earthquake probabilities for the san francisco bay region 2002-2031 working group 2002: chapter 6...
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Earthquake Probabilities for the San Francisco Bay
Region 2002-2031
Working Group 2002: Chapter 6Ved LekicEQW, April 6, 2007
BackgroundBackground
• Probabilities are weighted averages of Poisson, Brownian Passage Time, Time-predictable and Empirical probability models
• Mean probability and 95% confidence bounds
• Time period: 2002-2031• Regional and Individual Fault earthquake
probabilities
• Probabilities are weighted averages of Poisson, Brownian Passage Time, Time-predictable and Empirical probability models
• Mean probability and 95% confidence bounds
• Time period: 2002-2031• Regional and Individual Fault earthquake
probabilities
Regional Earthquake Probabilities
Regional Earthquake Probabilities
• 30 year probabilities of large earthquakes
• 30 year probabilities of large earthquakes
Regional Earthquake Probabilities
Regional Earthquake Probabilities
• Smaller quakes can be costly. 1987 M5.9 Whittier Narrows caused $350M in damage
• Historical record places bounds on probabilities of 6.0≤M<6.7 :• 1972-2001 lower bound 0.8 or Nexp = 1.6• 1850-1906 upper bound 0.99 or Nexp = 4.6
• SFBR model extended using Gutenberg-Richter with b = 0.9 predicts:• 0.96 [0.91 - 0.99] or Nexp = 3.3
• Smaller quakes can be costly. 1987 M5.9 Whittier Narrows caused $350M in damage
• Historical record places bounds on probabilities of 6.0≤M<6.7 :• 1972-2001 lower bound 0.8 or Nexp = 1.6• 1850-1906 upper bound 0.99 or Nexp = 4.6
• SFBR model extended using Gutenberg-Richter with b = 0.9 predicts:• 0.96 [0.91 - 0.99] or Nexp = 3.3
Regional Earthquake Probabilities
Regional Earthquake Probabilities
• Exposure times other than 30 years
• Similarities with Poisson Model prediction “reflect the distributed weights assigned to the “competing” models - which in turn stems from uncertainty about the effects of the” 1906 stress shadow.
• Exposure times other than 30 years
• Similarities with Poisson Model prediction “reflect the distributed weights assigned to the “competing” models - which in turn stems from uncertainty about the effects of the” 1906 stress shadow.
San Andreas FaultSan Andreas Fault
• Master fault carrying half of plate motion across the region
• 1906 had largest surface rupture of any continental strike slip earthquake
• High likelihood of floating earthquake (M6.9)
• Nearly uniform probabilities of rupture of each segment
• Loma Prieta stress change cause of SAP > SAS probabilities
• Master fault carrying half of plate motion across the region
• 1906 had largest surface rupture of any continental strike slip earthquake
• High likelihood of floating earthquake (M6.9)
• Nearly uniform probabilities of rupture of each segment
• Loma Prieta stress change cause of SAP > SAS probabilities
Hayward-Rodgers Creek (140 km)
Hayward-Rodgers Creek (140 km)
• Essentially two different and independent faults
• HS and HN experiencs significant aseismic creep
• Most likely to produce M≥6.7 quake• Uncertainties from:
• Depth extent of aseismic creep• Existence and position of HS-HN
segmentation point
• Essentially two different and independent faults
• HS and HN experiencs significant aseismic creep
• Most likely to produce M≥6.7 quake• Uncertainties from:
• Depth extent of aseismic creep• Existence and position of HS-HN
segmentation point
Calaveras Fault (123 km)Calaveras Fault (123 km)
• Southern two segments > 1/3 of plate motion across the SFBR and creep aseismically
• Surface breaking quake on CN between 1160 and 1425 a.c.e.
• Largest historical earthquakes in 1911 and 1984 (both M6.2)
• Segments thought to rarely link up• Uncertainties:
• Can creeping segments produce M≥6.7 since they also have high rates of moderate sized quakes?
• Southern two segments > 1/3 of plate motion across the SFBR and creep aseismically
• Surface breaking quake on CN between 1160 and 1425 a.c.e.
• Largest historical earthquakes in 1911 and 1984 (both M6.2)
• Segments thought to rarely link up• Uncertainties:
• Can creeping segments produce M≥6.7 since they also have high rates of moderate sized quakes?
Concord-Green Valley (56 km)
Concord-Green Valley (56 km)
• No large quakes in historical period• M5.4 on central Concord Fault in
1955• Aseismic slip present but
significance unknown source of uncertainty!
