Why earthquakes happen: causes, measurement and how to stay safe

Suddenly the ground starts shaking, fans swing and utensils fall — that is an earthquake. A big earthquake lasting a few seconds can destroy a whole city. Thousands of earthquakes are recorded around the world every year, but most are so mild they are not even felt. Here is why earthquakes happen, how they are measured and what to do during one.

The cause of earthquakes: moving plates

Earth's outer shell is broken into several large tectonic plates, which move a few centimetres every year. Where two plates meet, the rocks get stuck against each other and strain slowly builds up in them. When the strain becomes more than the rocks can bear, they suddenly break and slip, and the stored energy spreads out as waves. That is an earthquake. The Indian plate is moving north by about 5 centimetres a year and pushing into the Eurasian plate, which is why earthquakes are more common in the Himalayan region.

Focus, epicentre and seismic waves

• Focus: the point underground where the rocks break.
• Epicentre: the point on the surface directly above the focus; the shaking is strongest here.
• P waves: the fastest; they arrive first.
• S waves: arrive next and shake the ground up and down and side to side.
• Surface waves: the slowest, but they cause the most damage to buildings.

How is an earthquake measured?

Earthquakes are recorded by an instrument called a seismograph. To express an earthquake's strength, or "magnitude", Charles Richter created the "Richter scale" in 1935; today scientists use a similar "moment magnitude scale", but in everyday speech it is still called the Richter scale. On this scale, a rise of one whole number means the ground shakes about 10 times more and about 32 times more energy is released. So a magnitude 7 earthquake is about 32 times more powerful than one of magnitude 6.

Did you know? The most powerful earthquake ever recorded struck Valdivia in Chile on 22 May 1960, with a magnitude of 9.5. The tsunami it set off caused destruction thousands of kilometres away, in Japan and Hawaii.

Some of India's great earthquakes

India is divided into zones according to earthquake risk. The risk is highest in the Himalayan states, the north-east, Kutch in Gujarat and the Andaman and Nicobar Islands. In such areas buildings must follow the earthquake-resistant codes of the Bureau of Indian Standards (BIS).

What to do in an earthquake

• Drop, cover, hold on: drop to the ground, cover your head and neck under a sturdy table and hold on to it until the shaking stops.
• Keep away from windows, glass, cupboards and hanging objects.
• Do not use the lift; go out by the stairs once the shaking stops.
• If you are outside, move to an open space away from buildings, trees, electric poles and wires.
• If you are driving, stop in a safe place away from bridges and flyovers.
• Be ready for aftershocks, and do not pay attention to rumours.

An earthquake under the sea can also set off a tsunami — read The biggest tsunamis in history. How volcanoes and earthquakes are linked: How volcanoes form.

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