Influence of sediment dynamics and alluvial fan formation on paleoseismic studies in southern California, North America

Sourav Saha, Seulgi Moon, Nathan D. Brown, Ed J. Rhodes, Sally F. McGill, Bryan A. Castillo, Katherine M. Scharer, Devin McPhillips, & Doug Yule

Published August 13, 2019, SCEC Contribution #9535, 2019 SCEC Annual Meeting Poster #129 (PDF)

Paleoseismic data provide a record of past surface-rupturing earthquakes, but the fidelity of the record depends on the completeness of trench stratigraphy and the precision of geochronology. For example, a period of erosion might scour fault terminations from multiple earthquakes to a single stratigraphic level, leaving evidence of no more than one earthquake in the stratigraphy. On the southern San Andreas Fault System (sSAFS), California, USA, stratigraphic completeness may be of particular concern because many paleoseismic sites are located on alluvial fans, where sediment is deposited episodically and sometimes eroded. Classic cut-and-fill terrace sequences are an end-member example of this behavior, but even a discrete alluvial fan surface has likely experienced hiatuses of years to decades between depositional events. On the sSAFS, previous workers have calculated apparent recurrence intervals (RIs) from trenches on different fault strands and interpreted variable RIs in terms of differences in fault slip rate or other tectonic characteristics. Here, we explore the alternative hypothesis that apparent RIs depend on local depositional history and stratigraphic completeness. To this end, we are in the process of examining individual feldspar single-grain post-infrared infrared stimulated luminescence (pIR IRSL) ages from a trench site on the Banning Strand of the sSAFS at 18th Avenue. The paleoseismic site was in an alluvial fan-axial valley deposit. At least eight earthquake event horizons are identified and dated using 17 feldspar pIR IRSL ages, consisting of approximately 250 grains apiece. Fifteen of these ages show stratigraphic coherence and correspondence with the youngest detrital charcoal 14C ages at 1-sigma. Analysis of over-dispersion indicates that all but one of the samples are partially bleached; this is consistent with limited exposure at the surface and rapid sediment transport prior to deposition at the trench site. At least seven single-grain age sub-populations are identified using the finite mixture model. The ages of these subpopulations appear to correlate with the 10Be-derived ages of terraces in upstream cut-and-fill sequences, potentially recording incision followed rapid deposition a short distance downstream. These results indicate that there is a linkage between climate-driven selective sediment evacuation and aggradation in the study area, which needs to be considered when interpreting RIs from similar paleoseismic sites.

Key Words
Luminescence dating, San Andreas Fault System, Sediment dynamics, Recurrence interval, Paleoseismic, Southern California

Citation
Saha, S., Moon, S., Brown, N. D., Rhodes, E. J., McGill, S. F., Castillo, B. A., Scharer, K. M., McPhillips, D., & Yule, D. (2019, 08). Influence of sediment dynamics and alluvial fan formation on paleoseismic studies in southern California, North America. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)