Kato and Yoshida (2011) assumed that the strength of interplate megathrusts was maximized at intermediate depths (approximately 20 km) close to the nucleation site of the M9 earthquake.
Aftershock's ruptures are situated at depths greater than 5 km, close to the assumed fault plane of the mainshock (adopted from Asano and Iwata 2016), and three of them intersect the mainshock fault plane.
We show in Fig. 3c the furthest earthquake triggering from the Kumamoto region, observed in Hokkaido, at an epicentral distance of ~1650 km, close to Akan volcano, which is one of the most active in Hokkaido.
The rupture mainly propagated upward and northeastward (Fig. 4c), and another large-slip area ruptured approximately 4 s after the origin time at a depth of 5 km, close to the northeastern end of the Hinagu fault beneath the town of Mashiki, where a seismic intensity of 7 was recorded.
Instead, another MJMA 6.4 earthquake occurred at 00 03 JST on April 15 , 2016 about 2.5 h after the foreshock at a depth of 6.71 km, close to the southwestern end of the foreshock's rupture area (see the top-right panel in Fig. 2).
In our case, initial similarity was defined as the similarity at a distance of 150 km, to ensure that we did not calculate the similarity within just one 1° × 1° grid cell (approximately 110 km * 110 km close to the Equator) (Table 4).
