FUTURE LARGE EARTHQUAKES IN THE SAN FRANCISCO BAY REGION: LOOKING BACK TO LOOK AHEAD

Stress changes produced by the 1906 San Francisco earthquake had a profound effect on Bay Area seismicity, dramatically reducing it in the 20th century. Whether the San Francisco Bay Region (SFBR) is still within, is just emerging from it, or is out of the 1906 stress shadow is an issue with important implications for earthquake mechanics and seismic hazards. Historically the SFBR has not experienced one complete earthquake cycle--the interval immediately following, then leading up to and repeating, a 1906-type (M7.8 multi-segment rupture) San Andreas event. Paleoseismic data now provide a more complete view of the most recent pre-1906 SFBR earthquake cycle.

Paleoseismic efforts under the Bay Area Paleoearthquake Experiment (BAPEX) have developed a chronology of the most recent large earthquakes (MRE) on major SFBR faults. The San Andreas (SA), northern Hayward (NH), southern Hayward (SH), Rodgers Creek (RC), northern Calaveras (NC), San Gregorio (SG), and Green Valley (GVY) provide evidence of large events post-1600 AD and pre-1776 (founding of Mission Dolores). The timing of MREs, in years AD, follows. Age ranges are 2-sigma radiocarbon intervals; dates in parentheses are 1-sigma. MRE ages are: a) SA 1600-1670 (1630-1660), NH 1640-1776 (1635-1776); SH 1635-1776 (1685-1676); RC 1690-1776 (1715-1776); NC 1670-1830; SG 1640-1776 (1685-1776), and GVY 1630-1776. The most highly uncertain is the age of the penultimate multi-segment SA earthquake, with the possibility of several individual-segment events during this period. The NH/SH/RC/NC/SG sequence likely occurred between 1700 and 1776. Offset data, which reflect M, are limited but measured point-specific slip (RC, 1.8-2.3m; SG, 3.5-5m) and modeled average slip (SH, 1.9m) for the MREs indicate large magnitude earthquakes on the regional faults.

Major observations from paleoseismic data are: 1) during a maximum interval of 176 years (1600-1776), significant seismic moment was released in the SFBR by large (M≥6.7) surface-faulting earthquakes on the SA, RC, SH, NH, NC, SG, and GVY faults; 2) a regional cluster of large events occurred between 1700-1776; 3) there was an absence of large events between 1776-1838; and 4) the 18th century cluster contained the large earthquakes that lie ahead for the SFBR and are modeled in the 62% probability of occurrence by 2032.