濁川カルデラの内部構造

抄録

The Nigorikawa Caldera in southwest Hokkaido, Japan, is 3 km in diameter at the outer rim. Drilling data from 42 geothermal wells of up to -3,000 masl (m above sea level) has been used to study the internal structure of the caldera. Interpretation of the data shows an angular funnel shape, with a wide upper region (3×2.5 km) tapering to a narrower lower region (0.7×0.5 km). The shear zone is the same shape as the caldera, that is, rectangular with a NE-SW elongation. The caldera is infilled with vent-fill material, lake and alluvial deposits, landslide deposits, and post-caldera intrusions. The vent-fill material is a gray, non-welded lapilli tuff and tuff breccia, which homogeneously includes accidental lithics and shattered fragments, which were sheared during pyroclastic eruption, as well as accretionary lapilli occurring up to -824 masl. The vent-fill is intercalated with many lithic bands or lithic dominant zones that dip toward the caldera center. No large fault displacement can be recognized around the caldera wall. The Nigorikawa Caldera was formed ca 12,000 years ago by violent pyroclastic flow eruption, fall-back, and the following subsidence by compaction with degassing.