Work in progress to estimate a Latest Pleistocene slip rate for the Banning Strand of the San Andreas Fault near North Palm Springs

Sally F. McGill, Paula M. Figueiredo, & Lewis A. Owen

Published August 13, 2016, SCEC Contribution #6737, 2016 SCEC Annual Meeting Poster #125

Existing late Pleistocene slip rate estimates for the southern San Andreas fault reveal high rates of slip on the Mission-Mill Creek strand in the Indio Hills but very low rates on that strand farther northwest in the San Bernardino Mountains. The northwestward reduction in slip rate on the Mission-Mill Creek strand is likely accommodated by transfer of slip both northward to the Eastern California shear zone and southwestward to the Banning and Garnet Hill strands of the San Andreas fault. To better understand the likely rupture paths and slip distribution in future earthquakes on the southern San Andreas fault, it is important to know how the latest Quaternary slip rate has been partitioned among these fault strands. The published late Holocene slip rate of the Banning strand is relatively low, suggesting that it has played a relatively minor role in plate boundary slip over that time period. We present here our work in progress at a new slip rate site on the Banning fault near North Palm Springs, which will provide an opportunity to confirm or modify that conclusion and to test for variation in the slip rate of the Banning strand over time. An ancient channel wall of Mission Creek that incised about 1.2 m depth into an older, distal alluvial surface has been offset 148 ± 9 m by the Banning fault. In January 2016 we excavated three dating pits at the site after a lengthy permitting process. Two 2-m deep pits were excavated on the old alluvial surface into which Mission Creek incised, and one 1.5-m deep pit was excavated into the alluvial deposits that post-date the incision but that have been offset out of the path of the modern channel of Mission Creek. Twelve samples for optically stimulated luminescence dating were collected (4 samples from each pit), as well as seven samples for Be-10 depth profiles from each of the two pits on the incised alluvial surface. Samples are being processed at the Geochronology Laboratories at the University of Cincinnati and preliminary results for some samples may be available in time for the meeting. Six detrital charcoal samples from eolian and/or fluvial overbank deposits in the top 0.5 m in the pits are all less than 1000 years old. One detrital charcoal sample from 0.85 m depth in a pit on the incised surface south of the fault has a late Holocene age (2430 ± 25 radiocarbon years BP). The young age of this sample suggests that it must also be from fluvial overbank deposits that post-date the incision of Mission Creek into the underlying, older alluvial surface. Luminescence dates from well consolidated alluvial deposits near the base of the two deepest pits are expected to provide a maximum age for the incision of the offset channel wall and a minimum slip rate for this section of the Banning fault.

Key Words
Banning fault, slip rate, San Andreas fault, San Gorgonio Pass, Coachella Valley

Citation
McGill, S. F., Figueiredo, P. M., & Owen, L. A. (2016, 08). Work in progress to estimate a Latest Pleistocene slip rate for the Banning Strand of the San Andreas Fault near North Palm Springs. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)