Why do earthquakes happen?

Earthquakes happen when rock along a fault suddenly slips and releases stress that built up slowly. Most occur where tectonic plates meet, such as the Pacific Ring of Fire, but earthquakes can also occur inside plates and, more rarely, be triggered by human activity like wastewater injection.

Plates, faults and stress

Earth’s outer shell is split into plates that move a few centimetres a year. The edges snag on each other instead of sliding smoothly. Stress builds until the rock breaks and slips along a fault. The slip sends out seismic waves, which is the shaking you feel.

Three main fault types

• Strike-slip: blocks slide sideways past each other, as on California’s San Andreas Fault.
• Normal: one block drops down as the crust is pulled apart, common in the Basin and Range region of the western U.S.
• Reverse or thrust: one block is pushed up and over another. Subduction zones, where one plate dives beneath another, produce the largest earthquakes on Earth.

Why most earthquakes are on plate boundaries

Roughly 90% of earthquakes occur around the Pacific “Ring of Fire”, where several plates meet. Other active belts run through the Mediterranean, the Himalayas and the mid-ocean ridges. You can browse recent activity by place on our earthquakes by place pages.

Earthquakes away from plate edges

Old faults inside plates can still slip. The 1811–1812 New Madrid earthquakes in the central U.S. are a well-known example.

Human-induced earthquakes

Injecting wastewater deep underground, filling large reservoirs, and mining can change stress on existing faults and trigger earthquakes. USGS has linked a rise in earthquakes in parts of Oklahoma and Texas to wastewater disposal.