Subsurface Geology at the Lake Mikata and Nakayama Lowlands, Fukui Prefecture, Central Japan: Implications on the Late Pleistocene Activity for the Mikata Fault Zone

Abstract

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 (14 C) 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 ＊ 立命館大学グローバル・イノベーション研究機構(歴史都市防災研究センター) ＊＊ 兵庫県立人と自然の博物館 ＊＊＊ 京都大学大学院理学研究科地球惑星科学専攻

Extreme flood events and their frequency variations during the middle to late-Holocene recorded in the sediment of Lake Suigetsu, central Japan

“…Lake Suigetsu has a water depth of 34 m, which is relatively deep considering its small size (4.2 km 2 ). This large water depth is maintained by the movement of the Mikata Fault, which bounds the east side of Lake Suigetsu and allows the bottom of Lake Suigetsu to subside at a rate of ~25 m over the last ~20 kyr (Nakae et al, 2002; Okada et al, 2010). This rate roughly balances the sedimentation rate within the lake (~0.7 mm/yr) during the Holocene (Nakagawa et al, 2012).…”