causes and effects of earthquakes.pptx

7/29/2019 Causes and Effects of earthquakes.pptx

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Causes and Effects of

earthquakes



TYPES OR CAUSES OF EARTHQUAKES

Earthquakes can be divided into three categoriesbased on their mode of generation.

• Tectonic Earthquakes –

Convergent Plate Boundary• Subduction Zone Earthquakes

• Collision Zone Earthquakes

– Divergent Plate Boundary

– Transcurrent Plate Boundary

– Intraplate Earthquakes

• Volcanic Earthquakes

• Induced Earthquakes



Tectonic Earthquakes

• Tectonic earthquakes are by far the mostcommon earthquakes.

• These earthquakes are produced when the rocksfracture by various geological forces because of constant geological reshaping of the Earth.

• Tectonic earthquakes are most important tostudy the structure and interior of the Earth, andthey are of much social significance as they posegreatest hazard.

• Sensitive instruments register major tectonicearthquakes at large distances demonstratingthat such events disturb the whole Earth.



Types of Tectonic Earthquakes

Three basic types of plate boundary are characterized bydifferent modes of plate interaction:

1. Convergent boundaries, where lithosphere isthickened or consumed by sinking into the mantle.Subduction zones and alpine belts are examples of

convergent plate boundaries.2. Divergent boundaries, where two plates are moving

apart and new lithosphere is produced or oldlithosphere is thinned. Mid-oceanic ridges andcontinental rifts are examples of divergent

boundaries.3. Transcurrent boundaries, where plates move past

one another without either convergence ordivergence. Transform faults and other strike-slip

faults are examples of transcurrent boundaries.



• The basic idea in plate tectonics is that the Earth’soutermost part, called lithosphere, consists of severallarge and fairly stable slabs; these are called plates.

•

Boundaries of these plates are the seismic belts of theworld.

• The plates move horizontally relative to adjoiningplates on a layer of softer rock, called theasthenosphere.

• Lava is continually upwelling at the mid-oceanic ridges.• This rock slowly moves across the Earth’s surface as a

new sea-floor on either side of the ridge.

• In this way plates move like a conveyer belt over the

asthenosphere, and get cooled as they get furtheraway from the ridges.

• The mid-oceanic ridges are called spreading zones oraccreting plate boundaries or divergent plateboundaries



• Since the Earth probably remains the same sizeover quite a long period of geological time, themoving plates must be absorbed at some places;

these are convergent plate boundaries.• The burial ground of plates is believed to be the

ocean trenches, where the surface layers of rockplunge into Earth’s interior. This process is knownas subduction.

• The other type of convergent plate boundaryforms the continent-continent collision zone.

• The third type of plate boundary is the

transcurrent boundary, where the plates movepast one another, without either convergence ordivergence.

• Transform faults or strike-slip faults are theexamples of transcurrent boundaries.



Convergent Plate Boundary

• There are three types of convergent margins depending

upon the type of crust on the colliding plates:1. ocean-ocean convergent margins

2. ocean-continent convergent margins

3. continent-continent convergent margins

• There are two types of convergent boundaries, subductionzones and collision zones.

• Continental collision zones are rare (although extremelyimportant for producing major mountain ranges andplateaus).

•

Currently more than 90% of the world's convergentboundaries are subduction zones, which form at zones of oceanic-oceanic or oceanic-continental convergence.



• Convergence at these boundaries is

accommodated by underthrusting of one

lithospheric plate beneath another.

• The underthrust plate descends through the

asthenosphere and represents "consumed”

lithosphere, which balances the surface area

of new lithosphere created at midoceanicridges.



Subduction Zone Earthquakes

• As the oceanic plate bends downwards at the oceantrenches, fractures cause shallow earthquakes withinthe subducting plate as well as in the overriding plate,these are called interplate earthquakes.

•In the process of its downward movement, additionalforces are generated causing further deformation andfracturing, thus giving rise to deep focus earthquakes.

• The deeper earthquakes, which occur in the subductingplate, define a clear dipping seismic zone, called

Benioff zone.• On the average, frequency of occurrence of

earthquakes in this zone declines rapidly below 200 kmdepth.



