S-wave Seismic Reflection Profiling in the Northern Area of the Omiya Upland: Uplift Rate during Late Quaternary Raised by the Ayasegawa Fault in the Kanto-heiya-hokuseien (Northwestern Margin of the Kanto Plain) Fault Zone, Central Japan

Abstract

We conducted an S-wave shallow seismic reflection profiling to reveal subsurface structure deformed by a blind thrust across the Ayasegawa fault in the northern area of the Omiya upland, central Japan. The NW-SE trending and SW dipping Ayasegawa fault is located in the southeastern part of the Kanto-heiya-hokuseien fault zone along the northwestern margin of the Kanto Plain, and is one of the nearest active faults to the Tokyo metropolitan area. This seismic profiling consisted of two survey lines which are Line 1 with a length of about 0.5km and Line 2 with that of about 0.9km. An automated S-wave plank hammering system (Hanshin Consultants Co., Ltd.) was used as seismic source. Both the standard shot intervals and group intervals of geophones were 2m. The common mid-point seismic reflection data was acquired by a digital telemetry recording system (JGI, Inc.). Folded strata, which were showing the deformation zone, were clearly recognized in the obtained post-stack migrated, depth converted seismic section. The width of the deformation zone with anticlinal uplift is about 0.8km along the seismic section. The relative uplift rate in the southwestern side of the Ayasegawa fault is 0.12-0.14mm/yr at the crest of the anticlinal uplift inside the deformation zone, and is 0.08-0.09mm/yr outside the deformation zone.