Low-Frequency Earthquakes Track the Evolution of a Captured Slab Fragment at the Mendocino Triple Junction

David R. Shelly, Amanda M. Thomas, Kathryn Materna, & Robert J. Skoumal

Submitted September 7, 2025, SCEC Contribution #14441, 2025 SCEC Annual Meeting Poster #TBD

The transition between the San Andreas Fault and the Cascadia subduction zone at the Mendocino triple junction is critical to understanding the tectonics and seismic hazard associated with these fault systems. Despite its significance, this complex region, where the Pacific, Gorda, and North American plates meet, remains poorly understood. A primary uncertainty is the location and nature of the southern edge of Gorda subduction at depth. Contrasting models propose that the southern edge of the northward-migrating Gorda slab is trailed either by upwelling asthenosphere filling a slab gap or by a fragment of the former Farallon slab now captured by the Pacific plate. No direct constraints on the associated kinematics have been available to distinguish these models. Here, we analyze tidal sensitivity and P-wave motions of a recently identified zone of tectonic tremor and low-frequency earthquakes (LFEs), showing that these events are generated by dipping, strike-slip motion at depth. This slip matches the expected relative motion between the Pacific and Gorda plates, arguing that the LFEs occur on a previously unrecognized, buried extension of the Mendocino transform fault, validating the captured slab hypothesis. A zone of comparatively low seismic velocity between Gorda slab seismicity and the LFEs may represent North American plate accretionary prism material that was progressively subducted at the southernmost edge of Cascadia, with the primary subduction interface ~10 km above the slab at its southern edge near the coast. These findings lend plausibility to unconfirmed captured slab models proposed to underlie the San Andreas system farther south, while contrasting with typical assumptions that subduction faults follow the petrologic slab boundaries. Our results provide a new tectonic framework that describes the southern edge of Cascadia subduction, with implications for potential interactions between Cascadia and San Andreas faulting.

Key Words
tremor, tectonics, subduction, Cascadia, San Andreas

Citation
Shelly, D. R., Thomas, A. M., Materna, K., & Skoumal, R. J. (2025, 09). Low-Frequency Earthquakes Track the Evolution of a Captured Slab Fragment at the Mendocino Triple Junction. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)