Fault fabric as a function of fault maturity and its interaction with preexisting structures: Examples from the San Andreas plate boundary system

Debi Kilb, Vera Schulte-Pelkum, & Thorsten Becker

Accepted December 2024, SCEC Contribution #14172

Faults, in the idealized case, are represented as discrete surfaces separating homogeneous blocks of material. In reality, off-fault deformation, fault-related fabrics, and fault zones consisting of multiple individual strands suggest faulting processes encompass a 3D volume. Fault complexity and fault fabric strength may be associated with the maturity of a fault system, but the details of such relationships are unclear. Here, we explore these relationships using a receiver function method that detects contrasts in rock fabric as represented by contrasts in anisotropy. We target a range of faults within the San Andreas transform plate boundary system, including the San Andreas fault at Parkfield, the Hayward and Calaveras faults in central California, the San Jacinto fault zone in southern California, and faults in the eastern California shear zone in the Mojave. All faults show fault-parallel rock fabric within and below the seismogenic zone. We find a positive correlation between fault maturity and fault-parallel fabric strength, with minor faults in the eastern California shear zone showing the weakest fabric and the San Andreas fault at Parkfield the strongest. 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. In all but the least mature settings, fault fabric contrasts follow surfaces outlined by seismicity, continuing above and below the seismogenic zone, suggesting faults continue as shear zones beyond the brittle-ductile transition. Mapping with receiver functions that are sensitive to anisotropy contrasts thus allows outlining fault zones outside of their seismogenic portions, which can illuminate fault interactions and aid seismic hazard assessments and investigation of the plate boundary as a continuum.

Citation
Kilb, D., Schulte-Pelkum, V., & Becker, T. (2024, 12). Fault fabric as a function of fault maturity and its interaction with preexisting structures: Examples from the San Andreas plate boundary system. Oral Presentation at American Geophysical Union.