[S-GD02] Geodesy and Global Geodetic Observing System

[SGD02-13] Postseismic gravity change after the 2011 Tohoku Earthquake observed by superconducting gravimeters at Mizusawa, Japan

*Yoshiaki Tamura¹, Yuichi Imanishi², Ryuichi Nishiyama², Takahito Kazama³, Koji Matsuo⁴ (1.National Astronomical Observatory of Japan, Mizusawa VLBI Observatory, 2.Earthquake Research Institute, The University of Tokyo, 3.Graduate School of Science, Kyoto University, 4.Geospatial Information Authority of Japan)

Keywords:postseimic gravity change, superconducting gravimeter, 2011 Tohoku Earthquake, satellite gravimetry

In Mizusawa VLBI Observatory, NAOJ, we started continuous gravity observation with the superconducting gravimeters (SGs) since early 2009. First we used GWR Instruments' model TT70 #007 gravimeter and we replaced it with #016 on October 2017. Though the instrumental drift rate of the superconducting gravimeters are small, combination of SG observations and absolute gravity measurements are required in order to discuss long term gravity changes. We carried out absolute gravity measurements with the joint work of Earthquake Research Institute and Mizusawa VLBI Observatory every year since 2015. Fixing the SG drift rate by the absolute measurement data, we get the continuous gravity change data since 2014. The observed data were corrected with the ground water effect simulation results at first. Next the free air correction was given which is based on the GNSS observation result. The final observation result shows obvious increase of gravity with exponential decay. Estimating decay time constant and amplitude coefficient by applying nonlinear least squares method, we get the time constant of 648.9±17.6 day and the coefficient of -85.7±4.1 micro gal. On the other hand, the satellite gravity data GRACE/GRACE-FO gives different result. We calculated Mizusawa's gravity time series by using DDK3(240km) filter which is widely used. It shows similar gravity increase but the estimated decay time constant and amplitude coefficient are different. They are 3555.9 ± 763.8 day and 14.9±1.6 micro gal respectively. We consider that the difference is caused by the difference in spatial sensitivity of two techniques. The ground gravity measurements are sensitive to small scale (50km-100km) phenomena. The satellite gravity measurements are not sensitive to small scale (shorter than 200km) phenomena.
The SG observation in Mizusawa was partially supported by Prof's S. Okubo, S. Miura, H. Ikeda, and Y. Tanaka. We thank their financial support.