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 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 ~40 m-wide fault zone in interbedded alluvial sand and gravel, coarse-grained debris flow deposits, and clay-silt deposits. We present the first paleoseismic record for the Banning strand of the southern SAF. The most recent event occurred between 730 and 950 cal BP, potentially coincident with rupture of the San Gorgonio Pass Fault Zone. We interpret that five earthquakes have occurred since 3.3-2.5 ka and eight earthquakes have likely occurred since 7.1-5.7 ka. It is possible that additional events may have occurred without being recognized, especially in the deeper section the stratigraphy, which was not fully exposed across the fault zone. We calculate an average recurrence interval of 380 - 640 yrs based on four complete earthquake cycles between earthquakes 1 and 5. The average recurrence interval is thus equivalent to or less than the elapsed time since the most recent event on the Banning strand. The recurrence interval is similar to that of the San Gorgonio Pass Fault Zone (450-1850 years) but longer than that for the Mission Creek strand (~220 years).