Change in groundwater discharge rate around Lake Ezu before and after the 2016 Kumamoto Earthquake

Abstract

In the Suizenji and Lake Ezu regions in Kumamoto City, changes in groundwater discharge rate were measured both before and after the 2016 Kumamoto earthquake that occurred on April 14th and 16th. The study area was divided into four zones and only positive values in the rate of groundwater discharge within each zone were accumulated. As a result, discharge rate averaged approximately 410,000 m³/day from May 2015 to April 2016, and then increased significantly to approximately 620,000 m³/day from May 2016 to April 2018. The reasons for the large increase after the earthquake are that groundwater discharge to the lake greatly increased in the Kami-Ezu and Naka-Ezu zones. While lake seepage into groundwater greatly increased in the Shimo-Ezu zone. Since 2018, groundwater discharge decreased to average approximately 510,000 m³/day from May 2018 to April 2020, and became approximately 550,000 m³/day after May 2020. It is possible that the observed river discharge rates after the earthquake might be affected by restoration works carried out predominantly in the Kami-Ezu region.