Fig. 2: Distribution of hypocentres and aftershock observation stations.

a Plan view. Black circles are aftershocks that occurred from October 22 to December 15, 2016, for which focal mechanisms were determined. Grey circles were aftershocks that were manually picked. Locations of temporary seismic stations for one- and three-component seismographs and permanent stations (operated by the National Research Institute for Earth Science and Disaster Mitigation, NIED, and the Japan Meteorological Agency, JMA) are indicated by red triangles, green crosses, and dark blue rectangles, respectively. The cyan dots indicate the estimated mainshock fault. The possible locations of the northern edges of the WF are indicated by blue dots from Y = 6.1 to 6.9 km. The orange star indicates a quaternary volcano³⁹. The focal mechanism (in the lower hemisphere) of the mainshock determined by JMA is shown by a grey and white beach ball²⁶. The mean directions of σ₁ and σ₃ in this area, estimated from stress inversion analyses, are shown by black and red lines, respectively. The grey dashed rectangle indicates the area for which detailed analyses were performed. The orange square indicates the areas around the southern edge of the mainshock fault shown in Fig. 4. b Cross-section along the fault strike. The purple contours are the estimated slip distribution in 0.2 m increments, that overlap those of EF and WF (the case of the northern end of Y = 6.1). The red circle indicates the mainshock hypocentre determined by JMA. c Cross-section perpendicular to the fault strike. d Magnitude−frequency distributions of the aftershocks focal mechanisms of which were determined (black dots) and manually picked (grey circles). e Large scale map showing the aftershock region modified from Fig. 1 of Kawanishi et al.²⁵.