Abstract
Several active faults have been recognized based on displacement of geomorphic surfaces around the Konan terrace and Menuma lowland, Saitama Prefecture, northwestern Kanto Plain. In this area, a shallow damaging earthquake occurred (1931 Nishi Saitama earthquake, M=6.9) and the Median Tectonic Line (MTL) probably extends southeastward. However, the relationship between the active faults and the MTL is not well known, because the subsurface structure of them has not been investigated enough. We therefore conducted seismic reflection surveys and interpreted the seismic sections together with previously published geological and geophysical data. The following conclusions are reached:
(1) A similar deformation pattern of sedimentary layers is detected at 100m to 1km depth under both the flexure scarp of the Fukaya fault and its extension. This supports the southeast extension of the fault. The Fukaya fault has been active at least to the Kumagaya Ohashi of the Ara River based on the estimated ages of the deformed layers.
(2) Discontinuous reflectors are recognized at 100m to 1km depth near the Naranashi fault that has been inferred from surface geology. They are the material boundary MTL, which juxtaposes the Sanbagawa metamorphic rocks against the Neogene strata. But the material boundary MTL is not clear to the northeast of the Naranashi fault.
(3) The Fukaya fault and Konan fault are considered as a listric master fault dipping southwest and a secondary fault dipping northeast, respectively, and they have formed a popup. This is based on reflector dips, directions of fault displacement, stress field and fault lengths.
(4) The dip and strike of the Naranashi fault are similar to those of the source fault of the Nishi Saitama earthquake. This implies that the two faults correspond.
(5) A schematic model is proposed in which the Naranashi fault branches off northeastward at depths and continues upward to the Fukaya fault.
