10. what are seismic waves? does the velocity at which an earthquake wave travels change or stay constant as the wave passes through the earth? 11. what are the principal layers of the earth? what happens to earthquake waves when they reach the boundary between layers? 12. how do temperature and pressure change with increasing depth in the earth? be sure to explain the geothermal gradient 13. what is the moho? how was it first recognized? describe the differences between continental crust and oceanic crust