Nature · Earth Science
The restless movements of the crust
An earthquake is a sudden shaking of the ground caused by the abrupt release of energy in Earth's crust. They are the surface expression of plate tectonics — the slow drift of the rigid plates that make up Earth's outer shell, whose grinding edges build mountains and unleash quakes.
Earth's crust is broken into plates that move a few centimetres a year. Where they meet, stress builds until the rock suddenly slips along a fault, releasing energy as seismic waves. The point of slip is the focus; the spot directly above it on the surface is the epicentre.
Stress builds along a fault until rock slips at the focus; seismic waves radiate out and shake hardest at the epicentre above.
Where plates collide
When plates collide, one may dive beneath the other or they may crumple upward. These zones raise great mountain ranges and produce Earth's mightiest earthquakes.
Where plates pull apart
As plates separate, magma rises to fill the gap and create fresh crust, building undersea ridges and tearing open continental rift valleys.
Where plates grind past each other
Here plates scrape horizontally past one another. They lock, build stress, then slip abruptly — the cause of the famous quakes along faults like the San Andreas.
Ocean waves born of the seabed
When the seabed jolts upward in a quake, it shoves the whole water column, sending fast, low waves across the ocean that pile into towering walls at the shore.