About 50,000 earthquakes large enough to be felt occur around the world every year. About 100 of these are very big and can cause significant damage. Most of the major ones happen along fault zones where tectonic plates (giant rock slabs that make up Earth’s upper layer) collide or slide against each other. Earthquakes release enormous energy when they slip along these faults. Some of the energy is radiated away as seismic waves that can travel through the ground, sometimes to the surface. The rest is transferred directly to the ground in the form of shaking, called an aftershock.
Earthquakes occur when the rock in a fault suddenly breaks and there’s rapid motion on the fault. The spot underground where this happens is called the focus and the place on the surface above it is called the epicenter. Scientists know the size of an earthquake from looking at the recordings of its vibrations on a seismograph at a research station. The longer a line on the seismogram wiggles, the larger the earthquake was.
In some places on the planet, such as the ring of fire near the Pacific Ocean, earthquakes happen often. The plates on the edge of this ring grind together at a constant rate, producing tremors and small earthquakes regularly. But on other faults, like the San Andreas Fault, movement isn’t constant, and huge amounts of stress can build up for years until it gets released in a big earthquake.