[S-GD01] Geodesy and Global Geodetic Observing System

[SGD01-14] Modeling of seasonal gravity changes observed with iGrav SG at Teshikaga, eastern Hokkaido

*Taiki Marufuji^1,2, Kazunari Nawa², Hiroaki Takahashi³ (1.Graduate School of Science, Hokkaido University, 2.Geological Survey of Japan, AIST, 3.Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University)

Keywords:superconducting gravimeter, Teshikaga, land-water gravity change

Continuous high sensitivity relative gravity observation with iGrav SG (#17) superconducting gravimeter was carried out at the Teshikaga, eastern Hokkaido, from November 2018 to August 2022, to monitor tectonic and volcanic activities in the caldera area. Yamaga and Nawa (2020) analyzed the first two years data and found that gravity decreased slowly in winter and increased rapidly in spring due to snow cover and melting. We analyzed all the 4-year long data to examine the gravity change over a longer time scale. The effects of tides, atmospheric pressure change, and polar motion were removed from the original gravity data. Seasonal characteristic gravity changes from winter to spring suggested by Yamaga and Nawa (2020) were also detected in 4 times. The rate of gravity increases in the early spring in 2021 and 2022 were, however, smaller than those in 2019 and 2020. The G-water software (Kazama et al., 2015) was applied to estimate the gravity change due to underground water flow due to snow cover and melting. Theoretical gravity changes using parameters from Yamaga and Nawa (2020) could not explain the above differences. Proper parameters associated with infiltration process of snowmelt water and the saturated hydraulic conductivity of the soil were searched by try and error. Gradual snowmelt infiltration process with an approximately one-order larger hydraulic conductivity could well explain the 4 years seasonal gravity change observed in winter to early spring. Drift correction for iGrav SG using repeated absolute gravity measurements by FG5 (Imanishi and Nishiyama, 2020) will be required to extract gravity signal from tectonic and volcanic activities.

Acknowledgements: Rainfall and maximum temperature data at Teshikaga were from Japan meteorological Agency. This work was supported by JSPS KAKENHI Grant Number JP20K04093.