Abstract

Moment magnitude M with objective confidence-level uncertainties are estimated for felt San Francisco Bay region earthquakes using Bakun and Went-worth's (1997) analysis strategy for seismic intensity observations. The frequency-magnitude distribution is well described for M ≧ 5.5 events since 1850 by a Gutenberg-Richter relation with a b-value of 0.90. The seismic moment rate ΣM0/yr since 1836 is 2.68 × 1018 N-m/yr (95% confidence range = 1.29 × 1018 N-m/yr to 4.07 × 1018 N-m/yr); the seismic moment rate since 1850 is nearly the same. ΣM0/yr in the 56 years before 1906 is about 10 times that in the 70 years after 1906. In contrast, ΣM0/yr since 1977 is about equal that in the 56 years before 1906. 80% (1σ = 14%) of the plate-motion moment accumulation rate is available for release in earthquakes. The historical ΣM0/yr and the portion of the plate-motion moment accumulation rate available for release in earthquakes are used in a seismic cycle model to estimate the rate of seismic activity in the twenty-first century. High and low rates of future seismic activity are both permissible given the range of possible seismic-cycle recurrence times T and the uncertainties in the historical ΣM0 and in the percentage of plate motion available for release in earthquakes. If the historical seismic moment rate is not greater than the estimated 2.68 × 1018 N-m/yr and the percentage of the plate-motion moment accumulation available for release in earthquakes is not less than the estimated 80%, then for all T, the rate of seismic moment release from now until the next 1906-sized shock will be comparable to the rate from 1836 to 1905 when M 6 1/2 shocks occurred every 15 to 20 years.

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