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 the fault that has not ruptured in the last 200 years. 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 exposed a ~30 m wide fault zone in interbedded alluvial sand and gravel, coarse-grained debris flow deposits, and clay/silt deposits. We documented evidence for eight prehistoric earthquakes, and we note that evidence for the events higher in the section is stronger than the evidence for those that are deeper in the section. Based on the age of two charcoal samples that bracket the most recent event (MRE), it occurred sometime between 560 and 960 cal. BP. We interpret that three earthquakes have occurred since ~1.8 ka, four earthquakes since ~2.7 ka, and five earthquakes have likely occurred since 5.0 ka. These event rates assume the stratigraphic section is complete and there are no missed events. Three charcoal samples located at the base of the trench have calibrated ages of 10.7 to 11.8 ka. All dated samples are detrital charcoal, so we use the youngest dated sample to estimate the age of each layer. Dates from luminescence samples are pending and will help to address the degree of inherited charcoal and place additional age constraints on individual events. Using our existing radiocarbon ages, 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 equivalent to or less than the elapsed time since the MRE on the Banning strand. The average recurrence interval for the Banning section of the SAF appears to be intermediate between that for the San Gorgonio Pass thrust fault (˜1000 years) and that for the Mission Creek and Coachella sections (˜215 and ˜220 years, respectively).