Post-seismic hydrochemical changes in regional groundwater flow systems in response to the 2016 Mw 7.0 Kumamoto earthquake

An understanding of the detailed mechanisms and processes of hydrochemical changes in regional groundwater flow systems in response to large earthquakes has been hindered because of a lack of spatiotemporal datasets to describe these changes. To clarify these issues we documented hydrochemical changes observed during the 2016 M-w 7.0 Kumamoto crustal earthquake using a large amount of analytical records for an area of ca. 945 km(2) from 139 wells for 82 measured parameters. Spatiotemporal distribution patterns of these changes were characterized based on a factor notation calculated using a 10-year data trend. Our results suggest that alteration of hydrogeological properties, i.e., seismic rupturing, coseismic mountain water release, and release of soil pore waters, caused changes in hydrochemical signatures at the regional scale, most prominently reflected in elevated dissolved Si, diluted Cl-, and increased CO2 and organic compounds contents. Other hydrochemical anomalies detected at the local scale were induced by deep fluid upwelling and liquefaction. This study identified the combined mechanisms and processes that generate the overall water chemistry changes in active groundwater flow systems in Kumamoto. Our results provide new insights into coseismic hydrochemical changes that could be applied to other aquifer systems and, importantly, the use of groundwater resources during and after disasters.