Why do seismic waves travel faster through the upper mantle than in the core? Why does a seismic wave travel faster through the upper mantle than it does in the crust?
1 Answer
The densities and temperatures are different.
Explanation:
Seismic velocities depend on the material properties such as composition, mineral phase and packing structure, temperature, and pressure of the media through which seismic waves pass.
Seismic waves travel more quickly through denser materials and therefore generally travel more quickly with depth. Anomalously hot areas slow down seismic waves. Seismic waves move more slowly through a liquid than a solid.
Molten areas within the Earth slow down P waves and stop S waves because their shearing motion cannot be transmitted through a liquid. Partially molten areas may slow down the P waves and attenuate or weaken S waves. - from http://www.columbia.edu/~vjd1/earth_int.htm This page also has some very good graphics.
Some other graphics here: http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.html
