Japan Geoscience Union Meeting 2018

Presentation information

[EJ] Evening Poster

S (Solid Earth Sciences) » S-CG Complex & General

[S-CG59] Structure and evolution of Japanese islands - Formation of island arc systems and earthquake cycles

Tue. May 22, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Masanao Shinohara(Earthquake Research Institute, University of Tokyo), Masahiro Ishikawa(横浜国立大学大学院環境情報研究院, 共同), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Resilience)

[SCG59-P06] 2017 Deep seismic reflection profiling across the western part of the Hidaka collision zone and Ishikari foreland basin, Hokkaido, Japan

*Hiroshi Sato1, Tatsuya Ishiyama1, Naoko Kato1, Shinji Kawasaki2, Hidehiko shimizu2, Satoru Yokoi3, Susumu Abe3, Takeshi Sato4, Tetsuo No4, Shuichi Kodaira4 (1.Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo, 2.JGI, Inc., 3.Japan Petroleum Exploration Co., Ltd., 4.JAMSTEC)

Keywords:blind active fault, Ishikari foreland basin, Hidaka collision zone, deep seismic reflection profiling, velocity structure, Hokkaido

Integrated research project on the tsunami and earthquakes in the Sea of Japan aims to reveal the geometry of seisogenic and tsunamigenic source faults. For the sake of direct imaging of source faults, deep seismic reflection profiling has been performed across the western part of the Hidaka collision zone and Ishikari foreland basin. Beneath the Ishikari basin, active fault-related fold has been described, however, the deep geometry of faults is poorly understood. Geometry of source faults and velocity structure are significant for better esttmation of strong ground motions.

We collected deep seismic reflection data late June to middle July. A 68.5-km-long seismic line located across the western part of the Hidaka collision zone and extend along the Ishikari River. Seismic source was four vibroseis truck.Receiver system consist of off-line recorders (GSX and GSR, Geospace Inc.).Sampling rate was 4 msec. Receiver interval is 50 m and total fiexd 1358 channel is used. Shot interval was 50 m in the western half and 100m in the eastern part of the seismic line. Sweep frequency is 3 to 40 Hz. Number of sweep is 3 in the western part and 8 in the eastern part. To obtain deep image, Standard 50 sweeps was made every 4 km along the seismic line. A dynamite shot ( 100 kg of dynamite) was performed in the eastern end and 850 sweeps were made at the western end of the seismic line. Air-gun signals along the offshore extension of the seismic line were also recorded. Obtained seismic data were processed by CMP-method and MDRS method. P-wave velocity profile was analyzed by refraction tomography analysis.

Processed seismic section portrays the structural image down to 4.5 sec TWT (10 km in depth), reflection image is obtained down to 6 km in the eastern part of seismic section. In the western part of the Hidaka collision zone, crustal structure is marked by large amount of crustal shortening, including west-vergence thrust system. In the P-wave velocity profile, the western part of the Hidaka collision zone is characterized by thick low velocity rock units and top of Vp 5.5 km/s is located in 8-km-depth. On the contrary, beneath the Ishikari basin, Vp 5.5 km/s is located at 6 km in depth. In the Ishikari basin, deeper extension of fault, which forms the Nopporo anticline, is imaged down to 8 km in depth as eastward dipping thrust. This fault is located west of the eastern boundary fault of Ishikari lowland, forming an independent east dipping source fault.