The primary “tool” for investigating the physical properties of the Earth’s interior is seismology.

Seismology

Seismology = The study of seismic waves.

Seismograph = Instrument that records seismic waves.

Seismogram = The record of ground motion that is produced by a seismograph.

The Earth’s Interior

Deep wells and boreholes(4 to 12 km)

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

Earthquake in JapanMagnitude 8.0

September 25, 200319:50 UTC

Izmit Turkey Seismogram

How does a seismograph work?

To measure horizontal ground motion:

A heavy mass is decoupled from the Earth by means of a pendulum.

When the ground moves, the mass tends to remain stationary because of its inertia, but the support (frame) moves with the Earth.

The movement of the Earth relative to the stationary mass is recorded on a rotating drum.

To measure vertical motion:

The principle is the same, but the mass is suspended on a spring.

With a modern seismograph, the ground motion is also recorded on a computer.

AS1 Seismograph

Earthquake damage near the epicenter of a magnitude 5.1 earthquake 15 miles south of Plattsburgh, NY (near Au Sable Forks, NY) on Saturday, April 20, 2002.

AP Photo

AS1 Seismograph - Devlin Hall, Boston College

Earthquake in AlaskaMagnitude 7.9

November 3, 200222:12 UTC

Two main groups of seismic waves.

Body waves travel through the Earth’s interior.

Surface waves are guided by the outer layers of the Earth.

Two types of body waves.

P-waves = Primary Waves = first arrival

S-waves = Secondary Waves = second arrival

P-waves and S- waves are distinguished from each other by the way they propagate through the Earth.

P-waves push (compress) and pull (dilate) rocks in the direction the wave is traveling.

S-waves “shake” the material at right angles to their direction of travel.

P-waves can travel through solids, liquids, and gases because matter in all three of these states resists being compressed and will elastically spring back once the force is removed.

S-waves can only travel through solids, because fluids do not resist shearing motion. So, fluids do not transmit S-waves.

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

Continental Crust:P-wave Velocity = 6 km/sec“Granite”

Oceanic Crust:P-wave Velocity = 7 km/sec“Basalt”

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

Crust6-7 km/sec

Mantle8-13 km/sec

Outer Core8-10 km/sec

Inner Core10-11 km/sec

P-wave Velocitie

s