Abstract

The 2016 Mw 7.1 (Mj 7.3) Kumamoto earthquake, which occurred in central Kyushu Island, southwest Japan, produced an 40kmlong surface rupture along the Hinagu–Futagawa fault zone (HFFZ) that cuts across Aso caldera. Field investigations related to the Kumamoto earthquake, trench excavations across the Hinagu and Futagawa faults, and radiocarbon dating results reveal that (1) prior to the 2016 earthquake, at least two morphogenic earthquakes occurred in the past 2000  yrs on the Hinagu fault, and four events in the past 4000–5000 yrs on the Futagawa fault, suggesting an average late Holocene recurrence interval of 1000 yrs for morphogenic earthquakes within the HFFZ; (2) the most recent event occurred in the period between A.D. 850 and 1150; and (3) the penultimate morphogenic event took place in the period between A.D. 80 and 200. The average strike‐slip rates are estimated to be 0.50.7  mm/yr for the Hinagu fault and 1.72.7  mm/yr for the Futagawa fault. These results contradict previous studies that estimate recurrence intervals for morphologic earthquakes of 3600–11,000 and 8000–26,000 yrs on the target segments of the Hinagu and Futagawa faults, respectively. Our findings show that previous studies had significantly underestimated the slip rate and overestimated the recurrence intervals on the active faults of the HFFZ. Accordingly, these active faults of the HFFZ pose a much greater hazard than previously believed, and therefore, it is necessary to reassess the potential seismic hazard for densely populated areas of Kyushu, Japan.

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