• Some earthquakes are, however, as deep as 700 km in some of thesubduction zones, e.g. the Tonga Islands, Java Islands, the NewHebrides etc.

• Earthquakes, generally, with foci between 70-300 km deep are

called intermediate focus and those below this depth are calleddeep focus.

• Along with the deep focus earthquakes, the chemical compositionof rocks in the subducting plate changes.

• The molten fraction of rocks may be stored for a time in magmachambers, which are huge reservoirs underneath volcanic vents.

• From these pressure chambers magma moves upwards from timeto time as lava.

• Thus, a volcanic arc is a typical characteristic in a subduction zone.

• Further, interaction between the two plates, overridingcontinental plate and the subducting oceanic plate, near thetrench causes folding, faulting and thrusting in the continentalcrust, and makes an exposure of ophiolite (oceanic crust) at thesurface.



Collision Zone Earthquakes

• Tectonic earthquakes are also produced at the edges of the interacting plates where the plates collide head on,and rise as mountain chains, e.g. the Himalayan belt inIndo-Eurasian collision zone, the Alpine belt in the

eastern• Europe, the Zagros thrust system in Iran.

• The interplate earthquakes in collision zones areusually shallow (<100 km), and dominated by low anglethrusting, which is a manifestation of lithospheric

shortening.• These low angle faults/thrusts can generate large

earthquakes as we see in the Himalayan belt.



Divergent Plate Boundary

• Divergent margins typically begin on continental crust butquickly wind up on oceanic crust.

• There are several distinct stages in these margins.

• The process of pulling crust apart is called rifting.

• The second stage involves thinning of the continental crust.• The lower crust is ductile and stretches thinner, whereas

the upper crust is brittle.

• Brittle deformation includes the development of activenormal faults and consequently horsts and grabens.

• The next stage involves the development of a narrow oceanbasin.

• A mid-ocean ridge is where new ocean crust is continuouslybeing formed.



• Divergent plate boundaries are characterizedby much shallower seismicity (focal depth lessthan 30 km).

• The mid-oceanic ridges, the divergent zones,are the manifestation of shallow extensionalfaulting.

•

One of the interesting features of mid oceanicridges is that they are offset by lineamentknown as fracture zones.



Transcurrent Plate Boundary

•

Transcurrent boundaries, between horizontallyshearing plates, are of two types

• Transform faults , which offset ridge segments, and

• strike-slip faults that connect various combinations of divergent and convergent plate boundaries.

• Transcurrent plate boundaries are also characterized bymuch shallower seismicity (focal depth less than 30km).

• Transform faults are closely related to transcurrent

faults.• Both types of faults are strike-slip or side-to-side in

movement, however transform faults end at the junction of another plate boundary or fault type, whiletranscurrent faults die out without a junction.



Intraplate Earthquakes

• About 60 percent of the world’s seismicity is mapped by

the interplate earthquakes at the plate boundaries asstated above, and about 10 percent is mapped at thespreading zones.

• The world’s seismicity map, however, shows thatearthquakes do take place within the plate, far from the

plate boundaries as well.• These earthquakes are called intraplate earthquakes, and

they arise from the localised system of forces.

• The recurrence interval of intraplate earthquakes isgenerally much longer than those of the interplate

earthquakes• Hypocentres of the earthquakes in the intraplate zones

rarely occur deeper than 15-20 km.

• Deepest earthquakes indicate the base of the bottom of the fault.



Volcanic Earthquakes

• Volcanic earthquakes are defined as the earthquakeswhich occur or are linked up with the volcanic activity.

• These earthquakes occur in two ways.

• First, often before an eruption minor seismic activityincreases in the vicinity of the volcano.

• Some kilometres below the volcanic vent, very hotviscous magma moves sluggishly under high steampressure through a network of veins and arteries fromone storage chamber to another.

•

Due to this motion, various parts of the surroundingrock become hotter and more strained as the magmapushes them.

• These forces fracture the neighboring rocks, and thestrain is relieved by small or moderate earthquakes.



• Second, sometimes fault rupture precedes themotion of magma and eruption of lava.

• The earthquake waves from the rupturing faultmay shake up the molten material in the storagereservoir beneath the volcano.

