Shallow seismic section in the central Kanto plain, to the north of Tokyo metropolitan area, Japan

Shallow seismic reflection surveys were carried out in the central part of the Kanto plain, 40 km northwest of Tokyo, Japan. The survey target ranges about from 50 m to 500 m in depth. The final purpose of the surveys is to reveal the relationship between the subsurface structure and the distribution and flow of the underground water in this area. The survey lines were divided into Kawagoe1(length of CMP line: 8km), Okegawa1(5.3km), Shobu1(8.7km) and Kazo1(6.3km). They line up southwest to northeast from the Iruma upland to the Kazo lowland via the Arakawa lowland and Omiya upland. The survey lines cross the Arakawa fault, the Ayasegawa fault and the Kuki fault. They are considered active, but only the Aysegawa fault has fault topography and is confirmed by subsurface structure. The total length of the lines is about 27km and there is a 2 km long opening between the Okegawa1 and the Shobu1, because of dense population and heavy traffic. Survey parameters are as follows. Seismic source: one EnviroVib or MiniVib or Mini Impactor, shot interval: 2.5m or 10m, sweep frequency: 15-120Hz, sweep length: 13s, receiver: UltraMark2, receiver interval: 10m, elements: 6 bunching, recording instrument: DAS-1, number of channel: 144, listening length: 3.3s, spread: shots from the edge to the 48th point of 144 fixed receiver points, maximum offset: between 1440m and 960m. The data were processed to make seismic sections for each survey line by conventional CMP method. Then the sections were cut and pasted into a series of seismic sections. Many continuous reflectors are imaged between several ten meters and 1 km in depth in the whole seismic section. Reflectors are discontinuous below 1 km, probably because of lack of source energy. In the Iruma upland and Arakawa lowland, the Pliocene and Pleistocene units thicken northeastward, indicating that sedimentation has been synchronous with northeastward tilting of the underlying basement rocks. Undulation and bending of reflectors suggest basement faults in the Arakawa lowland. Near the northern edge of the Omiya upland, deformation, vertical gap, disturbance and tilting of reflectors are perceived around the flexure scarp of the Ayasegawa fault. This indicates that displacement of the Ayasegawa fault in the basement has been deformed the sediment layers over 2 km in width crossing the flexure scarp. In the Kazo lowland, neither deformation nor gap of reflectors are perceived around the inferred Kuki fault in the seismic section.