Seafloor geodetic evidence of slip deficit near the southwestern Kuril Trench

Understanding near-trench interseismic deformation at subduction zones is crucial for assessing the hazards of giant earthquakes, especially trench-breaking tsunamigenic earthquakes, such as the 2011 Tohoku earthquake. However, interseismic locking preceding such trench-breaking tsunamigenic events remain poorly constrained due to the limited sensitivity of onshore geodetic observations. Here, we present seafloor geodetic observations along the southwestern Kuril trench, a region with the potential for large coseismic ruptures reaching the trench, as inferred from historical tsunami records. Our results show high slip-deficit rates near the trench during 2019–2024 and suggest that a slip deficit of 20.5–30.0 m may have accumulated over the past ~400 years, considering the long-term low seismic activity in this region. These findings imply future recurrence of a megathrust earthquake (Mw ~8.8) with rupture reaching the Kuril trench. Furthermore, they highlight the importance of seafloor geodetic monitoring for seismic and tsunami hazard assessment at subduction zones. Seafloor geodetic monitoring suggests the southwestern Kuril trench offshore Hokkaido, Japan, has accumulated substantial slip deficits through shallow plate interface coupling, which could indicate potential for a rare megathrust earthquake