earthquakes 2

8/6/2019 Earthquakes 2

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Chapter 16

Earthquakes

Photo credit: USGS


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Effects ofEarthquakes

Tsunamis

World Distribution ofEarthquakes

First-Motion Studies ofEarthquakes


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Soil Liquefaction - 1964 Nilgata, Japan


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Surface Displacement - 1964 Alaska


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Ground Rupture, 1906 Olema, CA


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Fence Compression - Gallatin County, MT


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Buckled Concrete - 1971 San Fernando, CA


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Tsumani Generation


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Tsunami Devastation - 1964 AlaskaEarthquake


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Tsunami Animation


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Tsunami Wave Propagation Times


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Earthquakes and Plate Tectonics

Earthquakes at Plate Boundaries

Subduction Angle

Earthquake Prediction


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different types ofstructures are associated with each boundary type:

divergent/rifting: extensional (normal faulting)

convergent/collisional: compressional (thrust faulting)

transform/transcurrent: shear-dominated (strike-slipfaulting)


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Earthquake Distribution


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Relative Plate Motions and Boundaries


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Frequency-Intensity-Magnitude Relations


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Measuring Earthquakes

Three distinct methods to measure earthquakes. Twobased on energy and one based on intensity.

Richter Magnitude Scale: originally developed forsouthern California. Log scale, which has no upper

bound. Small earthquakes may yield negative values.Tends to be inaccurate at >7 magnitudes.

Moment Magnitude Scale: measurement ofthe amountofworkdone during the earthquake. Based on rockstrength,area ofrupture,and displacement during event.

Modified Mercalli Intensity Scale:based on thedamage associated with aparticular event at aparticularlocation. Ranges from I (less damage) to XII (moredamage).


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Earthquake First Motion Records


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Focal Mechanisms


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most subduction zones are in the Ring ofFire (so-called

because ofvolcanism) ofthe Pacificfrom: http://www.geo.lsa.umich.edu/~crlb/COURSES/270


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Convergent Boundary Earthquakes


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seismicity along subduction zones:

earthquakes are shallow, intermediate,and deep

but have systematic location

related to subducting slab

note: shallow adjacent to trench and deepfarthest awayfrom: http://www.geo.lsa.umich.edu/~crlb/COURSES/270


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location ofdowngoing slab as it dives into mantle

is defined by seismicity

earthquakes occuralong an inclined belt:

the Wadati-Benioffzonereaches maximum depth of~670 km

epicenters: location ofearthquake ruptureprojected to surface;

dip ofslab leads to observed seismicity patterns:

deeperfartherfrom trench

deep intermediate shallow

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


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seismicity ofsubduction zones

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


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Divergent Boundary Earthquakes


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earthquakes along

mid-ocean ridge are

shallower than those

along subduction zone

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


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TransformBoundary Earthquakes


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the western US is

somewhat anomalous

note absence of

deep earthquakes



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San Andreas Fault SystemSan Andreas Fault System

is composed ofseveral Distinctsegments, which may rupture

independently.

N. California segment ruptured

in 1906 San Francisco (>8 M) and

1989 Loma Prieta (>7 M) events.

S. California segment ruptured in

1857 Fort Tejon (>7 M) and 1992Landers/Big Bear (>7 M) events.

Central segment near Parkfield is

creeping and generates frequent,

Small events.


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Shallow vs. Deep Subduction Earthquakes

Continent-continent collision

zones have broad areas ofofrelatively shallow seismicity

Ocean-continent convergentmargins have earthquakes foci

that extend to great depths.

Mechanism tend to change from

extension to compression downdip.


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670-kmBoundary and Slab Events


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Wadati-Benioffzones:

different dips in

different locations

where slab bends:

earthquakes related to

normal faults

moderate depths:earthquakes related to

thrusting along

plate interface

these most destructive

at greater depth:

earthquakes related to

normal faulting again;

slab may be stretched

depth to which slab extends is controversial from: van der Pluijmand Marshak, 1997


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slab is cold and thus can have

earthquakes at greater depths

tomography (3D seismic):

note continuity ofblue slab

to depths on order of670 km

blue is fast

interpreted as slab



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Phase Changes and Slab Events


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


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Variable Slab Angle

Factors affecting slab

dipangle include:

Slab age

-younger -> hotter

and more buoyant

Coupling with upper

plate

Kinematics ofplate

boundaries


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Diachronous Convergence & Earthquake Events


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Historical Earthquake Magnitudes


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Eastern and Central US Seismicity


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US National Seismic Hazard Map

Source: USGS NEHRP


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New Madrid Seismic Zone

Source: Mattioli & Jansma, NEHRPproposal


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New Madrid Slip Rate vs. Recurrence Time

Source: Mattioli & Jansma NEHRP proposal; modified from Newman et al 1999

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