Abstract
To reveal the eastern extension of the Futagawa fault zone and its paleoseismicity, we excavated a trench across the surface rupture of the 2016 Kumamoto earthquake extended to the Aso caldera. The six-meter-deep trench at Kurokawa, the town of Minami-Aso exposed young sediments mainly composed of multiple massive loam units, volcanic sand layers, fall-out tephra units, and organic rich fine sediments that are evidently warped and cut by a group of numerous faults. The Kikai-Akahoya tephra layer of 7.3 ka exposed near the bottom of the trench, shows up to two-meter vertical offset, which suggests cumulative slip due to the multiple paleoseismic events. As a result of our detailed observation and radiocarbon dating, we interpret four paleoseismic events, the 2016 Kumamoto earthquake, the penultimate event occurred any time between 2,132 and 1,900 cal BP, antepenultimate event of 4,237-4.1 cal ka BP, and the oldest one of 7,300-4,090 cal BP. The average inter-event time is 1,400-2,400 years. Even though we eliminated the antepenultimate event as an uncertain event, the inter-event time would be 2,000-3,700 years. These estimates would be consistent with the ones estimated from other post-Kumamoto earthquake studies along the Futagawa fault zone in and out of the Aso caldera. It suggests that the similar type of the earthquakes of the 2016 Kumamoto earthquake might have repeatedly occurred during the Holocene period.
