Earthquakes, one of the most destructive natural phenomena, consist of rapid vibrations of rock near the earth's surface. Because of their unpredictable occurrence and enormous capacity of destruction, they have brought fear to mankind since ancient times. A single shock usually lasts no more than a few seconds, but a series of smaller quakes may last for as long as five minutes. The quake felt on the surface is always the result, not the cause of some underground geologic process, and in many cases the damage done is immense.The Greek word for shaking, and (when applied to the earth) earthquake, is seismos. Therefore, the science of earthquakes is called seismology (World Book Dictionary 1985). Earthquakes have been recorded as early as 526 A.C., but seismology is comparatively new. Until the 18th century, few accurate descriptions of earthquakes were recorded, and little was known about what caused them. When seismology was introduced it was learned that many earthquakes are the result of sea floor spreading, but most are caused by volcanic eruptions and plate tectonics. The plate tectonic theory explains that the earth is made up of 20 different plates that are always moving slowly past each other. This action pulls and compacts the plates, creating intense forces that cause the plates to break. This, in turn, causes earthquakes. The energy released then travels along fault lines in seismic waves (World Book Encyclopedia).Seismic waves are either P-waves (primary), or S-waves (secondary). P-waves create a relatively low wave train and arrive at the surface first. On the surface they create a push-pull effect, thus moving the surface up and down. Their low amplitude and vertical movement create an effect much like a dangling slinky. S-waves arrive second and are much more damaging. The high amplitude of S-waves, combined with their horizontal movement cause crippling effects on the earth’s surface and man made ...
