1. Earthquake Probabilities for the San Francisco Bay Region 2002-2031 Working Group 2002: Chapter 6 Ved Lekic EQW, April 6, 2007 Working Group 2002: Chapter 6 Ved Lekic EQW, April 6, 2007

2. 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 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

3. Regional Earthquake Probabilities 30 year probabilities of large earthquakes

4. 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

5. 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.

6. 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 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

7. San Andreas Fault

8. 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

9. Hayward-Rodgers Creek

10. 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?

11. Calaveras Fault

12. 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

13. Concord-Green Valley

14. 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

15. San Gregorio

16. 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

17. Greenville

18. 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

19. 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

20. 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 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

21. 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 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

22. Comparison of Probabilities

23. 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

24. Alternative Predictions

26. Choice of Rupture Model SAF and SG depend strongly on choice rupture model, while HRC and C do not

27. M-logA Relations Determining M from A is a significant but not dominant source of uncertainty

28. Aseismic 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

29. Aperiodicity Used in BPT Early in cycle, greater aperiodicity increases probabilities; late in cycle it decreases them Used in BPT Early in cycle, greater aperiodicity increases probabilities; late in cycle it decreases them