Detailed seismic velocity structure beneath the Hokkaido corner, NE Japan: Collision process of the forearc sliver

1. Introduction In south-eastern Hokkaido, the Kuril forearc sliver is colliding with the northeastern Japan arc due to the oblique subduction of the Pacific plate. This collision causes the formation of the Hidaka mountain range since the late Miocene (Kimura, 1986) and delamination of the lower-crust materials of the Kuril forearc sliver, which would be expected to descend into the mantle wedge below (e.g., Ito 2000; Ito and Iwasaki, 2002). In this study, we precisely investigated the three-dimensional seismic velocity structure beneath the Hokkaido corner to examine the collision of two forearcs in this area by using both of data from a dense temporary seismic network deployed in this area (Katsumata et al. [2006]) and those from the Kiban observation network, which covers the entire Japanese Islands with a station separation of 15-20 km. 2. Data and method The double-difference tomography method (Zhang and Thurber, 2003; 2006) was applied to a large number of arrival time data of 201,527 for P-waves and 150,963 for S-waves that were recorded at 125 stations from 10,971 earthquakes that occurred from 1999 to 2010. Grid intervals were set at 10 km in the along-arc direction, 12.5 km perpendicular to it, and 5-10 km in the vertical direction. 3. Results and discussion Inhomogeneous seismic velocity structure was clearly imaged in the Hokkaido corner at depths of 0-120 km. A high-velocity anomaly of P- and S- waves with a volume of 20 km x 90 km x 35km was detected just beneath the main zone of the Hidaka metamorphic belt at depths of 0-35 km. This high-velocity anomaly is continuously distributed from the depths of the mantle wedge to the surface. The western edge of the anomaly exactly corresponds to the Hidaka main thrust (HMT) at the surface. The highest velocity value in the anomaly corresponds to those of the uppermost mantle material (e.g. peridotite). The location of them at depths of 0-35km is also consistent with that of the Horoman-Peridotite belt, which is located at the southwestern edge of the main zone of the Hidaka metamorphic belt.On the other hand, a low-velocity anomaly of P- and S- waves with a volume of 80 km x 100 km x 50 km is distributed to the west of the Hidaka metamorphic belt at depths of 35-90km. This low-velocity anomaly seems to be continuously distributed from the continental crust of the NE Japan forearc. The velocity values of this low-V anomaly correspond to those of crustal materials, which is consistent with results of the tomographic study of Kita et al. [2010, EPSL] and Takanami et al. [1982]. Comparison with the results of seismic reflection surveys of Ito [2000] shows that the boundary between anomalous high-velocity mantle materials and low-velocity continental crustal materials just beneath the Hidaka main thrust (HMT) presently obtained is exactly consistent with the locations of reflection planes of their study. Moreover, our study also suggests that the anomalous low-velocity crustal materials at the mantle wedge depth appears to belong to the NE Japan forearc crust, which does not support the expectation of the previous studies that the delaminated lower-crust materials of the Kuril forearc sliver descends into the mantle wedge due to the collision.