Earthquakes are vibrations of the Earth that are caused by the rupture and sudden movement of rocks along faults or breaks within the earth's crust. The elastic-rebound theory is a model used to explain the origin of earthquakes. It illustrates the buildup of elastic strain from stresses within the earth's crust and the abrupt release of energy along faults as the rocks return or rebound to their initial state (Ristau 249; Hamblin and Christiansen 510). The point within the Earth where the earthquake's energy is generated by the initial slippage or movement is called the focus. The epicenter is the point on the Earth's surface that is directly above the focus (Hamblin and Christiansen 510).
An earthquake shock generates three types of seismic waves that travel at different speeds: primary waves, secondary waves and surface waves. Primary (P) waves are first to arrive at a seismograph and are a kind of compressional wave, so called because it transmits energy by compressing and dilating the material through which it moves. Next to arrive are secondary (S) waves, which are shear waves that cause particles to oscillate back and forth at right angles to the direction of wave travel. Shear waves cannot move through liquids. The last to arrive are surface waves that travel relatively slowly along the Earth's surface (Hamblin and Christiansen 511).
The destructive power, or intensity, of an earthquake is evaluated by the severity of ground motion at a certain location and is often reported in terms of subjective damage caused to human structures. Intensity is affected by several factors, the most prominent being the total amount of energy released, the distance from the epicenter, and the type of rock and its degree of consolidation (Hamblin and Christiansen 512). A popular scale for measuring earthquake intensity is the Mercalli scale (and its modified versions).
The magnitude of an earthquake is the objective and more precise measure of the amount of energy it released. It is based on direct measurements of the amplitude or size of seismic waves using specialized recording devices. The earliest earthquake magnitude scale is called the Richter scale. It has since been modified to better distinguish the differences in larger earthquakes. The most widely used magnitude scale today is called the moment magnitude scale (Hamblin and Christiansen 514).
Learning the science behind earthquakes is an important tool for better development of mitigation procedures when disaster strikes. Moreover, while the exact location and timing of an earthquake cannot be foretold or predicted, seismic risk can be evaluated to prepare for future earthquakes at certain areas.
Works Cited
Hamblin, W. Kenneth, and Eric H. Christiansen. Earth's Dynamic Systems. 10th Ed. New Jersey: Prentice Hall, Inc., 2004. Print.
Ristau, John. "Elastic Rebound Theory." Encyclopedia of Natural Hazards. Ed. Peter T. Bobrowsky. Netherlands: Springer, 2013. Print.
