PRELIMINARY AGES OF PREHISTORIC EARTHQUAKES ON THE BANNING STRAND OF THE SAN ANDREAS FAULT, NEAR NORTH PALM SPRINGS, CALIFORNIA

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 for this portion of the fault, 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 and the Banning strand has no available age control for any surface-rupturing, prehistoric earthquakes. We studied a paleoseismic trench that was excavated across the Banning strand by Petra Geosciences (33.9172°, -116.538°). The trench displayed a ~30 m wide fault zone in interbedded alluvial sand and gravel, coarse-grained debris flow deposits, and clay/silt deposits. We were able to document clear evidence for five prehistoric earthquakes and possible evidence for four additional events. The most recent event occurred sometime before deposition of a detrital charcoal sample that has a date range of 746-626 cal BP, and after sample with an age of 1119-864 cal BP. At least 3-4 earthquakes have occurred since a ~2.7 ka. At great depths, we document two events with strong evidence and two other possible event horizons with weak evidence. Two charcoal samples located at the base of the trench had calibrated ages of ~11.4 ka. All dates are based on detrital charcoal, so we use the youngest dated sample to constrain the age of each layer. 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 330 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 strand appears to be intermediate; shorter intervals than 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.