福井県，三方湖および中山低地の地下地質と三方断層帯の活動解明

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The Lake Mikata and Nakayama lowlands are situated along the central coast of Wakasa Bay, at the center of the Japan Sea side of Honshu Island, and fringed by the Mikata fault zone at the east side. The 1662 Kanbun Great Earthquake (estimated magnitude of 7.2 to 7.6) occurred there with vertical crustal movements up to 3 to 4 meters. Submerged topography with thick alluvial deposits in the lowlands might have been caused by Late Quaternary crustal movements of the Mikata fault zone. We analyzed the subsurface geology of the lowlands using many drilling cores recently obtained for scientific purpose such as the 60-m long MK09 core and highway construction (represented by NEXCO core). The main results are summarized below. (1) Subsurface geology beneath the eastern basin and eastern coast of Lake Mikata is clarified by detailed investigations of many drilling cores up to 100 m deep including the NEXCO core. Many radiocarbon (14C) dating and tephrochronological correlations have elucidated the standard chronostratigraphy during the Late Quaternary in this area. At least 100-m thick sediments have deposited continuously for about 130 kyr, recording climatic and seismic events etc. during the last glacial period. (2) The MK09 core is composed of a repeated coarsening-upward sequence with rapid facies changes at unit boundaries. We infer that these units reflect the rapid rise of a relative level of a paleo-lake and subsequent progradation of alluvial fans. The mean interval of individual units is estimated to be about 7.7 kyr from the sedimentary age of each unit, and is considered to be an upper limit of the recurrence interval of the Mikata fault zone. (3) The Mikata fault is recognized for observations of new outcrops and core inspections at the northeastern part of the Nakayama lowland. The vertical displacement across the Mikata fault is inferred to be 30-35 m, and probably reached further after formation of a higher terrace. (4) The concealed Mikata fault extending N-S parallel to the Mikata fault on the east, is estimated from our drilling data. On the basis of the displacements of tephra horizons such as the Aira-Tn volcanic ash beds (AT) in deposits of the Nakayama lowland, it is possible that activity of this fault is equivalent to or larger than that of the Mikata fault. Further detailed studies by deeper drilling cores and seismic refraction surveys are required to clarify the E-W direction underground structure across the Mikata fault zone. They will unravel major development histories of this lowland with many datable materials.