Abstract
This article provides the preliminary results of literature and field survey in terms of the long-term stability in and around the Horonobe area. The Horonobe area is situated in the Tenpoku Basin, and is dominated by the Neogene to Quaternary sedimentary sequences. Based on the time-staratigraphy and sedimentary analysis, depositional area has migrated from east to west in the basin. The distribution of hypocenters for micro-earthquake, geologic structures, and the Quaternary sediments indicate that present-day activity has been localized with the western part of the Horonobe area. In addition, the seismic reflection profiles of the Horonobe area show the growth structures of the fold-and-thrust belt of central Hokkaido suggesting that the ongoing-tectonics (neotectonics) in the study area has begun in three to two million years ago.
The Horonobe area has widespread distribution of the marine terrace deposits, which are correlated to the marine oxygen isotope stages (MIS) 9 through 5c. The elevation of these marine-terrace surfaces on axial part is higher than that of on limb in the Sarobetsu Anticline at the western part of the Horonobe area. The former shoreline of MIS 1 proceeded ca. 14 km away from that of MIS 7 although there is no great difference in global-scale sea-level in each stage. The active folds are distributed in area to be changed into land in MIS 1. In order to assess the long-term stability of geological environments in the test-field Horonobe area it is important to consider the faulting and folding effects to the uplifting, subsidence, migration of former shoreline and so on.
