Multicomponent Model of Crustal Stress at Cajon Pass with Implications for Stress Field Heterogeneity

Elliott C. Helgans, Karen M. Luttrell, & Bridget R. Smith-Konter

Submitted August 15, 2018, SCEC Contribution #8684, 2018 SCEC Annual Meeting Poster #259

Cajon Pass (CP) marks the primary junction between the San Andreas and San Jacinto fault networks. Highly segmented faulting in the region encourages investigation into whether the junction acts as a behavioral boundary on multi-fault ruptures. Stress maps, inverted from nearby focal mechanisms, feature a rotation in the stress field south of the junction. Identifying the causes of this rotation will help determine how stress modulation in CP affects future rupture behavior. We model the CP in situ stress field as the superposition of stress from three tectonic processes: the accumulation of stress on locked faults over variable loading times, the load of topography, and the far field geodynamic driving stress. Our model serves to quantify which processes dominate in CP over which length scales, and to identify potential heterogeneities in the geodynamic driving stress. Existing models for stress from locked faults and topography are used with loading times drawn from paleoseismic slip histories to derive a set of simple driving stress fields. We constrain the magnitude and orientation of the geodynamic stress field by comparing the forward model to the "modern in situ¬" stress field, inverted from a catalog of ~180,000 regional focal mechanisms. We find that individually neither stress from topography nor locked faults capture the heterogeneity or large-scale features present in the in situ field. To fully understand stress state across the region we consider contributions from locked faults, topography, and far field plate driving stresses together. We first examine the simple scenario of a series of homogenous regional geodynamic stress fields with a single set of load times at each fault segment. For each segment, as the orientation of the geodynamic stress field trends further east larger load times are required to best fit the model to the in situ field. Fit improves as the magnitude of the geodynamic stress increases until a threshold differential stress of ~30 MPa after which fit only marginally improves for all segments. We find that it is not geologically feasible to fit the stress orientations at each fault segment with a single load time. It is apparent that variably loaded fault segments, a heterogeneous geodynamic stress field, or likely both are required to fit the data. Ultimately, understanding the sources of stress heterogeneity in CP will aid in evaluating how stress variations across the pass affect through-going rupture probabilities.

Citation
Helgans, E. C., Luttrell, K. M., & Smith-Konter, B. R. (2018, 08). Multicomponent Model of Crustal Stress at Cajon Pass with Implications for Stress Field Heterogeneity. Poster Presentation at 2018 SCEC Annual Meeting.


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