the primary “ tool ” for investigating the physical properties of the earth ’ s interior is...
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The primary “tool” for investigating the physical properties of the Earth’s interior is seismology.
Seismology
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Seismology = The study of seismic waves.
Seismograph = Instrument that records seismic waves.
Seismogram = The record of ground motion that is produced by a seismograph.
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The Earth’s Interior
Deep wells and boreholes(4 to 12 km)
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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
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Earthquake in JapanMagnitude 8.0
September 25, 200319:50 UTC
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Izmit Turkey Seismogram
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How does a seismograph work?
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To measure horizontal ground motion:
A heavy mass is decoupled from the Earth by means of a pendulum.
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When the ground moves, the mass tends to remain stationary because of its inertia, but the support (frame) moves with the Earth.
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The movement of the Earth relative to the stationary mass is recorded on a rotating drum.
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To measure vertical motion:
The principle is the same, but the mass is suspended on a spring.
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With a modern seismograph, the ground motion is also recorded on a computer.
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AS1 Seismograph
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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
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AS1 Seismograph - Devlin Hall, Boston College
Earthquake in AlaskaMagnitude 7.9
November 3, 200222:12 UTC
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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.
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Two types of body waves.
P-waves = Primary Waves = first arrival
S-waves = Secondary Waves = second arrival
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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.
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S-waves “shake” the material at right angles to their direction of travel.
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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.
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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
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Continental Crust:P-wave Velocity = 6 km/sec“Granite”
Oceanic Crust:P-wave Velocity = 7 km/sec“Basalt”
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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
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Crust6-7 km/sec
Mantle8-13 km/sec
Outer Core8-10 km/sec
Inner Core10-11 km/sec
P-wave Velocitie
s