Learn · Hazards · 8 minute read
Earthquakes
Rock bends for centuries, then snaps in seconds. And why a magnitude 7 is not a little bigger than a 6.
On 18 April 1906, the ground in San Francisco moved several metres sideways in under a minute. Afterwards, a geologist called Harry Fielding Reid compared surveys from before and after. He saw that the land either side of the San Andreas Fault had been bending for years before it broke. His explanation is still the one we use.
Bend, bend, snap
Plates keep moving, but the rock along a fault between them sticks. So the rock either side bends, storing energy like a drawn bow. When the strain is more than the fault can hold, it slips all at once and the rock springs back. That sudden release sends out the waves we feel. Reid called it elastic rebound.
The place underground where the slip starts is the focus. The spot on the surface directly above it is the epicentre. Most earthquakes start in the top few tens of kilometres, but where one plate sinks under another they can start far deeper. The deepest large one on record began about 680 km down.
Magnitude is not a ruler
Magnitude measures how much energy an earthquake releases. It goes up in steps, but the steps are not equal. Each whole number up is about 32 times more energy than the one before. Two whole numbers up is about 1,000 times more. So a magnitude 7 is not a slightly bigger 6. It is 32 sixes.
The largest earthquake ever measured struck Valdivia in Chile on 22 May 1960: magnitude 9.5. Britain's largest known earthquake, under the Dogger Bank in the North Sea on 7 June 1931, was magnitude 6.1. It was felt across most of Britain and as far away as Norway and France. The Chilean one released roughly 126,000 times more energy (our own sum, from the factor of 32).
Magnitude and shaking are different things
An earthquake has one magnitude, but the shaking it causes varies from place to place. It is strongest near the epicentre and fades with distance. It also depends on the ground: soft, wet sediment shakes harder than solid rock. And how many people are hurt depends most of all on buildings. That is why earthquakes of similar size can have very different outcomes.
Where they happen
About 90% of the world's earthquakes happen around the edge of the Pacific, where plates meet. The UK sits in the middle of a plate. It still has earthquakes, and the British Geological Survey's instruments pick them up regularly, but almost all are too small to feel. See Moving plates for why.