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

Elliott C. Helgans

Published September 1, 2019, SCEC Contribution #10078

Earthquake processes in plate boundary settings are chiefly controlled by the in situ crustal stress field. Knowledge of the relative importance of various active processes acting on a fault system is necessary to understand the mechanics of faulting. This is of extreme importance to the Cajon Pass region of southern California, which may function as an earthquake gate, imposing control on large multifault ruptures. We model the in situ stress field at seismogenic depth in Cajon Pass by balancing the orientation of the modern stress field inferred from earthquake focal mechanisms against the superposition of the far field tectonic driving stress, the load of topography, and the accumulation of stress on locked faults over variable loading times. We incorporate existing models for stress accumulation rate from locked faults and topography with a set of simple driving stress models, in which we treat driving stress orientations, magnitudes, as well as effective loading times on locked fault segments as free parameters. We use this model to assess relative influences of each process to the modern field observed today as well as identify any potential heterogeneity in the plate driving stress. Our results indicate that driving stress orientation may rotate clockwise (from north-northwest to north-northeast) southward across Cajon Pass and predict in situ differential stress magnitudes between 59 and 93 MPa in this region, consistent with previous findings. We find that the modern stress field may be most strongly influenced by heterogeneity in driving stress orientation on the scale of tens of kilometers. Despite rake angles indicating a primarily strike-slip faulting regime across Cajon Pass for our optimal model with fault segment-scale rotations in driving stress, we observe a heterogeneous distribution of maximum shear stress predicted on fault surfaces. We predict shear stress is highest, ~60 MPa, on the northern San Andreas fault, and sharply decreases to ~30 MPa at the onset of the subparallel San Jacinto fault network. The observed variations in resolved shear stress on major Cajon Pass fault surfaces indicate that heterogeneity in driving stress orientation may inhibit multifault ruptures across the region.

Helgans, E. C. (2019). Multicomponent Model of Crustal Stress at Cajon Pass, Southern California with Implications for Stress Field Heterogeneity (Louisiana State University). https://digitalcommons.lsu.edu/gradschool_theses/4998/