Earthquakes, Tsunamis, and Mountain Building

Earthquakes result from the sudden release of built-up strain energy along a fracture in the crust known as a fault. Rock on either side of a fault can remain locked by friction for long periods even as tectonic forces continue to deform it elastically, storing energy much like a bent stick; when the accumulated stress finally exceeds the friction holding the fault together, the rock ruptures and snaps back toward its original shape, a process known as elastic rebound, releasing the stored energy as seismic waves. These waves radiate outward from the point of rupture, called the focus or hypocenter, with the point on the surface directly above it known as the epicenter. Several types of waves are generated: faster-moving primary, or P, waves, which compress and expand rock as they travel and can pass through liquids; slower secondary, or S, waves, which move rock side to side and cannot pass through liquids; and surface waves, which travel only along the outer layers but tend to cause the most structural damage because of their larger amplitude and longer duration.