Chapter 16 Earthquakes

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Chapter 16Earthquakes
Photo credit USGS
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Effects of Earthquakes

• Tsunamis
• World Distribution of Earthquakes
• First-Motion Studies of Earthquakes

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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 Alaska Earthquake
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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 of structures are associated with
each boundary type divergent/rifting
extensional (normal faulting) convergent/collisio
nal compressional (thrust faulting) transform/t
ranscurrent shear-dominated (strike-slip
faulting)
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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.
  Two based on energy and one based on intensity.
• Richter Magnitude Scale originally developed for
  southern California. Log scale, which has no
  upper bound. Small earthquakes may yield
  negative values. Tends to be inaccurate at gt7
  magnitudes.
• Moment Magnitude Scale measurement of the amount
  of work done during the earthquake. Based on rock
  strength, area of rupture, and displacement
  during event.
• Modified Mercalli Intensity Scale based on the
  damage associated with a particular event at a
  particular location. Ranges from I (less damage)
  to XII (more damage).

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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 of Fire
(so-called because of volcanism) of the
Pacific
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
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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 deep
farthest away
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
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location of downgoing slab as it dives into
mantle is defined by seismicity
earthquakes occur along an inclined belt the
Wadati-Benioff zone reaches maximum
depth of 670 km
deep
intermediate
shallow
from http//www.geo.lsa.umich.edu/crlb/COURSES/2
70
epicenters location of earthquake rupture
projected to surface dip of slab leads to
observed seismicity patterns deeper farther
from trench
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seismicity of subduction 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.htm
l
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Transform Boundary Earthquakes
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the western US is somewhat anomalous
note absence of deep earthquakes
from http//www.pmel.noaa.gov/vents/coax/coax.htm
l
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San Andreas Fault System
San Andreas Fault System is composed of several
Distinct segments, which may rupture
independently. N. California segment
ruptured in 1906 San Francisco (gt8 M) and 1989
Loma Prieta (gt7 M) events. S. California segment
ruptured in 1857 Fort Tejon (gt7 M) and 1992
Landers/Big Bear (gt7 M) events. Central segment
near Parkfield is creeping and generates
frequent, Small events.
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Shallow vs. Deep Subduction Earthquakes
Ocean-continent convergent margins have
earthquakes foci that extend to great
depths. Mechanism tend to change from extension
to compression downdip.
Continent-continent collision zones have broad
areas of of relatively shallow seismicity
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670-km Boundary and Slab Events
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Wadati-Benioff zones 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 Pluijm and Marshak, 1997
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tomography (3D seismic)
blue is fast interpreted as slab
note continuity of blue slab to depths on
order of 670 km
slab is cold and thus can have earthquakes at
greater depths
from http//www.pmel.noaa.gov/vents/coax/coax.htm
l
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Phase Changes and Slab Events
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all from http//www.geo.lsa.umich.edu/crlb/COURS
ES/270
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Variable Slab Angle

• Factors affecting slab
• dip angle include
• Slab age
• younger -gt hotter
• and more buoyant
• Coupling with upper
• plate
• Kinematics of plate
• 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, NEHRP proposal
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New Madrid Slip Rate vs. Recurrence Time
Source Mattioli Jansma, NEHRP proposal
modified from Newman et al. 1999