structure of the earth’s interior.pptx
TRANSCRIPT
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 1/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 2/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 3/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 4/29
• Ground Shaking
–
amplitude, duration, and damage increases in poorlyconsolidated rocks
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 5/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 6/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 7/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 8/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 9/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 10/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 11/29
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 12/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 13/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 14/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 15/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 16/29
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 17/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 18/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 19/29
• 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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 20/29
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 21/29
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.
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 22/29
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
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 23/29
How is an Earthquake’s Epicenter
Located?
• Three seismograph stationsare needed to locate theepicenter of an earthquake
• The intersection of thecircles locates theepicenter
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 24/29
Pakistan Meteorological Department 24
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 25/29
Pakistan Meteorological Department 25
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 26/29
Pakistan Meteorological Department 26
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 27/29
Pakistan Meteorological Department 27
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 28/29
Pakistan Meteorological Department 28
7/29/2019 Structure of the Earth’s Interior.pptx
http://slidepdf.com/reader/full/structure-of-the-earths-interiorpptx 29/29
Pakistan Meteorological Department 29