Modul 13 - Earthquake

Module 13Module 13EarthquakeEarthquake

Photo credit: USGS

EARTHQUAKEEARTHQUAKE

EARTHQUAKEEARTHQUAKEEarthquakeEarthquake A series of shock waves generated at a point (A series of shock waves generated at a point (the the

focusfocus) within the Earth”s crust or mantle) within the Earth”s crust or mantle The point on the surface of the Earth above the The point on the surface of the Earth above the

focus is called the focus is called the epicenterepicenter Three main types of wave motion are generated Three main types of wave motion are generated

by an earthquake: P-Waves; S-Waves; dan L-by an earthquake: P-Waves; S-Waves; dan L-WavesWaves


P-WavesP-Waves• High-frequencyHigh-frequency• Short-wavelengthShort-wavelength• Longitudinal wavesLongitudinal waves• Can be reflected andCan be reflected and

refracted refracted • Travel through theTravel through the

solid and the liquidsolid and the liquidpart of the Earthpart of the Earth

S-WavesS-Waves• High-frequencyHigh-frequency• Short-wavelengthShort-wavelength• Transverse wavesTransverse waves• Can be reflected andCan be reflected and

refractedrefracted• Travel through theTravel through the

solid part of the solid part of the Earth at varyingEarth at varyingvelocities.velocities.

• Propagated in allPropagated in alldirections fromdirections fromthe focusthe focus

L-WavesL-Waves• Low-frequencyLow-frequency• Long-wavelengthLong-wavelength• Transverse vibrationsTransverse vibrations• Confined to the outerConfined to the outer

skin of the crustskin of the crust• Responsible for most Responsible for most

of the destructiveof the destructiveforce of earthquakeforce of earthquake

Body WavesBody Waves Primary or Primary or CompressionalCompressional Secondary or ShearSecondary or Shear

Surface WavesSurface Waves Rayleigh (large vertical displacements)Rayleigh (large vertical displacements) Love (shear)Love (shear)

EARTHQUAKEEARTHQUAKESeismic Wave TypesSeismic Wave Types

Seismic Wave TypesSeismic Wave TypesB

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Wav

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aves


Primary (P) WavePrimary (P) Wave Secondary (S) WaveSecondary (S) Wave

Love WaveLove Wave Rayleigh WaveRayleigh Wave

3 m offset

1906 San Francisco Earthquake1906 San Francisco Earthquake

Elastic Rebound TheoryElastic Rebound TheoryEARTHQUAKEEARTHQUAKE

Offset Lettuce Rows - El Centro, CAOffset Lettuce Rows - El Centro, CA

Elastic Rebound TheoryElastic Rebound TheoryEARTHQUAKEEARTHQUAKE

Where does the offset occur?Draw the relative movement

Vp = k + 4/3µ

1/2 density

µ shear modulus (rigidity)

k bulk modulus (rigidity)

Vs = µ

1/2

because shear modulus (rigidity) for fluid is zero,S waves cannot propagate through a fluid

consequence of equations is that P waves are 1.7x faster than S

can infer physical properties from P and S waves

Equations for velocitiesEquations for velocitiesEARTHQUAKEEARTHQUAKE

Pancaked BuildingPancaked Building - 1985 Mexico City - 1985 Mexico City

Earthquakes don’t kill people - buildings do!

Effects of the EarthquakeEffects of the Earthquake

Soil Liquefaction - 1964 NiSoil Liquefaction - 1964 Niiigata, Japangata, Japan


Surface Displacement - 1964 AlaskaSurface Displacement - 1964 Alaska


Ground Rupture, 1906 Olema, CAGround Rupture, 1906 Olema, CA


Fence Compression - Gallatin County, MTFence Compression - Gallatin County, MT


Buckled Concrete - 1971 San Fernando, CABuckled Concrete - 1971 San Fernando, CA


Tsunami Devastation - 1964 Alaska EarthquakeTsunami Devastation - 1964 Alaska Earthquake


Tsumani GenerationTsumani Generation


Tsunami Wave Propagation TimesTsunami Wave Propagation Times


Vertical Component SeismometerVertical Component SeismometerLocating the EarthquakeLocating the Earthquake

Horizontal Component SeismometerHorizontal Component SeismometerLocating the EarthquakeLocating the Earthquake

First Arrivals – Seismographic RecordFirst Arrivals – Seismographic RecordLocating the EarthquakeLocating the Earthquake

Distance – Time RalationsDistance – Time RalationsLocating the EarthquakeLocating the Earthquake

P P vsvs S Wave Travel Time Curves S Wave Travel Time CurvesLocating the EarthquakeLocating the Earthquake

Earthquake Location by RangeEarthquake Location by RangeLocating the EarthquakeLocating the Earthquake

• Three distinct methods to measure earthquakes. Two Three distinct methods to measure earthquakes. Two based on energy and one based on intensity.based on energy and one based on intensity.– Richter Magnitude Scale: originally developed for Richter Magnitude Scale: originally developed for

southern California. Log scale, which has no upper southern California. Log scale, which has no upper bound. Small earthquakes may yield negative bound. Small earthquakes may yield negative values. Tends to be inaccurate at >7 magnitudes.values. Tends to be inaccurate at >7 magnitudes.

