Paleoseismology of the Zougahana Fault, Northern Aso Outer Rim, and Its Role in the Tectonics of Northern Kyushu

ABSTRACT The Kumamoto earthquake (MJMA 6.5 and 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, ∼14 km northeast of the Futagawa fault. In this study, we conducted topographic interpretations, field surveys, outcrop descriptions and interpretations, and coring investigations of the Zougahana fault. We clarified that at least four earthquakes 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 5760–2750 cal B.P., 8700–5000 cal B.P., and 9330–7550 cal B.P. based on tephra and radiocarbon ages in the sediments. However, we note the possibility of missing events around 2 ka. 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 ground motions. However, seismogenic rupture of the fault during an aftershock 1.5 hr after the mainshock cannot be ruled out, implying that secondary faults around Aso caldera, including the Zougahana fault, may have slipped because of 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.