Earthquakes

Seismicity in the Earth's crust is largely a function of the buildup of strain as a result of the movement of tectonic plates, which in turn results from the circulation of magma in the underlying mantle. Movement occurs during the slippage of rigid bodies along discontinuities [Page 144](fault planes) buried in the crust, which propagates elastic waves through the rocks and along the surface. Although at least 3,000 significant earthquakes occur each year, usually fewer than 20 of them will cause damage and casualties.

The configuration of crustal plate boundaries is one of the principal determinants of the geographical pattern of seismicity. For instance, the rim of the Pacific Ocean largely coincides with plate boundaries, many of which are engaged in dynamic movement that in exceptional ...