Paleoseismic events of the Itozawa fault prior to the 2011 Fukushima-ken-hamadori earthquake revealed from a borehole survey at Shionohira, Iwaki City, Fukushima Prefecture, Japan

Abstract

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.