(a) Spatiotemporal distribution of earthquakes around clusters A–C (see Fig. 8) obtained from the DONET hypocenter catalogue. Gray triangles indicate the timing when new stations are installed. (b) Spatiotemporal distribution obtained by using the P and S readings from only the eight stations that were installed before the 2011 Tohoku-oki earthquake. Left, map views; right, time series of earthquakes according to latitude. The vertical gray line in the right panels indicates the origin time of the 2011 Tohoku-oki earthquake.