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Purdue EAS 45000 - Earthquake seismology

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EarthquakeseismologyThe San Andreas fault in theCarrizo plain, CaliforniaOffset drainage along the SanAndreas fault, Wallace Creek,CaliforniaFence offset by the 1906 SanFrancisco earthquakeElastic strainaccumulation• (Most) faults are lockedbetween earthquakes• The area around faultsaccumulates elastic strainGPS-derived velocities in Southern California (1992-2000). Velocities are shown with respect to NorthAmerica. The active faults of California are shown inorange.The seismiccycle• Between earthquakes:– Faults are locked– Area around faults accumulate deformation• During an earthquake:– A fault slips suddenly– The deformation accumulated around the fault isreleased• After an earthquake:– Stresses around the fault are modified– Readjustments on the fault plane = aftershocksThe seismic cycleDuring an earthquake:– A fault slips suddenly– The deformation accumulated around thefault is released– Stresses around the fault are modifiedBetween earthquakes:– Faults are locked– Area around faults accumulate deformationAnim ation: R. Stein, USGSClick for earthquake cycle animationEarthquake seismology• Location of the earthquake (hypocenter)?• Frequency of similar earthquakes?• Focal mechanism?• Rupture mechanism?• Size?– Intensity– Magnitude– Moment– Energy release• Earthquake triggering?Locating earthquakesDifference in travel timefor P and S wavesincreases with increasingepicentral distance:! tS=DVStP=DVP" tS# tP= D1VS#1VP$ % & ' ( ) VP= 5.85 km / s VS= 3.38 km / s" D = tS# tP( )* 8.0Locating earthquakes• Errors:– Picking arrivals– Actual travel times are slightlydifferent from theoretical ⇒ locationis dependent on the Earth model used(global or local).• With at least 3 stations:– Calculate S-P time difference– Convert to distance– Draw circles centered on stations– Location = intersection of circlesEarthquake focalmechanisms• Earthquake = release ofaccumulated elastic energy bydisplacement on a fault• Problem: what type of fault motion?• Case of a strike-slip fault: particlemotion due to fault slip:– Blue quadrants: particles pushedaway from the focus ⇒compressional first motion = UP– Red quadrants: particles pulledtowards the focus ⇒ dilatationalfirst motion = DOWN• As a result, we obtain 4 quadrants:– 2 compressional quadrants: firstmotion down– 2 extensional quadrants: firstmotion updilatationalfirst motioncompressionalfirst motionfault planeauxiliary planecompressionalquadrantcompressionalquadrantextensionalquadrantextensionalquadranttensionaxiscompressionaxisEarthquake focalmechanisms• Earthquake = release of accumulatedelastic energy by displacement on afault• Problem: what type of fault motion?• Let’s assume an earthquake on areverse fault:– Compressional / tensional quadrants– Compressional quadrant: surface ispulled down ⇒ first motion DOWN– Tensional quadrant: surface is pushedup ⇒ first motion UP• If we map first motion, we can find:– 2 focal planes– P- and T-axisEarthquake focal mechanisms• Seismic rays travel away from the focus• Each ray “samples” a dilatational or compressional quadrant around thefocus• Seismic stations at different distances record up or down first motions• Rays along nodal planes?in cross-sectionfocal mechanism(stereonet proj.)Earthquake focal mechanisms• The “focal sphere”:– Center = earthquake hypocenter– In each quadrant: first motion identical• Seismic stations are at the surface,(usually) not underground– Rays bend upward and eventuallyreaches a seismic station at the surface– The important parameter is the initialtake-off angle– Take-off angle can be calculatedknowing the earth’s structure =>accuracy of focal mechanisms dependon our knowledge of the Earth structure(local, regional, global)Earthquake focalmechanisms• Strike-slip faulting:– Vertical focal planes– Horizontal P-axis and T-axis• Other types of faulting:– Focal planes will have a dip– P-axis and T-axis will have adip• For representation: focal sphere+ stereographic projection offocal planes and P-T-axis ⇒“Beach balls”In the horizontal plane:The focal sphere:Earthquake focal mechanisms• Focal mechanisms definethe type of faulting thatoccurred during theearthquake.• The actual fault plane isambiguous• Focal mechanisms cancombine these types offaulting.• Focal mechanisms in anactively deforming areacontain information aboutthe strain regimereversenormalstrike-slipEarthquake focal mechanismsEarthquake focalmechanisms• Eastern Mediterranean• Earthquake focalmechanism illustrate:– Strike-slip faulting– Reverse faulting– Extensional faulting• Compare with GPSvelocities(McClusky et al., JGR, 2000)Earthquake rupture• An earthquake usually breaks a segment of a fault• The rupture does not always reach the surface• The earthquake is followed by aftershocks:– Readjustments on the rupture plane– Help define the rupture planeAnimation http://www.scecdc.scec.org/northreq.htmlNorthridge earthquake, January 1994, M=7.2Earthquake rupture• Time and space history of a rupture, example of the Northridge earthquake• Slip on the rupture plane is not homogeneous• Asperities and barriersAnimation D. Wald, http://www.scecdc.scec.org/northrup.htmlEarthquake size• Shear forces on a faults ⇒moment• Hooke’s law relates stressand strain for elasticsolids: for shear,proportionality factor isrigidity µ! MO= 2bF"shear=µ#$shearwith$shear=d2band"shear=FA=FL # W% MO=µAdMoment = rigidity x displacement x rupture areaF-FbRupture area: A = L x WEarthquake magnitude• 1935: Richter worked on rankingearthquakes as a function of theirsize• First definition:– “Maximum amplitude recordedat 100 km from the epicenter”:– For local earthquakes: S-waveshave the largest amplitude– Correction for distance: Δ (=angular epicentral distance indegrees)• Richter magnitude scale:• Open scale• Largest magnitude recorded =Chile, 1960, MW=9.6 (MS=8.3)• Negative magnitudes arepossible…ML=log10(Amax) + 3 log10Δ - 2.92nomogram used to compute magnitude quickly by eyeEarthquake magnitude• ML = local magnitudes (~ 600 km from earthquake)• At larger distances:– Using surface waves (they have the largest amplitude)A=max. amplitude of vertical component in microns, T = period inseconds, D = angular distance in degrees.– Using body-waves (P-waves)– Ms – mb relationship:MS = log10(Amax/T)+1.66 log10Δ +


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