14:15 〜 14:30
[AHW25-03] Numerical Investigation of Fault and Deep-Seated Fluid Contribution to Thermal Structure in Iwaki, Fukushima
Iwaki city, Fukushima, located in the non-volcanic Tohoku region of Japan's Pacific coast, exhibits moderate to low geothermal potential. In particular, the Joban Yumoto area in the city is known for its hot springs where maximum water temperature exceeding 70oC was observed. A prior investigation in this area in Togo et al. (2014) suggested that the subducted zone might be a source of the hot water. However, the specific nature and causes of these elevated temperatures remain unclear. Our previous research indicated that the temperature distribution is strongly influenced by terrestrial heat flux, with changes in sedimentary permeability having little impact. Currently, we aim to conduct numerical simulations to explore the influence of faults and the ascent of deep-seated fluids on the thermal structure in the Joban area.
Simulations of hydrothermal processes were conducted using the TOUGH3-EOS1 simulator. The geological model was simplified into two layers: an upper sedimentary rock layer and a lower basement rock layer. Key faults, including the Yunodake, Fujiwara and Yumoto, were integrated in the model based on their surface locations. The structured grid accounted for surface topography. The model assumed uniform atmospheric conditions across the surface, with an average annual air temperature of 15oC. A uniform heat flux of 80 mW/m2 was applied at the base. Multiple numerical simulations were conducted, varying the permeability of the Yunodake, Fujiwara, Yumoto, and Shirasaka faults from 1.0E-13 to 1.0E-16 m2 to match temperature profiles from eight wells in the Joban area. In addition, we examined the effect of deep-seated fluid ascending along faults on the temperature distribution by introducing fluid inflow at the bottom of fault cells. The flow rate ranged from 1.0E-5 to 1.0E-8 kg/(s*m2), with a fixed enthalpy of 3.58E5 J/kg, equivalent to approximately 80°C. All other rock properties and boundary conditions remained constant across the simulations.
Our numerical analysis shows that simulations excluding deep-seated fluid ascent successfully reproduced the measured well temperatures, suggesting a minimal impact of fluid ascent on the thermal structure in the Joban area. On the other hand, temperatures near the faults were affected by fault permeability. In particular, reproducing the temperatures of a well near the Yunodake fault required a high permeability (above 1.0E-14 m2), implying that the area is cooled by the infiltration of meteoric water through the fault. Measured temperatures from other wells indicated limited effects from faults. Additionally, the simulation results suggested that the Yumoto fault can be the primary cause of the shallow high-temperature anomaly near the Joban Yumoto area.
The result from simulations indicates that the fault, including Yunodake and Yumoto, play a significant role that defining the thermal structure in the Joban area where deep-seated fluid ascent not required to explain the temperature profile of this area, although the fluid can be originated from grater depths as suggested in Togo et al. (2014). These insights improve will contribute for better understanding of hydrothermal structure in this region.
Acknowledgement: This research was performed by the commissioned research fund provided by F-REI (JPFR23070103, JPFR24070103). We thank the consortium members of the project for their help and advice on this study.
References:
Togo et al. Groundwater, possibly originated from subducted sediments, in Joban and Hamadori areas, southern Tohoku, Japan. Earth Planet Sp 66, 131 (2014).
Simulations of hydrothermal processes were conducted using the TOUGH3-EOS1 simulator. The geological model was simplified into two layers: an upper sedimentary rock layer and a lower basement rock layer. Key faults, including the Yunodake, Fujiwara and Yumoto, were integrated in the model based on their surface locations. The structured grid accounted for surface topography. The model assumed uniform atmospheric conditions across the surface, with an average annual air temperature of 15oC. A uniform heat flux of 80 mW/m2 was applied at the base. Multiple numerical simulations were conducted, varying the permeability of the Yunodake, Fujiwara, Yumoto, and Shirasaka faults from 1.0E-13 to 1.0E-16 m2 to match temperature profiles from eight wells in the Joban area. In addition, we examined the effect of deep-seated fluid ascending along faults on the temperature distribution by introducing fluid inflow at the bottom of fault cells. The flow rate ranged from 1.0E-5 to 1.0E-8 kg/(s*m2), with a fixed enthalpy of 3.58E5 J/kg, equivalent to approximately 80°C. All other rock properties and boundary conditions remained constant across the simulations.
Our numerical analysis shows that simulations excluding deep-seated fluid ascent successfully reproduced the measured well temperatures, suggesting a minimal impact of fluid ascent on the thermal structure in the Joban area. On the other hand, temperatures near the faults were affected by fault permeability. In particular, reproducing the temperatures of a well near the Yunodake fault required a high permeability (above 1.0E-14 m2), implying that the area is cooled by the infiltration of meteoric water through the fault. Measured temperatures from other wells indicated limited effects from faults. Additionally, the simulation results suggested that the Yumoto fault can be the primary cause of the shallow high-temperature anomaly near the Joban Yumoto area.
The result from simulations indicates that the fault, including Yunodake and Yumoto, play a significant role that defining the thermal structure in the Joban area where deep-seated fluid ascent not required to explain the temperature profile of this area, although the fluid can be originated from grater depths as suggested in Togo et al. (2014). These insights improve will contribute for better understanding of hydrothermal structure in this region.
Acknowledgement: This research was performed by the commissioned research fund provided by F-REI (JPFR23070103, JPFR24070103). We thank the consortium members of the project for their help and advice on this study.
References:
Togo et al. Groundwater, possibly originated from subducted sediments, in Joban and Hamadori areas, southern Tohoku, Japan. Earth Planet Sp 66, 131 (2014).