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 runalong 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 thesouthern 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.
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