PALEOSEISMOLOGY OF THE NORTHERN SAN ANDREAS FAULT AT VEDANTA MARSH SITE, OLEMA, CALIFORNIA

At Vedanta marsh located in Olema, CA, 60 km north of San Francisco, well-defined stratigraphy and abundant in situ organic material allow the determination of the first long, high-resolution multiple event record for the north coast segment of the San Andrea fault. During the 1906 San Francisco earthquake, the ground ruptured along the northeastern edge of the Vedanta marsh where historical offset measurements near the site were about 5m. Subsidence of the marsh was also observed at the time of the 1906 earthquake as a lane of water ponded against the rupture trace (Lawson, 1908). Eight trenches have been excavated across the fault during the past two field seasons. The main fault zone consists of a 2-m-wide zone of upward-branching fault splays within the marsh stratigraphy. A secondary fault zone located 5m to the east juxtaposes older sediment (>25 ka) against colluvium in apparent normal separation. The marsh stratigraphy consists of major peat layers interbedded with marsh clay and silt that interfinger eastward with colluvial gravels. A clear, vertical transition from predominantly fine-grained marsh deposits to coarse clastic sediment occurs at a depth of approximately 1 m. AMS radiocarbon dating of peat, macrofossils, and charcoal collected throughout the section provide excellent age control for event horizons. At least ten pre-1906 events were identified by evidences of outward-splaying, upward fault terminations and fissure fills. The 1906 earthquake ruptures a portion of the upper gravel and a possible late 19th century road fill. Deformation within each of the laterally correlative, upper three peat layers deposited about 300, 700, and 1100 yr B.P. exposed in all the trenches provide age constraints on the timing of four pre-1906 earthquakes. Earlier events were exposed in the one deep excavation and are confined between the section dated to approximately 1200 to 2500 yr B.P. Most of the event horizons terminate in or near major in situ peat layers that provide excellent organic material for radiocarbon analysis. Correlation among trenches and radiocarbon dates indicates a range of recurrence intervals between paleoearthquakes. Further analyses of the radiocarbon data will help us to clearly define the dates of these paleoearthquakes.