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To better understand the structural heterogeneities under the Hokkaido subduction zone and its implications in the generation mechanism of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake, three-dimensional (3-D) high-resolution seismic velocities (Vp, Vs) and Poisson's ratio (σ) images were determined using 273,078 high-quality travel-times of P- and S-wave source-receiver pairs from both onshore and offshore earthquakes. Our results reveal that the 2018 eastern Iburi earthquake likely occurred at a presumed blind thrust fault with sharply varying seismic parameters. A strong low-V and high-σ body extending to 120-km depth was clearly observed northwest of the 2018 hypocenter, indicating the presence of fluids in the mantle wedge associated with the Pacific slab dehydration. Sharp contrasted structures of Vp, Vs, and σ were clearly imaged across the hypocenter of the 2018 Iburi earthquake. The sandwiched rigid zone with high-V and low-σ anomalies in the hypocenter was considered to be the subsidence of brittle crust, which is thickened and deepened due to the colliding system of the northeastern Japan and the Kuril arcs. The mechanical strength in the source rocks with high-V and low-σ was stronger than in surrounding regions where high pore-fluid pressure accumulated. Such a fluid-pressurized regime beside the hypocenter reduced the rock effective stress on the source rocks, lowered the threshold for fault failure, and consequently led to brittle failure. Fluid intrusion in the back-arc seisomogenic layer played a key role in intraplate earthquake generation in the Hokkaido subduction zone and elsewhere.
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