How do earthquakes happen?

Earth’s outer shell is broken into large tectonic plates that slowly move, driven by heat from deep inside the planet. Where plates meet, they can lock together along cracks called faults while stress builds up over years. When the stress finally overcomes the friction holding the rocks in place, they slip suddenly, releasing energy as seismic waves that radiate outward and shake the ground — an earthquake. The point underground where it starts is the focus, and the spot directly above on the surface is the epicenter. Scientists measure the energy released using magnitude scales; each step up represents roughly 32 times more energy.