What Is an Earthquake?

This diagram shows an earthquake along a fault. The focus of the earthquake is where the energy is released underground. The epicenter is the spot on the Earth’s surface directly above the focus.
USGS

The expression “on solid ground” is often used to describe something as stable. Usually the solid ground underfoot seems very stable. But sometimes it is not.

"The ground seemed to twist under us like a top while it jerked this way and that, and up and down and every way," wrote a person describing the experience of being in the large 1906 earthquake in San Francisco, CA.

Earthquakes happen as large blocks of the Earth’s crust move suddenly past one another because of the force of plate tectonics. These blocks of the Earth’s crust meet at cracks called faults. Sometimes those pieces do not slide smoothly past one another. There can be friction along the fault – jagged edges that snag the blocks of rock. This makes it difficult for them to move past each other. Sometimes they get stuck together temporarily. When the pieces of rock overcome the snags, energy is released. The release of energy causes shaking at the ground surface.

The location inside the Earth where an earthquake begins is called the focus. The point at the Earth’s surface directly above the focus is called the epicenter. The strongest shaking happens at the epicenter.

Each year, more than a million earthquakes occur worldwide. Most of these are so small that people do not feel the shaking. But some are large enough that people feel them, and a few of those are so large that they cause significant damage.

Earthquakes can cause damage to things like buildings, bridges, and roads. Earthquakes can cause landslides and mudslides, too. If a large earthquake happens under the ocean it can cause a tsunami – a giant ocean wave or series of waves.

Scientists can figure out whether an earthquake is likely to happen in a place by studying plate tectonics, the faults underground, and the history of the area’s earthquakes. However, unlike weather events, earthquakes can not be forecast ahead of time.