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About Earthquake

What is an Earthquake?
An earthquake is the motion or trembling of the ground produced by sudden displacement of rock in the Earth's crust. Earthquakes result from crustal strain, volcanism, landslides, and collapse of caverns. Stress accumulates in response to tectonic forces until it exceeds the strength of the rock. The rock then breaks along a preexisting or new fracture called a fault. The rupture extends outward in all directions along the fault plane from its point of origin (focus). The rupture travels in an irregular manner until the stress is relatively equalized. If the rupture disturbs the surface, it produces a visible fault on the surface.Earthquakes are recorded by seismograph consisted of seismometer, a shaking detector and a data recorder. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale. In India Medvedev-Sponheuer-Karnik scale, also known as the MSK or MSK-64, which is a macroseismic intensity scale, is used to evaluate the severity of ground shaking on the basis of observed effects in an area of the earthquake occurrence. Due to earthquake seismic waves are generated and measurements of their speed of travel are recorded by seismographs located around the planet. There are two main types of seismic waves - body and surface waves. Demonstration of seismic waves are explained here Demo

Why earthquake occurs?
Earthquakes occur due to the sudden release of stored strain energy inside the earth crust. The earth is divided into several major and micro tectonic plates, some 50 miles thick, which move slowly and continuously over the earth's interior. Most earthquakes occur as the result of slowly accumulating pressure that causes the ground to slip abruptly along a geological fault plane on or near a plate boundary. The resulting waves of vibration within the earth create ground motion at the surface that vibrates in a very complex manner.

What are the various types of earthquake?
Classification of earthquake is based on several parameters. Based on scale of magnitude (M), earthquake may be of Micro (M < 3.5) or macro (M > 3.5) type. Depending up on the extent of energy released and strength of the ground shaking it may be of several types, like moderate strong, very strong, great and very great earthquake. Depending up on the scale of damage, the earthquake may be of various types, such as Less damaging earthquake, Moderate damaging earthquake, and catastrophic earthquake. Depending up on the focal depth (h) of the event, it could be shallow earthquake (d< 70 km); intermediate depth earthquake (70 < h < 300 km); the deep earthquake (300 < h < 700 km). Depending up on the location of events in different tectonic settings, earthquake may be of intra-plate, inter-plate, and sub-oceanic earthquake. Depending up on involvement of other agencies / phenomena with earthquake genesis, it may be of several types, such as Reservoir induced; Fluid-driven earthquake; Tsunamigenic earthquake, and volcanic earthquake. Depending up on the type of faulting involved during earthquake genesis, earthquake may be categorized into several categories, such as normal faulting, reverse faulting, thrust faulting, and mega-thrust earthquake. Depending up on the frequency content, the earthquake may be of Low-Frequency tremors or high – Frequency tremors. Depending up on the epicenter distance (distance between earthquake mainshock and the recording stations), the earthquake may be classified into Local, Regional and Global earthquake.
For more Information Click here(types of earthquakes).

What one should know about earthquakes?
Earthquakes can be violent, and they remain unpredictable. A comprehensive background note on the earthquakes for commoner is given in the earthquake tips prepared by several institutions of SAARC countries, like India, Bhutan, Nepal and Pakistan.

Intensity scale

It manifests the degree of damage, which gets diminished as we go away from the main shock source zone and the reverse is also true. There are several earthquake intensity scale, which can be referred from the relevant pages.

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