An earthquake is a sudden shaking of the ground. It happens when stress built up in rocks suddenly releases. The rocks snap along a fault line, sending seismic waves in all directions. The point underground where it starts is the hypocenter. Directly above it is the epicenter.

Most earthquakes occur at tectonic plate boundaries. Plates grind past each other, pull apart, or collide. The Pacific Ring of Fire, circling the ocean, hosts about 80 percent of the world's largest quakes. But quakes can strike far from plate edges, in zones of ancient weakness. The 1811 New Madrid earthquakes shook the central United States.

Damage depends on more than magnitude. Depth, distance, soil type, and building codes all matter. A shallow quake under a city with weak buildings is catastrophic. A deep quake offshore may cause little harm on land. Tsunamis, landslides, and fires often kill more people than the shaking itself. Early warning systems can give seconds of notice, enough to slow trains and stop surgery.

Key terms

• Fault line
• Epicenter
• Magnitude and intensity
• Aftershocks
• Seismic waves