Three Worlds to Explore Look Up? (Astronomy) Look Down? (Geophysics) Look at Surface? (Geology)

The Earth’s Interior Deep wells and boreholes (4 to 12 km)

The Earth: What’s it made of? Gravity Seismology Magnetism S N Heat Flow Astronomical Observations

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

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

What is a Seismic Wave? Seismic waves are vibrations that travel through the Earth carrying the energy released during an earthquake. Seismic waves travel outward in all directions, like ripples in a pond. Epicenter Seismic Waves

Seismology = The study of seismic waves. Seismograph = Instrument that records seismic waves. Seismogram = The record of ground motion that is produced by a seismograph.

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.

Slide courtesy of Seth Stein

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.

The Earth’s Interior Deep wells and boreholes (4 to 12 km)

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).

Slide courtesy of Seth Stein monthsweeksdayshoursminutes

Slide courtesy of Seth Stein monthsweeksdayshoursminutes

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

Magnitude 7.1 Earthquake in Taiwan - December 26, 2006

Slide courtesy of Seth Stein Radial Transverse Vertical Rayleigh Love Rayleigh

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