Gone are the days when occurrence of earthquakes used to be a rare phenomenon. Nowadays, one big earthquake usually occurs once a year. Alongside, numerous small tremors usually go unnoticed by most people. Earth's crust ranges from 3 to 45 miles deep (5 to 70 kilometers).
The crust is a thin, hard shell that floats on the denser, hotter rock of the mantle. The crust is divided into several pieces known as tectonic plates that are continuously in motion. An earthquake takes place when the pressure built up along a fault becomes stronger than the pressure that holds the rocks together. When the rocks on both the sides rip apart, they release pent up pressure. This release of energy varies with each earthquake. There is no such regularity that is noticed in the pattern of earthquakes. Sometimes there is one big earthquake, sometimes there are two or three together. Often after a big earthquake, many small earthquakes follow. These are a result of the crust adjusting to the main shock. When earthquakes occur in an ocean and powerful waves push up, they are termed as tsunamis. An earthquake's magnitude is ranked on the moment magnitude scale, not the Richter scale. A magnitude of 3 to 5 is considered minor, while a quake with a magnitude of 5 to 7 is moderate to strong and earthquakes from 7 to 8 are major. Scientists have not been successful in figuring out a sure shot way to detect the occurrence of earthquakes yet. Some of the famous earthquakes include: 1811-1812 — Missouri;1906 — San Francisco, California;1923 — Tokyo, Japan; 1960 — Chile; 1970 — Peru; 2004 — Indonesia; 2011 — Japan.
By: Anuja Arora