Question: why do earthqakes happen

Hannah Bentham answered on 14 Jun 2013:

Most earthquakes occur in the crust and happen due to plate tectonic forces. The crust we live on is broken up into smaller irregular shapes that are moving around at rates of 1-10 cm/yr. The boundaries of the plates will generate different geohazards, depending on how adjacent plates are moving with respect to each other.

When plates are moving alongside each other but in opposite directions, we call this transform plate boundaries or strike-slip faults. The irregular edges of the fault cause the plates to get stuck even though there are forces trying to move them. The energy builds up, until the point that the tectonic forces overcome the friction forces, resulting in an earthquake. For example, this occurs on the San Andreas fault in California, USA.

Similar forces occur at collision boundaries (e.g. subduction zones) and at normal and thrust faults which result from stretching or squashing of the crust within a crustal plate.