A 100-Year Average Recurrence Interval for the San Andreas Fault, Southern San Francisco Bay Area, California

Paleoseismic excavations at Mill Canyon and Arano Flat, two sites 0.6 km apart on the San Andreas fault near Watsonville, California, provide the first high-resolution chronology of large earthquakes on the Santa Cruz Mountains segment of the fault. At Mill Canyon, a 2-m-wide zone of faulting has deformed latest Holocene deposits consisting of well-sorted sand and gravel interbedded with poorly sorted, commonly organic-rich debris flows ponded behind a small shutter ridge. We found evidence for the 1906 San Francisco earthquake and three additional ground-rupturing earthquakes since about 1500 A.D.. Radiocarbon ages and pollen analyses indicate that the penultimate earthquake at this site occurred about 1700-1790 A.D.. This indicates that the 1838 San Francisco peninsula earthquake did not rupture this portion of the fault. At Arano Flat, faulting is expressed as a 1 to 2-m-wide zone that deforms alluvial fan deposits overlying well-bedded overbank deposits. We found evidence at this location for at least nine earthquakes since about 1000 A.D. We constrain earthquake ages using a chronological model incorporating AMS radiocarbon ages of 113 samples of detrital charcoal from 19 layers and stratigraphic ordering. The mean recurrence interval is about 105 years, while individual intervals range from about 10-310 years. Two offset features at Arano Flat provide slip-per-event and slip rate data. A partially buried channel containing bottles from 1887-1890 is offset 3.5 m. Given that we found no evidence at either site for the 1890 M 6.3 earthquake, which produced surface rupture on the San Andreas fault southeast of Parajo Gap, this entire slip may have occurred during the 1906 earthquake. This value is unexpectedly high compared to the geodetic estimate of 2.3-3.1 m for the slip at depth (Thatcher et al., 1997) or the geologic estimate of 1.7-1.8 m of surface slip at Wright's tunnel (Prentice and Ponti, 1997), about 33 km northwest of Arano Flat. A fold that formed during two earthquakes, most recently about 1400-1470 A.D., is offset about 10.5 m during the past five earthquakes. This yields a slip rate of 22.5 +/- 2 mm/yr, significantly higher than values previously used for this segment. Average slip for the four earthquakes prior to 1906 is 1.2-1.8m indicating M?7. Thus the mean recurrence interval is half the value used by the Working Group on California Earthquake Probabilities (WG 03) for earthquakes of this magnitude on the Santa Cruz Mountains segment.