of location. The lightbulb analogyma y be use d: For a lightbulb, magnitude corresponds to the wattage, which indicates the a bsolute size in terms of thepower it consume s. I ntensity corresponds to the appa rent brightness of the bulb to a viewer, which varies with location.The effects of earthquakes on the built environment are measured by means of various intensity scales. A 12-grademo dified Mercalli (MM) scale and MSK-76 intensity scale are use d predom inantly in the US an d Europe respectively. Anew 12-grade Europe an m acrose ismic scale (EMS), which is a m odification of the MSK scale, has bee n recentlydeveloped.

    Performance of low-rise bearing-wall houses in earthquakesTo beginning of document

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    Figure 13- Resisting earthquake loads through box action

    On the figure above case (a) features a structure that perform as a whole unit i.e. like a box. Case (b) shows adam age d structure, lacking b ox a ction be cause wall integrity is not be ing en forced.

    The resistance of the ho use subject to a n ea rthquak e is d efined by the inter-conne ctivity between structural com pone ntsas well as the individual com pone nt's strength, stiffness and ductility.Good connection betwee n different mem bers creates continuous load pa th for the ine rtia loads . Such load p ath is calledcomplete load path. On Figure 13, case (a) complete load path is achieved by:

    Providing wall to wall connection through goo d qua lity bond at corners a nd construction of ho rizontal roof band(bond beam)Providing walls to foun dations conne ction through construction of plinth ba nd (bon d bea m at plinth level)Provide connection be twee n walls an d roof ban d

    Individual me mbe rs of the house structure should a lso pos sess the necess ary strength, stiffness a nd ductility. Ductility isa m echanical property of the ma sonry wall and will be discussed la ter in the text. On Figure 13, case (a) imp rovedresistance is achieved by:

    Good qu ality maso nry to improve compressive s trength o f wallsGood qu ality maso nry to improve she ar strength o f wallsGood quality material and reinforcement (when using RC) for the horizontal bands to improve their bending andtensile s trength

    Im portant role in the m echanism of lateral (horizontal) load resistance plays the construction of stiff in its plane roof/floor. Stiff roof/ floor would allow distribution of inertia load in proportion to the wall stiffness. Stiff roof/ floor providesdiaphragm action. In case o f flexible roof/ floor the inertia loa d will be distributed o n tributary area b asis fo r each wall,which may po tentially lead to wall failure d ue to unfavourable load distribution between walls. O n Figure 14 below, isillustrated the importance of diaphragm action. For this purpose is used a simple model of square bearing-wall masonryhouse . In case(a) the roof is flexible and the ine rtia forces from its weight will be distributed to all fou r walls (on tributaryarea basis). As a consequence the weak out-of-plane walls will be loaded with the same amount as the strong in-planewalls.

    In case(b ) the roof is stiff and provides a diaphragm action. Therefore the inertia loa d from the roof will be distributedpredom inantly to the strong in-plane walls. Plea se no te that som e loa d will be also d istributed to the out-of-plan e walls.The figures shown on the sketch below are for illustrative purposes. The out-of-plane walls have some stiffness andtherefore would a ttract a sm all portion of the inertia loa d.

    Figure 14- Diaphragm action

    Earthquake resistant plan and layout for masonry wall low-rise housesAs was d iscussed e arlier, plain i.e. un reinforced ma sonry walls are vulnerable under horizontal load s. Experience frompast ea rthqua kes as well as research have estab lished that if certain principles are followed rega rding ge neral planningand layout, masonry performance can be greatly improved.

    Simple plan configurationRegular distribution of structural walls in both directionsAvoid non parallel systemsAvoid re-en trant corners and recess esVertical irreg ularities

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    Irregula r mass d istribution in plan an d ele vation

    Figure 15- Examples of dangerous configurations in elevation

    On Figure 15, case (a) is s hown an elevation o f a h ouse with weight irregula rity and Setbacks which in the event of a nea rthquak e m ay lead to local or total failure. On Figure 15, case (b) is sh own an e levation of a h ouse with irregulardime nsion from s torey to storey, discontinued structural m em bers, Wea k a nd/or flexible storey as well as weightirregularity. Such irregula rities ma y also le ad to lo cal or com plete collapse . On Figure 15, case (c) is shown an e levationof a house with weight irregula rity in plan that can cause twisting of the structure add ing m ore load to the la teral forceresisting walls.

    Classification and analysis of earthquake damageMaso nry wall house s like e very other structure are built from different mem bers (like fou ndations, walls, bond bea ms ,roof ). Thes e com pone nts ma y be a ssem bled in different ways which can g reatly affect the pe rformance o f the structure.

    Under ea rthqua ke mo tion, in the case o f very stiff eleme nts ass em bled toge ther with rigid connections the structure willrespond u niformly as a whole. Ho wever in real structures the comp onen ts are ela stic and the connections are se mi-rigid.

    Therefore, the e lastic stiffness, strength, failure mo de a nd ductility of each compon ent dete rmines its pe rformance whichin turn determines the house's performance as a whole. During a strong earthquake the various elements tend toex hibit their own dynam ic vibrations. I n case of incompa tible vibrations be twee n m em bers (in-plane wall and a cross wallfor instance) m em bers can sepa rate from ea ch other towards ultimate state.

    Therefore the e valuation of ea rthquake dam age should focus on the component behaviour to understand the dama gemechanism. Conse quently cracks and o ther signs of dam age, m ust be ana lysed ba sed on the component behaviour.For exam ple for a compo nent with tensio n she ar behaviour crack size is critical. For a comp onen t domina ted frombending and e xhibiting "bending " cracks the s ize of the crack is not as critical.

