Fig. 6

a Map of estimated focal mechanisms. Red-colored beach balls show shallow earthquakes. The gray-colored beach balls represent the solutions of triggered tremors with minimum residuals. Brown lines are the same as in Fig. 1. Red and blue segments in the map indicate minimum and maximum principal stress directions projected in the horizontal plain derived from the focal mechanisms of shallow earthquakes, respectively (Matsumoto et al. 2013). b Map of Vp perturbations at 20-km depth and schematic illustration of the spatial variation of Vp in the cross sections (Matsubara and Obara 2011). Red stars and bars are the same as in Fig. 3. The depth distributions of shallow earthquakes that their focal mechanisms were plotted in Fig. 6a were marked as black squares in the subplots. Black dots in both figures denote the earthquakes that occurred during 2010–2013 (same as in Fig. 3)