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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS004 Oral Presentation 1897
The Earthquake Cycle in the San Francisco Bay Area: 1600-2007
Dr. David Schwartz
IASPEI
Wiliam Lettis, James J. Lienkaemper, Suzanne Hecker, Keith Kelson, Thomas
Fumal, John Baldwin, Gordon Seitz, Tina Niemi
The San Francisco Bay Area lies within the Pacific-North American plate boundary. The historical record
of earthquakes in the Bay Area is considered complete at magnitude 5.5 back to 1850. The striking
contrast between the large number (29) of moderate magnitude earthquakes (M 5.6-6.4) from1850 to
1906 and their general absence between the M7.9 1906 San Francisco earthquake and the present day
is a fundamental observation leading to the concepts of the earthquake cycle, the acceleration of
regional seismicity prior to a great earthquake, and stress shadows. There is no accurate quantification
of either the number or magnitude of moderate events prior to 1850 for comparison. Paleoseismic
studies cannot easily identify moderate-size earthquakes, but they do extend the record of larger
surface-faulting events. Recent investigations of the regions major plate strike-slip faults now provide a
longer view of the Bay Area earthquake cycle. Preliminary paleoearthquake chronologies for the regions
major faults have been developed ; these have durations of 1000 to 3000 years. Although the
completeness of individual fault chronologies is variable, the paleoearthquake history across the entire
Bay Area fault system since 1600 is essentially complete. Evidence of surface faulting that is post-1600
and pre-1776 (founding of Mission Dolores) is found on the northern San Andreas (SAN), Santa Cruz
Mountains San Andreas (SAS), northern Hayward (NH), southern Hayward (SH), Rodgers Creek (RC),
northern Calaveras (NC), San Gregorio (SG), and Green Valley (GVY) faults. The timing of these, listed
in order of their mean radiocarbon age (with age-range uncertainties in parentheses), is: SH 1620
(1605-1645): SAS 1640 (1600-1670): GVY 1700 (1685-1776); NH 1705 (1670-1776); SG 1720 (1695-
1776); SH 1730 (1685-1776); RC 1740 (1690-1776); SAN 1750 (1720- 1776); and NC 1760 (1670-
1830). (Given dating uncertainties, the actual ordering of earthquakes may have been different). Offset
data, which reflect magnitude, are limited. However, measured point-specific slip (RC, 1.8-2.3m; SG,
3.5-5.0m; SAN 3.0m) and modeled average slip (SH, 1.9m) indicate large magnitude earthquakes on
these regional faults. Major observations of the Bay Area earthquake cycle are: 1) between 1600 and
1838 the San Andreas fault failed in a series of large earthquakes rather than as a single, multi-segment
1906-type rupture; 2) a regional cluster of large earthquakes occurred between 1670-1776 (and the
actual interval was likely shorter); 3) the estimated moment release of the cluster (irrespective of the
sequence of events) is comparable to the moment release of 1906; and 4) the cluster was followed by a
regional quiescence of large earthquakes (paleo stress shadow), with only two (1838, 1868) until
1906. The Bay Area paleoseismic record has the potential to extend these types of observations through
multiple earthquake cycles.
Keywords: earthquake cycle, clustering, paleoseismology