Precise Temperature Measurements and Long‐Term Observations Revealed an Almost‐Constant Temperature Zone in a Vertical 700‐m‐Deep Borehole in the Aso Volcanic Region

Abstract

To determine the subsurface temperature distribution and its relevance to groundwater flow in the deep subsurface, we repeated temperature depth profile measurements and conducted long‐term temperature observations in an ∼700‐m‐deep borehole penetrating the Futagawa fault along the Kiyama–Kashima graben in the Aso volcanic region, Japan from 2018 to 2022. Our precise measurements and long‐term observations revealed an almost‐constant temperature depth interval with a gradient of ∼1°C/km at ∼310–430 m. Considering our downhole camera observations, this almost‐constant temperature zone was formed by downward groundwater flow in the borehole from an upper confined aquifer to a lower unconfined aquifer. The upper aquifer is likely an extension of a known confined aquifer called the second aquifer. Conversely, the lower aquifer, connected to the outflow zone and separated from the second aquifer, likely represents a previously unrecognized underlying unconfined aquifer located in the hanging wall of the Futagawa fault that ruptured during the 2016 Mw 7.0 Kumamoto earthquake mainshock. This deep unconfined aquifer may be related to deep groundwater flow in the Aso volcanic region. In addition to the temperature measurements and long‐term observations, numerical simulation of computational fluid dynamics suggested the occurrence of local convection of the borehole water at depth intervals with the natural geothermal gradient, resulting in water temperature fluctuations of <±0.02°C. Borehole water convection, however, may not influence the overall temperature profile; i.e., the temperature of the borehole water may not differ significantly from the temperature of the wall rocks.