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What causes Earthquakes?

EarthquakeTip 1LearningEarthquake DesignandConstruction

Figure 3:Major Tectonic Plates on the Earths surface

The Earth and its InteriorLong time ago, a large collection of material

masses coalesced and formed the Earth. Large amountof heat was generated by this fusion, and slowly as theEarth cooled, the heavier and denser materials sank tothe center and the lighter ones rose to the top. Thedifferentiated Earth consists of the Inner Core (radius~1290km), the Outer Core (thickness ~2200km), theMantle (thickness ~2900km) and the Crust (thickness ~5to 40km). Figure 1 shows these layers. The Inner Core

is solid and consists of heavy metals (e.g., nickel andiron), while the Crust consists of light materials (e.g.,basalts and granites). The Outer Core is liquid in formand the Mantle has the ability to flow. At the Core, the

temperature is estimated to be ~2500C, the pressure~4 million atmospheres and density ~13.5 gm/cc; this is

in contrast to ~25C, 1 atmosphere and 1.5gm/cc on thesurface of the Earth.

The Circulations

Convection currents develop in the viscousMantle, because of prevailing high temperature andpressure gradients between the Crust and the Core,like the convective flow of water when heated in abeaker (Figure 2). The energy for the abovecirculations is derived from the heat produced fromthe incessant decay of radioactive elements in therocks throughout the Earths interior. These convectioncurrents result in a circulation of the earths mass; hotmolten lava comes out and the cold rock mass goesinto the Earth. The mass absorbed eventually meltsunder high temperature and pressure and becomes a

part of the Mantle, only to come out again fromanother location, someday. Many such localcirculations are taking place at different regionsunderneath the Earths surface, leading to differentportions of the Earth undergoing different directionsof movements along the surface.

Plate TectonicsThe convective flows of Mantle material cause the

Crust and some portion of the Mantle, to slide on thehot molten outer core. This sliding of Earths masstakes place in pieces called Tectonic Plates. The surfaceof the Earth consists of seven major tectonic plates and

many smaller ones (Figure 3). These plates move indifferent directions and at different speeds from thoseof the neighbouring ones. Sometimes, the plate in thefront is slower; then, the plate behind it comes andcollides (and mountains are formed). On the otherhand, sometimes two plates move away from oneanother (and rifts are created). In another case, twoplates move side-by-side, along the same direction orin opposite directions. These three types of inter-plateinteractions are the convergent, divergent and transformboundaries (Figure 4), respectively. The convergentboundary has a peculiarity (like at the Himalayas) that

sometimes neither of the colliding plates wants to sink.The relative movement of these plate boundariesvaries across the Earth; on an average, it is of the orderof a couple to tens of centimeters per year.

Figure 2:Local Convective Currents in the Mantle

Crust

Mantle

Outer Core

Figure 1:Inside the Earth

Inner Core

PacificPlate

Indo-Aust ral ian

Plate

North AmericanPlate

Antarct ic Plate

EurasianPlate

Afri canPlate

SouthAmerican

Plate

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IITK-BMTPC Earthquake Tip 1

What causes Earthquakes? page 2

Figure 4: Types ofInter-Plate BoundariesDivergent Boundary

Figure 5:Elastic Strain Build-Up

and Brittle Rupture

Stage B

Figure 7: Type of Faults

Dip SlipFaults

Strike SlipFaults

Figure 6: Elastic Rebound TheoryTime(years)

StrengthEnergyBuild-Up

Elastic

Stress

Energy

Release

CumulativeSlip

CASlip

AB

C

A

BC

Time(years)

The EarthquakeTectonic plates are made of elastic but brittle rocky

material. And so, elastic strain energy is stored in themduring the relative deformations that occur due to thegigantic tectonic plate actions taking place in the Earth.But, when the rocky material along the interface of theplates in the Earths Crust reaches its strength, itfractures and a sudden movement takes place there(Figure 5); the interface between the plates where themovement has taken place (called the fault) suddenlyslips and releases the large elastic strain energy storedin the rocks at the interface. For example, the energyreleased during the 2001 Bhuj (India) earthquake isabout 400 times (or more) that released by the 1945

Atom Bomb dropped on Hiroshima!!

The sudden slip at the fault causes the earthquakea violent shaking of the Earth during which largeelastic strain energy released spreads out in the formof seismic waves that travel through the body andalong the surface of the Earth. And, after theearthquake is over, the process of strain build-up at

this modified interface between the tectonic platesstarts all over again (Figure 6). Earth scientists knowthis as the Elastic Rebound Theory. The collection ofmaterial points at the fault over which slip occursusually constitutes an oblong three-dimensionalvolume, with its long dimension often running intotens of kilometers in case of significant earthquakes.

Types of Earthquakes and FaultsMost earthquakes in the world occur along the

boundaries of the tectonic plates as described aboveand are called Inter-plate Earthquakes (e.g., 1897 Assam(India) earthquake). A number of earthquakes alsooccur within the plate itself but away from the plateboundaries (e.g., 1993 Latur (India) earthquake); theseare called Intra-plate Earthquakes. Here, a tectonic platebreaks in between. In both types of earthquakes, theslip generated at the fault during earthquakes is alongboth vertical and horizontal directions (called Dip Slip)and lateral directions (called Strike Slip) (Figure 7),with one of them dominating sometimes.

Reading MaterialBolt,B.A., (1999), Earthquakes, Fourth Edition, W. H. Freeman and

Company, New York, USAhttp://earthquake.usgs.gov/faq/http://neic.usgs.gov/neis/general/handouts/

general_seismicity.htmlhttp://www.fema.gov/kids/quake.htm

Convergent Boundary

Transform Boundary

Stage A

Stage C

Slip

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This release is a property of IIT Kanpur and BMTPC NewDelhi. It may be reproduced without changing its contentsand with due acknowledgement. Suggestions/commentsmay be sent to: nicee@iitk.ac.in. Visit www.nicee.org orwww.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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Authored by:C.V.R.MurtyIndian Institute of Technology KanpurKanpur, India

Sponsored by:Building Materials and Technology PromotionCouncil, New Delhi, India

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