1. Earthquakes and the Interior  Earthquakes are definitely a geologic hazard for people living in earthquake regions, but the seismic waves generated by earthquakes are invaluable for studying the interior of Earth.  Earthquake waves indicate several things about Earth’s Interior: 1)Earth appears to be layered, according to composition and physical properties. 2)Density increases with depth. 3)Outer Core is a liquid. Reference: Pages

2.  If composition (or physical properties) change abruptly at some interface, then seismic waves will both reflect off the interface and refract (or bend) as they pass through the interface. Two cases of wave refraction can be recognized. 1) If the seismic wave velocity increases when passing into the rock below the interface, the waves will be refracted or bent upward relative to their original path. (At Moho) Earthquakes and the Interior

3.  If composition (or physical properties) change abruptly at some interface, then seismic waves will both reflect off the interface and refract (or bend) as they pass through the interface. Two cases of wave refraction can be recognized. 2) If the seismic wave velocity decreases when passing into the rock below the interface, the waves will be refracted down relative to their original path. (Mantle – Outer Core) Earthquakes and the Interior

4.  If the seismic wave velocities gradually increase with depth in the Earth, the waves will continually be refracted along curved paths that curve back toward the Earth's surface. Earthquakes and the Interior

5.  Scientists look for variations in Seismic wave velocities that correspond to changes in rock properties. These changes indicate that Earth has four major layers; 1) Earth Appears to Be Layered 1)Crust - very thin outer layer ranging in thickness, 5km for oceans and up to 60km for continents. 2)Mantle - Seismic wave velocities increase abruptly at the Moho. (boundary between crust and mantle) 3)Outer Core - At a depth of 2900 Km P-wave velocities suddenly decrease and S-wave velocities go to zero. This is the top of the outer core. As discussed above, the outer core must be liquid since S-wave velocities are 0. 4)Inner Core - At a depth of about 4800 km the sudden increase in P-wave velocities indicate a solid inner core.

6.  Scientists also contribute the increase in seismic wave velocity to an increase in density with depth into Earth’s interior. 2) Density Changes With Depth  The four main Layers of Earth’s Interior differs in composition and physical state. This contributes to the change in density within Earth. 1)Crust – Continental crust composed of lower density silicates, whereas the ocean crust is composed of higher density silicates. 2)Mantle – Increase in temperature and pressure causes the silicates rocks to increase in density with depth. 3)Outer Core – Consist of very dense nickel and iron in a molten (liquid) state. 4)Inner Core - Consist of very dense nickel and iron in a solid state.

7.  Scientists look for variations in Seismic wave velocities at the boundaries of each layer. These changes indicate that Earth has a liquid outer core with a solid inner core within. 3) Liquid Outer Core  The evidence for a liquid outer core was based on the behavior of P- and S-waves at the mantle-core boundary.  P-waves are bent (refracted) so to bend deeper into the outer core which results in a P-wave shadow Zone.  S-waves do not pass through the core and a S-wave shadow zone results on the opposite side of Earth.

8. 105 E 140 E P-wave Shadow Zone  P-waves are bent (refracted) at the mantle-outer core boundary and bend deeper into the outer core. This causes these P-waves to arrive at Earth’s surface some distance away from P-waves that do not travel through the Outer core.  This resulted in an area from 105 degrees to 140 degrees from the focus of an earthquake where no P-waves were detected. called a P-wave shadow Zone.  This is called a P-wave shadow zone.

9. S-wave Shadow Zone  The S-wave shadow zone occurs because no S-waves were detected after 105 degrees from the focus. Since no direct S- waves arrive in this zone, it implies that no S-waves pass through the core.  Therefore, the core or at least part of the core is in the liquid state, since no S-waves are transmitted through liquids.  Thus, the S-wave shadow zone is best explained by a liquid outer core. 105 E