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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

Earthquake Probabilities for the San Francisco Bay Region 2002-2031

Working Group 2002: Chapter 6

Ved Lekic

EQW, April 6, 2007

background
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
Regional Earthquake Probabilities
  • 30 year probabilities of large earthquakes
regional earthquake probabilities4
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 probabilities5
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
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
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
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
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
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
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
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
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
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
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
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
choice of rupture model
Choice of Rupture Model
  • SAF and SG depend strongly on choice rupture model, while HRC and C do not
m loga relations
M-logA Relations
  • Determining M from A is a significant but not dominant source of uncertainty
aseismic slip
Aseismic Slip
  • Used seismogenic scaling factor R:
    • Used to scale the area
    • Used to scale the slip rate
aperiodicity
Aperiodicity
  • Used in BPT
  • Early in cycle, greater aperiodicity increases probabilities; late in cycle it decreases them