3:30 PM - 3:45 PM
[HRE12-01] An overview of investigations of geological structure, temperature and groundwater level in boreholes penetrating the Futagawa faul
★Invited Papers
Keywords:Futagawa fault drilling, geological structure, depth profile of temperature , groundwater level observation
In solid-earth science areas including engineering geology, rock core sampling, borehole logging, downhole experiments, long-term monitoring at large depths in scientific drilling projects are important and useful approaches, for examples, to investigations of active faults, survey and development of geothermal resource, geological disposal of high level radioactive waste, carbon dioxide capture, utilization and storage (CCUS) etc. We give an overview of investigations of geological structure, depth profile of temperature and groundwater level change in scientific drilling boreholes penetrating the Futagawa fault which ruptured during the Mw 7.0 mainshock of the 2016 Kumamoto earthquake sequence.
The Futagawa fault drilling project (FFDP) with one mostly vertical borehole called FDB and two inclined penetrating boreholes was conducted in 2017–2018 by Kyoto University (2018; https://www.nsr.go.jp/data/000256426.pdf) mainly to obtain fresh fault rock samples from the Futagawa fault zone. Drilling occurred at the FFDP site in the town of Mashiki, Kumamoto where the surface rupture attained a maximum dextral slip (∼2.2 m).
By core and cutting descriptions, we identified >200 m of normal faulting displacement along the currently dextral strike-slip Futagawa fault (Shibutani et al., 2022 G3). Considering previous kinematic and chronological studies of the fault, we interpreted that the Futagawa fault dominantly slipped as a normal fault in a short period (∼300–87 ka) before switching to its current dominant strike-slip faulting regime ∼87 ka caused by a local change in the stress field associated with the termination of the Aso caldera-forming eruptions. In FDB, three damage zones were identified. The second one at ∼461 m depth was more strongly damaged than the others. In addition, the physical properties revealed different change patterns near the three damage zones and showed strongest deterioration at the 461-m damage zone. Based on geological and geophysical observations, we suggested that this damage zone is the primary candidate of the 2016 Kumamoto earthquake mainshock.
To obtain temperature profiles in FDB borehole at different elapsed times after the drilling, we have conducted downhole temperature measurements of a total 15 times from May 2018 to April 2022 (Lin et al., 2021 presented at the annual meeting of Japan Society of Engineering Geology). In depth intervals of 200–310 m and 430–650 m, the temperature increased with depth gradually and almost linearly, gave a higher geothermal gradient of approximate 50 °C/km than a typical worldwide geothermal gradient 20 °C/km. This 50 °C/km could be a representative value at locations approximate 10 km away from the Aso caldera rim mountains in the volcanic region. However, in a depth interval of 310–430 m the temperature showed a very small geothermal gradient of approximate 1 °C/km. We expected that this exceptional phenomenon might be caused by water flow. Thus, we developed a heating flow mater and applied it to FDB. As a result, a downward flow in the borehole was confirmed (Feng et al., 2023; will be presented in the same session of JpGU).
To reveal the great-depth groundwater dynamics in Aso volcanic region, we have conducted observations on groundwater level change at depths below 300 m in the FDB borehole for four years (Shibutani et al., 2022 Journal of the Japan Society of Engineering Geology). The patterns of groundwater level change in the FDB borehole and pre-existing observation wells by the Kumamoto Prefecture indicated that the second aquifer, which is mainly composed of Aso-1 to Aso-3 pyroclastic flow deposits, may continue to older sedimentary rocks strata and pre-Aso volcanic rocks at depths below 300 m. Taking data of core descriptions and image logs into consideration, we suggested that the Futagawa fault damage zone contributes for downward extension of the second aquifer, and for the active groundwater flow systems in the Kumamoto area.
The Futagawa fault drilling project (FFDP) with one mostly vertical borehole called FDB and two inclined penetrating boreholes was conducted in 2017–2018 by Kyoto University (2018; https://www.nsr.go.jp/data/000256426.pdf) mainly to obtain fresh fault rock samples from the Futagawa fault zone. Drilling occurred at the FFDP site in the town of Mashiki, Kumamoto where the surface rupture attained a maximum dextral slip (∼2.2 m).
By core and cutting descriptions, we identified >200 m of normal faulting displacement along the currently dextral strike-slip Futagawa fault (Shibutani et al., 2022 G3). Considering previous kinematic and chronological studies of the fault, we interpreted that the Futagawa fault dominantly slipped as a normal fault in a short period (∼300–87 ka) before switching to its current dominant strike-slip faulting regime ∼87 ka caused by a local change in the stress field associated with the termination of the Aso caldera-forming eruptions. In FDB, three damage zones were identified. The second one at ∼461 m depth was more strongly damaged than the others. In addition, the physical properties revealed different change patterns near the three damage zones and showed strongest deterioration at the 461-m damage zone. Based on geological and geophysical observations, we suggested that this damage zone is the primary candidate of the 2016 Kumamoto earthquake mainshock.
To obtain temperature profiles in FDB borehole at different elapsed times after the drilling, we have conducted downhole temperature measurements of a total 15 times from May 2018 to April 2022 (Lin et al., 2021 presented at the annual meeting of Japan Society of Engineering Geology). In depth intervals of 200–310 m and 430–650 m, the temperature increased with depth gradually and almost linearly, gave a higher geothermal gradient of approximate 50 °C/km than a typical worldwide geothermal gradient 20 °C/km. This 50 °C/km could be a representative value at locations approximate 10 km away from the Aso caldera rim mountains in the volcanic region. However, in a depth interval of 310–430 m the temperature showed a very small geothermal gradient of approximate 1 °C/km. We expected that this exceptional phenomenon might be caused by water flow. Thus, we developed a heating flow mater and applied it to FDB. As a result, a downward flow in the borehole was confirmed (Feng et al., 2023; will be presented in the same session of JpGU).
To reveal the great-depth groundwater dynamics in Aso volcanic region, we have conducted observations on groundwater level change at depths below 300 m in the FDB borehole for four years (Shibutani et al., 2022 Journal of the Japan Society of Engineering Geology). The patterns of groundwater level change in the FDB borehole and pre-existing observation wells by the Kumamoto Prefecture indicated that the second aquifer, which is mainly composed of Aso-1 to Aso-3 pyroclastic flow deposits, may continue to older sedimentary rocks strata and pre-Aso volcanic rocks at depths below 300 m. Taking data of core descriptions and image logs into consideration, we suggested that the Futagawa fault damage zone contributes for downward extension of the second aquifer, and for the active groundwater flow systems in the Kumamoto area.