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
EARTHQUAKEEARTHQUAKE
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
ody
Wav
esB
ody
Wav
es
Surf
ace
Wav
esSu
rfac
e W
aves
EARTHQUAKEEARTHQUAKE
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
Effects of the EarthquakeEffects of the Earthquake
Surface Displacement - 1964 AlaskaSurface Displacement - 1964 Alaska
Effects of the EarthquakeEffects of the Earthquake
Ground Rupture, 1906 Olema, CAGround Rupture, 1906 Olema, CA
Effects of the EarthquakeEffects of the Earthquake
Fence Compression - Gallatin County, MTFence Compression - Gallatin County, MT
Effects of the EarthquakeEffects of the Earthquake
Buckled Concrete - 1971 San Fernando, CABuckled Concrete - 1971 San Fernando, CA
Effects of the EarthquakeEffects of the Earthquake
Tsunami Devastation - 1964 Alaska EarthquakeTsunami Devastation - 1964 Alaska Earthquake
Effects of the EarthquakeEffects of the Earthquake
Tsumani GenerationTsumani Generation
Effects of the EarthquakeEffects of the Earthquake
Tsunami Wave Propagation TimesTsunami Wave Propagation Times
Effects of the EarthquakeEffects of the Earthquake
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)
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
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
from: http://www.pmel.noaa.gov/vents/coax/coax.html
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.
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
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
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
SSeismicity of subduction zoneseismicity of subduction zones
all from: http://www.pmel.noaa.gov/vents/coax/coax.html
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
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
from: http://www.pmel.noaa.gov/vents/coax/coax.html
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
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
Earthquake and Plate TectonicsEarthquake and Plate Tectonics
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