Interaction between seismic and aseismic slips along the Mendocino oceanic transform fault offshore northern California

Aseismic slip plays an important role in the earthquake cycle. On oceanic transform faults (OTFs), the majority of fault slip occurs aseismically which makes OTFs a natural laboratory to study the physical mechanism of aseismic slip and its interaction with seismic rupture. In this study, we apply state-of-the-art methods to analyze the aftershock sequences of three Mw > 6.5 earthquakes on the Mendocino OTF offshore California, including the 1994 and 2024 Mw 7.0 earthquakes, which are one of the largest OTF earthquakes so far. Compared with similar-magnitude continental strike-slip earthquakes, these large Mendocino OTF earthquakes have an order of magnitude fewer aftershocks, suggesting limited inter-event stress triggering. Nevertheless, their aftershock zones expanded with logarithmic time substantially beyond the co-seismic rupture zones, likely reflecting propagating aseismic slip transients. This behavior suggests that low-seismic-coupling segments can host aseismic slip transients triggered by earthquakes in neighboring high-seismic coupling segments while limiting the propagation of these seismic ruptures. In addition, the aftershocks of the 1994 and 2024 Mw 7.0 events, together with abundant background seismicity and previously identified repeating earthquakes, surround the rupture zones of the 2024 Mw 7.0 event and a Mw 6.1 earthquake in 2021, which suggests heterogeneous embedding of seismic slip patches within a predominantly aseismic slip zone in the eastern Mendocino OTF. Our observations reveal heterogeneous slip behavior and complex interaction between seismic and aseismic slips along the Mendocino OTF possibly due to variation in fault zone serpentinization and/or fluid-filled porosity.