What causes earthquakes?

Earth's outer shell is broken into tectonic plates that move a few centimeters a year, driven by heat circulating in the mantle beneath. Where plates meet, they grind past, pull apart, or collide, and the rock along those boundaries does not slide smoothly. Friction locks the surfaces together while stress accumulates, sometimes for centuries, until the strain exceeds what the rock can hold and it fails suddenly. That abrupt slip releases stored elastic energy as seismic waves, which is the shaking you feel. The point underground where the rupture begins is the hypocenter, and the spot on the surface directly above it is the epicenter.

Magnitude is reported on a logarithmic scale, which means the numbers are far less linear than they sound. Each whole number step represents about ten times greater ground motion amplitude and roughly 32 times more energy released, so a magnitude 7 is not slightly worse than a 6 but dramatically worse, and a magnitude 8 releases about a thousand times the energy of a 6. The older Richter scale has largely been replaced in professional use by the moment magnitude scale, which better describes very large earthquakes. Damage depends on more than magnitude: depth, distance, local soil conditions that can amplify shaking, and building construction all matter enormously, which is why similar magnitudes produce vastly different death tolls in different countries.

Not all earthquakes occur at plate boundaries. Intraplate earthquakes happen within plates along ancient faults, including the New Madrid zone in the central United States, which produced severe events in 1811 and 1812. Human activity also induces earthquakes, most notably through wastewater injection associated with oil and gas operations, which drove a documented increase in seismicity in Oklahoma and neighboring states. Prediction of specific earthquakes remains beyond current science, though probabilistic forecasting for regions over decades is well established, and early warning systems that detect the first fast-moving waves can deliver seconds of notice before the damaging shaking arrives.