What are Earthquakes?
As tectonic plates move around they generate stress. We think of rocks as being hard and brittle but they can actually bend quite a bit before they break. This stress builds up in the rocks over time. An earthquake is the vibration of Earth produced by the rapid release of energy from the build up of all this stress.
Recall from the plate tectonics lesson that earthquakes are commonly located on plate boundaries. We can classify earthquakes by how deep they occur within the Earth's crust:
- Shallow - (up to 70 km below the surface)
- Intermediate - (70 to 300 km)
- Deep - (greater than 300 km)
TerminologyFocus – aka Hypocenter: Where the release of energy occurs
Faults - fractures in the ground along which apreciable displacement occurs.
Epicenter - the point on the Earth's surface directly above the focus
Elastic Rebound - This gradual accumulation and release of stress and strain is now referred to as the "elastic rebound theory" of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy. (source)
Foreshocks - small earthquakes that happen in the area where the mainshock will be.
Mainshock - the largest earthquake in a series of quakes; sometimes is preceeded by a foreshock but is always followed by aftershocks.Aftershock - earthquakes that occur after the mainshock as the ground is settling back down.