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

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

Published December 11, 2018, SCEC Contribution #9030

The Cajon Pass region 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 functions as an earthquake gate, or behavioral boundary on multi-fault ruptures. Stress maps, inverted from nearby focal mechanisms, feature a rotation in the regional stress field immediately south of the junction. Identifying the causes of this rotation will help determine how stress modulation in the pass affects future rupture behavior. We model the Cajon Pass in situ stress field as the superposition of stress from three distinct 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 stressing processes dominate in Cajon Pass over which length scales, as well as identify potential heterogeneities in the geodynamic driving stress. Existing models for stress from locked faults and topography are used in conjunction with loading times drawn from paleoseismic slip histories to derive a set of simple driving stress field geometries. We constrain themagnitude and orientation of the geodynamic stress field by comparing the forward model to inverted stress orientations from a catalog of ~180,000 regional earthquake focal mechanisms, regarded as the modern in situstress field. The in situstress field in Cajon Pass displays considerable heterogeneity in both the maximum horizontal stress azimuth and the dominant style of faulting. Fault regimes and in situstress field orientations vary across Mojave, San Bernardino, Claremont, and Clark fault segments as well as individually along single segment strikes. We find that individually neither stress from topography nor locked faults capture the heterogeneity or large-scale features present in the in situ field, and thus are only partly responsible for the modern Cajon Pass stress 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 simplest scenario of a series of homogenous regional geodynamic stress fields with a single set of loading times along each fault segment. We observe a positive relationship between driving stress orientation and loading time. For each fault segment, as the geodynamic stress field trends further east larger loading times are required to best fit the model to the in situfield. 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 fault segments. For low differential stress values, <5 MPa, a dichotomy exists where the Mojave and San Bernardino, and Clark and Claremont segments can be fit together with respective driving stress azimuths ~-10º and 20º East of North. As fits improve and differential stresses increase the best fit lines diverge and each fault segment is best fit with a different loading time and driving stress azimuth. 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 a combination of the two are required to fit the data. Ultimately, understanding the sources of stress heterogeneity in Cajon Pass 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, 12). Multicomponent Model of Crustal Stress at Cajon Pass with Implications for Stress Field Heterogeneity. Poster Presentation at Fall AGU Meeting.