What is an earthquake? An earthquake is what happens when two blocks of the earth suddenly slip past one another.

The surface where they slip is called the fault or fault plane.

[Photo:USGS]

The location below the earth's surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows.

Scientists can't tell that an earthquake is a foreshock until the larger earthquake happens.

The largest, main earthquake is called the mainshock.

Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock.

Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!