Earthquake


Ajit Kumar AJIT KUMARWISDOM IAS, New Delhi.

An earthquake is the sudden, rapid shaking of the earth, caused by the breaking and shifting of subterranean rock as it releases strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the earth, as the huge plates that form the earth’s surface slowly move over, under and past each other. Sometimes, the movement is gradual. At other times, the plates are locked together, unable to release accumulated energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage.
Earthquakes are quantified either in terms of magnitude or intensity. The strength of an earthquake, or strain energy released by it is usually measured by a parameter called “Magnitude” determined from the instrumental (Seismograph) records.
Intensity of an earthquake is a subjective measure of the force of an earthquake at a particular place as determined by its effects on persons, structures and earth materials. The intensity at a point, depends not only upon the strength of the earthquake (i.e. magnitude) but also upon the distance from the earthquake to the point and the local geology at that point. While the intensity decreases with the distance from the epicenter, magnitude for an earthquake remains the same, irrespective of where it is measured.
The epicenter of an earthquake is the point on the surface of the earth vertically above the place of origin (hypocentre) of an eartquake. This point is expressed by its geographical latitude and longitude.
Hypocentre or Focus of an earthquake is the point within the earth from where seismic waves orignate. Focal depth is the vertical distance between the hypocentre and epicentre.
No scientific technique is available anywhere in the world so far to predict the occurrence of earthquakes with reasonable degree of accuracy with regard to space, time and magnitude.
Classification of the earthquakes
Earthquakes can be classified as below:
Slight -Magnitude upto 4.9 on the Richter Scale.
Moderate- Magnitude 5.0 to 6.9
Great -Magnitude 7.0 to 7.9
Very Great -Magnitude 8.0 and more.
Causes of earthquakes in India
 From the concept of plate tectonics, the Indian plate is moving in a north-north-east direction and colliding with Eurasian plate along the Himalayan mountain range. This collision is responsible for the formation of faults such as Main Boundary Thrust, Main Central Thrust, etc in and along the Himalaya. Almost all the major earthquakes in India and its adjoining region occur along these faults. In brief, earthquakes occur due to forces of geological origin along weak planes called, faults. The earthquakes occurring near plate boundaries such as Himalaya are known as interplate earthquakes. Apart from earthquakes from the Himalayan belt, there have been earthquakes of relatively lesser magnitude and lesser frequency in the Peninsular India also. Such earthquakes are known as intraplate earthquakes and are attributed to the stresses building up in this area on account of compressive forces arising out of north-northeastward movement of the Indian plate.
 Seismic Zones of India
Bureau of Indian Standards, based on various scientific inputs from a number of agencies, has grouped the country into four seismic zones viz. Zone-II, III, IV and V. Of these, Zone V is the most seismically active region, while zone II is the least. The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones is as follows:
Seismic Zone Intensity on MM scale
II (Low intensity zone) VI (or less)
III (Moderate intensity zone) VII
IV (Severe intensity zone) VIII
V (Very severe intensity zone) IX (and above)
Broadly, Zone-V comprises of entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar islands. Zone-IV covers remaining parts of Jammu & Kashmir and Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan. Zone-III comprises of Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhatisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamilnadu and Karnataka. Zone-II covers remaining parts of the country.
 Seismic Microzonation
The ‘Seismic Hazard and Risk Microzonation’ (SHRM) is a process of classifying the given geographic domain into small units of likely uniform Hazard (H) level (Peak Ground Acceleration - PGA, Spectral Acceleration - Sa), nature of hazard (susceptibility to liquefaction and slope failure) and Risk. The objective of Seismic Microzonation is to provide (a) probabilistic estimate of the hazard for each microzone due to earthquake shaking, (b) extent of likely damage to the built environment (dwellings, community structures, lifelines, industrial structures, monuments, heritage structures, etc.) and define damage ratio and people living in structures susceptible to damage, (c) retrofitting measures for the existing structures to render them safe and (d) specific guidelines for designing and construction of earthquake resistant structures belonging to microzones.
Tsunami
Tusnami‟ is a Japanese word with English translation, “harbour wave”. When an earthquake of large magnitude occurs under the sea, due to large displacements of the sea floor, the water column is disturbed and sea waves, called „Tsunamis‟, are generated. Tsunamis may also be generated by submarine landslides, volcanic eruptions, explosions and meteorites. Tsunamis can savagely attack coastlines, causing devastation to property and loss of life. These waves travel long distances and when they reach shallow waters, their amplitude increases. At any given place, the amplitudes may range from a few meters to a few tens of meters depending upon several factors including the magnitude of the event, the type of faulting, depth of water column and bathymetry of the coast.
 
 
 
 

Wednesday, 05th Feb 2014, 08:22:05 PM

Add Your Comment:
Post Comment