8.1 – What is an Earthquake?

Do Now What is the difference between the focus and the epicenter of an earthquake?

Do Now What is the difference between the focus and the epicenter of an earthquake? The focus is the point where the earthquake originates under the surface and the epicenter is the location on the surface directly above the focus.

Key Words Seismic Originates Elastic Vibrations

Vocab Words Earthquake Focus Epicenter Fault Elastic Rebound Hypothesis Aftershock Foreshock

Earthquakes An earthquake is the vibration of Earth produced by the rapid release of energy. They are caused by slippage along a break in Earth’s crust.

Focus The point within Earth where the earthquake starts is called the focus. The released energy radiates in all directions from the focus in the form of waves. These seismic waves radiate through Earth. The focus is where the earthquake originates.

Epicenter The location on the surface directly above the focus is called the epicenter. The earthquakes does not originate from this point, since the epicenter is on the surface, and earthquakes originate under the surface.

Focus & Epicenter

Faults Faults are fractures in Earth where movement has occurred. Earthquakes are usually associated with large fractures in Earth’s crust and mantle and are called faults.

Faults

Cause of Earthquakes Before the great 1906 earthquake in San Francisco, we did not understand the causes and effects of earthquakes. When the forces within Earth deform rocks on either side of a fault, the rocks are forced to bend and store elastic energy. Elastic energy is the same kind of energy that is stored when you stretch a rubber band. Eventually, the resistance caused by internal friction that holds the rocks together is overcome. The rock slips at the weakest point (the focus). The vibrations we call an earthquake occur as the rock elastically returns almost to its original shape.

Elastic Rebound Hypothesis Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded, it suddenly breaks, causing the vibrations of an earthquake.

Aftershocks and Foreshocks An aftershock is a small earthquake that follows the main earthquake. Usually much weaker than the main earthquake. They can also cause damage; especially to structures already damaged by the main earthquake. A foreshock is a small earthquake that comes before the main earthquake. These can happen days or even years before a main earthquake.

San Andreas Fault The most studied fault system in the world. Some areas have a slow, gradual movement. Other areas stay locked in place for hundreds of years and this is why they believe there is a strong chance for a major earthquake along the fault in the future.

Group Activity Create a diagram that shows the difference between the focus and the epicenter of an earthquake.

Videos http://www.youtube.com/watch?v=VSgB1IWr6O4