• In a way, similar to the violent shaking of a bottleof soda pop, steam and gas that have previouslydissolved in the magma begin to boil off andaccelerate the escape of lava and gaseousmaterial.

• This, in turn, would disturb the unstableequilibrium of the magma below the vent, andstimulate local volcanic earthquakes.



Induced Earthquakes

• Some earthquakes are induced; this means

that human activity triggers the earthquakes.

• These are:

1. Reservoir Induced Seismicity (RIS)

2. Nuclear explosion

3. Rock burst, fluid injection, etc.



Reservoir Induced Seismicity

• Reservoir induced earthquakes have beenreported in conjunction with the impoundmentof water or rapid water level changes behindlarge dams.

• It is unlikely that simply weight of the water,which would only add a tiny fraction of the totalstress at a depth 3 or 5 km below the surface,causes the RIS.

• A more likely explanation is that the pore pressure increases because of the hydrostatichead of the reservoir.

• RIS is mostly shallow focus, less than 6 km.



Nuclear Explosion

• The other man-made earthquake is the

nuclear explosion.

• The generation of the seismic waves due to

the nuclear activity cause the shaking of the

earth and producing the earthquake.



Rock Burst, Fluid Injection etc.

• The third type of induced earthquakes aregenerated by rock burst, fluid or stream injectionetc.

•

Collapsing of the roof of mines and caverns, thesocalled phenomena of rock-burst, occurs due toinduced stress around the mine workings, and itproduces seismic waves.

• Fluid or steam injection process in the oil fieldsfor secondary production also causes minortremors



Effects of earthquakes

• Damage to Man-made Structures

• Effects on Ground

• 5.1 Shaking and ground rupture

•

5.2 Landslides and avalanches• 5.3 Fires

• 5.4 Soil liquefaction

• Effects on Water (Ground and Surface)

• 5.5 Tsunami

• 5.6 Floods



Damage to Man-made Structures

• An earthquake may cause injury and loss of

life, road and bridge damage, general property

damage (which may or may not be covered by

earthquake insurance), and collapse ordestabilization (potentially leading to future

collapse) of buildings. The aftermath may

bring disease, lack of basic necessities, andhigher insurance premiums.



• Description of earthquake damages includes the degree of damage made on small weak mud-hut to modernstructures.

•Masonry structures, however, have been classified into fourcategories to devise the intensity scale.

• Some damages are caused by the effect of shaking notdirectly on the masonry structure but on the foundation.

• Warping or fissuring can make a considerable damage even

to exceptionally strong building.• Relative motions of the two sides of an active fault may

destroy a structure of any kind by bending or crushing.

• Other engineering structures like railways, bridges, damsetc. are also subject to severe damage by shaking, slumpingor faulting.

• Proper earthquake-resistant design for large or tallstructures is still vigorously debated in the engineeringprofession.



• Causes of most earthquake-damages are due to thehorizontal motion rather than the vertical part of themotion.

•Vertical motion operates against gravity, whereas horizontalmotion meets no such resistance.

• Earthquake effects are often evaluated exclusively in termsof acceleration.

• With an amplitude of one inch and a frequency of one per

second, we have an acceleration of 0.1 Gal, which isconsidered as damaging.

• If such motion continues only few seconds, it will notdamage ordinary structure, but if the motion continues for15 or 20 seconds, as it may in a great earthquake, damage

may be much greater.• Long duration, reasonably high acceleration and

considerable amplitudes are the combination which causemaximum damage in buildings and loss of lives.



Masonry structures

• Masonry A. Good workmanship, mortar, and design;reinforced, especially laterally, and bound together by usingsteel, concrete, etc.; designed to resist lateral forces.

• Masonry B. Good workmanship and mortar, reinforced, butnot designed in detail to resist lateral forces.

• Masonry C. Ordinary workmanship and mortar; no extremeweaknesses like failing to tie in at corners, but neitherreinforced nor designed against horizontal forces.

• Masonry D. Weak materials, such as adobe; poor mortar;

low standards of workmanship; weak horizontally.



Effects on Ground

• The primary effects on ground by anearthquake are regional warping, faulting,

offsets, fissures, cracks, changes in elevation

or depression, changes in coast line etc.• The geographical extent of these

macroseismic effects can be vast, may be upto

1000 km for a great earthquake.• Liquefaction of ground is often seen in alluvial

deposits.



Shaking and ground rupture

• Shaking and ground rupture are the main effectscreated by earthquakes, principally resulting inmore or less severe damage to buildings andother rigid structures.

• The severity of the local effects depends on thecomplex combination of the earthquakemagnitude, the distance from the epicenter, andthe local geological and geomorphological

conditions, which may amplify or reduce wavepropagation.

• The ground-shaking is measured by groundacceleration.



• Specific local geological, geomorphological, andgeostructural features can induce high levels of shakingon the ground surface even from low-intensity

earthquakes.• This effect is called site or local amplification.

• It is principally due to the transfer of the seismicmotion from hard deep soils to soft superficial soils.

• Ground rupture is a visible breaking and displacementof the Earth's surface along the trace of the fault,which may be of the order of several metres in the caseof major earthquakes.

• Ground rupture is a major risk for large engineeringstructures such as dams, bridges and nuclear powerstations and requires careful mapping of existing faultsto identify any which are likely to break the ground

surface within the life of the structure.



Landslides and avalanches

• Earthquakes, along with severe storms,volcanic activity, coastal wave attack, andwildfires, can produce slope instability leading

to landslides, a major geological hazard.• Landslide danger may persist while emergency

personnel are attempting rescue.

• Landslides occur in the form of earth slumps,earth flows and earth avalanches are mostnotable.



Fires

• Earthquakes can cause fires by damaging

electrical power or gas lines.

• In the event of water mains rupturing and a

loss of pressure, it may also become difficult

to stop the spread of a fire once it has started.

• For example, more deaths in the 1906 San

Francisco earthquake were caused by fire than

by the earthquake itself.



Soil liquefaction

• Liquefaction of ground is often seen in alluvial deposits.Solid sand formations, if saturated with water, aretransformed into a state of suspension during the vibrationof seismic waves, and behave as a viscous liquid.

• Soil liquefaction may cause rigid structures, like buildingsand bridges, to tilt or sink into the liquefied deposits.

• This can be a devastating effect of earthquakes causingtotal collapse of structures, buildings etc.

• For example, in the 1964 Alaska earthquake, soil

liquefaction caused many buildings to sink into the ground,eventually collapsing upon themselves.



Effects on Water Tsunamis

•Tsunamis are long-wavelength, long-period sea wavesproduced by the sudden or abrupt movement of large volumesof water.

• In the open ocean the distance between wave crests cansurpass 100 kilometers (62 mi), and the wave periods can vary

from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373 –497 miles per hour), dependingon water depth.

• Large waves produced by an earthquake or a submarinelandslide can overrun nearby coastal areas in a matter of

minutes.• Tsunamis can also travel thousands of kilometers across open

ocean and wreak destruction on far shores hours after theearthquake that generated them.

• Most destructive tsunamis are caused by earthquakes of

magnitude 7.5 or more.



• Tsunamis, the long waves on the ocean, are produced by large earthquakes.

• The height of tsunamis, sometimes, increases greatly

and they crash down upon the shore with disastrouseffects.

• A comparison of historical earthquakes and tsunamisshows that a large tsunami washing onto a stretch of populated coastline is likely to be much more

destructive than the shaking from all but exceptionallylarge earthquakes.

• Most tsunamis are caused by fault rupture along asubmerged fault.

•The killing tsunami of the December 26, 2004 Sumatraearthquake is the most recent example, that wasgenerated due to the mega thrust event (M 9.3) in theIndian ocean subduction zone.



Floods

• A flood is an overflow of any amount of waterthat reaches land.

• Floods occur usually when the volume of waterwithin a body of water, such as a river or lake,

exceeds the total capacity of the formation, andas a result some of the water flows or sits outsideof the normal perimeter of the body.

• However, floods may be secondary effects of

earthquakes, if dams are damaged. Earthquakesmay cause landslips to dam rivers, which collapseand cause floods.

causes and effects of earthquakes.pptx

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