Holocene paleoseismic history of the Yunodake fault ruptured by the 2011 Fukushima-ken Hamadori earthquake, Fukushima Prefecture, Japan

The 11 March 2011 Mw 9.0 Tohoku earthquake, which occurred along the Japan Trench, induced an unusual shallow normal-faulting earthquake: the 11 April 2011 Fukushima-ken Hamadori earthquake (Mw 6.6) near the Pacific coast in southeastern Fukushima Prefecture. Remarkable surface ruptures appeared along the NW-trending Yunodake fault, the NNW-trending Itozawa fault (west) (also known as the Shionohira fault). Although the Yunodake fault is mapped with a normal down-to-the-SW sense of slip, its paleoseismic history is not clear. A trenching survey on the northwestern part of the Yunodake fault is reported here that reveals its Holocene paleoearthquake history. The trench wall beneath the 2011 ruptures exposed fissures, open cracks, stratigraphic offsets, and a succession of cumulatively inclined Holocene sediment layers. Detailed observations of the trench walls and radiocarbon dating reveal that the two latest previous surface-rupturing earthquakes occurred between 800 and 6200 years ago and between 5900 and 6700 years ago respectively. The estimated recurrence interval of the Yunodake fault is 800 to 5900 years at this location.