Earthquake magnitude, explained

An earthquake’s magnitude measures how much energy was released at its source. Its intensity describes how hard the ground shook in a particular place. A quake has one magnitude but many intensities: strong near the centre, weaker far away.

A scale of tens and thirty-twos

Magnitude scales are logarithmic. Each step of one whole number means the waves recorded on a seismometer are ten times larger — and the energy released is about 32 times greater. So a magnitude 7 releases about 32 times as much energy as a 6, and about 1,000 times as much as a 5.

Charles Richter devised the first magnitude scale in 1935, for earthquakes in southern California. For large quakes, seismologists today use the moment magnitude scale, introduced in 1979, which is worked out from the size of the fault that slipped, how far it moved and the stiffness of the rock. News reports still say “Richter scale” out of habit; the numbers are close for moderate quakes.

How often

The US Geological Survey (USGS) groups quakes like this, with how many happen worldwide in an average year:

• Great — magnitude 8 or more: about one a year
• Major — 7 to 7.9: about 15 a year
• Strong — 6 to 6.9: more than 100 a year
• Moderate — 5 to 5.9: more than 1,000 a year

Why depth matters

A shallow earthquake — less than about 70 km below the surface — usually shakes the ground much harder than a deeper one of the same magnitude. How much damage follows depends on more than the quake: the soil, the way buildings are built, the time of day and how ready people are.

India’s earthquakes

The Indian plate is moving north into the Eurasian plate by about 4 to 5 centimetres a year. The collision raised the Himalayas and keeps loading the faults beneath them. Some of the region’s worst quakes:

• Assam–Tibet, 15 August 1950 — magnitude 8.6
• Bhuj, Gujarat, 26 January 2001 — magnitude 7.7
• Indian Ocean, 26 December 2004 — magnitude 9.1, off Sumatra; its tsunami killed more than 10,000 people in India and many more around the ocean
• Gorkha, Nepal, 25 April 2015 — magnitude 7.8

Since 2007, the Indian Tsunami Early Warning Centre at INCOIS in Hyderabad has watched for undersea quakes that could send waves towards India’s coasts.

What nobody can do — and what you can

No one can predict the day, place and size of an earthquake; claims that someone has are false. What works is preparation: buildings designed for the shaking expected where they stand, and knowing what to do. When the ground shakes, drop to the ground, take cover under a sturdy table, and hold on until it stops. Expect aftershocks.

Our earthquake page maps every quake of magnitude 4.5 or more worldwide, and 2.5 or more in and around India, from the USGS’s live feeds.