April 26, 2016
I. Machida, M. Ono, and A. Marui
Institute for Geo-Resources and Environment
There are two major aquifers* beneath Kumamoto City and surrounding areas: the first shallower aquifer and the second deeper aquifer, the latter of which is the most important groundwater resource for the area. In the vicinity of the Lake Ezu, the base of the first (shallower) aquifer locates about 20m below sea level, and that of the second (deeper) aquifer locates about 200m below sea level (Figure 1).
Previous studies have shown that the major recharge area* of the second aquifer is the middle course of Shirakawa River (“Recharge area” in Figure 2). The distribution of hydraulic head in the second aquifer shows that the most groundwater flows southwestward from the recharge area (blue arrows in Figure 2). Since the fault whose rupture has reached the ground surface this time runs south of Kumamoto City, it is not very likely that the earthquakes generate drastic changes in the conditions of the groundwater resources in the area as a whole. However, crustal expansion or contraction and strong seismic vibration may cause temporal changes in groundwater level and turbidity. Careful observation needs to be continued.
The Figures below are included in “7. Kumamoto Area” of the Water Environmental Map series published by GSJ (2014). Original maps were published by Kumamoto City, Kumamoto Prefecture (1995).
* Aquifer: a geological layer that contains groundwater resource.
* Recharge area: a land area where surface water infiltrates into an underground aquifer