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)

Crust gm/cm 3 Mantle gm/cm 3 Outer Core gm/cm 3 Inner Core gm/cm 3 Density of Water 1 gm/cm 3

Earthquake in Japan Magnitude 8.0 September 25, :50 UTC

Izmit Turkey Seismogram

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.

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”

Two types of surface waves. Rayleigh waves: Discovered by Lord Rayleigh (1887). Love waves: Discovered by A.E.H. Love (1911).

Data from station MBWA (Marble Bar, Western Australia) last updated at Thu 10/30/03 12:53 GMT

In the case of surface waves, the ground motion is concentrated near the surface of the Earth, and the wave motion is similar to that of water waves in the ocean.

In the case of water waves, a particle of water (or a toy boat) at the surface moves in a circular pattern, but stays at the same location. At greater depth, the water particles also move in circles, but the circles are smaller.

Rayleigh wave motion is similar to water wave motion, except that for Rayleigh waves, the circles are “retrograde”. < >

Love waves have side-to-side ground motion, similar to S-waves, except that the ground motion decays with depth in a manner similar to Rayleigh waves and water 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”

Crust gm/cm 3 Mantle gm/cm 3 Outer Core gm/cm 3 Inner Core gm/cm 3 Density of Water 1 gm/cm 3

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