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Timing of Late Holocene Paleoearthquakes on the Northern San Andreas Fault at the Fort Ross Orchard Site, Sonoma County, California

¹ William Lettis & Associates, Inc.
1777 Botelho Dr., Suite 262
Walnut Creek, California 94596
kelson{at}lettis.com
 (K.I.K.)
² 4 Floor, No. 3, Avenue 334
Jong-Jung Road, Xin-Dian City
Taipei County, Taiwan (R.O.C.) 231
ashleystreig{at}hotmail.com
 (A.R.S.)
³ Center for Neotectonic Studies
University of Nevada
Reno, Nevada 89557
koehler{at}seismo.unr.edu
 (R.D.K.)
⁴ Sun Pro Engineering Consultants Co., Inc.
4F-1, No.368, Changchun Rd., Jhongshan District
Taipei City 104, Taiwan (R.O.C.)
kangkanghk{at}hotmail.com
 (K.-H.K.)

Paleoseismic trenching within Fort Ross State Historic Park provides data on the late Holocene rupture history of the North Coast segment of the northern San Andreas fault. The 1906 earthquake ruptured through the Fort Ross Orchard site, which is characterized by a narrow shutter ridge and associated linear trough containing latest Holocene sediments. Trenches across the northeast-facing fault scarp exposed sediments interpreted as scarp-derived colluvium and possible fissure-fill deposits, and tentative upward fault truncations that provide evidence of three possible surface ruptures prior to 1906. Coarse-grained scarp-derived colluvial sediments were deposited after individual surface-rupturing earthquakes that predate the 1906 rupture. Radiocarbon analyses of 31 detrital radiocarbon samples collected from the colluvial deposits constrain the timing of earthquakes over the past approximately 1000 years. Based on stratigraphic ordering and a statistical comparison of radiocarbon dates using the OxCal program, we estimate (at a 95% confidence level) that three pre-1906 surface ruptures at the Orchard site occurred at A.D. 1660–1812, A.D. 1220–1380, and A.D. 1040–1190. Previous trenches at the nearby Fort Ross Archae Camp site are consistent with these dates and further suggest the occurrence of an earlier event between A.D. 555 and 950. Collectively, the Fort Ross Orchard and Archae Camp sites suggest pre-1906 ruptures at A.D. 1660–1812, A.D. 1220–1380, A.D. 1040–1190, and A.D. 555–950. The time windows for these ruptures are consistent with results from other sites on the North Coast segment of the San Andreas fault. However, additional information on the late Holocene history of rupture events on adjacent fault segments is needed to evaluate whether the long-term behavior of the San Andreas fault involves a mix of large, 1906-type ruptures and shorter, segment-specific ruptures.

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