日高衝上断層系前緑部・馬追丘陵西緑を横切る反射法地震探査

概要

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Due to the convergence between the Eurasian and North American/Okhotsk plates and possibly due to the westward motion of the Kuril fore arc sliver, a N-S trending fold-and-thrust belt was formed in the western part of the Hokkaido axial zone. In the frontal part of this thrust system, the Umaoi hills were formed as a fault-related fold. To reveal the activity of the thrusting in late Cenozoic and the mechanisms forming the Umaoi hills, high-resolution shallow seismic reflection profiling was undertaken across the western part of the Umaoi Hills. The seismic source was a Mini-vibrator truck, and shot and receiver intervals were 10 m. On the western flank of the Umaoi hills, west-dipping flexures are recognized on the geomorphic surfaces of late Quaternary ages. The seismic reflection profile demonstrates growth folding in the western flank of the Umaoi hills. The basal horizon of the growth strata almost corresponds to the top of the Nina Formation dated at 3.5 Ma. Thus the uplift of the Umaoi hills began after the deposition of the Nina Formation and continued in the late Quaternary. Near the summit of the Umaoi hills, a west dipping active fault (Izumizato fault) is cropping out. A seismic reflection profile shows that the Izumisato fault is a bedding-slip fault and the fault is not connected directly with the subsurface master thrust.
東京大学の論文