Structure of the Earth’s Interior.pptx

7/29/2019 Structure of the Earth’s Interior.pptx

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SEISMIC WAVES AND GROUND

SHAKING

• The body waves (P and S-waves), when movethrough the layers of rock in the crust, arereflected and/or refracted at the interfacesbetween the rock types or layers.

• When P and S-waves reach the surface of theground, most of their energy is reflected backinto the crust.

• Thus, the surface is affected simultaneously byupward and downward moving waves.

• After a few shakes, a combination of two kinds of waves is felt in ground shaking.



• A considerable amplification of shaking occursnear the surface.

• This surface amplification enhances the shakingat the surface of the Earth.

• On the other hand, earthquake shaking belowground surface, say in the mine, is much less.

• Again combination of two kinds of waves in

shaking is not felt at sea.• The only motion felt on ship is from the P-waves,

because S-waves cannot travel through waterbeneath the ship.



• The horizontal and transverse motion of the

Love waves, and elliptical and retrograde

motion of the Rayleigh waves cause severe

damage to the foundations of engineering

structures and buildings.



• Ground Shaking

–

amplitude, duration, and damage increases in poorlyconsolidated rocks



Seismic wave behaviour in the Earth’s

Interior

• Even the most elementary seismic waves recorded by aseismograph station cannot be described anddiscussed without having first a working model of theearth’s interior through which the waves travel.

•For seismological purposes it is convenient to assumethe earth to be constituted of crust, mantle and core.

• This major division was established from the analysis of recorded seismic waves and provides a reasonableworking model.

• The mantle-crust as well as the core-mantle boundaryare distinct discontinuities in seismic wave velocitiesand efficient reflectors/refractors of the incidentseismic energy.



Crust

• The mantle-crust boundary, generally called theMohorovicic discontinuity (often abbreviated asM or Moho).

• It separates rocks at the base of the crust fromthe underlying mantle rocks.

• Compressional-wave velocities at the base of thecrust is about 6.5 km/s, and in the underlingmantle rocks, it is about 8 km/s.

• The average thickness of the crust varies fromabout 25 to 40 km below the continents but maybe as large as 60 to 70 km under high mountains.



• Under the deep ocean, the crust is much thinner,

only about 5 km.

• In studies of nearby earthquakes, epicentral

distance less than 1000 km, we often assume a

crust consisting of two horizontal layers of

approximately the same thickness, separated bythe Conrad discontinuity.

• The upper layer represents granitic rocks,

whereas the lower layer consists of basaltic rocks.• For a typical crustal model under the deep ocean

we usually omit the granitic layer.



Mantle

• The earth’s mantle extends from Moho to core-mantle boundary at 2900 km depth.

• The whole of the mantle is now considered to beessentially solid and to a large extent radially

homogenous.• The compressional wave velocity increases from

about 8 km/s just beneath the Moho to 13.7km/s at the core-mantle boundary.

• The mantle may be subdivided into the uppermantle, including the non-crustal lithosphere andthe asthenosphere, and the lower mantle.



• The upper mantle extends to a depth of about

700 km, where the velocity gradient suddenly

decreases, and contains several discontinuities.

• There is unquestionable seismological evidence

of interfaces, e.g., at depths of 400 and 650 km.

• They all are less precisely determined than the

Moho.

• Because of this factor some works prefer to work

with models containing transition zones or layersof a thickness of the order of, say, 50 km rather

than with definite of sharp discontinuities.



• It is assumed that within the transition zonesthe velocities increases with depth more

rapidly than in the surrounding layers.• Recent research provides good evidence that

the 650 km discontinuity is sharp e.g. short-

period sharp reflections in P’dP’.• While the 400 km discontinuity is not sharp.

• One of the important features of the uppermantle is the world-wide existence of a low-velocity layer (LVL) between about 100 and250 km below the surface.





• Within the LVL, the rocks are partially molten, therigidity is low, the attenuation is the largest of thewhole mantle and seismic wave velocities fall

about 6% when compared with the velocity justunder the Moho.

• The lower mantle extends from some 700 kmdepth to the core-mantle boundary at 2900 kmdepth, first recognized by R.D. Oldham in 1906.

• Whereas this boundary was accurately located byB. Gutenberg in 1913.

• Seismic velocities in the lower mantle increase

gradually with increasing depth although at asignificantly lower rate than in the upper mantle.

• There are no distinct reflectors/refractors in thelower mantle.



Compressional wave Shadow Zone

• Seismic waves arriving at a distance beyond 10°

up to about 30° mainly travel through the upper

mantle (Moho to 410 km) and through the

transition zone to the lower mantle (410-660km).

• At epicentral distances between about 30° and

100°, the P and S waves travel through the lowermantle that is characterized by a rather smooth

positive velocity and density gradient.



• The seismograms are clearly structured with Pand S arrivals, followed by multiple surface andcore-mantle boundary (CMB) reflections on

conversions.• The existence of the great velocity reduction

across the CMB causes seismic wave energy todiffract into the geometric shadow zone at

distances greater than 100°.• Beyond 100°, only P-wave enters outer core, and

reaches surface.

• There is a dramatic reduction of P-wave velocity,

from 13.7 km/s in the lower most mantle to 8.0km/s in the upper outer core.

• This P-wave forms a core shadow.



• Oldham (1906) first observed that a P-wave arrivingdue to an earthquake was late, in comparison with theexpected arrival time.

•

He proposed the existence of core of lower velocitythan outer region, and predicted the presence of ashadow zone.

• Gutenberg (1914) verified that there was a shadowzone for P between = 103° and = 142° with strong

arrivals just beyond 142°.• Gutenberg estimated the depth to the core boundary

as 2900 km, which stood unchallenged.

• The shadow zone of the core is not complete, there

being arrivals of P-waves of small amplitude throughthe entire zone.

• Lehmann (1935) suggested that these arise from innercore of higher velocity within the main core.





Core

• Below the core-mantle boundary is the core of

the earth with an approximate radius of 3500 km.

• The boundary represents a sharp discontinuity in

physical properties such as fall of thecompressional-wave velocity from 13.7 to 8.1

km/s and cessation of shear waves.

•In spite of great observational efforts, no shearwaves that have travelled through the core have

yet been identified on seismograms.



• It is generally accepted that shear waves ceaseto exist at this depth due to the fluid character

of the core.• Seismic wave studies led to a subdivision of

the core into an outer core, which in relationseismic waves act as a liquid and the innercore which acts as a solid.

• Some early works claimed originally that theinner and outer core were separated by a

transition layer about 150 km thick withinwhich the compressional wave velocitydeclines sharply.



• Recent studies do not show this transition

layer and advocate the existence of a rather

sharp discontinuity in the compressional-wave

velocity at the bottom of the outer core.

• The compressional-wave velocity in the inner

core is significantly higher than that in the

surrounding outer core.





Seismic wave behavior

– P waves arrive first, then S waves, then L and R

– After an earthquake, the difference in arrival times at a

seismograph station can be used to calculate the distance from

the seismograph to the epicenter.



Seismic waves are useful for

• 1. determining size & location of earthquakes

• 2. monitoring volcanic activity

•

3. monitoring nuclear explosions• 4. probing interior of the Earth



How is an Earthquake’s Epicenter

Located?

• Three seismograph stationsare needed to locate theepicenter of an earthquake

• The intersection of thecircles locates theepicenter



Pakistan Meteorological Department 24
















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Structure of the Earth’s Interior.pptx