# What is an Earthquake? Chapter 8, Section 1.

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What is an Earthquake? Chapter 8, Section 1

Earthquakes Earthquake – vibration of Earth produced by the rapid release of energy Focus – the point within Earth where the earthquake starts The released energy radiates in all directions from the focus in the form of waves Epicenter – the location on Earth’s surface directly above the focus Earthquakes are usually associated with large fractures in Earth’s crust and mantle called faults Fault – fractures in Earth where movement has occurred

Earthquakes

Concept Check What is a fault?
A fault is a fracture in Earth where movement has occurred.

Cause of Earthquakes Forces within Earth slowly deform the crustal rocks on both sides of a fault, seen by the bending of the rocks Eventually, the resistance caused by the internal friction holding the rocks together is overcome The rocks slip at their weakest point (the focus) This small movement causes other areas along the fault to slip, until most of the built-up energy is released The slippage allows the deformed rock to return to its original form. The earthquake occurs as the rock elastically returns almost to its original shape

Elastic Rebound Hypothesis
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 Earthquakes most often happen along existing faults, and occur when the frictional forces on the fault surface are overcome

Elastic Rebound Hypothesis

Aftershocks and Foreshocks
Most of the movement along a fault occurs during the main earthquake, however additional movement may occur before and after along this fault and adjoining faults Aftershocks – smaller earthquakes that occur as a result of movements along the fault after a major earthquake Foreshocks – small earthquakes that often come before a major earthquake

Aftershocks and Foreshocks
Aftershocks as a result of November 15, 2006 earthquake off the coast of Japan

Assignment Read Chapter 8, Section 1 (pg. 218-221)
Do 8.1 Assessment #1-9 (pg. 221)