RE+-+Causes+of+Earthquakes

What causes earthquakes?

Earthquakes occur at plate margins because of friction - the plates try to move and get stuck. Pressure builds up as the plates keep trying to move. If the pressure is released slowly, we hardly notice the movement. But if the pressure is released suddenly, it can send out huge pulses of energy - causing the Earth's surface to move violently.

Earthquakes occur at all four plate margins:
 * destructive margins - when one plate tries to sink below the other. The pressure resulting from the sinking of the subducting plate and its subsequent melting can trgger strong earthquakes as this pressure is periodically released.
 * collision zones - where the plates are pushing together. Strong earthquakes
 * constructive margins - when both plates are moving apart. These earthquakes tend to be less severe than those at destructive plate margins. The friction and pressure caused by the plates moving apart is less intense than at destructive plate margins.
 * conservative margins - when plates moving alongside each other get stuck. These are very strong earthquakes. Here, where the plates slide past each other, the plates tend to stick for periods of time. This causes stresses and pressure to build. The release of the pressure occurs in a sudden, quick release of the plates and the result is an earthquake.

Measuring the strength of earthquakes

There are two main methods as detailed below: