Paleoseismic studies of the Northern San Andreas Fault at Vedanta Marsh Site, Olema, California

The northern San Andreas fault (SAF) ruptured in 1906 and generated the Great San Francisco Earthquake. This study involves collection and interpretation of paleoseismic data from the North Coast segment of the northern SAF at Vedanta marsh, Olema, California, to determine the timing of prehistoric large earthquakes, coseismic slips, and earthquake recurrence on this fault segment. These important parameters will test the existing earthquake models and add new data to the San Francisco Bay Area earthquake probability analysis.; Late Holocene sediments deposited at Vedanta marsh preserve a continuous record of prehistoric earthquakes. Excavations into the marsh provided exposures of the sediments across the SAF zone. Well-defined marsh stratigraphy and abundant in situ organic material allow the determination of the first long, high-resolution, event-by-event record of earthquakes for the northern SAF. Evidence for twelve earthquakes, including the 1906 earthquake, have been identified from the main fault zone based on fault outward splays, fault upward termination, fissures, colluvial wedges, and soft-sediment deformation. All of these features occurred since the deposition of a unit that is approximately 3000 years old.; The age of eleven pre-1906 seismic events are well bracketed by radiocarbon dates and age modeling using the OxCal radiocarbon analysis program. The average recurrence interval at the Vedanta site is ∼250 years. However, individual recurrence intervals are quite irregular, ranging from as short as 53 years to as long as 605 years. From comparison with other sites on the northern S AF, I interpreted four pre-1906 events may have ruptured the entire North Coast segment, including earthquakes in the following age ranges: AD 1670--1740; AD 1290--1380; AD 1100--1165; and AD 650--710.; A buried paleochannel has been right-laterally offset 7.8--8.3 m by coseismic slip of the 1906 and the penultimate earthquakes. Historical record of the 1906 coseismic slip measured near the excavation site was ∼5 m. If we assume 5 m of 1906 slip at the marsh site, then the coseismic slip of the penultimate event is between 2.8--3.3 m. Timing (AD 1670--1740) and coseismic slip of the penultimate event indicate that the northern SAF may rupture in sequences of closely timed earthquakes on shorter segments, and does not support the assumption that the fault has failed as a single, long rupture similar to 1906 in mid 1600s.