EARTHQUAKE EVENTS AND RECURRENCE INTERVAL ON THE NORTHERN SAN ANDREAS FAULT AT VEDANTA MARSH SITE, OLEMA, CA

The Vedanta marsh is an exceptional site because predominately fine-grained, organic-rich deposits have continuously buried the Northern San Andreas fault during the Holocene. The trench exposures show evidences for over 10 earthquakes on the San Andreas fault over about 3000 years. The main fault zone consists of a 2-m-wide zone of upward-branching fault splays within the marsh stratigraphy, where major peat layers interbed with marsh clay and silt that interfinger eastward with colluvial gravels. A clear transition from predominantly fine-grained marsh deposits to coarse clastic sediment occurs at the depth of approximately 1 m. The 1906 earthquake ruptured a portion of the upper gravel. Historic offset measurements from the 1906 ground rupture earthquake near the site were about 5 m. At least 10 pre-1906 events were identified by evidences of outward-splaying, upward fault terminations and fissure fills. Most of the event horizons terminated in or near the peat layers. Correlation among trenches and radiocarbon dates indicate the roughly recurrence interval is around 250 years. Although the evidence of the Penultimate Event is elusive due to the obliteration of 1906 event and bioturbation, subtle evidences observed in different trenches support the event could occur around 17th century.