5:15 PM - 6:30 PM
[SGD01-P01] Detection of a coseismic groundwater response localized near the site of the Mizunami Underground Research Laboratory using a horizontal gravimeter array
Keywords:gravity, groundwater, Mizunami Underground Research Laboratory
In order to monitor the gravity change due to the backfilling of the Mizunami Underground Research Laboratory (MIU), the gPhone#153 gravimeter has been operated at the Mizunami Geoscience Agency (MGA) since before the backfilling, and the gPhone#130 gravimeter has been operated at the Togari observation shelter (TGR) since March 2020. The horizontal distance between MGA and MIU is approximately 150 m, and the horizontal distance between TGR and MIU is approximately 300 m. The horizontal distance between MGA and TGR is approximately 500 m. The absolute gravity measurement by FG5 had been carried out once a month at MGA, but it was terminated in October 2020 when the He-Ne laser was exhausted. By taking the difference between the two gPhone data, it should be possible to detect the local gravity change caused by the MIU backfilling work. It was thought that tidal and atmospheric disturbances could also be canceled out by taking the difference, but in reality, oscillations with several-microGal amplitude remained. Therefore, the Baytap08 was applied to each data to remove tidal and atmospheric responses. Sensor drift and oscillation (natural and human-made) disturbances were manually corrected to obtain residual gravity values. Due to the recent relocation of #130, the sensor-drift rate fluctuated significantly. And due to other construction works near each gravimeter, the overall noises were scattered. The gravity increase associated with the seismic groundwater response (maximum amplitude of about 4 microGal, duration of about 12 days) triggered by the July 22 Alaska Peninsula earthquake (Mw 7.8) was detected only in #153 during the analysis period from April to September 2020. The residual gravity values of the difference between #153 and #130 are shown in the figure. S represents a step noise (probably human-made), and E represents the seismic groundwater response detected here. Immediately after step noise, a nonlinear drift continues for several days or more. Absolute gravity measurements show a slight decrease of approximately -0.5 microGal/month during this analysis period, which seems to reflect the crustal uplift after the 2011 Tohoku Earthquake mainly. Therefore, it should be noted that the residual gravity values in the figure should still include the sensor drift component. Note that the MIU backfilling work during this period was from 500m depth to 400m depth, and its direct attraction effect is still less than 1 microGal (Honda et al., 2019, BUTSURI-TANSA). Seismic groundwater (confined) response has been detected several times by gravity observation at the MGA. The water level always increased, and gravity always decreased. When a seismic groundwater response occurs, the groundwater level (or pore pressure) increases significantly only in the vicinity of MIU, while the water level decreases in the surrounding area. (Tanaka et al., 2006, Gcubed; Takeuchi et al., 2015, J. Nuclear Fuel Cycle and Environment) We have been interpreting the gravity decreasing as caused by the contribution of surrounding groundwater that surpasses the localized groundwater. Here, the relation between groundwater and gravity cannot be confirmed because of the lack of the TGR350m borehole water level data and the unclarity of the surrounding borehole water level changes. A seismic groundwater response of Mw 7.8 at an angular distance of 5200 km is close to the detection limit (Asai, 2006, D. thesis). Something like hydraulic environment changes caused by the MIU backfilling and the Linear Shinkansen construction (4km upstream of the groundwater flow) might affect gravity response's reversal. We want to increase the number of cases in the future to investigate the cause of the localized gravity change source location and the reversal.