Earthquakes - an overview
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An earthquake is manifested as a shaking of the ground resulting from a series of shock waves generated following the brittle failure of rocks within the earth's crust or upper mantle. The failure comes about due to the build up of stress which occurs because of the constant movement of blocks of the earth's crust known as the lithospheric plates. Failure occurs at a point, or in a fairly small zone, known as the focus with the epicentre being the point on the earth' surface directly above this focus. However, once failure has occurred, movement may persist along a zone of weakness - known as a fault - for a considerable distance, occasionally as much as 1000 km).
Many earthquakes occur each year, on average greater than 800,000, but most are small and not felt by humans. A severe earthquake, with a magnitude of greater than 8.0, can be expected every 8 to 10 years. Yet, a significant number of smaller earthquakes, which are still capable of destruction, occur each year.
Earthquakes show a marked spatial distribution. The vast majority are located within narrow zones which correspond to the boundaries of the plates. These plates are in continuous movement relative to each other, thought to be driven by convective processes in the earth's mantle, the region of rocks beneath the crust which are heated to the point of becoming soft or plastic.