the evolution of regional seismicity between large earthquakes david d. bowman california state...
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The Evolution of Regional SeismicityBetween Large Earthquakes
David D. BowmanCalifornia State University, Fullerton
Geoffrey C. P. KingInstitut de Physique du Globe de Paris
Charles G. SammisUniversity of Southern California
All California Earthquakes M≥6.51950-1995
Slipping Fault Slipping FaultSeismic Slip Seismic Slip
Creep at Depth
FutureEarthquake
Calculate from motion on all adjacent faultsplus creep at depth
Stress Change From Loading a Locked Patch on a Simple Fault
FutureEarthquake
Creep at Depth
=
Negative Slip
Where are pre-earthquake stresses?
Accelerating Seismicity in Stress Accumulation Regions
slip 0 unitsslip 0 units slip 0 units
slip 0 units
slip 0.25 units slip 0 units
slip 0.25 units
slip 0.25 units
slip 0.5 unitsslip 0.5 units
slip 0.5 units
slip 0 unit
slip 0.5 units
slip 0.5 units
slip 0.5 unitsslip 1 unit
slip 1 unitslip 0.75 unit
slip 0.75 units
slip 0.75 units
slip 1 unit slip 1 unit slip 1 unit
slip 1 unit
EarthquakeStatic stress (Coulomb)
changes during the earthquake cycle
Stress FieldFrom Previous History of EQs
Stress FieldFrom Loading
CurrentStressField
= +
• Stress distant from the fault must approach,but not exceed, the failure stress
• Stress must be low along the strike of the fault(or rupture would continue)
• Coulomb field + background field can notexceed the failure stress (except locally)
• Stress distant from the fault must approach,but not exceed, the failure stress
• Stress must be low along the strike of the fault(or rupture would continue)
Coulomb field immediatelybefore the event
Coulomb field immediatelyafter the event
- Positive Coulomb stressesmust have a correspondingnegative value in thebackground field
• Coulomb field + background field can notexceed the failure stress (except locally)
• Stress distant from the fault must approach,but not exceed, the failure stress
• Stress must be low along the strike of the fault(or rupture would continue)
Background stress field• Stress must be low along the strike of the fault(or rupture would continue)
Characteristics of the Background Stress Field(for a simple model)
The start of theearthquake cycle
The end of theEarthquake cycle
Stress Change
Stress Level Relative to the Failure Stress
Tectonic Memory Stress
Eart
hquake
Eart
hquake
25
0
-25
bars
0
-50
bars
failurestress
Stress Change vs. Stress Through the Earthquake Cycle
An earthquake occurs when stress rises above the failure level
A BFault
Str
ess
Failure Stress
A BFault
Str
ess
Magnitude of the event depends on the size of the stress concentration
This allows the calculation of theFrequency-Magnitude relation
FailureStress
Creating a Synthetic Catalog
Immediately after the earthquake
25% of the cycle
50% of the cycle
75% of the cycle
Immediately before the earthquake
failure stress
-50 0bars
Stress and Seismicity Through the Seismic Cycle
fault
Aftershocks Stress shadows
Start of theEarthquake
Cycle
Approachingthe
Earthquake
Immediatelybefore theEarthquake
Immediatelyafter the
Earthquake
75% of theEarthquake
The end ofthe
Earthquake cycle
Earthquake
Seismicity in the Earthquake Cycle
• Accelerating Moment Release over a broad spatial region before large EQs
• a-value increase before a large event anddecreases after the event
• Evolution of the frequency-magnitude statistics (Gutenberg-Richter relation)
• Stationary (time-independent) b-value
• 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”
Implications of Regional Stress Accumulation Model
• Region size scales with size of the “predicted” earthquake
California Seismicity 1912-2001 M>3.5
Earthquake
Late in theEarthquake cycle
Immediatelybefore theearthquake
Immediatelyafter the
earthquake
Build-up to the Earthquake
San Fernando EarthquakeModel
Cum
ula
tive
Be
niof
f S
trai
n
Time Time
Which cumulative moment release curve is fora REAL seismicity sequence?
Accelerating Seismicity
Earthquake
Late in theEarthquake cycle
Immediatelybefore theearthquake
Immediatelyafter the
earthquake
Build-up to the Earthquake
Evolution of Gutenberg-Richter Scaling Beforethe 1987 Superstition Hills Earthquake
126˚W
126˚W
124˚W
124˚W
122˚W
122˚W
120˚W
120˚W
46˚N 46˚N
48˚N 48˚N
50˚N 50˚N
Two large nearby events show accelerating moment release
The regions overlap, approximating the evolution of the seismicity over 2.5 cycles
Seismicity in the Pacific Northwest
Pacific Northwest Seismicity
Seismicity in the model & Pacific Northwest
Cumulative Benioff Strain
Evolution of the frequency-magnitude statistics
Evolution of the frequency-magnitude statistics
• 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
Looking Forward:
Pre/re-prints available at:http://geology.fullerton.edu/faculty/dbowman