井戸沢断層の2011年福島県浜通りの地震に先行する断層活動：福島県いわき市塩ノ平地区におけるボーリング調査

抄録

We conducted a borehole survey across the western strand of the Itozawa fault at Shionohira, Iwaki City, Fukushima Prefecture where normal faulting surface rupture occurred at the April 11, 2011 Fukushima-ken hamadori earthquake (Iwaki earthquake, Mj7.0). Extracted core samples expose bedrock conglomerate, fluvial sand and gravel deposits, and artificial fill, from lower to upper, in ascending order. We found evidence for cumulative vertical displacement on the top of pre-2011 artificial fill, river channel gravel layer, and bedrock. Radiocarbon age was yielded from the organic-rich unit deposited on downthrown side. It suggests that one of the pre-2011 surface rupturing earthquakes occurred about 50,000 years ago. Small amount of vertical displacement at the pre-2011 earthquakes imply that coseismic slip and its deformation style at the paleo-earthquakes are different from those of the 2011 Iwaki earthquake.