Conjugate faulting in the Gorda plate and its influence on the southern Cascadia subduction thrust

Bo Rong, Weiqiang Zhu, & Roland Bürgmann

Submitted September 7, 2025, SCEC Contribution #14722, 2025 SCEC Annual Meeting Poster #TBD

Fault zones in the oceanic lithosphere can play important roles in the subduction process and may act as fluid pathways, thereby influencing slip behavior on the megathrust. Despite their importance, intraplate fault zones are often insufficiently understood, especially in complex tectonic settings such as near triple junctions. In northern California, the Mendocino triple junction (MTJ) is characterized by exceptionally high seismicity rates, yet the structures and properties of fault networks in this region remain poorly understood due to the complex deformation process. To illuminate the distributed faulting, we construct a high-resolution earthquake catalog of this region from 2000 to 2024 by applying a machine-learning-based workflow, QuakeFlow. We focus on the fault structural complexity in the subducting Gorda plate and divide the seismicity into intraslab and plate interface categories based on the slab geometry. In the lower Gorda plate, we observe abundant conjugate faults and infer that they may result both from reactivation of structures formed at the Gorda Ridge and newly formed faults due to high stress levels associated with the impinging moving Pacific plate. Near the subduction interface at depths of 15 - 20 km, we detect several earthquake swarms that share similar fault structures and show spatial migrations with a hydraulic diffusivity of about 0.015 m2/s if assuming a role of underlying pore pressure diffusion. The swarm activities may indicate a hydrated Gorda subduction thrust with elevated upper plate permeability. The seismic evidence across different depths collectively suggests a highly fractured Gorda plate that facilitates the transport and release of subducted fluids. Our results have important implications for understanding the fault mechanics and megathrust behavior in the MTJ.

Citation
Rong, B., Zhu, W., & Bürgmann, R. (2025, 09). Conjugate faulting in the Gorda plate and its influence on the southern Cascadia subduction thrust. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology