earthquakes 1.pdf

8/13/2019 Earthquakes 1.pdf

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EarthquakesEarthquakes


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An earthquake is a trembling or shaking of

the ground caused by the sudden release of

energy stored in the rocks beneath earths

surface.The shaking during an earthquake is caused

by seismic waves.


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Tectonic forces acting deep in the earth

may put a stress on the rock, which may

bend or change in shape (strain).

If ou bend a stick of wood our handsput a stress (the force per unit area) on the

stick; its bending (a change in shape) is the

strain.


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Like a bending stick, rock can deform only

so far before it breaks. When a rock breaks,

waves of energy are released and sent out

through the earth.

These are seismic waves, the waves of

energy produced by an earthquake.

It is the seismic waves that cause the

ground to tremble and shake during anearthquake.


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The sudden release of energy when rock

breaks may cause one huge mass of rock to

slide past another mass of rock into a

different relative position. The break

between the two rock masses is a fault.

The classic explanation of why earthquakes

take place is called the elastic rebound

theory.


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Elastic Rebound TheoryElastic Rebound Theory

It involves the sudden release of

progressively stored strain in rocks, causing

movement along a fault.

Tectonic forces act on the rocks for man

decades.

Initially, the rock bends but does not break.

More and more energy is stored in the rock

as the bending becomes more severe.


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Elastic Rebound TheoryElastic Rebound Theory

Eventually, the energy stored in the rockexceeds the strength of the rock.

and the rock breaks suddenly, causing anearthquake.

Two masses of rock move past oneanother along a fault.

The strain on the rock is released; theenergy is expended by moving the rock intonew positions and by creating seismic

waves.


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CausesCauses

Most earthquakes are associated with

movement on faults, but in some quakes,

the connection with faulting may be

difficult to establish. Some quakes occur on buried thrust faults;

Unknown faults usually with no surface

displacement.


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CausesCauses

Earthquakes also occur during explosive

volcanic eruptions

and as magma forcibly fills underground

ma ma chambers rior to man eru tions Such quakes are not associated with fault

movement at all.


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CausesCauses

Another cause has been recently postulatedfor deep earthquakes (100 to 670 kilometersbelow the surface), essentially all of whichare found on cold, subducting plates sliding

own n o e man e.Although the down-going plates are colder

than the surrounding rock, the hightemperature and pressure at depth suggest

that the rock in the plates should behave ina ductile way rather than breaking in thebrittle manner of near-surface rocks.


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CausesCauses

The suggested cause for such deep quakes

is mineral transformations within the down-

going rock, as pressure collapses one

mineral into a denser form. Lab experiments have also shown bodies of

the new, denser minerals along fractures.

Whether the process occurs on a large scale

to produce large quakes is still unknown.


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CausesCauses

Similar suggestions for the cause of deep

quakes include the dehydration of water

containing serpentine and the conversion

of serpentine into glass. Both of these processes occur suddenly on

small fractures in lab experiments.


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TerminologyTerminology

Focus/Hypocenter

The point within the earth where seismic

waves first originate.

The point of initial breakage and

movement on a fault.

Rupture begins at the focus and then

spreads rapidly along the fault plane.


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TerminologyTerminology

Epicenter

The point on the earths surface directly

above the focus


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Seismic WavesSeismic Waves

Seismic waves are generated by the release

of energy during an earthquake. They

travel through the earth like waves travel

through water.

The study of seismic waves and

earthquakes is called seismology, which is a

branch of geophysics.


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Seismic wavesSeismic waves

The propagation velocity of the waves

depends on density and elasticity of the

medium. Velocity tends to increase with

depth, and ranges from approximately 2 to

8 km/s in the Earth's crust up to 13 km/s

in the deep mantle.

Two types

Body waves Surface waves


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Body wavesBody waves

Body waves are seismic waves that travel

through the Earths interior, spreading

outward from the focus in all directions.

Two t es of bod waves P- waves (primary)

S- waves (secondary)


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P waves

a compressional (or longitudinal) wave in

which rock vibrates back and forth parallel

to the direction of wave ro a ation.Because it is a very fast wave, traveling

through near-surface rocks at speeds of 4

to 7 kilometers per second, a P wave is the

first (or primary) wave to arrive at arecording station following an earthquake.


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S waves

A slower, transverse wave that travels

through near-surface rocks at 2 to 5

kilometers er second. S wave is propagated by a shearing motion

much like that in a stretched, shaken rope.

The rock vibrates perpendicular to the

direction of wave propagation.


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Both P waves

and S waves pass

easily through

solid rock.

P wave can also

pass through a

fluid (gas or

liquid), but an Swave cannot.


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Surface wavesSurface waves

The waves that travel along the Earth'ssurface.

They are called surface waves because theydiminish as they get further from the

sur ace.

In general, surface waves cause moreproperty damage than body waves becausesurface waves produce more ground

movement and travel more slowly, so theytake longer to pass.


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Two types

Love wave

Rayleigh wave


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Surface WavesSurface Waves

Love wavesThey are like S waves that have no vertical

displacement. The ground moves side to sidein a horizontal plane that is perpendicular tothe direction the wave is traveling or

propagating.

Like S waves, Love waves do not travelthrough liquids and would not be felt on abody of water.

Because of the horizontal movement, Lovewaves tend to knock buildings off theirfoundations and destroy highway bridgesupports.


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Rayleigh waves Rayleigh waves behave like rolling ocean waves

as they cause rolling motion on earths surface.Unlike ocean waves, Rayleigh waves cause the

to the direction the wave passes.

Rayleigh waves tend to be incrediblydestructive to buildings because they produce

more ground movement and take longer topass.


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