Fault Fabric in Relation to Seismicity and Fault Geometry in the San Andreas - Cascadia Transition Zone

Debi Kilb, & Vera Schulte-Pelkum

Submitted September 7, 2025, SCEC Contribution #14626, 2025 SCEC Annual Meeting Poster #TBD

The transition zone from the San Andreas system transform plate boundary to the Cascadia subduction zone in northern California is a present-day snapshot of the long-term northward migration of the Mendocino triple junction. This offers an opportunity to study the transition in deformation style in space as a possible proxy for the transition over time. We image rock deformation fabric by analyzing receiver function arrivals from contrasts in anisotropy, mapping strike, depth, and strength of fabric contrasts. In Central California and the Bay Area, rock fabric is parallel to present-day major strike-slip fault traces and Community Fault Model (CFM) surfaces. However, while present-day fault surfaces are dominated by steep to vertical dips, the rock fabric shows consistent intermediate dips to the northeast. We interpret this as an indication that the present-day vertical strike-slip motion may have initiated on intermediate-dipping formerly convergent faults that proceeded to steepen over time in the transform regime. Farther north in the onshore portion of the Mendocino triple junction, mapped surface faults and the CFM show NW-SE striking faults, while seismicity and receiver function results show an additional set of conjugate ENE-WSW striking features. We explore relationships between fabric contrasts from receiver functions, CFM fault surfaces, and seismicity from published as well as two preliminary machine learning-based catalogs by other groups to disentangle the structure of the subducting Gorda plate and the overriding plate.

Key Words
Cascadia, San Andreas, Mendocino Triple Junction, Rock Fabric, Receiver Functions, CFM, ML, Gorda Plate

Citation
Kilb, D., & Schulte-Pelkum, V. (2025, 09). Fault Fabric in Relation to Seismicity and Fault Geometry in the San Andreas - Cascadia Transition Zone. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology