Distribution of brightness and hypocenters. The top panel is a map view at a depth of 13 km, and the bottom panel is the cross section along P to Q. White circles show the location of probable scatterers with the highest brightness values. Black open circles and red stars denote hypocenters of earthquakes that occurred from March 2011 to September 2013 and low-frequency earthquakes from January 1998 to May 2011, respectively. Colored circles represent the locations of pressure sources estimated from the grid search (Figure 8a). The color of the circles indicates the hydraulic diffusivities. Inverted triangles indicate station locations. Dashed lines show locations of reflectors estimated by Hori and Hasegawa (1991).