Why Does the Ground Shake?

Step 1 of 5

Earth's surface is broken into moving plates.

The crust is a set of huge slabs drifting a few centimetres a year — about the rate your fingernails grow. They grind against each other constantly.

Step 2 of 5

Friction stops them sliding smoothly.

At the boundaries the rock locks together. The plates keep being pushed, but the join does not move — so strain builds up in the rock.

Step 3 of 5

The rock bends like a loaded spring.

Rock is not rigid over years. It flexes and stores energy — sometimes for centuries — exactly like a spring being wound tighter.

Step 4 of 5

Then it slips all at once.

The join gives way and the plates jump metres in seconds. All the stored energy is released as vibrations racing through the rock.

Step 5 of 5

Those waves are what you feel.

Fast pressure waves arrive first as a jolt. Slower side-to-side waves follow and do most of the damage, because buildings sway rather than compress.

Each step up the magnitude scale is about 32 times more energy. A 7 releases a thousand times a 5.