How an earthquake occurs

An earthquake is the sudden shaking of the ground caused by the release of strain energy stored in the earth's crust, which travels as seismic waves.

How an earthquake occurs

1. The crust is divided into tectonic plates which move slowly (cm per year) by mantle convection. India moves toward Eurasia by about 4-5 cm/yr.
2. At plate boundaries and faults, rocks are locked by friction, so strain accumulates and the rocks deform elastically.
3. When the stress exceeds the strength of the rock or the friction on the fault, the rocks suddenly slip.
4. The stored elastic energy is released as seismic waves from the focus (hypocentre); the point on the surface above it is the epicentre.

Causes: tectonic (most important), volcanic, reservoir-induced, mining and explosions.

Elastic rebound theory (H. F. Reid, 1906, after the San Francisco earthquake)

1. Rocks on both sides of a fault are strained slowly by plate motion; they bend and store elastic energy like a spring.
2. When the strain exceeds the rock strength, rupture occurs at the weakest point.
3. The rocks spring back (rebound) to a less strained position, and the fault moves; the energy is released as seismic waves.
4. The process then repeats, which is why earthquakes recur.

 (a) original   (b) strained   (c) after rupture
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