What actually happens when the ground shakes in an earthquake?

An earthquake is the shaking of Earth's surface after a sudden release of energy in the rocky outer shell of the planet. That energy spreads out as seismic waves, and the waves are what rattle your cupboards.

The story starts underground. Rock in Earth's crust is under constant slow pressure from the moving plates, and it holds that pressure the way a bent ruler does. Along a crack called a fault, the two sides are stuck together. Stress builds slowly until the rock suddenly breaks and slips. The spot where the break begins is the hypocenter, also called the focus. The epicenter, the spot you see on news maps, is just the point on the ground straight above it.

So the quake you feel is not the break itself but its messengers. The break spreads along the fault, and the waves travel outward through the rock to wherever you happen to be standing.

Earthquakes are far more common than most people think. Instruments detect around 500,000 a year, and about 100,000 are strong enough to be felt. And they are not spread evenly: about nine in ten happen around the Pacific Ocean, in the belt nicknamed the Ring of Fire, where the plates grind past and under each other.

Often what kills is not the shaking itself but what it does to buildings. In 1556 in Shaanxi, China, many people lived in homes dug into soft hillsides, and when the ground shook those homes collapsed. More than 100,000 people died. In Shaanxi, the homes themselves were the danger.