Seismic activity of the San Francisco Bay region

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.