オフ・フォールト古地震学的手法から推定される完新世中期の国府津―松田断層帯の活動履歴

抄録

　 Middle Holocene palaeoseismic history of Kozu-Matsuda Fault Zone, central Japan, was studied by an off-fault paleoseismological survey. The Kozu-Matsuda Fault Zone is located between the Ashigara Plain and the Oiso Hills, and is a reverse fault uplifting the Oiso Hills relatively with quite high slip rate (～3.5m/ky). We conducted analyses on sedimentary facies, diatom fossil assemblages and tephra layers and radiocarbon dating of three boring cores, GS-ASG-3, M3, B-11 cores, in southeastern part of the Ashigara Plain. In the study area, freshwater marsh or channel environment occurred until ca. 7.9-8.1ka. This suggests that regressive stage began before ca. 7.9-8.1ka because sea-level rising became slower than before. Comparison of sedimentary environmental changes of boring cores suggests two coseismic subsidence events. These events were characterized by drastic environmental changes from freshwater marsh to tidal flat in the regressive stage in common among boring cores. Timings of these events were suggested to be ca. 7.6ka and ca. 6.8-6.9ka respectively. In addition, both of these events are corresponding to the coseismic events reported previously around the Oiso Hills.