Earthquake Probabilities for the San Francisco Bay Region 2002-2031

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Earthquake Probabilities for the San Francisco Bay Region 2002-2031. Working Group 2002: Chapter 6 Ved Lekic EQW, April 6, 2007. Background. Probabilities are weighted averages of Poisson, Brownian Passage Time, Time-predictable and Empirical probability models

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### Earthquake Probabilities for the San Francisco Bay Region 2002-2031

Working Group 2002: Chapter 6

Ved Lekic

EQW, April 6, 2007

Background
• 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
• 30 year probabilities of large earthquakes
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
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.
San 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
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
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?
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
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
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
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
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
Earlier 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
Improvements 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
• 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 Model
• SAF and SG depend strongly on choice rupture model, while HRC and C do not
M-logA Relations
• Determining M from A is a significant but not dominant source of uncertainty
Aseismic Slip
• Used seismogenic scaling factor R:
• Used to scale the area
• Used to scale the slip rate
Aperiodicity
• Used in BPT
• Early in cycle, greater aperiodicity increases probabilities; late in cycle it decreases them