h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Earthquakes are not generated due to the thinning of crustal material. Instead, as the continental plates move past one another along faults, or in regions where there is some strain present. Friction prevents motion of the blocks against one another, until the strain exceeds the frictional forces which hold the blocks immobile. When the blocks begin to move, water and ground rock along the fault serve form a lubricating mixture which facilitates movement along the fault plane, causing a large release of energy. This energy is absorbed as motion in the surrounding rock and forms the longitudinal and pressure waves which we feel as earthquakes.