Late Quaternary Geomorphology of the Tonami Plain and Activity of the Tonami-Heiya Fault Zone, Toyama Prefecture, Central Japan

Abstract

The Holocene alluvial fans and flood plains formed by the Sho and Oyabe Rivers spread out in the Tonami plain from the central through the northern parts in the Toyama Prefecture, central Japan. Along the foot of the surrounding mountains and hills, higher, middle and lower terraces of late Pleistocene-Holocene in age are distributed. These terraces have been displaced, by the reverse dip-slip activities of Tonami-heiya fault zone in a sense of upheaval in the mountains side, even during the Holocene time. We examined stratigraphic cross section utilizing borehole data and geomorphologically analyzed 5 m-DEM data in order to elucidate the fault trace of the Isurugi fault which Tonami-heiya fault zone. As the results, the northern segment of Isurugi fault seems to run along the northeastern foot of Hodatsu Hills and extends underground through the lower-most Oyabe River into the Toyama Bay. Consequently, its total length reaches about 30 km. In the southern segment, a continuous fault scarplet was recognized to cut across the lower dissected fans. The slip-rate of Isurugi fault is estimated to be 0.31 - 0.64 m/kyr. In the Hokuriku region, reverse faulting and related folding with strike in a NE-SW direction have occurred during the late Quaternary. The hinge line of block movement due to the activities of the Tonami-heiya fault zone is revealed to have shifted from the mountain side into the plain side within the Holocene time. In conclusion, the Quaternary folding and faulting associated with the crustal warping at a wavelength of about 20 km is currently in progress, causing both the subsidence of Tonami plain and the upheaval of surrounding mountains and hills.