Fig. 5

Change in travel time and amplitude throughout the observation period. The changes in travel time (top three panels with yellow background) are calculated according to Ikuta et al. (2002) and changes in amplitude (bottom three panels with green background) are obtained by our method. The two panels in the middle illustrate atmospheric temperature and rainfall observed at Gunka station, which is the nearest meteorological station of the Japan Meteorological Agency. Blue shading shows the periods of the water injection experiments. The red vertical lines show the time of the earthquakes mentioned in the text. The gap between horizontal lines with a pair of triangles in the left shows the mean error of the amplitude change calculated by Eq. (6). The colors of the lines and triangles correspond to sensors in the legend on the right