Distanced measured in degrees from epicenter.

Travel-time versus Distance Curves

Crust6-7 km/sec

Mantle8-13 km/sec

Outer Core8-10 km/sec

Inner Core10-11 km/sec

P-wave Velocitie

s

VP = P-wave velocity

VS = S-wave velocity

D = density

Differences in density and other physical properties affect the velocity of seismic waves.

Granite (and Average Continental Crust)

6.0 km/sec

3.5 km/sec

2.5 km/sec

3.0 km/sec

Crust2.6-3.1 gm/cm3

Mantle3.3-5.7 gm/cm3

Outer Core10-12 gm/cm3

Inner Core13-14 gm/cm3

Density of Water1 gm/cm3

€

VP =K + 4

3G

D

€

VS =G

D

K = bulk modulus = resistance of material to squeezing

G = shear modulus = resistance of material to shearing

D = density

€

VP =K + 4

3G

D

€

VS =G

D

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

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VP =K + 4

3G

D

€

VS =G

D

G = 0for liquids

Outer Core of Earth is liquid

GraniteBasaltPeridotite

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

Crust:Granite/Basalt

Mantle:Peridotite

Outer Core:Liquid Iron

Inner Core:Solid Iron

Composition of Earth’s Interior

Why have geophysicists concluded that the Earth’s core is primarily composed

of iron?

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.

Solar System

Meteorites

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

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.

Origin of the Solar

System

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.

Stony MeteoriteIron Meteorite

Types of Meteorites

Iron Meteorites – Composed of iron and nickel.

Stony Meteorites – Composed of dense rock similar to “peridotite”.

Composition of Meteorites

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.

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).

Heavy minerals (iron) sank to the center of Earth to form the Core, and lighter minerals floated to the top forming the Crust.

What causes the Earth’s magnetic field?

Dynamo Theory of Earth’s Magnetic Field

Rapid motion of liquid iron circulating in the outer Core

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

Rock

Metallic Core

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).

Crust:Granite/Basalt

Mantle:Peridotite

Outer Core:Liquid Iron

Inner Core:Solid Iron

Composition of Earth’s Interior

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.

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

Environmental Geophysics

Depth to Bedrock Subsurface Structure Subsurface Paths for Flow of Groundwater

250 - 600 m/sec

300 - 900 m/sec

2000 - 6000 m/sec

VP

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