ISNS 3359 Earthquakes and Volcanoes(aka shake and bake)Lecture 6: Locating EQs, EQ Magnitude and Intensity

Fall 2005

Development of SeismologySeismology: study of earthquakesEarliest earthquake device: China, 132 B.C.Instruments to detect earthquake waves: seismometersInstruments to record earthquake waves: seismographsCapture movement of Earth in three components: north-south, east-west and verticalOne part stays as stationary as possible while Earth vibrates: heavy mass fixed by inertia in frame that moves with the Earth, and differences between position of the frame and the mass are recorded digitally

WavesAmplitude: displacementWavelength: distance between successive wavesPeriod: time between wavesFrequency: number of waves in one second (1/period)

Seismic WavesSeismic waves come in two families: those that can pass through the entire Earth (body waves) and those that move near the surface only (surface waves)Body waves: faster than surface waves, have short periods (high frequency 0.5 to 20 Hz), most energetic near the hypocenterTwo types of body waves:P waves and S wavesb

Body WavesP (primary) wavesFastest of all wavesAlways first to reach a recording station (hence primary)Move as push-pull alternating pulses of compression and extension, like wave through Slinky toyTravel through solid, liquid or gasVelocity depends on density and compressibility of substance they are traveling throughVelocity of about 4.8 km/sec for P wave through graniteCan travel through air and so may be audible near the epicenter

Body Waves

Body WavesS (secondary) wavesSecond to reach a recording station (after primary)Exhibit transverse motion shearing or shaking particles at right angles to the waves path (like shaking one end of a rope)Travel only through solidsS wave is reflected back or converted if reaches liquid Velocity depends on density and resistance to shearing of substance Velocity of about 3.0 km/sec for S wave through graniteUp-and-down and side-to-side shaking does severe damage to buildings

Seismic Waves

Seismic Waves and the Earths InteriorWaves from large earthquakes can pass through the entire Earth and be recorded all around the worldWaves do not follow straight paths through the Earth but change velocity and direction as they encounter different layersFrom the Earths surface down:Waves initially speed up then slow at the asthenosphereWave speeds increase through mantle until reaching outer core (liquid), where S waves disappear and P waves suddenly slowP wave speeds increase gradually through outer core until increasing dramatically at inner core (solid)

Seismic Waves and the Earths Interior

Surface WavesSurface wavesTravel near the Earths surface, created by body waves disturbing the surfaceLonger period than body waves (carry energy farther)Love wavesSimilar motion to S waves, but side-to-side in horizontal planeTravel faster than Rayleigh wavesDo not move through air or waterRayleigh wavesBackward-rotating, elliptical motion produces horizontal and vertical shaking, which feels like rolling, boat at seaMore energy is released as Rayleigh waves when the hypocenter is close to the surfaceTravel great distances

Sound Waves and Seismic WavesSeismologists record and analyze waves to determine where an earthquake occurred and how large it wasWaves are fundamental to music and seismologySimilarities:More high frequency waves if short path is traveledTrombone is retracted, short fault-rupture length (small earthquake)More low frequency waves if long path is traveledTrombone is extended, long fault-rupture length (large earthquake)

Seismic VelocitySeismic velocity is a material property (like density).There are two kinds of waves Body and Surface waves.There are two kinds of body wave velocity P and S wave velocities.P waves always travel faster than S waves.Seismic velocities depend on quantities like chemical composition, pressure, temperature, etc.Faster Velocities Lower temperatures Higher pressures Solid phases

Slower Velocities Higher temperatures Lower pressures Liquid phases

Locating the Source of an EarthquakeP waves travel about 1.7 times faster than S wavesFarther from hypocenter, greater time lag of S wave behind P wave (S-P)(S-P) time indicates how far away earthquake was from station but in what direction?

Locating the Source of an EarthquakeNeed distance of earthquake from three stations to pinpoint location of earthquake:Computer calculationVisualize circles drawn around each station for appropriate distance from station, and intersection of circles at earthquakes locationMethod is most reliable when earthquake is near surface

Fig. 4.23

Solution to epicenter and hyopcenterMathematically, the problem is solved by setting up a system of linear equations, one for each station. The equations express the difference between the observed arrival times and those calculated from the previous (or initial) hypocenter, in terms of small steps in the 3 hypocentral coordinates and the origin time. We must also have a mathematical model of the crustal velocities (in kilometers per second) under the seismic network to calculate the travel times of waves from an earthquake at a given depth to a station at a given distance. The system of linear equations is solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times. The process begins with an initial guessed hypocenter, performs several hypocentral adjustments each found by a least squares solution to the equations, and iterates to a hypocenter that best fits the observed set of wave arrival times at the stations of the seismic network.

