2 EarthquakesEarthquake – vibration of Earth produced by the rapid release of energyFocus – the point within Earth where the earthquake startsThe released energy radiates in all directions from the focus in the form of wavesEpicenter – the location on Earth’s surface directly above the focusEarthquakes are usually associated with large fractures in Earth’s crust and mantle called faultsFault – fractures in Earth where movement has occurred
4 Concept Check What is a fault? A fault is a fracture in Earth where movement has occurred.
5 Cause of EarthquakesForces within Earth slowly deform the crustal rocks on both sides of a fault, seen by the bending of the rocksEventually, the resistance caused by the internal friction holding the rocks together is overcomeThe 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 releasedThe slippage allows the deformed rock to return to its original form.The earthquake occurs as the rock elastically returns almost to its original shape
6 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 earthquakeEarthquakes most often happen along existing faults, and occur when the frictional forces on the fault surface are overcome
8 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 faultsAftershocks – smaller earthquakes that occur as a result of movements along the fault after a major earthquakeForeshocks – small earthquakes that often come before a major earthquake
9 Aftershocks and Foreshocks Aftershocks as a result of November 15, 2006 earthquake off the coast of Japan