Spatiotemporal variation of seismicity in cluster A. (a) Distance versus time plot for cluster A. We measured the distance from the deep part of initial seismic activity in May 2011. The four curves shown indicate the theoretical position of the propagating pore pressure front for the cases of hydraulic diffusivities of D = 0.01, 0.02, 0.05, and 0.1 m2/s, respectively. Background colors labeled (6a to 6d) denote the time periods for which hypocenter distributions are shown in Figure 6. (b) Hypocenter distribution in cluster A. Gray symbols show earthquakes that occurred during the period in (a). Colored circles indicate earthquakes in the short time intervals shown in (c) and (d). Stars represent the locations of pressure sources for the events with corresponding colors. (c, d) Distance versus time plots for the two time periods shown at the top of each figure. Distances were measured from the estimated pressure sources (stars in (b)). Dashed lines indicate the theoretical position of the propagating pore pressure front for hydraulic diffusivities of D = 0.1, 0.5, and 1.0 m2/s, respectively. Red lines indicate the theoretical curve for the optimal D value.