A 2500-YEAR RECORD OF EARTHQUAKES ALONG THE NORTHERN SAN ANDREAS FAULT AT VEDANTA MARSH, OLEMA, CA

At Vedanta marsh, 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. Five trenches were excavated across the fault during the 2001 field season. 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. We exposed a 4 m section consisting of six major peat layers interbedded with fluvial gravel and 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 and is probably caused by historical logging and landuse changes in the watershed. AMS radiocarbon dating of more than 70 samples of peat, macrofossils, and charcoal collected throughout the section indicate that the base of the exposed section is about 2500 yr B.P. We recognize evidence for up to ten earthquakes using 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. An offset channel traced to the west side of the fault and a buried tree are future targets of study, as they provide excellent piercing points to determine slip of paleoearthquakes. Extension and refinement of the record of earthquakes at this site is anticipated with additional field exploration and continued radiocarbon dating and analyses.