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The 2016 Kumamoto–Oita earthquake sequence involving three large events (Mw ≥ 6) in the central Kyushu Island, southwest Japan, activated seismicities in two volcanic areas with unusual and puzzling spatial gaps after the largest earthquake (Mw 7.0) of April 16, 2016. We attempt to reveal the seismic process during the sequence by following seismological data analyses. Our hypocenter relocation result implies that the large events ruptured different faults of a complex fault system. A slip inversion analysis of the largest event indicates a large slip in the seismicity gap (Aso gap) in the caldera of Mt. Aso, which probably released accumulated stress and resulted in little aftershock production. We identified that the largest event dynamically triggered a mid-M6 event at Yufuin (80 km northeast of the epicenter), which is consistent with existence of the 20-km long zone where seismicity was activated and surface offset was observed. These findings will help us study the contribution of the identified complexity in fault geometries and the geotherm in the volcanic areas to the revealed seismic process and consequently improve our understanding of the seismo-volcano tectonics.Graphical abstract(Left) Map view of the fore- and aftershock distribution and the coseismic slip distribution of the mainshock (Mw 7.0) indicated by the color image. Circles denote event locations, colored by event depth. The inset shows the fault slip model with its epicenter, indicated by the star. The blue line indicates the rim of the caldera of Mt. Aso based on the peak of the topography. (Middle left) Cross sections of the relocated earthquakes at locations (A–A′ to I–I′) indicated in the left panel. Blue and red circles are for earthquakes before and after, respectively, the mainshock. Gray lines indicate the fault model for the slip inversion analysis. The brown line on the top of each panel corresponds to the surface trace of active faults (Nakata and Imaizumi in digital active fault map of Japan. University of Tokyo Press, Tokyo, 2002). (Middle right) Map for epicenters (gray circles) and seismic stations (triangles). The epicenter of the triggered event identified in this study marked by the star. Red dots in the inset map denote epicenters of this earthquake sequence and the dotted box indicates the area of the main map. (Right) Vertical component of acceleration seismograms highpass-filtered (HPF) at 16 Hz and sorted by distance from the hypocenter of the mainshock (the bottom is closer). All traces are drawn in red after the arrival of the P-wave from the triggered event. At OIT009, the seismic waves from the event γ are invisible due to too strong waves from the triggered event. The maximum absolute value for each trace is denoted on the right.

The 2016 Kumamoto–Oita earthquake sequence: aftershock seismicity gap and dynamic triggering in volcanic areas