Paleoseismic events and shallow subsurface structure of the central part of the Futagawa fault, which generated the 2016 Mw 7.0 Kumamoto earthquake

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The mainshock (Mw 7.0) of the 16 April 2016 Kumamoto earthquake sequence caused a ~30-km-long surface rupture along the Futagawa fault and the northern Hinagu fault in central Kyushu, Japan. A large number of distributed secondary surface ruptures also appeared in a wide area around the primary surface rupture. However, whether the primary and secondary faults ruptured at the same time during paleoseismic events on the Futagawa fault is not well-known. To tackle this problem, paleofaulting events on both primary and secondary faults must be accurately and precisely dated. In this study, we conducted a trenching survey at Futa, in the central part of the primary Futagawa fault, where a 100-m-high, 400-m-wide fault scarp was previously identified, and excavated five closely spaced trenches on the slope of the fault scarp to date paleofaulting events on the primary fault precisely and accurately. Then, by examining fissure fills and the total displacement accumulated on each fault, we identified a total of eight surface rupturing events, including the 2016 event. Although we did not find evidence of all events in all trenches, we identified these events by combining the results from the five trenches; thus, the reliability of our findings is greater than if the events had been identified in only a single trench. We estimated that three paleofaulting events occurred at 2150–1460, 4310–2940, and 6030–4360 cal yr BP, after deposition of the Kikai-Akahoya tephra (7.3 ka). Our results confirm the age of the penultimate event on the Futagawa fault to be around 2 ka, as previously reported, corresponding to the ages of penultimate events on secondary faults as well as to the age of ground-shaking related phenomena identified at archeological sites. This correspondence strongly suggests that the primary and secondary faults moved simultaneously during the penultimate event on the Futagawa fault. In addition, cumulative deformation along the Futagawa fault after deposition of the Aira-Tn tephra (30 ka) resulted in the formation of a small graben structure at the trench site. This shallow subsurface structure is partly responsible for the 400-m-wide fault scarp at Futa, where the slip distribution is complex due to the presence of thick lava and the immaturity of the Futagawa fault.