11:45 AM - 12:00 PM
[SCG56-05] Coseismic temperature transient in borehole on the flank of Mt. Fuji
★Invited Papers
Keywords:Borehole temperature monitoring, Earthquake in western Kanagawa
On August 8-10, 2024, temperature measurement was conducted in a groundwater observation well south of Kawaguchi Lake (elevation 862 m, bottom depth 121 m below ground, groundwater level 32.8 m, cased with screen at 95-115 m). The lithology inferred from the nearby boreholes is lava down to a depth of 60 m and volcanic sand and gravel below that (equivalent to the Paleo-Fuji ejecta layer). The borehole temperature profile was obtained by slowly lowering the thermistor sensor. Two aquifer zones were detected, with temperatures of 10.7°C at 40-60 m below the surface and 9.8°C deeper than 90 m (Figure 1). The temperature decreases linearly between 60 m and 90 m, interpreted as a steady-state solution of the thermal diffusion.
During the observation, a M5.3 earthquake occurred at 19:57 on Aug. 9, 2024 in the western Kanagawa (35.41°N, 139.17°E, depth ~14 km). Our observation site (~40 km WNW of the epicenter) also felt a shake of about intensity 3. Four self-recording thermometers were in operation in the borehole. The temperature at depths of 66 m and 81 m rose by 2~3 mK in response to the earthquake (Figure 2), whereas temperature did not change at 95.5 m and 51 m. The temperature change appeared in two stages: the first one was a step-like temperature increase ~4 minutes after the event with different amplitudes. The second one appeared 30 minutes to 1 hour after the event, smooth in shape. Transients terminated after 30 minutes to 1 hour, after which it seemed to return to the original level.
Possible causes of temperature increase can be the movement of water in the borehole (e.g. Shimamura & Watanabe, 1981 Nature; Kitagawa & Koizumi, 2000 JGR). It should have been caused by the descent of water, since the temperature gradient is negative between 66 m and 81 m. Based on the temperature gradient (-30mK/m), this can be interpreted as a 7cm descent of the water in the borehole. Water probably flowed out of the screened section at the bottom of borehole into the surrounding formation. In this presentation, the poroelastic response to the earthquake will also be discussed more in detail.
This study was conducted as part of an observational training course at the Department of Earth and Planetary Science, The University of Tokyo. We thank the owner of the borehole, Hotel Regina, for allowing the observation. We thank Takashi Uchiyama of the Mont Fuji Research Institute, Yamanashi Prefecture, for providing information about the borehole and assistance in making measurements possible.
During the observation, a M5.3 earthquake occurred at 19:57 on Aug. 9, 2024 in the western Kanagawa (35.41°N, 139.17°E, depth ~14 km). Our observation site (~40 km WNW of the epicenter) also felt a shake of about intensity 3. Four self-recording thermometers were in operation in the borehole. The temperature at depths of 66 m and 81 m rose by 2~3 mK in response to the earthquake (Figure 2), whereas temperature did not change at 95.5 m and 51 m. The temperature change appeared in two stages: the first one was a step-like temperature increase ~4 minutes after the event with different amplitudes. The second one appeared 30 minutes to 1 hour after the event, smooth in shape. Transients terminated after 30 minutes to 1 hour, after which it seemed to return to the original level.
Possible causes of temperature increase can be the movement of water in the borehole (e.g. Shimamura & Watanabe, 1981 Nature; Kitagawa & Koizumi, 2000 JGR). It should have been caused by the descent of water, since the temperature gradient is negative between 66 m and 81 m. Based on the temperature gradient (-30mK/m), this can be interpreted as a 7cm descent of the water in the borehole. Water probably flowed out of the screened section at the bottom of borehole into the surrounding formation. In this presentation, the poroelastic response to the earthquake will also be discussed more in detail.
This study was conducted as part of an observational training course at the Department of Earth and Planetary Science, The University of Tokyo. We thank the owner of the borehole, Hotel Regina, for allowing the observation. We thank Takashi Uchiyama of the Mont Fuji Research Institute, Yamanashi Prefecture, for providing information about the borehole and assistance in making measurements possible.