Indian collision with Asian underbelly led to quake: Dam and massive reservoir near epicentre may have contributed to geological changes that weakened fault zones
Less obvious, however, is that the whole of India feels the effect of the collision. There is stress and compression in the rocks, which is occasionally relieved in the form of earthquakes that occur far from the Himalaya mountains, such as the one that hit the towns of Umbarga and Khilari, some 450km (280 miles) east of Bombay in Maharashtra state, this week.
According to James Jackson, of the Department of Earth Sciences at Cambridge University, the event may be associated with the presence of a massive reservoir nearby. 'Big reservoirs can induce earthquakes,' Dr Jackson said, 'although the mechanism is not well understood.'
The Koyna dam near Khilari, a small town with a population of about 20,000, was full at the time of the earthquake but did not burst. The weight of water in the reservoir can contribute more stress but - more important - some of the water will percolate into subterranean fault zones and weaken them, Dr Jackson said. This is not so much a matter of the water acting as lubricant as actually weakening the material itself.
The Bombay meteorological bureau said there was a succession of five tremors that shook western and southern India. The first, at 3.56am, measured at least 6.4 on the Richter scale. In Golden, Colorado, the US Geological Survey described the earthquake as the largest in the area since 11 December 1967, when a shock of 6.5 magnitude was felt some 270km (170 miles) west of the epicentre of this week's quake.
According to Chris Browitt of the British Geological Survey's Global Seismology Unit, 'Earthquakes of this magnitude happen about 80 times every year across the world.' This earthquake proved especially damaging because it was geologically shallow, thus allowing a great deal of earth movement to affect a highly populated area at the surface of the Earth's crust.
There is no direct relationship between the magnitude of an earthquake, as measured on the Richter scale, and the damage that results. Much larger earthquakes can cause many fewer casualties if they happen in remote areas. The death toll from this week's earthquake in India is high partly because it struck at night, when most people were indoors, and because buildings in the affected area were not designed to withstand seismic events and collapsed, crushing their occupants.
But Dr Jackson pointed out that in 1897 and 1905 there were 'enormous earthquakes' of magnitude 8 or greater along the foothills of the Himalayas. The event in Maharashtra state 'is a lot smaller', he said. He warned that there would be huge casualties if another magnitude 8 event were to strike northern India again.
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