– Moment Magnitude Scale: measurement of the Moment Magnitude Scale: measurement of the amount of work done during the earthquake. Based amount of work done during the earthquake. Based on rock strength, area of rupture, and displacement on rock strength, area of rupture, and displacement during event.during event.

– Modified Mercalli Intensity Scale: based on the Modified Mercalli Intensity Scale: based on the damage associated with a particular event at a damage associated with a particular event at a particular location. Ranges from I (less damage) to particular location. Ranges from I (less damage) to XII (more damage). XII (more damage).

Measuring the EarthquakeMeasuring the Earthquake

Magnitude-Description-Intensity-Frequency RelationsMagnitude-Description-Intensity-Frequency RelationsMeasuring the EarthquakeMeasuring the Earthquake

1886 Charleston, SC earthquake

Measuring the EarthquakeMeasuring the EarthquakeModified Modified Mercalli Mercalli Intensity Index (1931)Intensity Index (1931)

Measuring the EarthquakeMeasuring the EarthquakeHistorical Earthquake MagnitudesHistorical Earthquake Magnitudes

• Earthquakes at Plate Boundaries• Subduction Angle

Earthquake and Plate TectonicsEarthquake and Plate Tectonics

Earthquake DistributionEarthquake DistributionEarthquake and Plate TectonicsEarthquake and Plate Tectonics

Relative plate motion and boundariesRelative plate motion and boundariesEarthquake and Plate TectonicsEarthquake and Plate Tectonics

different types of structures are associated with each boundary type:divergent/rifting: extensional (normal faulting)convergent/collisional: compressional (thrust faulting)transform/transcurrent: shear-dominated (strike-slip faulting)


Divergent Boundary and EarthquakesDivergent Boundary and EarthquakesEarthquake and Plate TectonicsEarthquake and Plate Tectonics

EEarthquakes alongarthquakes alongmid-ocean ridge aremid-ocean ridge areshallower than thoseshallower than thosealong subduction zonealong subduction zone

from: http://www.pmel.noaa.gov/vents/coax/coax.html

Divergent Boundary and EarthquakesDivergent Boundary and EarthquakesEarthquake and Plate TectonicsEarthquake and Plate Tectonics

Earthquake and Plate TectonicsEarthquake and Plate TectonicsTransform Boundary and EarthquakesTransform Boundary and Earthquakes

The western USThe western USis somewhat is somewhat anomalousanomalousNote: absence ofNote: absence ofdeep earthquakesdeep earthquakes


Earthquake and Plate TectonicsEarthquake and Plate TectonicsTransform Boundary and EarthquakesTransform Boundary and Earthquakes

Earthquake and Plate TectonicsEarthquake and Plate TectonicsConvergent Boundary and EarthquakesConvergent Boundary and Earthquakes

Shallow vs. Deep Subduction EarthquakesShallow vs. Deep Subduction Earthquakes

Continent-continent collisionContinent-continent collisionzones have broad areas of zones have broad areas of of relatively shallow seismicityof relatively shallow seismicity

Ocean-continent convergentOcean-continent convergentmargins have earthquakes focimargins have earthquakes focithat extend to great depths.that extend to great depths.Mechanism tend to change fromMechanism tend to change fromextension to compression extension to compression downdip.downdip.


deep intermediate shallow

• Epicenters: Epicenters: location of earthquake rupture projected to location of earthquake rupture projected to surface;surface;

• DDip of slab leads to observed seismicity patterns:ip of slab leads to observed seismicity patterns: deeper deeper farther from trenchfarther from trench

• LLocation of downgoing slab as it dives into mantleocation of downgoing slab as it dives into mantle i is defined s defined by seismicityby seismicity..

• EEarthquakes occur alongarthquakes occur along an inclined belt:an inclined belt: the Wadati-Benioff the Wadati-Benioff zonezone r reaches maximum depth of ~670 kmeaches maximum depth of ~670 km

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270


SSeismicity of subduction zoneseismicity of subduction zones

all from: http://www.pmel.noaa.gov/vents/coax/coax.html


Seismicity of Japan and Kuril Island:Seismicity of Japan and Kuril Island:1975 - 19951975 - 1995

Seismicity Fiji IslandsSeismicity Fiji IslandsRegion: 1977 - 1997Region: 1977 - 1997

Seismicity of Peru-BoliviaSeismicity of Peru-BoliviaBorder Region: 1977 - 1997Border Region: 1977 - 1997

slab is cold and thus can haveslab is cold and thus can have earthquakes at greater depthsearthquakes at greater depths

Tomography (3D seismic)Tomography (3D seismic)

note continuity of blue slabnote continuity of blue slab to depths on order of 670 kmto depths on order of 670 km

blue is fast…blue is fast… interpreted as slabinterpreted as slab



SSeismicity along subduction zones:eismicity along subduction zones: earthquakes are shallow, intermediate, and deepearthquakes are shallow, intermediate, and deep but have systematic locationbut have systematic location related to subducting slarelated to subducting slabb shallow adjacent to trench and deep farthest awayshallow adjacent to trench and deep farthest away

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270


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Modul 13 - Earthquake