Preliminary ages of prehistoric earthquakes on the Banning Strand of the San Andreas Fault, near North Palm Springs, California

RM 216

The southernmost section of the San Andreas Fault (SAF) is the only section of that fault that has not ruptured historically. It is not known whether this long quiescent period reflects a long average recurrence interval, or whether the current interseismic interval is longer than average. Near Indio, the SAF splits into 3 strands; limited paleoseismic work has been conducted on the Mission Creek and Garnet Hill strands. We studied a paleoseismic trench on the Banning strand, which has no available age control for any surface-rupturing, prehistoric earthquakes. We were able to document clear evidence for five prehistoric earthquakes and possible evidence for four additional events. The most recent event occurred sometime between 560 and 936 calibrated years before present (BP). At least 3-4 earthquakes have occurred since ~2700 years ago. At greater depths, we document two events with strong evidence and two other possible events with weak evidence. From existing dates, we calculate a maximum average interval of 720 yrs based on three complete earthquake cycles between earthquakes 1 and 4 or a minimum average interval of 350 yrs based on limiting ages for earthquakes 1 and 5. This makes the average interval equal to or less than the current open interval on the Banning strand. Compared to nearby paleoseismic sites, the Banning appears to be intermediate; it has a shorter interval than that published for the San Gorgonio Pass Thrust (~1000 yrs) but longer than the Mission Creek (~215 yrs) and Coachella sections (~220 yrs) of the SAF