An earthquake consists of vibrations of the Earth's surface that follow a release of energy in the Earth's crust (Stein, pp. 215-285). The energy release may be from slippage along a geologic fault or fault zone or movement of magma associated with volcanic activity. Pressure in the Earth may build up and result in bending of geologic units, then sudden breaking and "snapping" to a new orientation. In the process of breaking, vibrations called seismic waves are generated. These waves travel outward from the source of the earthquake along the surface and through the Earth at different velocities depending on the materials that they move through.
Earthquake vibrations (seismic waves) can be initiated by a number of subsurface actions including movement along faults, and volcanic activity. Some activities of man including use of explosives and mechanical methods (e.g. weight dropping and sledge hammer pounding) can cause seismic waves. The most powerful earthquakes are caused by large-scale earth movements along faults. Activities of man are sometimes used to create seismic waves for geophysical subsurface surveys using either the seismic refraction or reflection methods.
The Epicenter and Focus of an Earthquake
The drawing above illustrates the epicenter and focus of an earthquake. The focus is the point or center where the energy release starts. The epicenter is the point on the Earth's surface directly above the focus of the earthquake. When the energy release occurs, seismic waves travel away from the focus in all directions.
The photo (source: morgueFile.com) on the left illustrates typical earthquake damage to loose blocks of stone or masonry. Structures constructed of many materials are prone to damage from the shaking caused by seismic waves, usually close to the epicenter of an earthquake. Earthquake resistant construction techniques and materials can be used to minimize damage from intense seismic vibrations.
Some Useful Links
The above discussion provides basic information and illustrations for earthquakes and seismic waves. To further explore this subject, excellent publications or the resources of the internet can be explored. Following are some hyperlinks that can further assist with the evaluation and description of earthquakes and sesimic waves:
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