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Home » Geological Disasters » Earthquake

Earthquake is a sudden shaking of ground due to natural causes (rock displacements, landslide, avalanche, volacanic eruption, meteoritic impact, sub-marine sea faulting, etc) so to refer it as natural earthquake. The shaking of ground may be due to several other man-made agencies, such as, explosions due to chemical blasts or nuclear blasts or rock burst due to mining activities, and reservoir induced earthquakes. Occurrence of earthquake is still a puzzle for the entire scientific community as its generating mechanism does not follow a thumb rule. It can hit anywhere at any point of time and that is why earthquake is regarded as most unpredictable, uncontrollable and unfathomable cause of bringing disasters to both flora and fauns. Scale of disaster due to earthquake is dictated by several earthquake parameters. Among which are earthquake magnitude (extent of energy release during the earthquake), focal depth or hypocenter (the depth at which the rupture initiates), the epicenter location of the earthquake (the vertical projection of the hypocenter to the surface). Thus bigger magnitude earthquake at shallower depth (< 30 km) with its epicenter location near the densely populated region can inflict a severe damage to both property and person, whilst the reverse may lead to relatively less damaging effects in the earthquake prone zones. In addition to that earthquake occurrence time and building typology also play their important roles in influencing the degree of damage due to earthquakes in the region. It is rightly said that “earthquake does not kill the people; it is structure which kills the people”! Several pieces of studies revealed that upper crust of the earth up to a depth of 18 km from the surface is seismically active because of its brittle behavior, while lower crust between 18 km and 33 km depth inside the earth is seismically less active due its ductility under high temperature. The boundary between the lower crust and upper crust is referred to as Conrad discontinuity, while the boundary between the lower Crust and the upper Mantle is referred to as Mohorovicic discontinuity, in short it is mentioned as Moho discontinuity, which varies from shallower (10 – 12 km) beneath the Ocean to deeper (70 – 80 km) beneath the Himalaya. The entire crust and upper Mantle constitute “Lithosphere”, whose thickness is found to varying from 100 km to 150 km. It is a solid lithosphere which floats on the semi-viscous layer of the earth called Aesthenosphere, a part of the lower Mantle. The lithosphere constitutes an important ingredient of Earth’s physiographic that divides entire Earth into several tectonic blocks to explain the geodynamics of the Earth and seismogenesis in diverse tectonic environ. Earthquake can hit either within the same tectonic block / plate (intra-plate earthquake) or at the boundary of two or more tectonic plates (plate-boundary earthquakes). The plate-boundary earthquakes are frequent (e.g., earthquakes in Himalayan region; earthquakes in the Andaman – Nicobar region; earthquakes in the Ring of Fire zone, Alaska, Japan, Chile ), while the intra-plate earthquakes are less frequent (e.g., earthquakes in stable continental region of India; the 1819 Kutchch earthquake; 1993 Latur earthquake; 2001 Bhuj earthquake). The great damaging earthquakes are generally accompanied with secondary effects, such as tsunamis, liquefactions, landslides, volcanic (lava and mud) eruptions; nuclear emissions, and fires that may multiply the degree of losses due earthquake many folds. The recent occurrence of the 2004 tsunami-genic Sumatra – Andaman earthquake (Mw 9.3) and the 11th March 2011 Japan tsunamigenic earthquake (Mw 9.0)(For more details,click here) are appropriate example to understand how a huge loss of both people and property was mainly due to secondary effect of tsunamis rather than that of primary earthquake shakings.
School Children,Ahmedabad,India

Occurrence of damaging Earthquakes are a major problem for the mankind, killing more than 17000 persons per year in the twentieth century. It causes multifaceted damages and destruction to life and property. What to do about earthquake risk and how to mitigate earthquake induced damages, particularly in the developing countries, has been a challenging issue and still need a coordinated effort to tackle the regional menace that occur during the killer earthquakes. Assessing the earthquake risk and determining mitigation alternatives varies from country to country. Reduction in the earthquake risk involves the use of knowledge, methodologies adopted based on the geopolitical boundaries. More so on multi-objective-multi-stake holder decision to be taken. Making a decision on what to do, depends on the site specific data collected from affected region, including multifaceted role played by geosciences, engineering, disaster planning and response, techno-legal, insurance and economics.

Several earthquakes in the recent past remind us about the high level of seismic hazard and risk prevailing in the country. About 59% of India's land area is under the threat of moderate to severe earthquake shaking intensity VII and higher. During the last two decades, several major earthquakes have resulted in over 100,000 deaths in South Asian nations, especially located in the vicinity of Himalaya and in the oceanic subduction zone of Andman – Nicobar Island region . The regions far away from the Himalaya and other inter-plate boundaries, which were once considered to be relatively safe from strong shaking, have also experienced several devastating earthquakes. The huge losses of life and property in the earthquake-prone areas of the region have shown that the built-environment is extremely fragile, and our ability to respond to these events is extremely inadequate. It has been found that the casualties were caused primarily due to the collapse of buildings that have usually no earthquake-resistant features. This emphasizes the need for strict compliance of town planning bye-laws and earthquake-resistant building codes in India.

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