    Most dam age patterns in ma sonry wall low-rise h ouse s can fit in the following groups:

    Separation of walls a t corners an d T intersectionsDiagona l cracks s tarting at wall openingsOut-of-plane partial or complete collapse of walls

    Diagona l and X type of cracksCracking betwee n walls a nd roof/floorDiagona l cracks b etween wall ope nings

    The beha viour of the load-be aring ma sonry walls when resisting lateral loa ds is g reatly influenced from the verticalloads. This is d ue to the fact that the s hea r resistance o f walls increa ses substantially when vertical loa ds increa se. I nthe case of single storey houses with light roof (for ex am ple timbe r construction) the she ar strength of the walls is no tused but rather the walls (o r its comp onen ts) rotate and rock. Apart from the low vertical load a rea son for that is thewindow and/or door ope nings in the wall. The open ings in the walls pa rtition the wall compo nent in vertical com pone ntscalled piers and horizontal ones called sprandels.

    On Figure 16 bellow are sh own comm on crack pa tterns a t openings in in-plane walls when vertical loa ds a re low. Thepattern depend s on the ratio betwee n the width of the adjoining wall compon ents forming L sha pe a round ope ning.

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    Figure 1 6- Crack pa tterns a t wall ope ning when vertical loa ds a re low

    Figure 17- C rack pa tterns in in-plane walls when resisting ea rthquak e loa ds

    On Figure 1 7, case(a ) ab ove a re shown com mo n crack pa tterns in in-pla ne walls when vertical loa ds a re low, which isoften the case in one strorey house s with light roofs.On Figure 17, case (b) abo ve are sho wn comm on crack pa tterns in in-plane walls for two storey house . In this case thevertical loads are significant in the first storey walls therefore the piers in this wall exhibit shear X cracks which show thatthe component failed in shear. On the second storey of this example house, is assumed that the vertical loads are low(light roof), a nd therefore X s hape shea r cracking d id not o ccur.

    Ductile or brittle performanceThe inertia load s that the structure of the h ouse resist may reach the strength capacity of the compo nents causingdisplacem ent in the structure.Whe n the strength of compone nts is rea ched they ma y crack a nd fail without being able to carry mo re load or dissipateene rgy. Whe n this hap pens the structure is termed to ha ve a b rittle be haviour. Brittle structures a re not suitable fo rea rthquak e resistance. Whe n the strength of compone nts is reached and they may crack, but are able to carry mo re

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    .structures to resist earthquak es. Figure 18, case (a) disp lays the e arthquake response of a well constructed wall of ade quate strength a nd ductility. The ductility allows for dissipation o f ea rthqua ke ene rgy and reduction of ine rtia loa ds.The ade quate strength distribution a llows for ben ding of the wall an d a voids stress concentration. Such wall survives theearthquake without excessive damage or failure.

    Figure 18, case(b) displays the earthquake response of a of inadequate strength and lacking ductility. The inadequatestrength distribution p revents bending of the wall a nd crea tes stress concentration. T he lack of d uctility does n't allowdissipation of e arthquake ene rgy and reduction of inertia loa ds. As a result the wall fails.

    Figure 18- Ductile and unductile respo nse

    Efficient way to provide a dequa te strength and ductility of the ma sonry walls is through reinforcement. For the s hea rresistance of m aso nry walls is im portant both horizontal, called be d-joint reinforcem ent a s well as vertical reinforcem ent.

    On Figure 19 (a) is s hown the respons e of un reinforced ma sonry wall where the structure is be ing dam age d due to itsinade quate strength a s well a s ductility.

    On Figure 19 (b) is sho wn the respo nse of reinforced m aso nry wall where bo th horizontal bed-joint reinforcem ent an dvertical reinforcem ent is u sed for wall construction. The vertical reinforcem ent sh ould be anchored in the horizontalbands, like the roof beam and the plinth beam .The result is a structure o f ade quate strength a s well as ductility to resist the e arthquake loads .

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    Figure 19- I mproving pe rformance of walls through reinforcing

    ReferencesTo beginning of document

    1) Introducing and Dem onstrating Earthquake Engineering Rese arch in Schools, Earthqua ke Engineering Rese archCentre, University of Bristol, IDEERS

    2) This dynamic Earth : The story of plate tectonics, Robe rt Tilling and Ja cquelyne Kious, USGS, This dynam ic Earth

    3) USGS Earthquake haza rds program g lossary, http://earthquake.usgs.gov

    4) David Fanella and Javeed Munshi, Design of low-rise concrete buildings for earthquake forces, Portland CementAssociation,1998

    5) Arya, A., Guidelines for Earthquake Res istant Non-Engineered C onstruction published by the International Asso ciationfor Earthquake Engineering(IAEE), 1986 To kyo, Japa n

    6) Murty, C., IITK-BMTPC Earthquake Tips Series, The National In formation Ce nter of Earthquake Engineering (NICEE) at

    Indian Institute of Technology Kanpur (IIT Kanpur), 2004, EQ Tips

    7) D'Ayala, D., Seism ic vulnerability and s trengthe ning o f historic building, in Lalitpur, Nepal, University of Bath, England

    8) Nationa l Earthquake Informa tion Cen ter, U.S. Geological Survey, http://neic.usgs.gov

    9) Arya, A., Recent developm ents towards ea rthquak e risk reduction in India , Special edition- Seismolog y 2000

    10) Arya, A., Guidelines- Im roving ea rthqua ke resistance of housing, Building Materials & Technology Promo tion Council(BMTPC), Ministry of Urban Developm ent an d Po verty Alleviation, India