Holocene slip rates along the Mojave Section of the San Andreas fault

Elaine K. Young, Eric S. Cowgill, & Katherine M. Scharer

Submitted August 15, 2018, SCEC Contribution #8702, 2018 SCEC Annual Meeting Poster #269

The ∼100-km long Mojave section of the San Andreas fault is positioned within a double restraining bend of the fault in southern California. The Holocene slip rate on the Mojave San Andreas Fault (MSAF) is not well known, despite its importance for understanding earthquake hazard, apparent discrepancies between geologic and geodetic slip rates along this fault section, and potential long-term fault interactions in southern California. Here we use detailed surficial geologic mapping, stratigraphic analysis, and radiocarbon dating of charcoal to characterize and determine ages of landforms offset by the MSAF at two slip-rate sites. To bracket the age of the offset landform, we date deposits emplaced before and after the feature formed. At site X-12, a beheaded channel that formed between ∼1700 and ∼550 calBP is offset ∼50 to ∼60 m. At site Ranch Center, a channel that formed between ~4200 and ~3700 calBP is deflected ~70 to ~100 m behind a shutter ridge. Data from Ritter Ranch suggests an absolute minimum rate of 21 mm/yr [1] since ~3 ka. In combination, the results at these MSAF sites suggest an increase in average slip rate from 17-27 mm/yr since ~4 ka (Ranch Center), to ≥29 mm/yr over the last ~1.7 ka (X-12). Our MSAF results may support a kinematic model for southern California faults [2] that predicts a shared slip history for the San Andreas fault, the Garlock fault, and faults in the Los Angeles basin that is anticorrelated with periods of increased slip in the Eastern California Shear Zone. The increase in average slip rate at ~1.7 ka is similar to the slip history for the Garlock fault [3]. The rate of 17-27 mm/yr since ~4 ka is lower than the long term (413 ka) average slip rate of 30-37 mm/yr [4], but the post ~1.7 ka rate of ≥29 mm/yr is in agreement or higher, suggesting variations in slip rate over time. The Holocene slip rates for the MSAF reported here are compatible with geodetic slip rates modeled using a viscoelastic crust [5] but are still generally higher than those modeled with an elastic crust.

[1] Weldon, 2009, USGS/NEHRP #G09AP00012 (UO #238610) Report
[2] Dolan et al., 2007, Geology. v.35, p. 855
[3] Dolan et al., 2016, EPSL. v. 446, p. 123
[4] Matmon et al., 2005, GSAB. V. 117 p. 795
[5] Chuang and Johnson, 2011, Geology. v. 39, p.627

Key Words
San Andreas Fault, Slip Rate, Holocene, Radiocarbon

Citation
Young, E. K., Cowgill, E. S., & Scharer, K. M. (2018, 08). Holocene slip rates along the Mojave Section of the San Andreas fault. Poster Presentation at 2018 SCEC Annual Meeting.


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