This figure shows the change in the hydraulic gradient after earthquakes. After earthquakes with magnitudes of 7 to 8 (shown as (1) – (3) in the figure), the hydraulic gradient temporarily changed and was then restored with time before the earthquake. On the other hand, after the magnitude-9 earthquake (4), the hydraulic gradient changed but was not restored before the earthquake. The change in the hydraulic gradient during earthquake (5), whose epicenter is close to the study area, did not recover before the earthquake, too.