Internal Structure Of Earth With Respect To Seismic waves

2.  Introduction
 Internal structure of
earth
 The crust
 The mantle
 The core
 Seismic waves
 Types of seismic
waves
 Seismic velocity

3. A Schematic view of interior of earth
CRUST MANTLE OUTER
CORE
INNER
CORE

4. Deep parts of Earth are studied indirectly, however,
largely through the branch of geology called geophysics.
 Geophysics includes the study of seismic waves
and Earth’s magnetic field, gravity, and heat.
 Together, they create a convincing picture of what
makes up Earth’s interior.
Introduction
 There are two important ways of learning about Earth’s
interior.
1.seismic reflection
2.Seismic refraction

5. P-WAVES:
The P-Waves are primary waves, longitudinal or compressional waves
that travel in earth’s interior. P-waves can pass through solid, liquid
and gas.
S-WAVES:
The S-waves are the secondary waves or shear waves. These can
only pass through solid.
1-BODY WAVES:
These waves travel in the interior of earth.
TYPES OF SEISMIC WAVES

6. 2-Surface waves:
 These waves travel along the Earth's surface.
 They travel more slowly than seismic body waves.
 In large earthquakes, these waves have an
amplitude of several centimeters.
 Rayleigh waves:
 Rayleigh waves that travel as ripples with motions that are similar to those of
waves on the surface of water.
 These waves was predicted by John William Strutt, Lord Rayleigh, in 1885.
 They are faster than body waves.
 The Rayleigh waves depends on their frequency and wavelength.
 Love waves :
 Love waves are horizontally polarized shear waves.
 They are named after A.E.H. Love
 They usually travel slightly slower than Rayleigh waves and have
the largest amplitude and more distructive.

7. Composition
Density
Temperature
Pressure
Medium.
●Seismic waves travel more quickly through denser
materials.
● Seismic waves move more slowly through a liquid
than a solid.
● Hot areas slow down seismic waves.
Seismic velocities depend on the
material properties such as:

8.  These reflected waves are recorded on a seismogram, which
shows the amount of time the waves took to travel down to the
boundary, reflect off it, and return to the surface.
Seismic reflection:
The return of some of the energy of seismic waves to Earth’s
surface after the waves bounce off a rock boundary.
 From the amount of time necessary for the round trip,
geologists calculate the depth of the boundary.
Seismic refraction:
 The bending of seismic waves as they pass from one material to
another.
 As a seismic wave strikes a rock boundary, much of the energy
of the wave passes across the boundary & it changes direction.
 This refraction occurs only if the velocity of seismic waves is
different in each layer.

9. By seismic study earth has following layers:
1-The crust
Continental crust
Oceanic crust
2-The mantle
Upper mantle
Lower mantle
3-The core
Outer core
Inner core

10. Studies of seismic waves have shown that the
crust is thinner beneath the oceans as compare
to the continents.
The uppermost layer of earth is called crust.
 Seismic waves travel faster in oceanic crust than in
continental crust.
On this basis crust is divided into two types.
Continental crust
Oceanic crust
 Seismic P waves travel through oceanic crust at about 7km/s
& about 6km/s through continental crust.

11.  Lithosphere
 The combination of uppermost mantle and crust above the
asthenosphere is called the lithosphere.
 The lithosphere is free to move over the weak asthenosphere.
 The tectonic plates are, in. fact, lithospheric plate
The boundary that separates the crust from the
mantle is called the Mohorovic˘ic´ discontinuity.
Moho Boundary:

13. Properties of the Crust:
Continental Crust:
►Depth 20 to 70 km, average 30 to 40 km
► Composition: felsic, intermediate, and mafic
igneous, sedimentary, and metamorphic rocks.
► Age: 0 to 4 B.y.
► Summary: thicker, less dense, heterogeneous,
 Oceanic Crust:
► Depth to Moho: ~7 km
► Composition: mafic igneous rock (basalt &
gabbro) with thin layer of sediments on top
► Age: 0 to 200 M.y.
► Summary: thin, more dense, homogeneous,

14. The Mantle:
 The next layer below the crust is mantle.
 Its thickness extended to a depth of 2890km.
 It is more denser than crust
 It consist of dense silicate rocks
both p-waves & s-waves travel through the mantle
 depths of around 100 km to 250 km arrive later than expected
(because pressure is increasing) indicating a zone of low seismic wave
velocity.

15. Velocity of seismic waves in mantle:
P-waves velocity:
Its velocity is about 8km/s in the upper
mantle and increases up to 13km/s in lower
mantle
 S-waves velocity:
S-waves velocity profile in the mantle is
similar to the p-wave profile. S-waves velocity
time cannot be measured as accurately as p-
waves travel time.

16. The core
The central or innermost portion of
earth below the mantle is core
 it lies at depth of 2900km up to
6370km
 the core is divided into two layers
outer core
inner core
Outer core:
The outer core begins at a depth of 2,890 to 5,150 km.
 The temperature at outer core is about 3700˚C
Inner core:
 The inner section begins at a depth of 5,150 to 6,360km.
 The temperature of inner core is about 4300˚C.

17. Seismologists first determined that the
back and-forth motion of S-waves does not
travel through liquid.
 Then, seismologists noticed that S-waves
do not travel through Earth’s center.
 This observation led to the discovery
that Earth’s core must be at least partly
liquid.

18. Velocity of seismic waves in core:
 P-waves velocity in outer
core is 8.1km/s and in inner
core is 11.3km/s.
 The outer core doesn’t
transmit S-waves and is
therefore assume to be
fluid/liquid.

19. At arc distances of between
about 103° and 143° no P
waves are recorded.
 No S waves are record beyond
about 103°.
 Gutenberg (1914) explained
this as the result of a molten core
beginning at a depth of around
2900 km.
Low velocity zone: