Low-frequency earthquakes track the motion of a captured slab fragment.

Accurate tectonic models are essential for assessing seismic hazard and fault interactions. However, the plate configuration at the complex Mendocino triple junction, where the San Andreas Fault and the Cascadia subduction zone meet, remains uncertain. We analyzed fault slip associated with a recently identified zone of tectonic tremor and low-frequency earthquakes (LFEs) near the southern edge of the subducting Gorda slab. Based on tidal sensitivity and P-wave first motions, we show that the LFEs are generated by dipping, strike-slip motion. This suggests that a former Farallon slab fragment, now captured by the Pacific plate, is translating northward beneath westernmost North America. This geometry effectively extends the slab interface fault, challenging prevailing interpretations of slab window formation and creating a potential unaccounted earthquake hazard in this region. Editor's summary: At triple junctions, where three tectonic plates meet, the mixture of slab motions makes earthquake hazard assessments a challenge. The Mendocino triple junction absorbs stress between the Gorda and two of Earth's largest plates, the North American and Pacific, creating a zone of high seismicity that affects risk along the adjacent San Andreas and Cascadia fault zones. Shelly et al. applied first-motion analysis with tidal sensitivity tests to small earthquakes in the region. A pattern of slip emerged indicating that at the triple junction, remnants of an older plate may be moving with the Pacific plate beneath North America. —Angela Hessler [ABSTRACT FROM AUTHOR]