Super-Deep Seismic Reflection Profiling in the Southern Part of Hidaka Collision Zone, Hokkaido, Japan

The Kuril Arc has been colliding against the Northeast Japan Arc around the Hidaka mountain range, Hokkaido, Japan. Furthermore the Pacific plate is subducting north-northwestward beneath the Hidaka Collision Zone (HCZ). Recent seismic reflection experiments in this area have revealed that the delamination-wedge structure is formed by Arc-Arc Collision beneath the HCZ (Ito et.al.,1998,2000,Tsumura,1999). However the relationship between the delaminated lower portion of the lower crust and the subducting Pacific plate has remained unclear. In order to reveal the detailed 3-dimensional structure of the HCZ, super-deep seismic reflection experiments were conducted in the southern part of the HCZ, Samani-cho, Hokkido, in September, 2000. Two seismic lines were deployed: Line1 is parallel to the subducting trend of the Pacific plate (NW-SE), and Line2 is nearly parallel to the trend of the Arc-Arc Collision (NE-SW). Line1 is almost perpendicular to Line2. The length of Line1 and Line2 is about 16km and 8km, respectively. Five vibrators were operated, obtaining seismic data of the both lines simultaneously. Here we report the results from 2D seismic profiles. The main features recognized from this survey can be summarized as follows: For Line1: 1) At shallower portion, southeast-dipping reflections are truncating horizontal reflections, which are seen at 3 sec two-way travel time (TWT) in the northwestern half of the survey line. 2) A northwest-dipping strong reflection at 14 sec TWT descends down from the southeastern end of the survey line to the center, whereas gently southeastern-dipping reflection is seen at 14 sec TWT in the northwestern half. 3) Intermittent events are found at 7 sec. , 9 sec. , 12 sec. , and 16 sec TWT. For Line2: 4) Above 5 sec TWT, horizontal reflections are predominant but complicated. 5) There are gently southwest-dipping reflections at 8 to 9 sec TWT, nearly horizontal reflections at 13 sec to 16 sec TWT, but they are discontinuous and unclear. The northwest-dipping reflector at 14 sec seen in Line1 probably corresponds to the upper boundary of the Pacific plate, which is suggested by the study of seismic activity in this area and shows the subducting trend of the Pacific plate. The existence of the reflector derived from the subducting Pacific plate indicates that seismic waves generated by the vibrators passed through the delaminated lower portion of the lower crust. This provides us good prospects that the further improvement of data processing must reveal the relationship between the delaminated lower portion of the lower crust and the subducting Pacific plate.