Seismic Structure Characterizing the Large Shallow Slip Zone of the 17th‐Century Kuril Earthquake

The southern Kuril Trench subduction zone experienced a M9‐class megathrust earthquake in the 17th century, and another is highly probable within the next 30 years. This earthquake likely exhibited large coseismic slip at the shallower plate boundary fault, causing a devastating tsunami, while the deeper plate boundary experienced smaller slip, similar to the 2011 Tohoku‐oki earthquake. To investigate the structural factors behind the contrasting slip behaviors, we conducted a controlled‐source seismic survey across the Kuril Trench axis. We found that the boundary between the largely‐slipped‐shallow and small‐slipped‐deeper faults aligns with a Vp transition zone in the overriding plate, characterized by sharp landward Vp increase from the low‐Vp frontal wedge (0–30 km from the trench) to the high‐Vp island arc crust (>60 km), and associated with discontinuous near‐horizontal reflectors zone (RZ) above the plate boundary, exhibiting negative polarity. These findings suggest that the slip behavior boundary correlates with sudden trenchward rigidity reduction along the plate boundary, as found in other areas where seismic rupture extended to the trench. The negative‐polarity reflections in the RZ, together with the plate boundary, imply high‐pore‐pressure created by abundant fluid supply from accreted or underplated sediments consolidation and/or dehydration, and oceanic crustal crack cementation. This suggests a weak fault strength at the plate boundary beneath the transition zone, indicating weak mechanical coupling similar to the shallow fault in Tohoku‐oki. Similar to the Tohoku earthquake, a slip‐to‐the‐trench rupture may be triggered by releasing large strain energy accumulated along the deeper fault beneath the high‐rigidity crust.