Newly imaged shape of the deep seismic zone within the subducting Pacific plate beneath the Hokkaido corner, Japan‐Kurile arc‐arc junction

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Abstract
The geometry of the deep seismic zone associated with the subducting Pacific plate beneath the North American plate in the Hokkaido corner, Japan‐Kurile arc‐arc junction has been investigated based on the hypocenters accurately relocated by a dense local seismographic network with three‐dimensional P and S wave velocity structures. The model suggests that the lateral changes in the dip of the subduction zone are more rapid than previously believed. We found that the deep seismic zone in the Pacific plate dips at 20° to 30° on the side of the Japan arc, the dip angle increased in a narrow region around 143°–144°E by 10° to 20°, and the dip was 40° to 50° on the side of the Kurile arc. In the transitional zone around 143°–144°E an unusual distribution of microearthquakes has been discovered within the Pacific plate, which was located on a near‐vertical plane with an area of 50 × 100 km2 with strike that was perpendicular to the Kurile Trench axis. This feature was named the Tokachi‐Oki slab‐cracking zone. We suggest that this is a slab‐cracking zone torn by the extensional stress due to the rapid lateral changes in the dip of the subduction zone. The slab‐cracking zone may possibly grow into the large disruption or tear of the slab in the future.

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