Posted by nshirodkar on August 27th, 2010An earthquake is caused by an abrupt gush of energy in the Eargh's crust and it creates seismic waves that are the waves of energy traveling through the center of the Earth. The frequency, kind and strength of the earthquake are referred to as seismicity or seismic activity of an area. A seismometer is a device which measures the earthquake. The unit of earthquake is moment magnitude (or the old Richter magnitude). Earthquake of moment magnitude 3 or less is considered to be unnoticeable, and that of moment magnitude 7 is taken to be very serious, causing heavy damage. The largest earthquake occurred in 1960, which was of 9.5 magnitude and it occurred in Chile. The intensity of quake is measured by the modified Mercalli scale.
There are about 500,000 earthquakes every year, out of which 100,000 are sensitized actually. Most of the earthquakes (nearly 90%) take place in the circum-Pacific seismic belt, which is 40,000 km long and is known as 'Pacific Ring of Fire'. World's nearly 81% of the major earthquakes are amongst these. This ring has 452 volcanoes and over 75% of world's active and dormant volcanoes are concentrated in this ring. The most seismic region after it is the Alpide belt, which ranges from Java to Sumatra through the Himalayas, the Mediterranean and out in the Atlantic. 5 to 6% of earthquakes and 17% of the world's largest earthquakes occur in this region.
Though mostly, earthquakes occur due to the activity in the Earth's tectonic plates (broken and moving pieces of Earth's lithosphere), human activities can produce earthquake too. Four major human activities that produce earthquake are construction of buildings and dams, drilling and injecting liquid into wells, oil drilling and coal mining. The best example of this is the Sichuan earthquake of May, 2008 in China's Sichuan Province. It resulted in 69,227 casualties and was the 19th world's deadliest earthquakes. The Zipingpu Dam is considered to have altered the pressure of the fault of 1,650 feet (503 m) away; and this presure perhaps increased the power of the earthquake and sped up the rate of motion for the fault. Another greatest earthquake induced by human activity was in Dec.1989, of 5.6 Richter scale, in Australia, where the city of Newcastle was constructed over a large sector of coal mining area. The earthquake was resulted from a fault which was reactivated because millions of tons of coal was removed in the mining activity.
Shaking and land rupture are the major effects of earthquakes causing severe damage to buildings and other structures. Other consequences are severe storms, volcanic activity, coastal wave attack, slope instability resulting into landslides, fires, soil liquefaction, tsunami, floods and tides.
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Joined: August 11th, 2010
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