Refining the south-central San Andreas fault slip rate at the millennial timescale using reconstructed offset paleo-channels in the Carrizo Plain, California

James B. Salisbury, Ramon Arrowsmith, John Sims, Lisa Grant Ludwig, & Sinan O. Akciz

In Preparation November 23, 2016, SCEC Contribution #7154

Geologic slip rates of active faults are essential for seismic hazard analysis and their comparison with decadal geodetic measurements can be used to assess the constancy of strain accumulation and earthquake-modulated strain release. Inherent to the proper interpretation of geologic fault slip markers is a thorough understanding of the geomorphic processes responsible for the formation and preservation of such landforms. We investigated these processes at the Phelan Creeks site along the San Andreas Fault (SAF), 1.6 km southeast of Wallace Creek in the Carrizo Plain—a region with simple fault geometry, the highest millennial-scale slip rate in California (>3 cm/yr), a Mw 7.8 surface rupture in 1857, and abundant well-preserved geomorphic slip markers at several length scales. The Phelan Creeks area consists of three downstream channel complexes on the southwest side of the SAF that emanated from a pair of feeder channels (“Little” and “Big” Phelan Creeks) on the northeast side of the SAF. These channel complexes record offsets of ~16.5 m (average of the two modern channel offsets), ~125 m (partially infilled abandoned paleo-channel), and ~238 m (mostly infilled abandoned paleo-channel). We pieced together sedimentologic data from 24 excavations that record a detailed ~7,000-year history of channel processes during progressive offset and eventual abandonment of channel complexes along the semi-arid reach and provide a simplified tectono-stratigraphic model to describe the geomorphic response of stream channels to continued strike-slip faulting. Additionally, we dated sediments from multiple reconstruction stages throughout the ~238 m of cumulative offset to fill a spatio-temporal gap in age-offset constraints at Wallace Creek and show that there is minimal slip-rate variability along the south-central SAF during the past 7,000 years (13,000 years if we include dates and offsets from Wallace Creek).

Citation
Salisbury, J. B., Arrowsmith, R., Sims, J., Grant Ludwig, L., & Akciz, S. O. (2016). Refining the south-central San Andreas fault slip rate at the millennial timescale using reconstructed offset paleo-channels in the Carrizo Plain, California. Journal of Geophysical Research, (in preparation).