Abstract

Our research describes historical observations of the 1906 earthquake rupture trace in Marin County, documents the style of subsurface deformation, and summarizes our interpretation of the number and timing of pre-1906 ground-rupturing earthquakes exposed in 18 trenches excavated across the northern San Andreas fault (NSAF) at the Dogtown site. Dogtown is located immediately southeast of what was known as the Strain Ranch in 1906. The trenches reveal two stratigraphic sections of late Holocene age separated by a well-constrained depositional hiatus. The upper section, deposited in the past 200–250 yr, is separated from the lower section, which was deposited between A.D. 250 and 650, by a hiatus during the interval A.D. 650–1700. We see evidence for two events in the upper section—Event I (1906) and Event II, a likely penultimate earthquake, plus a minimum of two events in the lower section (Event[s] A and B). In the lower stratigraphic section, the oldest exposed potential surface-faulting evidence (Event B) is constrained to the interval between A.D. 330 and 580. While there is sound evidence for ground-rupturing earthquakes in the 1100 yr long interval between Event B and Event II, pedogenic processes have obscured their number and timing. Faulting evidence at the Dogtown site suggests the penultimate earthquake occurred between A.D. 1695 and 1776. Data from the Dogtown site, although equivocal, suggest that the time interval (130–210 yr) between the 1906 earthquake and the penultimate earthquake on the San Andreas Fault north of San Francisco may be shorter than previously documented.

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