introduction seismic waves - university of … · • introduction • seismic waves ... records...

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• Introduction• Seismic Waves

– Locating Earthquakes– Probing Earth’s interior

• Effects of Earthquakes• Earthquake Risks and Predictions• Waiting for the BIG ONE

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San Francisco CA San Francisco CA -- San Andreas Strike Slip FaultSan Andreas Strike Slip Fault

M=7.0 Loma Prieta EQ, 1989 Bay Bridge collapse

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M=8.3 San Francisco, April 18, 19063,000 casualties

Bam, IranMw = 6.6

December 26, 200330,000 casualties/30,000 injured

http://earthquake.usgs.gov/recenteqsww/Quakes/uscvad.htm

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IntroductionIntroduction• An earthquake is a trembling of Earth caused by

sudden release of stored energy, usually along faults.

San Andreas fault, CA

IntroductionIntroduction• Earthquakes are also associated with

volcanic activity

Eruption of Mt. St. Helens

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IntroductionIntroduction• Earthquakes confined to a few narrow belts

IntroductionIntroduction• The focus of an earthquake is the location within

the earth where the earthquake originates.

• The epicenter is the point on the earth’s surface directly above the focus.

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– Location and size of Earthquakes– Probing Earth’s Interior


Seismic WavesSeismic Waves

• Seismic Waves are the waves of energy produced by an earthquake.

• Seismograms reveal two main groups of seismic waves

• Seismometer– Suspended weight that

detects wave motion

• Seismograph– Drum/computer that

records wave motion

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• Body waves• P-waves - travel through solids and fluids• S-waves - only travel through solids

– Slower than P waves

–Surface waves • Slowest• Most damaging

Body WavesBody Waves

Surface WavesSurface Waves

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First P wave First S wave

SeismogramSeismogram1906 San 1906 San FranscicoFranscico EarthquakeEarthquake


Seismic waves are useful for:

1. determining size & location of determining size & location of earthquakesearthquakes

2. monitoring volcanic activity

3. monitoring nuclear explosions

4. probing interior of the Earthprobing interior of the Earth

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Locating an EarthquakeLocating an Earthquake• The difference between P and S wave velocity

provides method to locate epicenter–– Travel time curvesTravel time curves

First P wave First S wave10 min. time difference

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Locating an EarthquakeLocating an Earthquake• Epicenter located using seismograms from 3 3 stations

Measuring the Size of EarthquakesMeasuring the Size of Earthquakes

MagnitudeMagnitude - measure of the amount of energy released during an earthquake– Modified Mercalli scale

• observed damage– Richter scale (MM)

• calculate energy released from amplitude of waves– Moment magnitude scale (MMww)

• energy from rock strength, surface rupture area, displacement along fault

• logarithmic scale--each unit corresponds to roughly a 10-fold increase in released energy

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First P wave First S wave

M = 7.0

M = 8.3

Richter magnitudeRichter magnitude / EQ frequencyEQ frequency / Energy releasedEnergy released

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Earth’s Internal Structure• The Core

– P-wave Shadow Zones S-wave Shadow Zone

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Effects of EarthquakesEffects of Earthquakes

• Ground motion• Landslides• Ground displacement• Liquefaction• Tsunamis• Aftershocks

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Effects of EarthquakesEffects of Earthquakes• Ground motion•• LandslidesLandslides•• Permanent ground displacementPermanent ground displacement

Effects of EarthquakesEffects of Earthquakes

• Ground motion• Landslides• Ground displacement•• LiquefactionLiquefaction

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Effects of EarthquakesEffects of Earthquakes• Ground motion• Landslides• Ground displacement• Liquefaction

•• Tsunamis Tsunamis -- seismic sea wavesseismic sea waves

Effects of EarthquakesEffects of Earthquakes• Ground motion• Landslides• Ground displacement• Liquefaction• Tsunamis •• AftershocksAftershocks

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Earthquake Prediction and RiskEarthquake Prediction and Risk

Scientific techniques being explored

• Microseisms, small strains near fault• Changes in properties of rock near fault• Water levels, Radon in wells• Surface tilt and changes of elevation• Patterns of past Earthquakes

– In space and time

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Earthquake Prediction and RiskEarthquake Prediction and Risk

•• Microseisms, small strains near faultMicroseisms, small strains near fault•• Example: Example: ParkfieldParkfield, California, California

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San Andreas fault

Earthquake Prediction and RiskEarthquake Prediction and Risk• Microseisms, small strains near fault•• Changes in properties of rock near faultChanges in properties of rock near fault•• Water levels in wellsWater levels in wells

Water well monitoring along the San Andreas fault

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Earthquake PredictionEarthquake Prediction

•• Surface tilt and elevation changesSurface tilt and elevation changes

Lasers measure very small changes in distance and angles

GlobalPositioning System (GPS) measurementsof motion via satellite

Earthquake Prediction and RiskEarthquake Prediction and Risk•• Microseisms, small strains near faultMicroseisms, small strains near fault

•• Example: Example: ParkfieldParkfield, California, California

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Earthquake PredictionEarthquake Prediction

• Animal behavior• Foreshocks•• Patterns of earthquakes in time and spacePatterns of earthquakes in time and space

–– Seismic GapsSeismic Gaps

Seismic Risk Map

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Waiting for the Waiting for the BIG ONEBIG ONE

TheThe San Andreas fault

•• 1906 San Francisco M 8.31906 San Francisco M 8.3•• 1857 Fort 1857 Fort TejonTejon M=7.6M=7.6• 1994 Northridge M=6.4 • 1989 Loma Prieta M=7.0

•• With respect to great With respect to great EQsEQs, , southern section has been southern section has been inactive longerinactive longer——–– Is this region due for a Is this region due for a

great earthquake?great earthquake?

•• M 8 quake releases 32 times M 8 quake releases 32 times the energy of M 7 quake !the energy of M 7 quake !

Earthquake distributionEarthquake distribution•• narrow belts reflect plate boundary behaviornarrow belts reflect plate boundary behavior

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Earthquakes at Plate BoundariesEarthquakes at Plate Boundaries• Divergent

– depth: shallow

• Transform • depth: shallow-intermediate < 20 km

• Convergent• depth: shallow - intermediate - very

deep (< 670 km)• define BenioffBenioff zones

introduction seismic waves - university of … · • introduction • seismic waves ... records...

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