Spatiotemporal variation of hydraulic diffusivities. (a) Spatial distribution of pressure sources estimated by the grid search method. The colors of the circles represent the hydraulic diffusivities. The size of the colored circles is inversely proportional to the RMS residual for the fitting of the diffusion model. Gray symbols are earthquakes that occurred from March 2011 to September 2013. (b) Temporal changes in hydraulic diffusivity. The circles are plotted at the time of pressure increase, as estimated by the grid search. The gray line indicates the cumulative number of earthquakes in cluster A.