[M-IS08] Interdisciplinary studies on pre-earthquake processes

[MIS08-P03] The fluctuation of the underground Rn flux near the surface

*Kazuhide Nemoto¹, Haruna Kojima², Chie Yoshino³, Shu Kaneko², Katsumi Hattori³, Toshiharu Konishi⁴, Ryuichi Huruya⁵ (1.Faculty of Science, Chiba University, 2.Graduate School of Science and Engineering, Chiba University, 3.Graduate School of Science, Chiba University, 4.OHYO KOKEN KOGYO Company Limited, 5.COM SYSTEM Incorporated)

In recent years, radon anomalies preceding large earthquakes have been reported, which are interpreted as modulation of Rn gas emanation due to stress changes in the earth’s crust. For this verification, we are conducting continuous observations of underground radon concentration (GRC) in Asahi, Chiba Prefecture (N35.77, E140.69, June 2014-) and Miho Village, Ibaraki Prefecture (N35.99, E140.29, September 2020-), Japan. We have developed the method to estimate the underground Rn flux (GRF) by using the MSSA (Multi-channel Singular Spectrum Analysis) to remove the effects of weather factors of temperature and atmospheric pressure. GRF indicates the upward flow of Rn gas near the ground surface. However, GRF may be affected by heavy precipitations. We conducted a statistical analysis of the relationship between the fluctuation of GRF and precipitation. As a result, we found that GRF significantly increased after heavy rainfall events with 20 mm or more in 2 hours. This is caused by both the increase in load due to rainwater and the infiltration of rainwater near the station. Similar tendency is given for Miho station even though the data are very short. Excluding the influence of the above precipitation effects at the Asahi station, we compare the variation of GRF with the seismic activity near the station, we found there are cases where the GRF increases for reverse fault type earthquakes within an epicenter distance of 50 km from the station. This result suggests the possibility of GRF fluctuation due to the earthquake. In the future, it is necessary to quantitatively evaluate the relationship between the fluctuation of crustal stress fields and that of GRF at Miho and Asahi stations.