INVESTIGATION OF QUATERNARY FAULT DEFORMATION AND COSEISMIC SLIP OF THE 1906 EARTHQUAKE IN MARIN COUNTY USING THE LAWSON REPORT AND UNPUBLISHED DOCUMENTS

The San Andreas fault rupture of the 1906 San Francisco earthquake in Marin County, California was documented by G.K. Gilbert, J.C. Branner, and others and is well preserved within the Point Reyes National Seashore. Published and unpublished historical photographs, notebooks, air photos, and maps were analyzed to document the exact location and style of deformation of the earthquake and the morphological changes in the landscape. Relocated 1906 photographs, survey data, and historic maps were compiled into a digital GIS database. Analysis of the position of a pre- and post-earthquake meander loop, fence location, and utility poles adjacent to the famous 20-foot (6.1 m) offset of Sir Francis Drake Boulevard built across the head of Tomales Bay, 0.8 km west of Point Reyes Station indicate coseismic slip between 4.4-7.5 m. A new coseismic slip of 5 m for the 1906 earthquake is also documented from historical notes at the Vedanta wind gap site. High-resolution topographic data from RTK GPS and conventional surveying yields a additional offset estimates of 6-8 m cumulative offset along the 1906 trace for this site. Matching of ridge morphology for another location suggests 6.5 m along the ruptured fault trace and up to 22 m of cumulative dextral slip on a second trace that remained inactive in 1906. Furthermore, we hypothesize that left-stepping surface fault patterns of the 1906 earthquake in the area of a stratigraphic high at Five Brooks imply inactive fault systems along an Eastern Boundary fault south of this location. The result of detailed mapping show processes of erosion and landslides greatly imprint on today's fault characteristics and have implications on fault patterns of future earthquakes at Point Reyes.