• Only M6.0 to M6.7 are likely
• No large quakes in historical period• M5.4 on central Concord Fault in
1955• Aseismic slip present but
significance unknown source of uncertainty!
• Only M6.0 to M6.7 are likely
San Gregorio (175 km)San Gregorio (175 km)
• Unlikely any activity in historical era (small probability of 1838)
• SGS under water; SGN large slip events• Multiple traces under Montery Bay• Golden Gate segmentation point uncertain• Possibility of linking SGN with SAN
neglected• Uncertain slip rate, past seismicity, effect
of 1906
• Unlikely any activity in historical era (small probability of 1838)
• SGS under water; SGN large slip events• Multiple traces under Montery Bay• Golden Gate segmentation point uncertain• Possibility of linking SGN with SAN
neglected• Uncertain slip rate, past seismicity, effect
of 1906
Greenville Fault (23-63 km)
Greenville Fault (23-63 km)
• Central part had M5.8 and M5.4 quakes in 1980
• Paleoseismic events of unknown magnitude occurred
• Unknown whether norther and southern segments rupture together or separately
• Central part had M5.8 and M5.4 quakes in 1980
• Paleoseismic events of unknown magnitude occurred
• Unknown whether norther and southern segments rupture together or separately
Mt Diablo Thrust (20-30 km)
Mt Diablo Thrust (20-30 km)
• Blind thrust fault resulting from crustal shortening within a fold-and-thrust belt
• Treated as a single earthquake source
• Blind thrust fault resulting from crustal shortening within a fold-and-thrust belt
• Treated as a single earthquake source
Background Earthquakes
Background Earthquakes
• Faults: slip rates < 1 mm/yr; undiscovered; poorly characterized
• Significant seismicity in SFBR occurs on uncharacterized faults (Wesson 2002)
• Third-most-important source region
• Faults: slip rates < 1 mm/yr; undiscovered; poorly characterized
• Significant seismicity in SFBR occurs on uncharacterized faults (Wesson 2002)
• Third-most-important source region
Earlier StudiesEarlier Studies
• WG88 & WG90 used magnitude threshold of M≥7
• WG88: SAF and Hayward; 0.5 30-year probability for each; time-predictable probability model
• WG90: SAF, Hayward and Rodgers Creek; 0.67 30-year probability
• WG88 & WG90 used magnitude threshold of M≥7
• WG88: SAF and Hayward; 0.5 30-year probability for each; time-predictable probability model
• WG90: SAF, Hayward and Rodgers Creek; 0.67 30-year probability
Improvements in WG02Improvements in WG02• Inclusion of overall moment budget (36-43
mm/yr)• Inclusion of aseismic creep and 1989 shadow
lower probabilities on SAF and Hayward• Inclusion of background seismicity, fault
segmentation, multi-segment ruptures, and other faults
• Multiple probability models (especially BPT)• Different treatments of 1906 stress-shadow
• Inclusion of overall moment budget (36-43 mm/yr)
• Inclusion of aseismic creep and 1989 shadow lower probabilities on SAF and Hayward
• Inclusion of background seismicity, fault segmentation, multi-segment ruptures, and other faults
• Multiple probability models (especially BPT)• Different treatments of 1906 stress-shadow
Sensitivity of Results: 1906 shadow
Sensitivity of Results: 1906 shadow
• Uncertainty about 1906 stress change contributes to half of total uncertainty
• 2 approaches to incorporating 1906:• BPT - underestimates stress shadow; upper bound• Empirical - faults in stress shadow; lower bound
• 2 approaches neglecting 1906:• BPT without fault interactions & Poisson model
• Time-predictable model for SAF used information on slip in 1906
• Uncertainty about 1906 stress change contributes to half of total uncertainty
• 2 approaches to incorporating 1906:• BPT - underestimates stress shadow; upper bound• Empirical - faults in stress shadow; lower bound
• 2 approaches neglecting 1906:• BPT without fault interactions & Poisson model
• Time-predictable model for SAF used information on slip in 1906
Choice of Rupture ModelChoice of Rupture Model
• SAF and SG depend strongly on choice rupture model, while HRC and C do not
• SAF and SG depend strongly on choice rupture model, while HRC and C do not
M-logA RelationsM-logA Relations
• Determining M from A is a significant but not dominant source of uncertainty
• Determining M from A is a significant but not dominant source of uncertainty
Aseismic SlipAseismic Slip
• Used seismogenic scaling factor R:• Used to scale the
area• Used to scale the slip
rate
• Used seismogenic scaling factor R:• Used to scale the
area• Used to scale the slip
rate