Chapter 19 - Earthquakes 19.1- Forces within Earth

Earthquakes Result of movement of crust produced by Plate tectonics Stress = total force acting on crustal rocks Builds up overtime Overcomes strength of rocks and causes movement along fractures in rock Earthquake = vibrations caused by movement in crust

http://earthquake.usgs.gov/learning/kids/sciencefair.php Stress Three types Compression – decrease volume of material Tension – pulls material apart Shear – causes material to twist Strain = deformation of material in a response to stress

Deformation Elastic – when a material is compressed, bent, or stretched at low stress Material can return to “normal” when stress is zero Example: Pulling on a rubber band

Deformation Plastic – stress builds up past elastic limit Permanent deformation – stays changed even when stress is zero Point of Rupture = failure Example: Pulling rubber band until it breaks

Deformation Most materials exhibit varying degrees of plastic and elastic deformation http://www.uwsp.edu/geo/faculty/ritter/images/lithosphere/Earth_Structure/plastic_deformation.jpg

Faults Any fracture or system of fractures along which Earth moves Happens along weak regions Fault plane = surface along which movement takes place

http://facweb.bhc.edu/academics/science/harwoodr/Geol101/study/Images/ReverseFault.gif Faults Reverse Fault – form as a results of horizontal and vertical compression that squeezes rock Seen near convergent plate boundaries Two areas end up closer together after faulting

Faults Normal fault – movement partly horizontal and partly vertical http://facweb.bhc.edu/academics/science/harwoodr/Geol101/study/Images/NormalFault.gif Faults Normal fault – movement partly horizontal and partly vertical Horizontal movement pulls rocks apart and stretches crust Vertical movement causes one side of rock to move down relative to the other side Two areas end up farther apart after faulting

Faults Strike-slip fault – caused by horizontal shear http://facweb.bhc.edu/academics/science/harwoodr/GEOL101/Study/Images/StrikeSlipRLFault.gif Faults Strike-slip fault – caused by horizontal shear Movement is mainly horizontal in opposite direction Example: San Andreas Fault Creates offset features after movement

Earthquake waves Seismic waves = vibration of the ground produced during an earthquake Primary waves = squeeze and push rocks in same direction wave is traveling Secondary waves = cause rocks to move at right angles relative to wave direction Slower than P waves Surface waves = cause most destruction and take longest to past Slowest wave, only travel of Earth’s surface

Seismic Waves http://www.seismo.unr.edu/ftp/pub/louie/class/100/waves.GIF

Earthquake Focus = point where waves originate Epicenter = point on surface directly above focus http://stevekluge.com/geoscience/images/epicenter.jpg

In-Class Assignment/Homework