The Evolution of Regional Seismicity

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The Evolution of Regional Seismicity Between
Large Earthquakes David D. Bowman California
State University, Fullerton Geoffrey C. P.
King Institut de Physique du Globe de
Paris Charles G. Sammis University of Southern
California
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All California Earthquakes M6.51950-1995
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Stress Change From Loading a Locked Patch on a
Simple Fault
Calculate from motion on all adjacent faults plus
creep at depth
Future Earthquake
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Where are pre-earthquake stresses?
Future Earthquake
Creep at Depth

Negative Slip
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Accelerating Seismicity in Stress Accumulation
Regions
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Static stress (Coulomb)changes during the
earthquake cycle
Earthquake
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Stress Field From Previous History of EQs
Current Stress Field
Stress Field From Loading


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Characteristics of the Background Stress
Field(for a simple model)
Background stress field
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Stress Change vs. Stress Through the Earthquake
Cycle
The start of the earthquake cycle
The end of the Earthquake cycle
Earthquake
Stress Change
Tectonic Memory Stress
Earthquake
Stress Level Relative to the Failure Stress
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Rough Stress field - Eqs move in
An earthquake occurs when stress rises above the
failure level
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Creating a Synthetic Catalog
Magnitude of the event depends on the size of
the stress concentration
This allows the calculation of the Frequency-Magni
tude relation
Failure Stress
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Stress and Seismicity Through the Seismic Cycle
Immediately after the earthquake
25 of the cycle
50 of the cycle
75 of the cycle
Immediately before the earthquake
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Seismicity in the Earthquake Cycle
The end of the Earthquake cycle
Start of the Earthquake Cycle
Approaching the Earthquake
Immediately before the Earthquake
Immediately after the Earthquake
75 of the Earthquake
Aftershocks
Stress shadows
Earthquake
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Implications of Regional Stress Accumulation Model
Off-fault aftershocks occur in regions of
elevated static stress change due to the
earthquake
Main fault is seismically quiet for most of the
seismic cycle
Mogi doughnuts
Accelerating Moment Release over a broad
spatial region before large EQs
Region size scales with size of the predicted
earthquake
Evolution of the frequency-magnitude statistics
(Gutenberg-Richter relation)
Stationary (time-independent) b-value
a-value increase before a large event
and decreases after the event
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California Seismicity 1912-2001 Mgt3.5
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Build-up to the Earthquake
Earthquake
Immediately before the earthquake
Immediately after the earthquake
Late in the Earthquake cycle
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Which cumulative moment release curve is fora
REAL seismicity sequence?
Cumulative Benioff Strain
Time
Time
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AMR Model vs. Observed
Accelerating Seismicity
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Build-up to the Earthquake
Earthquake
Immediately before the earthquake
Immediately after the earthquake
Late in the Earthquake cycle
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Evolution of Gutenberg-Richter Scaling Beforethe
1987 Superstition Hills Earthquake
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Seismicity in the Pacific Northwest
Two large nearby events show accelerating moment
release The regions overlap, approximating the
evolution of the seismicity over 2.5 cycles
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Pacific Northwest Seismicity Statistics
Pacific Northwest Seismicity
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Seismicity in the model Pacific Northwest
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Benioff Strain in the model PNW
Cumulative Benioff Strain
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Evolution of the frequency-magnitude statistics
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Frequency-magnitude stats in model PNW
Evolution of the frequency-magnitude statistics
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Looking Forward
Better calculation of the noise functions -
incorporate stress transfer? More complex
fault geometries - simulate real fault
networks Additional Testing on real data -
Test on earthquakes from other regions (Greece,
Turkey, China, etc) - False alarm rate?
Relationship to Time-Dependent Hazard Analysis
Pre/re-prints available at http//geology.fullert
on.edu/faculty/dbowman