Imaging northern California faults across the brittle- ductile transition using receiver function anisotropy, seismicity, and focal mechanisms

Vera Schulte-Pelkum, & Debi Kilb

Accepted January 2025, SCEC Contribution #14173

We develop contraints on the geometry of major faults and fault junctions in the San Andreas plate boundary system: The San Andreas fault at Parkfield, the Hayward-Calaveras junction in the East Bay fault system, and the San Andreas-Calaveras junction. We combine constraints on stress and geometry from seismicity, rock fabric contrasts imaged with anisotropy-aware receiver functions (RF), and machine-learning (ML) improved focal mechanisms. All faults show fault-parallel rock fabric within and below the seismogenic zone. Fault fabric is not usually symmetric about the fault, particularly in places with significant pre-existing fabric. For example, foliation near Parkfield dips to the northeast on both sides of the San Andreas fault, and fabric contrasts are stronger on the northeast side of the fault. We propose this stems from an interaction with subduction-related inherited dipping fabric that localized the proto-San Andreas, followed by gradual steepening of the fault to vertical. At the Hayward-Calaveras fault junction, our results show fabric contrasts that appear to track the projection of the merged steeply dipping fault plane to below the brittle-ductile transition. They suggest that a shear zone forming the downward extension of the merged fault plane exists below the seismogenic depth range to depths of close to 30 km. The implication is that the Hayward-Calaveras in the East Bay fault system may be a crustal scale feature. Foliation strikes at the imaged fabric contrasts are parallel to near-parallel to the strikes of the fault surfaces. A change in imaged strikes to near fault-perpendicular at the latitude of a pronounced shallowing of the fault junction line in depth suggests complexity in the deformation signature at depth due to the 3-D geometry of the fault junction line.

Citation
Schulte-Pelkum, V., & Kilb, D. (2025, 01). Imaging northern California faults across the brittle- ductile transition using receiver function anisotropy, seismicity, and focal mechanisms. Oral Presentation at Northern California Earthquake Hazards workshop.