2. Travel-time versus Distance Curves
Distanced measured in degrees from epicenter.

3. P-wave Velocities Crust 6-7 km/sec Mantle 8-13 km/sec Outer Core
Inner Core
10-11 km/sec

4. VP = P-wave velocity
VS = S-wave velocity
D = density

5. Granite (and Average Continental Crust)
6.0 km/sec
2.5 km/sec
3.5 km/sec
3.0 km/sec
Differences in density and other physical properties affect the velocity of seismic waves.

6. Density of Water
1 gm/cm3
Crust
gm/cm3
Mantle
gm/cm3
Outer Core
10-12 gm/cm3
Inner Core
13-14 gm/cm3

7. K = bulk modulus
= resistance of material
to squeezing
D = density
G = shear modulus
= resistance of material
to shearing

8. Seismological Results
Values of K, G and D at a given point in the Earth’s interior.
Search for materials with appropriate values of K, G and D
Estimates of composition of different parts of the Earth’s interior

9. Outer Core of Earth is liquid
G = 0
for liquids
Outer Core of Earth is liquid

10. Basalt Granite Peridotite
Peridotite = a dense rock that is thought to be a major component of the Earth’s Mantle.

11. Composition of Earth’s Interior
Crust:
Granite/Basalt
Mantle:
Peridotite
Outer Core:
Liquid Iron
Inner Core:
Solid Iron

12. Why have geophysicists concluded that the Earth’s core is primarily composed of iron?

13. Why have geophysicists concluded that the Earth’s Core is primarily composed of iron?
Iron is an abundant element in the Solar System (meteorites).
The Core has the right density for iron (10 to 13 gm/cm3).
The Core has the right VP and VS for iron.
Liquid iron in the Outer Core - Earth’s magnetic field.

14. Solar System

15. Meteorites
Meteor Crater, Arizona (about 0.75 miles across and about 560 feet deep). The crater was created approximately 50,000 years ago.

16. Meteorites
Meteorites are the remains of rocky and metallic material that fall to the Earth from inter-planetary space.
These meteorites are thought to be representative samples of material that formed the early solar system, and they are believed to have solidified at about the same time as the Earth.

17. Origin of the Solar System

18. Meteorites provide information about the types of materials that were available to form the Earth and the other planets when the Solar System formed 4.6 billion years ago.
Thus, by combining information about meteorites, with results of seismic studies, we can estimate the composition of the Earth’s interior.

19. Types of Meteorites
Iron Meteorite
Stony Meteorite

20. Composition of Meteorites
Iron Meteorites – Composed of iron and nickel.
Stony Meteorites – Composed of dense rock similar to “peridotite”.

21. The Earth’s crust contains a much smaller % of iron than meteorites do.
Heavy minerals (iron) sank to the center of Earth during the early history of the Earth to form the Core, and lighter minerals floated to the top forming the Crust.

22. Heavy minerals (iron) sank to the center of Earth to form the Core, and lighter minerals floated to the top forming the Crust.
Therefore, the Core of the Earth is believed to be mainly iron (similar to iron meteorites).
and
The Mantle is believed to be mainly peridotite (similar to stony meteorites).

23. What causes the Earth’s magnetic field?

24. Dynamo Theory of Earth’s Magnetic Field
Rapid motion of liquid iron circulating in the outer Core

25. Venus, our nearest neighbor in space, may be structurally similar to Earth, also having an iron core.

26. Rock
Metallic Core

27. But, Venus is spinning very slowly (once in 243 days).
So, one of the important ingredients of the dynamo theory is absent.
Thus, Venus would not be expected to have a magnetic field (and none has been observed).

28. Composition of Earth’s Interior
Crust:
Granite/Basalt
Mantle:
Peridotite
Outer Core:
Liquid Iron
Inner Core:
Solid Iron

29. Variations in the seismic velocity are depicted by warm (low velocity) or cool (high velocity) colors. Large scale patterns correlate with features at Earth's surface. Lower velocities are seen beneath mid-ocean ridges, high velocities are detected beneath regions of past or present subduction.

30. Computer simulation of P-waves from underground nuclear explosion in Nevada, propagated westward under the Sierra Nevada to stations in California.

32. Environmental Geophysics
Depth to Bedrock
Subsurface Structure
Subsurface Paths for Flow of Groundwater
m/sec
m/sec
m/sec VP

33. Image of upper Mantle seismic structure at 100 km depth beneath North America. Blue is high velocity Mantle and red is low velocity Mantle.