Paleoseismology of the Zougahana fault, northern Aso outer rim, and its role in the tectonics of northern Kyushu

The Kumamoto earthquake (Mj 6.5, Mj 7.3) occurred on 14 and 16 April 2016, with the epicenter on the Futagawa and Hinagu faults, central Kyushu Island, southwest Japan. Differential interferometric synthetic aperture radar analyses detected many phase discontinuities in the source region. Phase discontinuities and ruptures were also confirmed at the Zougahana fault on the northeastern rim of Aso caldera, approximately 14 km northeast of the Futagawa fault. In this study, we conducted topographic interpretations, field surveys, outcrop descriptions/interpretations, and coring investigations of the Zougahana fault. We clarified that at least four earthquake events have occurred since 10 ka, including the 2016 earthquake. Among the events preceding the 2016 earthquake, the ages of the most recent three were estimated to be 5780–2750 cal BP, 9020–7200 cal BP, and 9430–8450 cal BP based on tephra and radiocarbon ages in the sediments. However, we note the possibility of missing events during 4350–1010 cal BP and 8690–5020 cal BP. Comparison with the paleoseismic history of the primary Futagawa fault and surrounding secondary faults suggests that the Zougahana fault is among the secondary faults that have repeatedly interacted with the Futagawa fault. The distribution of aftershocks and our analysis of seismic intensity and Coulomb stress change indicate that the trigger for the Zougahana fault may have been strong seismic activity. However, secondary faults around Aso caldera, including the Zougahana fault, may have slipped due to various triggers. Furthermore, within the transtensional tectonics model, which views the complex of volcanoes, grabens, and strike-slip faults that fan out across central Kyushu as a single unit, the Zougahana fault may not be a simple secondary fault on the surface, but rather part of the main shear structure.