Causes Of Earthquake
Earthquakes occur from the deformation of outer, brittle portions of "tectonic plates", the earth's outermost layer of crust and upper mantle. Due to the heating and cooling of the rock below these plates, the resulting convection causes the adjacently overlying plates to move, and, under great stresses, deform. The rates of plate movements range from about 2 to 12 centimeters per year. Sometimes, tremendous energy can build up within a single, or between neighboring plates. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake. An earthquake begins when the plates push against earth other and the pressure builds. Cracks start to appear in walls and roads. The ground may shiver (tremor). Where the rocks snap seismic waves (body waves) make the ground move. The first ones ( primary ) travel through rock, volcanic lava, water and even air. They take about 20 minutes to go from one side of the earth to the other. The next waves (secondary) move only through solid rock. Body waves that reach the surface make the ground move like waves on the ocean. These are the most deadly. The quake starts with a violent jolt at the epicenter followed by more tremors. Aftershocks may occur as everything is setting into a new position.
Plate Tectonics, the Cause of Earthquakes
The plates consist of an outer layer of the Earth, the lithosphere, which is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. The map below locates earthquakes around the globe. They are not evenly distributed; the boundaries between the plates grind against each other, producing most earthquakes. So the lines of earthquakes help define the plates:
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| EARTHQUAKE AROUND THE GLOBE |
In cross section, the Earth releases its internal heat by convicting, or boiling much like a pot of pudding on the stove. Hot asthenospheric mantle rises to the surface and spreads laterally, transporting oceans and continents as on a slow conveyor belt. The speed of this motion is a few centimeters per year, about as fast as our fingernails grow. The new lithosphere, created at the ocean spreading centers, cools as it ages and eventually becomes dense enough to sink back into the mantle. The subducted crust releases water to form volcanic island chains above, and after a few hundred million years will be heated and recycled back to the spreading centers.
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There are three main plate tectonic environments: extensional, transform, and compressional. Plate boundaries in different localities are subject to different inter-plate stresses, producing these three types of earthquakes. Each type has its own special hazards.
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VOLCANICITY
Volcanic activity is considered to be one of the major cause of earthquake. In fact, volcanicity & seismic events are so intimately related to each other that they become cause & effect for each other. The explosive violent gases during the process of volcanicity, try to escape upward & hence they push the crustal surface from below with great force & thus cause severe earth tremour of high magnitude.
HUMAN ACTIVITIES
Fault rupture is not the only cause of earthquakes; human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily. While earthquakes caused by human activities may be harmful, they can also provide useful information. Prior to the Nuclear Test Ban Treaty, scientists were able to analyze the travel and arrival times of P waves from known earthquakes caused by underground nuclear test blasts. Scientists used this information to study earthquake waves and determine the interior structure of the Earth.Scientists have determined that as water level in a reservoir increases, water pressure in pores inside the rocks along local faults also increases. The increased pressure may cause the rocks to slip, generating earthquakes .Koyna earthquake is a good evidence of reservoir-induced earthquakes in India.
