An earthquake is a series of shock waves generated following the brittle failure of rocks within the earth's crust or upper mantle as a result of a build up of stress. Many earthquakes occur each year, on average greater than 800,000, but most are small and not felt humans. A severe earthquake, with a magnitude of greater than 8.0, can be expected every 8 to 10 years. But a significant number of smaller earthquakes, which are still capable of destruction, occur each year.
Earthquakes show a marked spatial distribution .
The vast majority are located within narrow zones which correspond to the boundaries of the lithospheric plates . These plates are blocks of the earth's crust which are in continuous movement relative to each other.