The southern Kuril Trench subduction zone experienced a M9‐class megathrust earthquake in the 17th century, and another is highly probable
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. Plain Language Summary: To understand the large shallow slip that triggered tsunamis in the 17th‐century M9‐class Kuril earthquake, we estimated the crustal structure in its source area and identified a distinct change in the overriding plate at the boundary of the deep small‐slip and shallow large‐slip faults of the 17th‐century earthquake. This transition zone exhibits a distinct lateral Vp gradient, unlike the trenchward frontal sedimentary wedge and the landward island arc crust, coexisting with discontinuous near‐horizontal reflectors zone (RZ), and its negative‐polarity reflectivity, together with the plate boundary, implies high pore‐fluid‐pressure. Low rigidity in the transition zone and frontal wedge indicates prominent tsunamigenic slip behavior of the plate boundary fault beneath these structures. The spatial correspondence of the high‐pore‐pressure RZ with the boundary of deep small‐slip and shallow large‐slip faults mirrors the situation observed in the Tohoku‐oki earthquake, showing a similar RZ located at the boundary of the deep‐strong and shallow‐weak coupling regions of the megathrust, suggesting that the RZ likely marks the updip limit of a strongly coupled zone along the megathrust. We infer that the transition zone played a key role in controlling the large shallow slips of the 17th‐century Kuril earthquake, similar to the mechanisms proposed for the Tohoku‐oki earthquake. Key Points: Lateral P‐wave velocity transition was identified above shallow–deep megathrust boundary involved in the 17th‐century Kuril earthquakeNegative‐polarity reflectivity in transition zone suggests potential high pore pressure in low rigidity body above shallow plate boundaryLow rigidity and potential high pore pressure in transition zone suggest weak mechanical coupling on plate boundary updip from this zone [ABSTRACT FROM AUTHOR]
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