PRELIMINARY EVALUATION OF PALEOEARTHQUAKE TIMING, NORTHERN SAN ANDREAS FAULT, FORT ROSS, CA

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, linear shutter ridge and associated linear trough within which latest Holocene sediments have accumulated. Five trenches across the northeast-facing fault scarp show that the site preserves evidence of the 1906 and three pre-1906 ruptures. Trenches excavated in 2000 show that scarp-derived colluvial deposits shed into the linear depression from the bedrock-cored shutter ridge are as old as about AD 230. The scarp-derived colluvium and tentative upward fault truncations provide evidence of three possible surface ruptures prior to 1906. Radiometric analyses of charcoal fragments, and historical archives from Russian occupation of the fort, suggest that the penultimate surface rupture occurred after about AD 1640 but before Russian arrival in AD 1812. Trench relations and radiocarbon dates also suggest that earlier surface ruptures may have occurred between about AD 780 and 1640, and between about AD 230 and 1290. Additional trenches excavated in 2004 exposed a series of four scarp-derived colluvial deposits that also record episodic uplift and scarp degradation. Three of these colluvial deposits pre-date the 1906 rupture, and should provide better constraint on the timing of past earthquakes. Interpretation of an additional eleven radiometric analyses of charcoal fragments from faulted sediments is underway. Initial interpretation of data from the 2004 trenches suggest that the penultimate rupture occurred prior to about AD 1780, the third-most-recent rupture occurred between about AD 980 and 1380, and the fourth-most-recent rupture occurred prior to AD 1160. Collectively, data from the two trenching efforts suggests ruptures occurred in AD 1906, between about AD 1640 and AD 1780, between about AD 980 and 1380, and prior to AD 1160. The tentative time windows for these ruptures are consistent with results from nearby studies on the North Coast segment of the fault, and could be refined through additional radiometric analyses and statistical treatment.