[SCG59-P06] 2017 Deep seismic reflection profiling across the western part of the Hidaka collision zone and Ishikari foreland basin, Hokkaido, Japan
Keywords:blind active fault, Ishikari foreland basin, Hidaka collision zone, deep seismic reflection profiling, velocity structure, Hokkaido
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.