Earthquakes shake the ground surface, can cause buildings to collapse, disrupt transport and services, and can cause fires. They can trigger landslides and tsunami.
Earthquakes occur mainly as a result of plate tectonics, which involves blocks of the Earth moving about the Earth's surface. The blocks of rock move past each other along a fault. Smaller earthquakes, called foreshocks, may precede the main earthquake, and aftershocks may occur after the main earthquake. Earthquakes are mainly confined to specific areas of the Earth known as seismic zones, which coincide mainly with ocean trenches, mid-ocean ridges, and mountain ranges.
The point of origin of an earthquake is called the focus. The epicentre is the point on the Earth's surface directly above the focus. Most earthquake foci are within a few tens of kilometres of the Earth's surface. Earthquakes less than 70 km deep are classified as shallow-focus. Intermediate-focus earthquakes are 70-300 km deep, and deep-focus earthquakes more than 300 km deep. Shallow-focus earthquakes occur in all of the Earth's seismic zones, but intermediate- and deep-focus earthquakes are almost exclusively associated with seismic zones near ocean trenches.
The destructiveness of an earthquake depends on the size, the depth (shallow ones are more destructive) and the location. Earthquake size can be stated in terms of the damage caused (the intensity) or the amount of ground motion and the energy released by the earthquake (related to the Richter magnitude).