Earthquakes, also called temblors, shake the Earth millions of times each year. They are caused by the movement of the outer layer of the planet, its crust. Most are so small people don’t feel them, but powerful ones can cause landslides and tsunamis that threaten lives.
The outer layer of the Earth is a bit like the skin on a hard-boiled egg. It’s smooth and solid, but it isn’t one piece – it’s made up of giant puzzle pieces that wrap around the viscous, or slowly flowing, mantle layer below. The pieces are called tectonic plates, and they are constantly shifting as they drift on the plate boundary zones. Sometimes a huge amount of stress builds up between two plates, and when it is released, an earthquake happens.
Most earthquakes happen near fault zones where the plates meet. But they can also occur far away from these places if the plates are stretched or squeezed. The resulting vibrations, called seismic waves, can be felt thousands of miles away. The strength of an earthquake is measured in magnitude, based on how much energy the seismic waves release at the point where they strike the Earth. Scientists also rate the intensity of a quake, using a number scale based on how much shaking occurs and the effects it has on people, buildings, and other structures.
The location where the quake starts is called its hypocenter. A spot directly above it on the surface is called the epicenter. The distance from the hypocenter to the epicenter can be determined by measuring how long it takes for a seismic wave to travel to that location from the epicenter.