Investigating strain transfer along faults in Joshua Tree National Park, CA, with possible implications for along strike variations in southern San Andreas Fault slip rate

Katherine A. Guns, Richard A. Bennett, Kimberly D. Blisniuk, & Sally F. McGill

Published August 14, 2018, SCEC Contribution #8490, 2018 SCEC Annual Meeting Poster #238

The pattern of and relationship between elastic and permanent strain accumulation adjacent to the southern San Andreas Fault (SSAF) zone in the Eastern Transverse Ranges Province (ETR) are currently only poorly understood, hampering assessments of earthquake potential and associated hazards to southern California. To address this issue, we are integrating geodetic and tectonic geomorphologic methods to investigate how plate boundary strain is accommodated within the ETR, which lies between the SSAF and the Eastern California Shear Zone (ECSZ). We focus our studies on the five east-trending, left-lateral strike slip faults of the ETR within Joshua Tree National Park (JTNP). These faults each have measured cumulative offsets ranging between 1 km to 20 km. If active, these faults could be facilitating contemporary rotation of crustal blocks in the ETR, with implications for along strike variation of SSAF slip rate and the kinematics of strain transfer between the SSAF and ECSZ. Existing block models present a variety of slip rate estimates for these faults, from as low as zero to as high as 7 mm/yr, suggesting a gap in our understanding of what role they play in the larger system. To determine whether present-day block rotation along these faults contributes to strain accumulation in the region, we apply 10Be surface exposure dating methods to observed offset channel and alluvial fan deposits in order to estimate fault slip rates along two faults in the ETR. We employed detailed geomorphic mapping, LiDAR data analysis, and clast count studies to identify and quantify Quaternary (at least latest Pleistocene) offsets at 3 sites along the Blue Cut Fault and 1 site along the Smoke Tree Wash Fault. Initial dating (N=7) at one of the sites, indicates a preliminary fault slip rate of 0.5—1.75 mm/yr for the Hexie Mountains section of the Blue Cut Fault, though further 10Be dating is needed. In addition, we present an updated GPS velocity field derived from two local campaign networks, one within JTNP and one spread out over the San Bernardino Mountains area. New geologic slip rate data, combined with an updated velocity field, will allow us to refine our elastic fault block models and to test different hypotheses about how strain is being distributed and transferred through this complex system. Evaluating how strain is distributed will help us better understand earthquake recurrence along a section of the SSAF that is thought to be overdue for a major earthquake.

Key Words
geodesy, geologic slip rates, SSAF, ECSZ, strain transfer

Citation
Guns, K. A., Bennett, R. A., Blisniuk, K. D., & McGill, S. F. (2018, 08). Investigating strain transfer along faults in Joshua Tree National Park, CA, with possible implications for along strike variations in southern San Andreas Fault slip rate. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology