Figure 8: The Tohoku Earthquake, magnitude 9.0, occurred on March 11, 2011. There are several time periods (grey horizontal line) without data due to logger problems before the main shock and regional electric power shutoff. (a) Time evolution of number of pulse-like signal per hour of type A (green). The unit is shown on the left hand side. Almost all type A variations occurred after the main shock. (b) Time evolution of type B (red) including B-1, B-2, and B-3. The type B variation is suggested to be particularly useful for imminent prediction. The bold continuous curve attached by shows the cumulative number of B-type variations. The unit of the cumulative number is shown in the right hand side. (c) Time evolution of type C (blue). This type occurred only after the main shock when the after-effect stage of the seismic cycle.