相対的海水準の急激な上昇イベントが示唆する養老断層系の完新世活動―濃尾平野西縁におけるボーリングコアの解析から―

Abstract

A 6000-year history of relative sea-level change on the western Nobi plain, at the footwall side of the Yoro fault, was reconstructed from three drilling cores based on analyses of sedimentary facies, grain-size distributions and electrical conductivity (EC), as well as 20 14C age determination. The Yoro fault comprises the northern part of the Yoro fault system. These cores show a prograding delta sequence. Two intervals characterized by fine deposits, mainly composed of fine sand, and high EC value are identified in sandy delta front deposits showing a general coarsening upward sequence. In synchronous with these changes in the delta front, a rapid rise of EC value is recognized in the abandoned channel fill deposits on the delta plain. These data suggest that temporal sea-level rise events occurred during the progradation process of the delta system. A probable cause of these events is coseismic subsidence. These subsidence events are dated at around 5600-4700 cal BP and 4600-3800 cal BP, respectively. The above-mentioned subsidence events broadly correspond with previously known faulting events at the Kuwana fault to the south of the Yoro fault. Evidence for the faulting of the Yoro and Kuwana faults associated with two historical earthquakes (AD 745 and AD 1586) accompanied by coseismic subsidence and synchronicity of the subsidence events identified in this study and faulting events at the Kuwana faults suggest that subsidence events identified in this study are caused by faulting at the Yoro fault. These results are consistent with the notion that the Yoro and Kuwana faults comprise a behavioral segment in the Yoro fault system.