Abstract
The 2016 Kumamoto earthquake, whose main shock was an M7.3 event on April 16, 2016, 28 h after a foreshock of M6.5, caused severe damage in and around Kumamoto Prefecture, Japan. It also caused postseismic hydrological changes in Kumamoto Prefecture. In this study, we analyzed daily streamflow data collected by eight observation stations from 2001 to 2017 in regions that experienced strong ground motion during the 2016 Kumamoto earthquake. We also surveyed 11 water springs in the region several times after the main shock. Streamflow had no or slight change immediately after the earthquake; however, large increases were recorded at some of the eight stations following a heavy rainfall that occurred 2 months after the earthquake. A decrease in the water-holding capacity of the catchment caused by earthquake-induced landslides can explain this delayed streamflow increase. Conversely, earthquake-related changes to the spring flow rate were not so clear. Water temperature and chemical composition of spring waters were also hardly changed. Only the concentration of NO3−, which is usually considered to be supplied from the surface, changed slightly just after the earthquake. These results show that the postseismic hydrological changes were caused mainly by earthquake-induced surface phenomena and that there was little contribution from hydrothermal fluid.
Highlights
Large earthquakes are well known to sometimes cause wide hydrological changes in regions affected by strong ground motion (e.g., Waller 1966)
We report the postseismic hydrological changes related to the 2016 Kumamoto earthquake
The minimum envelope of the flow rate appears to have increased at some stations, especially at JNN, following the main shock of the 2016 Kumamoto earthquake
Summary
Large earthquakes are well known to sometimes cause wide hydrological changes in regions affected by strong ground motion (e.g., Waller 1966). In several cases of such changes, streamflow and spring flow increased in lowlands and the water table dropped in highlands. Rojstaczer and Wolf (1992) and Rojstaczer et al (1995) comprehensively explained those changes related to the 1989 Loma Prieta earthquake with permeability enhancement. Sato et al (2000) used permeability enhancement to explain postseismic groundwater changes after the 1995 Kobe earthquake. An analysis of postseismic hydrographs of rivers affected by several large earthquakes, including the 1989 Loma Prieta earthquake, showed that the hydrological changes were caused by Because Japan is relatively rich in water resources, people may have not paid much attention to earthquake-related hydrological changes.
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