Scientists from the USGS, UC Davis and the University of Colorado Boulder analyzed low-frequency earthquakes in the Mendocino Triple Junction region, where the San Andreas fault meets Cascadia. These earthquakes, thousands of times weaker than those we feel on the surface, follow the movement of a slab fragment that has broken off from the North American Plate and is being pulled down with the Gordana Plate under North America[1][4][5]. The new model describes five moving parts, including two not visible from the surface, and explains the shallowness of the 1992 earthquake because the subduction zone is less deep than previously thought[1][5]. Low-frequency events occur on a tilted strike-slip surface at a depth that corresponds to the assumed movement between the Pacific and Gordana plates, thereby confirming the hypothesis of a trapped fragment of the former Farallon plate[4]. The fractures do not twist along the leading edge of the subducting plate as previously thought, and the plate boundary is not where it was thought[1][5]. The study provides a new tectonic framework for the southern Cascadia margin with implications for interactions with the San Andreas Fault[4]. The research was supported by the National Science Foundation and was published in Science on January 15, 2026[1].