What causes earthquakes to happen?

Earthquakes happen because Earth's outer shell is split into huge moving plates. Their rough edges sometimes lock together while the rest of the rock keeps pressing forward. When the stuck edge finally slips, stored energy races through the ground, and the surface shakes.

Those plates creep only a little each year, so the strain can hide for a long time before anyone feels it. Scientists use seismographs to record the passing waves, which helps them locate where the break began deep underground.