Shallow seismic reflection profiling across the central part of the Tokachi active fault zone, Hokkaido, Japan

Abstract

The Tokachi active fault zone, a chain of active faults and flextures 80 km long from north to south, exists as the topographic boundary between the Tokachi Plain and the eastern uplands, Hokkaido, Japan. We carried out shallow seismic reflection profiling using a mini-vibrator to clarify the implications between surface deformation and subsurface structures of this fault zone at the most highly active area of the Makubetsu Plateau. The results obtained are as follows. Two distinct flexures are recognized in the seismic reflection profile. The western flexure is about 2 km wide and has at least a 0.25 to 0.30 second vertical displacement, whose sense is concordant with the deformation of the surface of the upper fluvial terraces geomorphologically identified as the Tobatsugawa fault. The eastern one also has the same deformation pattern of fluvial terraces as that geomorphologically identified as the Itaira fault. However, both geomorphic flexures are below one-sixth to one-third of the subsurface flexural zone in width. The seismic reflection method is necessarily required, to evaluate the net displacement of active flexural zone, in addition to morphotectonic observations