Strain rate dependence on crustal rheology for the Cajon Pass, California

Lauren Ward, Bridget R. Smith-Konter, Xiaohua Xu, & David Sandwell

Published August 13, 2018, SCEC Contribution #8457, 2018 SCEC Annual Meeting Poster #260

While the dependency of fault strain rates on locking depth and slip rate variations are reasonably well-understood, our understanding of the sensitivity of fault loading to rheologic controls is still developing. Furthermore, how strain rates evolve throughout earthquake cycles and whether they are inclined to promote or inhibit earthquake gate behavior of conditionally opening or halting ruptures of neighboring segments likely depends on several factors, including lithosphere rheology and fault zone strength. The Cajon Pass, uniquely positioned between the junction of the southern San Andreas and San Jacinto fault segments, has been hypothesized as an earthquake gate; the Cajon Pass is also situated within a region of modest, but potentially important, crustal structure variation that may influence strain accumulation rate of key faults. Using the growing archive of GPS and InSAR measurements spanning the North American-Pacific plate boundary, we provide updated estimates of present-day surface strain rates using a new physical model (Sandwell and Smith-Konter, 2018) that extends our modeling capabilities to include the effects of spatial heterogeneities in crustal rigidity. For this work, we adopt a simplified representation of crustal rigidity derived from preliminary heat flow estimates and LAB depths. Incorporating reasonable uncertainties in these estimates, we explore the sensitivity of strain rate along the Mojave, North San Bernardino and San Jacinto Valley fault segments from a suite of crustal rigidities. For example, crustal rigidities along the Mojave segment (~30 GPa) may increase by ~25% south of the Cajon Pass, which may lower strain rates by 10-15% along the San Andreas San Bernardino segment and the San Jacinto Claremont segment. Moreover, the implication of an evident change in crustal rigidity south of the Cajon Pass lends support for the idea that strain rate reductions impede through-going ruptures in some cases. Future integration of rheology and crustal structure models provided by the CRM and CTM will help to refine simulations of earthquake cycle strain accumulation, as well as our understanding of the probability of large, multi-segment and multi-fault ruptures of the southern San Andreas Fault System.

Key Words
Cajon Pass, Crustal Rheology, Strain Rate, Earthquake Cycle

Citation
Ward, L., Smith-Konter, B. R., Xu, X., & Sandwell, D. (2018, 08). Strain rate dependence on crustal rheology for the Cajon Pass, California. Poster Presentation at 2018 SCEC Annual Meeting.

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