Fig. 4

Sustained groundwater level increases in the Oi well. The groundwater level changes from the 11 events are plotted of epicenter distance, from the bottom to the top in the figure. The start times of the groundwater level increases are aligned to be at the center of the time axis. The amplitudes of the groundwater level increases are plotted at the same scale. The red down-pointing and up-pointing triangles mark the calculated arrival times of the P and S waves, respectively. The blue triangle indicates the Rayleigh wave arrival time shown in Fig. 2c