Buildings collapse during an earthquake.Buildings collapse during an earthquake. An earthquake happens when the tectonic plates which form the Earth’s outer layer suddenly slip past each other, snap or make some other rapid movement, especially along their edges or at cracks (faults). There are about 100,000 earthquakes each year. Scientific devices called seismometers on continuous “quake watch” detect them all. Nine out of ten of these earthquakes are too small to cause damage, or they occur in very remote regions of the world. Another 30 to 40 cause small-scale problems, while up to about 20 cause major damage and a few make headlines. Every five or ten years, a massive quake results in great loss of life and devastates a wide area.

A subduction zoneA subduction zone

How an earthquake happens

The outer layer of the Earth is made up of a number of giant slabs, called tectonic plates. These are always on the move. This movement is very slow—no more than about a centimetre a year—but the pressure is enormous. When plate edges grind against one another, they send out shock waves through the ground. We feel these as vibrations, and call them earthquakes. Most are small tremors that do no damage. But when plates “lock” together, pressure starts to build up in the rocks under the ground. Eventually that pressure becomes too much for the rock to withstand. They suddenly snap, causing a major earthquake.

Across the world there are about 100,000 earthquakes each year. Japan alone has about 5000.

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