Magnitude of EarthquakesRichter scaleDevised in 1935 to describe magnitude of shallow, moderately-sized earthquakes located near Caltech seismometers in southern CaliforniaBigger earthquake greater shaking greater amplitude of lines on seismogramDefined magnitude as logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake, with corrections made for distanceFor every 10 fold increase in recorded amplitude, Richter magnitude increases one number

Magnitude of EarthquakesRichter scaleWith every one increase in Richter magnitude, the energy release increases by about 45 times, but energy is also spread out over much larger area and over longer timeBigger earthquake means more people will experience shaking and for longer time (increases damage to buildings)Many more small earthquakes each year than large ones, but more than 90% of energy release is from few large earthquakesRichter scale magnitude is easy and quick to calculate, so popular with media

Magnitude of Earthquakes

Magnitude of Earthquakes

Magnitude of Earthquakeshttp://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif21,688 earthquakes recorded by NEIC in 1998

Magnitude of Earthquakeshttp://www.iris.iris.edu/volume2000no1/RevFigure2.big.gif21,688 earthquakes recorded by NEIC in 1998

Other Measures of Earthquake SizeRichter scale is useful for magnitude of shallow, small-moderate nearby earthquakesDoes not work well for distant or large earthquakesShort-period waves do not increase amplitude for bigger earthquakesRichter scale:1906 San Francisco earthquake was magnitude 8.31964 Alaska earthquake was magnitude 8.3Other magnitude scale:1906 San Francisco earthquake was magnitude 7.81964 Alaska earthquake was magnitude 9.2 (100 times more energy)

Other Measures of Earthquake SizeTwo other magnitude scales:Body wave scale (mb): Uses amplitudes of P waves with 1 to 10-second periodsSurface wave scale (ms):Uses Rayleigh waves with 18 to 22-second periodsAll magnitude scales are not equivalentLarger earthquakes radiate more energy at longer periods not measured by Richter scale or body wave scaleRichter scale and body wave scale significantly underestimate magnitudes of earthquakes far away or large

Moment Magnitude ScaleSeismic moment (Mo)Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakesCalculated using rocks shear strength times rupture area of fault times displacement (slip) on the faultMoment magnitude scale uses seismic moment:Mw = 2/3 log10 (Mo) 6 Scale developed by Hiroo Kanamori

Foreshocks, Main Shock and AftershocksLarge earthquakes are not just single events but part of series of earthquakes over yearsLargest event in series is mainshockSmaller events preceding mainshock are foreshocksSmaller events following mainshock are aftershocksLarge event may be considered mainshock, then followed by even larger earthquake, so then re-classified as foreshock

Magnitude, Fault-Rupture Length and Seismic-Wave FrequenciesFault-rupture length greatly influences magnitude:100 m long fault rupture magnitude 4 earthquake1 km long fault rupture magnitude 5 earthquake10 km long fault rupture magnitude 6 earthquake100 km long fault rupture magnitude 7 earthquake

Magnitude, Fault-Rupture Length and Seismic-Wave FrequenciesFault-rupture length and duration influence seismic wave frequency:Short rupture, duration high frequency seismic wavesLong rupture, duration low frequency seismic wavesSeismic wave frequency influences damage:High frequency waves cause much damage at epicenter but die out quickly with distance from epicenterLow frequency waves travel great distance from epicenter so do most damage farther away

Ground Motion During EarthquakesBuildings are designed to handle vertical forces (weight of building and contents) so that vertical shaking in earthquakes is typically safeHorizontal shaking during earthquakes can do massive damage to buildingsAccelerationMeasure in terms of acceleration due to gravity (g = 9.8 m/s2)Weak buildings suffer damage from horizontal accelerations of more than 0.1 gIn some locations, horizontal acceleration can be as much as 1.8 g (Tarzana Hills in 1994 Northridge, California earthquake)

Periods of Buildings and Responses of FoundationsJust as waves have natural frequencies and periods, so do buildingsPeriods of swaying are about 0.1 second per story1-story house shakes at about 0.1 second per cycle30-story building sways at about 3 seconds per cycleBuilding materials affect building periodsFlexible materials (wood, steel) longer period of shakingStiff materials (brick, concrete) shorter period of shaking

Periods of Buildings and Responses of FoundationsVelocity of seismic wave depends on material it is moving throughFaster through hard rocksSlower through soft rocksWhen waves pass from harder to softer rocks, they slow downMust therefore increase their amplitude in order to carry same amount of energy greater shakingShaking tends to be stronger at sites with softer ground foundations (basins, valleys, reclaimed wetlands, etc.)

Periods of Buildings and Responses of FoundationsIf the period of the wave matches the period of the building, shaking is amplified and resonance resultsCommon cause of catastrophic failure of buildings

Earthquake Intensity What We Feel During an EarthquakeMercalli intensity scale was developed to quantify what people feel during an earthquakeUsed for earthquakes before instrumentation or current earthquakes in areas without instrumentationAssesses effects on people and buildingsMaps of Mercalli intensities can be generated quickly after an earthquake using peoples input to the webpage http://pasadena.wr.usgs.gov/shake

Earthquake Intensity What We Feel During an Earthquake

What To Do Before and During an EarthquakeBefore an earthquake:Inside and outside your home, visualize what might fall during strong shaking, and anchor those objects by nailing, bracing, tying, etc.Inside and outside your home, locate safe spots with protection under heavy table, strong desk, bed, etc.During an earthquake:Duck, cover and holdStay calmIf inside, stay insideIf outside, stay outside

Mercalli Scale VariablesMercalli intensity depends on:Earthquake magnitudeBigger earthquake, more likely death and damageDistance from hypocenterUsually (but not always), closer earthquake more damageType of rock or sediment making up ground surfaceHard rock foundations vibrate from nearby earthquake body wavesSoft sediments amplified by distant earthquake surface wavesSteep slopes can generate landslides when shaken

Mercalli Scale Variables

Mercalli Scale VariablesMercalli intensity depends on:Building styleBody waves near the epicenter will be amplified by rigid or short buildingsLow-frequency surface waves are amplified by tall buildings, especially if on soft foundationsDuration of shakingLonger shaking lasts, more buildings can be damaged

Design of Buildings in Earthquake-Prone AreasEliminate resonance:Change height of buildingMove weight to lower floorsChange shape of buildingChange building materialsChange attachment of building to foundationHard foundation (high-frequency vibrations) build tall, flexible buildingSoft foundation (low-frequency vibrations) build short, stiff building

Design of Buildings in Earthquake-Prone AreasFloors, Roofs and TrussesGive horizontal resistance by transferring force to vertical resistance elementsShear WallsDesigned to receive horizontal forces from floors, roofs and trusses and transmit to groundLack of shear walls typically cause structures like parking garages to fail in earthquakes

Design of Buildings in Earthquake-Prone AreasBracingBracing with ductile materials offers resistanceMoment-resisting framesDevices on ground or within structure to absorb part of earthquake energyUse wheels, ball bearings, shock absorbers, rubber doughnuts, etc. to isolate building from worst shaking

A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971Earthquake magnitudeM 6.6, with 35 aftershocks of magnitude 4.0 or higherDistance from epicenterBulls-eye damage patternFoundation materialsNot a major factorBuilding styleSoft first-story buildings were major problemHollow-core bricks at V.A. Hospital caused collapseCollapse of freeway bridges

Fig. 4.32

A Case History of Mercalli Variables: The San Fernando Valley, California, Earthquake of 1971Duration of shakingLasted 12 seconds (at that magnitude, can last from 10 to 30 seconds), relatively short timeLower Van Norman Reservoir 11,000 acre-feet of water behind earthen dam, above homes of 80,000When shaking stopped, only four feet (of original 30) of dam was still standing above water levelAnother few seconds of shaking might have caused catastrophic flood

Fig. 4.29

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Fig. 4.33a

Fig. 4.33b

Fig. 4.34

Problem Set 1, Journals, Term PaperProblem set 1 and term paper format will be available on WebCT by tomorrow morning.Problem set 1 will now be due on Sept 20, rather than Sept 15.Submit a topic for approval for term paper by Sept 22, if you have not already done soJournals - to reiterate - track 1 event (EQ or V) per week for four weeks and submit a journal on what you find. Length and format is up to you, but all work (other than figures) should be original and all sources cited

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