How an earthquake begins

Most earthquakes occur along a fault line, which is a fracture in the earth's rocky outer shelter where the plates or sections of rock repeatedly slide past one another. They usually begin deep in the ground.

When the two plates converge and subduct, the subduction is not smooth. The rocks are under enormous stress as the plates moving in opposite directions get stuck by tremendous friction. Large amounts of stored energy get built up and the rocks are forced to bend. The stress in the rocks beomes so tremendous that the rocks break, snap and jerk free into new positions along a fault line. The stored energy is released in the form of seismic waves which makes the ground vibrate.

From the focus, the fracture travels like a spreading crack along the fault. The speed at which the fracture spreads depends on the type of rock. The average speed is about 3 kilometres per second in granite or any other strong rock.

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