What causes earthquakes?
Earthquakes happen when rocks along a fault suddenly slip after stress builds up from the slow movement of tectonic plates. The released energy travels through the ground as seismic waves.
Tectonic plates move only a few centimeters a year, but their edges often get stuck. Stress builds up over years or centuries until the rocks break and slip, sometimes several meters in seconds. The point where the slip starts underground is the focus, and the spot on the surface above it is the epicenter.
Most earthquakes occur along plate boundaries, such as the Ring of Fire around the Pacific and the zone stretching from the Mediterranean through Türkiye, Iran and the Himalayas. Earthquakes can also be caused by volcanic activity, mining and filling large reservoirs.
Measuring earthquakes
Scientists measure an earthquake's size with the moment magnitude scale. Each whole number step means about 32 times more energy released. The largest earthquake ever recorded was a magnitude 9.5 in Chile in 1960.
Did You Know?
Undersea earthquakes can lift or drop the seafloor and trigger tsunamis, such as the 2004 Indian Ocean tsunami and the 2011 tsunami in Japan.