SCEC2021 is online September 12-17!  Registered participants click here.

Late Quaternary slip rates for the southern Elsinore fault in the Coyote Mountains, southern California from analysis of alluvial fan landforms and clast provenance, soils, and U-series ages of pedogenic carbonate

Thomas K. Rockwell, Eulalia Masana, Warren D. Sharp, Petra Stepancikova, M Ferrater, & Regina Mertz-Kraus

Published January 15, 2019, SCEC Contribution #7276

We studied offset alluvial fans along the Elsinore fault in the south-central Coyote Mountains to resolve an average medium-term late Quaternary slip rate (ca. 40-150 ka) for this major western strand of the San Andreas fault system in southern California. We mapped the alluvial fans using high-resolution DEMs combined with field observations of fan-surface morphology and the character of the soils developed in each fan remnant, collected clast assemblage data to determine the source of each fan to its source canyon, and applied U-series dating of pedogenic carbonate to estimate minimum ages of the fan surfaces. Using a set of 40 U-Th dates on pedogenic carbonate, we confirm the utility of the technique for dating Late Pleistocene fans in arid regions, suggest significant age variation among Late Pleistocene fans grouped on the basis of soils and geomorphic criteria, and document complexities in dating soils developed on pre-Late Pleistocene fans.
We estimate that the southernmost segment of the Elsinore fault has sustained a slip rate of at least 2.4±0.4 mm/yr for the past 60-70 ka and probably for the past 150 ka. Since displacement in the most recent surface rupture increases northwest of our slip rate sites, we consider this rate to be a minimum for the southern Elsinore fault, with the rate likely close to 3 mm/yr in the central part of the range. These new data confirm that slip gradients along individual fault segments must be considered when estimating pre-Holocene slip rates for seismic hazard estimates. Our new results show that the southern Elsinore fault accounts for about 6% of the total relative motion between North America and the Pacific lithospheric plates in southern California. Assessment of previous estimates of slip in the most recent event suggests earthquakes of about Mw6.8 and, when combined with our slip rate data, a recurrence of such events about every thousand years.

Key Words
Elsinore fault, slip rate, pedogenic carbonate dating, U-series dating

Citation
Rockwell, T. K., Masana, E., Sharp, W. D., Stepancikova, P., Ferrater, M., & Mertz-Kraus, R. (2019). Late Quaternary slip rates for the southern Elsinore fault in the Coyote Mountains, southern California from analysis of alluvial fan landforms and clast provenance, soils, and U-series ages of pedogenic carbonate. Geomorphology, 326, 68-89. doi: 10.1016/j.geomorph.2018.02.024.


Related Projects & Working Groups
Magnitude-Frequency Estimates From Paleoseismic Data Using the Stringing Pearls Methodology, San Andreas-San Jacinto Fault Syste, Earthquake Geology, SoSAFE Working Group