Identification of changes in subsurface temperature and groundwater flow after the 2016 Kumamoto earthquake using long-term well temperature–depth profiles

Repeated measurements of temperature–depth profiles were conducted in January 2017, June or November 2017, and February 2018 at 10 observation wells in the Kumamoto area, Japan, after the 2016 M_w7.0 Kumamoto earthquake to evaluate changes in post-earthquake subsurface temperature. These observation data were compared with pre-earthquake temperature data from the same wells measured during 2001 and 2009–2012 by previous studies. The observation and analysis of temperature–depth profiles are useful for studying groundwater flow but have not been used to determine coseismic changes in groundwater flow. This study detected earthquake-related changes in groundwater flow using observations and analyses of temperature–depth profiles and is the first to report on this type of evaluation of coseismic groundwater flow changes. The results indicated a general long-term subsurface warming from 2001 to 2018. Anomalous decreases and increases occurred in subsurface temperature at the western foot of the Aso caldera rim and western Takuma Plateau, respectively. The observed temperature decrease can be explained by the post-seismic release of mountain groundwater characterized by lower temperatures. A type curve match analysis indicated that increasing temperature anomalies were associated with downward groundwater flow. These results were consistent with a recently proposed coseismic groundwater drawdown model along new rupture systems. Although the first measurement was performed nine months after the earthquake, the groundwater level change data allowed the temperature measurements to successfully detect anomalies caused by hydrological changes that began immediately after the earthquake. However, these signals disappeared 14–19 months after the earthquake and were diminished by the original groundwater flow systems that control the local subsurface temperature. These results demonstrate the applicability of the observation and analysis of temperature–depth profiles to identify post-seismic changes in groundwater flow, which are required for the management of groundwater resources during earthquake disasters and reconstruction.