The San Francisco earthquake

Earthquake is a shaking of the domains go up. harmonize to the theory of scale tectonics, the earths crust is formed by a number of large plates that move very slowly in various directions on the earths surface. Most earthquakes occur in effect(p) a boundary between two plates. As one plate pushes past or over a nonher, great stresses build up in the swing along the edges of the plates because friction prevents them from sliding past each other. Eventually, the stresses plough great enough to rupture the swing. The edges of the plates slip a short outdo in opposite directions, causing an earthquake.The greater the stresses that have built up, the greater the resulting earthquake is. Some earthquakes are caused by the movement of lava beneath the surface of the earth during volcanic activity. The cause of earthquakes that occur in regions far from volcanoes or plate boundaries is uncertain. An earthquake is accompanied by three types of waves that vibrate the earths surfacepr imary, secondary, and surface waves. The primary wave alternately compresses and expands the rock of the earths interior. This wave moves through the earth at a speed that varies greatly.Under some conditions it is about five miles per second. The secondary wave shakes the rock sidewise as it advances. It travels through the interior of the earth about three-fifths as fast as the primary wave. While the Surface wave is the or so damaging, resembles the waves produced when a stone is dropped in a puddle. The surface of the table has not vibrated more than a fraction of an inch. This is why a slight panic in the earth can create havoc on the surface. Moreover, earthquakes toppled inherent cities, killed thousands of persons, and caused disastrous fires and oceanic waves (tsunamis).However, most(prenominal) earthquakes, of which there are thousands each year, are relatively weak and cause little or no damage. No part of the world is entirely free of earthquakes, but they occur most frequently in areas in which the earths crust is stillness changing. These areas, called earthquakes belts, include the shores of the Pacific Ocean and an area extending from south-central Asia to the Mediterranean Sea. study earthquakes often leave visible signs of their power, altering features on the earths surface. Reelfoot Lake in northwestern Tennessee was created by a series of earthquakes centered on New Madrid, Missouri, in 1811-12.The San Francisco earthquake of 1906 was associated with the great San Andreas Fault in California, a fracture in the earths surface that can be traced for more than 500 miles. An earthquake, even a small one, may be accompanied by a low, rumbling sound like thunder. The cycle of a mild quake feels like that caused by the passing of a heavy train. Large quakes are usually followed by many littler quakes called aftershock. Aftershocks can occur during a period of several months after the genuine quake.