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earthquakes earthquakes what is an earthquake ground shaking caused by sudden release of energy stored in rocks beneath surface tectonic forces produce stresses on rocks that exceed elastic limits and cause brittle failure rupture along a fault plane seismic waves released from breaking point what happens along the fault elastic rebound theory 3 m offset 1906 San Francisco earthquake offset lettuce rows El Centro CA earthquakes hypocenter focus location of first rupture along fault epicenter point on Earth s surface above rupture seismic waves radiate from focus hypocenter earthquakes seismic waves energy released from earthquake two types of seismic waves body waves travel outward from focus through body of Earth surface waves travel away from epicenter on surface of Earth body waves P primary waves compressional body wave vibration is back and forth in direction wave travels slinky fast 4 7 km s first to arrive pass through solids and fluids S secondary waves shear body wave vibration is perpendicular to direction the wave travels rope slower 2 5 km s secondary arrival pass only through solids body waves P wave displacement parallel to wave motion S wave displacement normal to wave motion from http www personal umich edu vdpluijm gs205 html P primary waves faster than S secondary waves body waves why are P waves faster than S waves why don t S waves travel through fluids look at equations for velocities V primary wave k 4 3 V secondary wave 1 2 1 2 density shear modulus rigidity k bulk modulus compressibility numerator of P wave velocity numerator of S wave velocity or shear modulus is zero for a fluid fluids cannot support shear velocity of S wave in fluid is zero surface waves slowest seismic waves Love waves side to side motion of ground surface cannot travel through fluids Rayleigh waves ground moves in elliptical path In direction opposite to direction of travel of wave very destructive to buildings measuring earthquakes instruments record arrival of seismic waves at sites calculation of size magnitude and location focus seismometers instruments that detect seismic waves seismographs devices that record motion detected by seismometers seismograms paper or digital records of seismic wave vibrations vertical component seismometer measures vertical motion of Earth s surface very heavy weight on spring ground moves but weight stays at same level pencil attached to weight writes on rotating paper horizontal component seismometer measures horizontal motion of Earth s surface sample seismogram P S L R arrivals of P S Love Rayleigh waves respectively from same earthquake at same seismometer note that P is first S is second surface waves Love and Rayleigh are last locating earthquakes P and S waves leave focus hypocenter at same time and head toward seismograph stations A and B P waves travel faster than do S waves P waves get farther and farther ahead of S waves with distance and time from earthquake difference in arrival time is greater at station B than at A locating earthquakes we know relationship of velocities of P and S waves thus we can generate a travel time curve to estimate distance from focus hypocenter travel time curve S wave curve separation between 2 curves increases with time and distance P wave curve 3 minute difference at 2000 kms 8 minute difference at 5300 kms locating earthquakes how do we determine location of epicenter use 3 stations determine distance to epicenter for each station draw circles with center at station and distance to epicenter as the radius constrain location of epicenter by intersection of 3 circles locating earthquakes example use arrival times of S and P waves on 3 seismograms and travel time curve from http vcourseware5 calstatela edu cgi bin 3 seismograms from Japan S Korea Akita Pusan Tokyo from http vcourseware5 calstatela edu cgi bin travel time curve find time of arrival of S and P waves for each site calculate difference in P and S wave arrival time for each site from http vcourseware5 calstatela edu cgi bin 3 seismographs S P interval Tokyo 44 sec Pusan 56 sec Akita 71 sec distance Tokyo 434 km Pusan 549 km Akita 697 km from http vcourseware5 calstatela edu cgi bin draw circles whose centers are stations and radii are distances from S P difference distance Tokyo 434 km Pusan 549 km Akita 697 km epicenter is intersection of 3 circles Kobe Japan hypocenter focus can be determined as well from http vcourseware5 calstatela edu cgi bin measuring earthquake size two approaches intensity measure of earthquake effects on people structures Modified Mercalli Intensity Scale I not generally felt XII total devastation magnitude amount of energy released by earthquake Richter Scale developed for southern California no upper bound accurate at 7 less accurate for small events Moment Magnitude more objective uses rock strength area of rupture and amount of displacement slip along fault Modified Mercalli Intensity Index 1886 Charleston SC earthquake relationship of Mercalli Intensity to Richter magnitude moment magnitude and energy released note for any year small big determining magnitude from seismograms measure maximum amplitude of S wave body wave magnitude other methods exist using other waves from http vcourseware5 calstatela edu cgi bin measure amplitude of S waves on 3 seismograms these are the same as earlier example Akita Pusan Tokyo 30 mm 90 mm 170 mm Akita Pusan Tokyo from http vcourseware5 calstatela edu cgi bin figure to left plots relationship between distance from epicenter magnitude amplitude of wave on seismogram from http vcourseware5 calstatela edu cgi bin distance from earlier Tokyo 434 km Pusan 549 km Akita 697 km amplitude Akita Pusan Tokyo 30 mm 90 mm 170 mm magnitude 6 8 from http vcourseware5 calstatela edu cgi bin location and size of earthquakes in the US occur everywhere but more common in Alaska Pacific Northwest and California note concentration in eastern Arkansas western Tennessee and Kentucky southern Illinois southeastern Missouri New Madrid Seismic Zone earthquake risk in the US assumes that future earthquakes will occur where they have in the past New Madrid Seismic Zone effects of earthquakes ground shaking most familiar topples buildings fire from broken gas mains fallen electric wires landslides triggered by shaking liquefaction water saturated soil behaves like fluid can no longer support structures displacement of land surface movement of blocks along fault pancaked building 1985 Mexico City earthquakes


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