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Using Focal Mechanisms to Distinguish Between Alternative Active Fault Configurations of the Southern San Andreas Fault at the San Gorgonio Pass

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip I

Type: Oral

Date: 4/22/2021

Presentation Time: 10:30 AM Pacific

Description: 

Debate persists on the activity of the northern pathway of the southern San Andreas fault (SAf) through the San Gorgonio Pass and recent studies propose alternative dips for faults within the southern pathway based on interpretations of seismic imaging. To delineate between plausible models, we assess the match of interseismic forward model results from four alternative 3D active subsurface fault configurations for the SAf (two variations of the southern pathway and two variations of active geometry of the northern pathway) to interseismic geodetic surface velocities and slip sense of off-fault microseismicity as well as geologic slip rates. Slip rates from the models generally all fit the geologic data with equal mismatch. Model predicted surface velocities differ by less than the GPS station uncertainties and cannot resolve activity of the northern pathway. Interseismic focal mechanisms occur at a range of depths making them more sensitive to variations in deformation below the locking depth during the interseismic period than GPS velocities at Earth’s surface. Thus, the focal mechanism catalog provides additional and valuable constraints on crustal stress that drives slip. Stress states inverted from focal mechanisms generally show north-south compression for the region, consistent with expectations. Where the forward model results for microseismicity slip sense differ, the single-pathway model predicts mostly strike-slip microseismicity and matches the observed focal mechanisms better than the dual pathway model, which predicts mostly reverse slip microseismicity. We show that interseismic focal mechanisms from off-fault microseismicity record the crustal stress state such that they can be used to delineate between alternative active fault configurations. Furthermore, stress states derived from focal mechanisms can be inverted for the fault slip distribution to provide additional insight on active fault configuration and locking depth.

Presenting Author: Hanna Elston

Student Presenter: Yes

Authors