Earthquakes Susan Bilek Associate Professor of Geophysics New
Mexico Tech How to figure out the who, what, where, why (or the
location, size, type)
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Topics for Today Earthquake location Earthquake magnitude Focal
mechanisms and moment tensors Relationship to tectonics
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Earthquake Location One of the first pieces of information
determined for an earthquake Location estimated by measurements of
P, S, other phase arrival times Time for arrival to reach station
will depend on distance away from station, and the velocity of the
wave With known(-ish) velocity structure, use time measurements to
estimate distance from station Combine with many observations (many
stations) to solve inverse problem for location
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Simple Location Example IRIS
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More details Equation solved: t s = time at station s t e =
origin time at earthquake e v = velocity of seismic waves through
given medium x e, y e, z e = x, y, z position of earthquake x s, y
s, z s = x, y, z position of recording station Set up system of
equations, use (many) arrival times and known station locations,
specified velocity, solve for earthquake location
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Sources of Error Figs from Shearer Velocity uncertainties:
Earth is made of rocks!! so earth not constant velocity (or even
layer cake velocities) Picking errors: Some data can be hard to
pick phase arrivals consistently
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Earthquake Magnitude
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Estimating m b in the 1st 5 s of P wave Estimating P-wave
magnitude
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Seismic Moment and Moment Magnitude Better estimates of
earthquake size seismic moment M o Relates size to fault and
earthquake parameters M o = AD (A: area of fault that slipped
(=LxW), D: amount of slip, : rigidity of fault) Get numbers like
1x10 18 Nm or 1x10 25 dyn-cm Convert to moment magnitude, get
similar form as m b, M s
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2011 Japan earthquake contributed large amount of moment to the
overall global totals Japan 2011 Moment release 1906- 2011
Magnitude 8+ events contribute the most in moment, with the 5 M 9
events being the largest components
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Fault Geometry and Focal Mechanisms: Using seismology to
describe faults Shearer
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Basic Types of Faulting Stein and Wysession
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Fault Geometry
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Direction of motion on fault -- produces compressional or
dilatational first arrivals at stations
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LW 8.23 Lay and Wallace
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Stereonet used for plotting earthquake first motions to
determine focal mechanism Plot azimuth, takeoff angle, polarity at
station Find planes that separate up (compressional) and down
(dilatational) motions
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Example focal mechanism and data Note small amplitude 1st
motions near nodal planes
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Build double couple forces - motion on fault plane or auxillary
plane
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Earthquake locations and focal mechanisms around the world why
this sort of pattern? Relates to Plate Tectonics!
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Hayward Fault, CA Large strike-slip fault east of the San
Andreas in San Francisco Bay area
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30-100 km ~350 km ~700 km
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Thrust mechanism earthquakes along Sumatra subduction zone
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http://gcc.asu.edu/patty/BQ2/c.php BirthQuake! What is your
birthquake (what kind of fault? What sort of tectonics?)