Slide 1Slide 2elastic rebound theoryoffset lettuce rows - El Centro, CASlide 5Slide 6Slide 7Slide 8Slide 9Slide 10measuring earthquakesvertical component seismometerSlide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Modified Mercalli Intensity IndexSlide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33pancaked building - 1985 Mexico Cityground rupture, TaiwanSlide 36surface displacement - 1964 Alaskabuckled concrete - 1971 San Fernando, CAsoil liquefaction - 1964 Nigata, JapanSlide 40Slide 41Slide 42Slide 43tsunami wave propagation times from Hawaiitsunami - 1964 Alaska earthquakeSlide 46Slide 47earthquakes and plate tectonicsSlide 49Slide 50Slide 51Slide 52Slide 53New Madrid Seismic ZoneSlide 55Slide 56Slide 57Slide 58Slide 59Modified Mercalli Intensity map for New Madrid events in 1811-1812Slide 61Slide 62Slide 63Slide 64Slide 65Slide 66Slide 67Slide 68Slide 69Slide 70Slide 71wave ray pathsvelocities of seismic waves in Earthradial pattern of ray paths from focusSlide 75Slide 76Slide 77Slide 78Slide 79Slide 80Slide 81Slide 82P wave shadow zone (from refraction)Slide 84Slide 85Slide 86Slide 87earthquakesearthquakeswhat is an earthquake?ground shaking caused by sudden release ofenergy stored in rocks beneath surfacetectonic forces produce stresses on rocks thatexceed elastic limits and cause brittle failure(rupture along a fault plane)seismic waves released from breaking pointelastic rebound theory3 m offset1906 San Francisco earthquakewhat happens along the fault?offset lettuce rows - El Centro, CAearthquakeslocation of firstrupture along faulthypocenter (focus)point onEarth’ssurfaceabove ruptureepicenterseismic waves radiate from focus (hypocenter)earthquakestwo types of seismic wavesseismic waves -- energy released from earthquakebody wavestravel outward from focus through body of Earthsurface wavestravel away from epicenter on surface of Earthbody wavescompressional body wavevibration is back and forthin direction wave travels(slinky)fast (4-7 km/s); first to arriveP (primary) wavespass through solids and fluidsshear body wavevibration is perpendicular todirection the wave travels (rope)slower (2-5 km/s); secondary arrivalS (secondary) wavespass only through solidsdisplacement parallel to wave motiondisplacement normal to wave motionP waveS wavefrom: http://www.personal.umich.edu/~vdpluijm/gs205.htmlP (primary) waves faster than S (secondary) wavesbody wavesV (primary wave) = k + 4/3µ 1/2• why are P waves faster than S waves?• why don’t S waves travel through fluids?densityµshear modulus (rigidity)kbulk modulus (compressibility)V(secondary wave) = µ 1/2body waveslook at equations for velocitiesµ, or shear modulus, is zero for a fluid (fluids cannot support shear)…velocity of S wave in fluid is zero…numerator of P wave velocity > numerator of S wave velocitysurface wavesside to side motion ofground surfaceLove wavescannot travel through fluidsground moves in elliptical pathIn direction opposite todirection of travel of waveRayleigh wavesvery destructive to buildingsslowest seismic wavesmeasuring earthquakes• calculation of size (magnitude) and location (focus)• instruments record arrival of seismic waves at sitesseismometersinstruments that detect seismic wavesseismographsdevices that record motion detected by seismometersseismogramspaper or digital records of seismic wave vibrationsvertical component seismometermeasures 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 paperhorizontal component seismometermeasures horizontal motion of Earth’s surfacesample seismogramP, S, L, R: arrivals of P, S, Love, Rayleigh waves, respectively from same earthquake at same seismometernote that P is firstS is secondsurface waves (Love and Rayleigh) are lastP waves travel faster than do S waveslocating earthquakesP and S waves leave focus (hypocenter) at same timeand head toward seismograph stations A and BP waves get farther and farther ahead of S waves with distance and time from earthquakedifference in arrival time is greater at station B than at Athus we can generate a travel-time curve toestimate distance from focus (hypocenter)locating earthquakeswe know relationship of velocities of P and S wavestravel-time curveP wave curveS wave curveseparation between2 curvesincreases withtime and distance3 minute differenceat 2000 kms8 minute differenceat 5300 kmslocating earthquakeshow 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 circlesuse arrival times of S and P waves on 3 seismogramsand travel-time curvelocating earthquakesexamplefrom: http://vcourseware5.calstatela.edu/cgi-bin3 seismograms from Japan/ S. KoreaAkitaTokyoPusanfrom: http://vcourseware5.calstatela.edu/cgi-bintravel-time curvefind time of arrival ofS and P wavesfor each sitecalculate difference inP and S wavearrival timefor each sitefrom: http://vcourseware5.calstatela.edu/cgi-binS-P interval:Tokyo: 44 secPusan: 56 secAkita: 71 sec distanceTokyo: 434 kmPusan: 549 kmAkita: 697 km3 seismographsfrom: http://vcourseware5.calstatela.edu/cgi-bindraw circles whose centersare stationsand radii are distancesfrom S-Pdifference distanceTokyo: 434 kmPusan: 549 kmAkita: 697 kmepicenter is intersection of 3 circles (Kobe, Japan)hypocenter (focus) can be determined as wellfrom: http://vcourseware5.calstatela.edu/cgi-binmeasuring earthquake sizetwo approachesintensitymeasure of earthquake effects on people/structuresModified Mercalli Intensity ScaleI (not generally felt) -XII (total devastation)magnitudeamount of energy released by earthquakeRichter ScaleMoment Magnitudedeveloped for southern California; no upper bound; accurate at > 7; less accurate for small eventsmore objective; uses rock strength, area of rupture, and amount of displacement (slip along fault)1886 Charleston, SC earthquakeModified Mercalli Intensity Indexrelationship of Mercalli Intensity to Richter magnitudemoment magnitude and energy releasednote for any year # small >>> # bigmeasure maximum amplitude of S wave (body wave magnitude) (other methods exist using other waves)determining magnitude from