Late Quaternary slip rates from offset alluvial fan surfaces along the Central Sierra Madre fault, southern California

Austin Hanson, Reed J. Burgette, Katherine M. Scharer, & Nikolas Midttun

Published August 15, 2016, SCEC Contribution #6909, 2016 SCEC Annual Meeting Poster #096

The Sierra Madre fault (SMF) is an east-west trending reverse fault system along the southern flank of the San Gabriel Mountains near Los Angeles, California. The ~140 km long SMF is separated into four segments; we focus on the multi-stranded, ~85 km long Central Sierra Madre fault (CSMF). The CSMF lacks a well-characterized geologic slip rate, and the goal of our work is to constrain the slip rate of the CSMF using offset alluvial fan terraces dated by terrestrial cosmogenic nuclide (TCN) and infrared-stimulated luminescence (IRSL). Our main study area includes three catchments along the western CSMF, where we modified previous mapping (Crook et al., 1987) using field observations and analysis of lidar DEMs to delineate terrace flights in greater detail. Vertical separation estimates are derived from 30-40 m wide swath profiles of topography extracted from 0.5-3 m resolution lidar DEMs, analyzed in 2 m wide subswaths. Dip-slip displacements with uncertainties are calculated following the methods of Thompson et al. (2002), which include a range of estimates for the vertical separation, hanging wall and footwall surface slopes, fault dip, and the location of the fault tip relative to the scarp surface. For four surfaces we have TCN depth profiles and complementary IRSL ages that provide an independent maximum age of the deposit and/or a minimum age from fine-grained material that post-dates terrace emplacement. For three additional terraces, we have IRSL but no TCN ages. Preliminary analysis of TCN data yields model ages that range from ~20-65 ka and are consistent with the order of terrace abandonment. IRSL indicate that the fine-grained material on top of the terraces was deposited rapidly following terrace abandonment. Preliminary slip rate estimates are ≤ 1 mm/yr in two locations along the western CSMF. Although averaged over a longer time period than previous studies, this slip rate may only span 3-4 earthquake cycles suggested by paleoseismic work on the CSMF. Our slip rate estimates are at the lower end of the UCERF3 estimate of 1-3 mm/yr, which suggests that some portion of the strain budget assigned to the CSMF has been accommodated elsewhere over the late Quaternary.

Key Words
Cental Sierra Madre fault, slip rate, cosmogenic, luminescence

Hanson, A., Burgette, R. J., Scharer, K. M., & Midttun, N. (2016, 08). Late Quaternary slip rates from offset alluvial fan surfaces along the Central Sierra Madre fault, southern California. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology