New observations on the northern San Andreas Fault system: Fast slip rates on the Santa Cruz Mountain section of the San Andreas fault and creeping on the San Gregorio fault?

Kimberly Blisniuk, San Jose State University
Katherine Guns, University of California, Berkeley
Hudson Washburn, San Jose State University
Carol Prentice, United States Geological Survey
Roland Bürgmann, University of California, Berkeley

To better assess seismic hazard and fault behavior in the San Francisco Bay Area on the San Andreas Fault system, field observations and high-resolution lidar topography data are applied to investigate the San Andreas fault in the Santa Cruz Mountains and the San Gregorio fault near Moss Beach. Field mapping and age constraints on offset landforms reveal slip rates of ~20 mm/yr since ~ 8 ka on the San Andrea fault and creep rates of 0.5 to 1.6 mm/yr on the San Gregorio fault. Mapping at Sanborn County Park near Saratoga reveals a progression of alluvial fans and debris flows offset by ~60 m since 3 ka and ~140-60 m since 7-8 ka. The ages of offset landforms here are constrained with 10Be exposure dating of sandstone surface clast. On the San Gregorio fault, tree lines planted by German immigrants circa. 1915 and a sidewalk curb constructed circa 1966 are offset by 50-160 mm and 40-80 mm, respectively. These new observations underscore our current understanding of the San Andreas fault system in this region revealing faster than expected slip rates on the San Andreas fault in the peninsula and evidence of creeping on the San Gregorio fault


Quantifying rates of slip