The 2011 Tohoku-Oki offshore subduction earthquake (M w 9.0) triggered many normal-type earthquakes inland in northeastern Japan. Among these were two very similar normal-faulting earthquakes in 2011 (M w 5.8) and 2016 (M w 5.9), which created surface ruptures along the newly named Mochiyama fault within the southern Abukuma Mountains, northeastern Japan, where no active faults had been previously mapped by interpretation of aerial photographs. We conducted field surveys in this area immediately after both earthquakes, and we performed trench excavations and observations of fault fracture zones after the 2016 event. These activities were complemented by an interferometric synthetic aperture radar analysis that mapped the areas of deformation and locations of surface discontinuities for both events. The combined results document the coseismic behavior of the Mochiyama fault during both events. Subtle tectonic geomorphic features associated with the fault were evident in a lidar digital elevation model of the area, and layered structures of gouge were documented in the field. These lines of evidence indicate repeated activity at shallow crustal levels and the possibility of Quaternary activity. In addition, our trench excavations revealed at least one faulting event before 2011. Our comparison of paleoseismic records on this and two other normal faults in the Abukuma Mountains suggests that great earthquakes in the Japan Trench supercycle of 500-700 years do not consistently trigger ruptures on these faults, and the case of 2011, in which the Tohoku-Oki megathrust earthquake triggered all three faults, is a rare occurrence.

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図 1 中国地方西部の主要な活断層の分布 島根県西部から山口県中南部にかけて大原湖 弥畝山西断 層系を構成する活断層が存在する (金折・遠田 1) を一部修正) ． 広島県中部には五日市断層帯が存在し 2) ， 3) ，その南西に岩 国断層帯が存在する 2) ， 4) ， 5) ．山口県北西部には菊川断層帯が 存在する 4) ， 6) ．さらに，瀬戸内海には周防灘断層群 7) ～9) と安 芸灘断層群 2) ， 1 0) ， 1 1) が確認されている． ＊山口大学大学院理工学研究科 Gr a d u a t eS c h o o lo fS c i e n c e a n dE n g i n e e r i n g , Ya ma g u c h i Un i v e r s i t y (会員)

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