Seismic%20Sources

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Seismic Sources
CEE 431/ESS465
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Seismic Sources
Identification Geologic evidence Field
reconnaissance Trench logging Test pits,
borings Airphoto interpretation Remote
sensing Geophysics Historical seismicity
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Seismic Sources
Fault types
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Seismic Sources
Source zones May consist of mapped fault (known
fault geometry) Areas of high seismicity Areas of
shallow or outcropping bedrock Areas of sparse
surficial vegetation May consist of diffuse zone
(unknown fault geometry) Areas of low
seismicity Areas with significant sediment
cover Areas with dense vegetation cover
California
Washington
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Seismic Sources
Fault activity Active fault? NRC definitions
require Movement at or near the ground surface
at least once within the past 35,000 yrs or
movement of a recurring nature within the past
500,000 yrs Macroseismicity instrumentally
determined with records of sufficient precision
to demonstrate a direct relationship with the
fault or A structural relationship to a capable
fault according to the previous two
characteristics, such that movement on one could
reasonably be expected to be accompanied by
movement on the other
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Seismic Sources
Magnitude indicators M f(E) E should increase
with increasing dimensions of rupture surface
Wells and Coppersmith (1994) Strike slip Mw
5.16 1.12 log L Reverse Mw 5.00 1.22 log
L Normal Mw 4.86 1.32 log L All Mw 5.08
1.16 log L
Wells and Coppersmith (1994) Strike slip Mw
3.98 1.02 log A Reverse Mw 4.33 0.90 log
A Normal Mw 3.93 1.02 log A All Mw 4.07
0.98 log A
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Seismic Sources
Segmentation Earthquake segments - those parts
of a fault that have ruptured during individual
earthquakes
Wasatch fault zone - Utah 8 segments Provo
segment (70 km) longest
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Seismic Sources
Segmentation
Segment length (or area) can constrain
magnitude Segments bounded by discontinuities Geom
etric discontinuities - abrupt changes in strike,
stepovers, gaps Structural discontinuities -
fault bifurcations, zones of increased structural
complexity, intersections with other
structures Behavioral discontinuities - changes
in slip rates, senses of displacement, creeping
vs. locked behavior
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Seismic Sources
Puget Sound area
Shallow events
Deep events
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Seismic Sources
Puget Sound area
Known crustal faults
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Seismic Sources
Puget Sound area
Seattle fault
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Seismic Sources
Puget Sound area
Intraplate source
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Seismic Sources
Puget Sound area
Interplate source