Fault imaging past the brittle-ductile transition: Relationship between fault fabric and maturity from anisotropic receiver function analysis of the San Andreas plate boundary system

Vera Schulte-Pelkum, Debi Kilb, & Thorsten W. Becker

Published September 8, 2024, SCEC Contribution #13810, 2024 SCEC Annual Meeting Poster #036 (PDF)

We use conversions from anisotropic contrasts observed in receiver functions at dense fault-crossing seismic networks to image fault-related fabric within the San Andreas Plate Boundary system. The method is sensitive to the depth of a contrast in dipping foliation and resolves the strike and dip direction of the stronger fabric across the contrast.
In the San Francisco Bay Area, four dense station lines oriented perpendicular to surface traces of the Hayward and Calaveras faults are used to investigate subsurface fault junction geometry and fault interaction with rock fabric. The strongest fabric contrasts are seen below the brittle-ductile transition inferred from the depth of seismicity, suggesting the fault continues as a shear structure below the seismogenic zone. The conversions show rapid changes with depth consistent with a steeply dipping fault that continues below the Hayward-Calaveras connection in the upper and middle crust and cluster at several depths where the fault may interact with pre-existing fabric. Most imaged fabric strikes and sense of dip align with those in the fault model, with a hint of orthogonal structures at one location.
We compare the fabric contrast amplitudes observed at these stations with other fault-crossing networks at Parkfield, in the San Jacinto fault zone, and in the Mojave. In all cases, nearly all arrivals show fabric strikes parallel to the surfaces in the fault model, interestingly dipping in the same direction on both sides of the fault. We observe a progressive strengthening of fabric contrast amplitude with fault maturity and cumulative displacement. Conversions at the San Andreas Fault at Parkfield have nearly double the amplitude of those found for the Hayward-Calaveras faults and the San Jacinto Fault Zone and are four times stronger than those from a line of stations crossing minor strike-slip faults in the Mojave. We interpret these observations as interactions of fault fabric development with pre-existing distributed rock fabric and reactivation of structures that may precede the present-day transform regime. These results have implications for our understanding of fault system evolution and general strain localization behavior at plate boundaries.

Key Words
faults, shear zones, anisotropy, receiver functions, fault models

Citation
Schulte-Pelkum, V., Kilb, D., & Becker, T. W. (2024, 09). Fault imaging past the brittle-ductile transition: Relationship between fault fabric and maturity from anisotropic receiver function analysis of the San Andreas plate boundary system . Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology