Introduction

We present photomosaics and logs of the walls of trenches excavated for a paleoseismic study at Arano Flat, one of two sites along the San Andreas fault in the Santa Cruz Mountains on the Kelley-Thompson Ranch. At this location, the fault consists of a narrow zone along the northeast side of a low ridge adjacent to a possible sag pond and extends about 60-70 meters across a broad alluvial flat. This site was a part of Rancho Salsipuedes begining in 1834 and was purchased by the present owner’s family in 1851.

We found evidence for nine earthquakes during the past 1000 years. The average recurrence interval it about 105 years. The most recent earthquake was the 1906 San Francisco earthquake. Faulting during this earthquake is poorly expressed in most exposures except on the northwest wall of Trench 3 were it appears as a small sand-filled fissure.