Earthquakes: Sudden movement of the earth resulting from release of energy when rocks that have been under stress break or move.

Earthquake practicalities: Earthquakes are not common, but when they do occur they can be very disruptive to humans, for example, in the years 1900-1989 1.3X106 people killed in earthquakes. That's 14,000 people per year - lot of people.

The largest of the last century occurred in T'ang Shan, China 1976 250,000 people killed

Elastic rebound: We've briefly discussed brittle and ductile deformation. What do these terms really mean?

Deformation in the Earth:

How an object deforms depends on:

Energy in elastic deformation: When an elastically deforming object bends, energy is being stored in it. Thus, as a pencil bends until you hear it break, the sound you hear is caused by the energy being released. Stresses in the lithosphere bend rocks, storing energy in them. When they break, that energy is released and propagated as waves. In an earth quake you can hear sound and feel waves moving through the Earth

Faults: It goes without saying that when a mass of rock breaks, a fracture forms within it. Fractures along which rock masses move relative to one another are called faults. We will have more to say about them in later lectures. For now, understand that most earthquakes occur when:

Earthquake location terms:

Types of energy released when earthquake occurs - waves

Wave speed: Each wave type travels with a characteristic speed.

Thus, if you stand some distance from an earthquake epicenter, you feel P waves first, then S waves, then surface waves. The farther away you are, the longer the lag time between these waves. This lag can be used to locate the epicenter, provided three observers at separate locations keep track.
When an earthquake occurs, observers at seismic stations note the lag time between arrival of P and S waves. From this, they can determine the distance to the epicenter. Seismologists then plot circles with radii of these distances from each station. Their intersection is the epicenter.
Methods for measuring earthquakes

Causes of earthquake damage

Where do earthquakes occur: We've already observed that roughly 90% occur along plate boundaries