Firdaus Khan
About 80% of the seismic energy is released by earthquakes occurring along the plate boundaries. These earthquakes are called as interpolate earthquakes, due to the fact that they are directly associated with forces related to the interaction of the plates. Thus earthquakes occurring along the Circum-Pacific belt, Mid-Atlantic ridge and Alpide-Himalayan belt fall under the category of interpolate earthquakes. Sporadically, earthquakes also occur at rather large distances from the respective plate margins, such as in central USA (New Madrid, 1812), north-eastern continental China (Tangshan, 1976) and central India (Latur, 1993). These, so called intraplate earthquakes, show a diffuse geographical distribution and their origin is still not well understood. These earthquakes can be large and because of their unexpectedness and infrequency can cause major disasters.
Earthquakes caused by the sudden release of accumulated strain energy, due to interaction of two or more lithospheric plates, are distinguished as tectonic earthquakes in contrast to volcanic earthquakes. Volcanic earthquakes are caused by sudden opening of channels in crustal rocks, rapid changes of motion of magma, excessive accumulation of gas pressure in the crust, roof collapses of subterranean channels emptied by magma and other associated phenomena of volcanic eruptions. Close to active volcanoes, so called volcanic tremors are frequently detected which are result of long duration, continuous volcanic vibrations.
There is another class of earthquakes called plutonic earthquakes, whose focus is far underground, sometimes as deep as 700 km. They comprise only 5-10 % of all recorded events but are responsible for the present day knowledge of wave motion and nature of the interior of the earth. The tectonic, plutonic and volcanic type earthquakes belong to the category of natural seismic sources.
There is also a variety of man -made seismic sources such as industrial or military (nuclear) explosions and various types of cultural noises (traffic, industry, construction works, etc.) which are examples of controlled seismic sources where place, time of occurrence and source intensity are determined in advance or are, at least, predictable. Other types of man-made seismic sources are induced or triggered events caused by reservoir loading, mining activities and fluid injection etc. Two plausible mechanisms are available to explain triggered events. Firstly, by changes in local elastic stresses (loading, unloading) caused by removal of large volumes or rocks as in mining and quarrying operations and by reservoir impounding. Secondly, by an increase of pore and fracture pressure, e.g. due to fluid injection, which in turn decreases the rock strength (it acts as a lubricant) and may thus give rise to an increase in local seismicity. It is obvious that the activity in the controlled seismic sources is confined to the source region and hence will be of very shallow nature. Although the physical explanations of reservoir induced seismicity (RIS) are still imperfectly understood but the impounding of large reservoirs can obviously affect both the local elastic stress as well as the fluid pressure. The seismic activity in Koyna region of Maharashtra is a unique example of RIS, which has been continuing for the last about three decades.
Earthquakes may also be classified as shallow-focus, intermediate-focus and deep-focus depending upon their focal depths. Shallow-focus earthquakes, which constitute about 80% of total activity, have their foci at a depth between 0 and 70 km. and occur at oceanic ridges, collision and subduction zones and transform faults. Intermediate-focus earthquakes (focal depth between 71 and 300 km.) and deep-focus earthquakes (focal depth greater than 300 km.) occur at subjection zones. Most earthquakes originate within the crust. At depth beneath the Moho, the number falls abruptly and dies down to zero at a depth of about 700 km.
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