What is an Earthquake?What is an Earthquake?

Chapter 8, Section 1Chapter 8, Section 1

EarthquakesEarthquakesEarthquake – Earthquake – vibration of Earth produced by the vibration of Earth produced by the rapid release of energyrapid release of energyFocusFocus – the point within Earth where the – the point within Earth where the earthquake startsearthquake startsThe released energy radiates in all directions The released energy radiates in all directions from the focus in the form of wavesfrom the focus in the form of wavesEpicenter – Epicenter – the location on Earth’s surface the location on Earth’s surface directly above the focusdirectly above the focusEarthquakes are usually associated with large Earthquakes are usually associated with large fractures in Earth’s crust and mantle called faultsfractures in Earth’s crust and mantle called faultsFault –Fault – fractures in Earth where movement has fractures in Earth where movement has occurredoccurred

EarthquakesEarthquakes

Concept CheckConcept Check

What is a fault?What is a fault?

A fault is a fracture in Earth where A fault is a fracture in Earth where movement has occurred.movement has occurred.

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

Elastic Rebound HypothesisElastic Rebound Hypothesis

Elastic Rebound Hypothesis – Elastic Rebound Hypothesis – most most earthquakes are produced by the rapid earthquakes are produced by the rapid release of elastic energy stored in rock release of elastic energy stored in rock that has been subjected to great forces. that has been subjected to great forces. When the strength of the rock is When the strength of the rock is exceeded, it suddenly breaks, causing the exceeded, it suddenly breaks, causing the vibrations of an earthquakevibrations of an earthquakeEarthquakes most often happen along Earthquakes most often happen along existing faults, and occur when the existing faults, and occur when the frictional forces on the fault surface are frictional forces on the fault surface are overcomeovercome

Elastic Rebound HypothesisElastic Rebound Hypothesis

Aftershocks and ForeshocksAftershocks and Foreshocks

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

Aftershocks and ForeshocksAftershocks and Foreshocks

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

AssignmentAssignment

Read Chapter 8, Section 1 (pg. 218-221)Read Chapter 8, Section 1 (pg. 218-221)

Do 8.1 Assessment #1-9 (pg. 221)Do 8.1 Assessment #1-9 (pg. 221)