Large Mendocino transform fault earthquakes’ foreshock and aftershock characteristics

Compared to continental strike-slip faults, oceanic transform faults (OTFs) are thought to mainly slip aseismically and host significantly more foreshocks likely triggered by precursory aseismic slip which enhance the mainshocks' short-term predictability. However, long-term high-resolution observational constraints remain limited. In December 2024, one of the largest ever OTF earthquakes occurred offshore California on the Mendocino OTF. Here we show that compared to similar-magnitude continental strike-slip earthquakes, this moment magnitude (Mw) 7.0 earthquake has an order of magnitude fewer aftershocks which suggests limited inter-event stress triggering. Nevertheless, the aftershock zone expanded with logarithmic time substantially beyond the mainshock's co-seismic rupture zone, hence likely reflects propagating aseismic slip transients. However, foreshock activity within the mainshock's rupture zone is limited and does not indicate any accelerating aseismic slip in the preceding 30 days. The 2016 Mw 6.6 and 1994 Mw 7.0 Mendocino OTF earthquakes share similar aftershock and foreshock characteristics. The 15 historical Mw>5.5 mainshocks also have few foreshocks on average. Our results demonstrate that low-seismic-coupling OTF segments can host aseismic slip transients triggered by earthquakes on neighboring segments while inhibiting these seismic ruptures’ propagation, and enhanced foreshock activity is not a general characteristic of OTFs despite prevalent aseismic slip.