Earthquakes are sudden movements of the Earth’s surface caused by movement on faults under the surface. The shaking from an earthquake creates seismic waves that travel through the Earth, affecting rock layers below and above ground.

Most earthquakes happen at fault zones, where giant rock slabs called tectonic plates, which make up the Earth’s outer layer (lithosphere), move slowly past each other. The edges of the plates are rough and stick to each other, causing immense stress that is sometimes released quickly as an earthquake. The energy of these earthquakes is transmitted through the Earth as seismic waves, which can shake buildings and even move people.

The strength of an earthquake is measured by its magnitude and intensity. The magnitude of a quake is based on the amount of energy released, and is determined by data from seismographs. It is represented by Arabic numbers, and a quake with a magnitude of 3 or less is considered minor; 5 to 6.9 is moderate; and 7 to 8.9 is major.

The intensity of an earthquake is how much it shakes and how fast the movement occurs. Intensity is based on the effect an earthquake has on people, plants and structures, and can be described by words such as “light,” “strong” or “disturbing.” Both the magnitude and intensity of a quake are reported on a seismogram.