[S-SS13] Progress in environmental seismology

[SSS13-06] Influence of seismic station environment on microtremor amplitude reductions during the COVID-19 pandemic: Inspections using MeSO-net data

*Takumi Hayashida¹, Masayuki Yoshimi², Suzuki Haruhiko³, Shinichiro Mori⁴, Takao Kagawa⁵, Masayuki Yamada⁶, Koji Ichii⁷ (1.IISEE, Building Research Institute, 2.Geological Survey of Japan, AIST, 3.OYO Corporation, 4.Ehime University, 5.Tottori University, 6.NEWJEC Inc., 7.Kansai University)

Keywords:Microtremor, Seismic noise, MeSO-net, COVID-19, Human activity

With the increase in the number of continuous seismic observation stations, discussions have been ongoing to characterize the relationship between microtremors (cultural noise) and human activities (socioeconomic activities). Many papers have reported microtremor amplitude reductions during the COVID-19 pandemic, reflecting restriction of social activities (e.g., Lecocq et al., 2020; Piccinini et al., 2020; Xiao et al., 2020). In Japan, some papers indicated the decrease and recovery of microtremor amplitudes after March 2020 in the Tokyo metropolitan area (Yabe et al., 2020; Nimiya et al., 2021). On the other hand, few reports focus on the local environment around the observation station on the microtremor amplitude reductions. Hayashida et al. (2022, JSCE) discussed the relationship between the time variation of microtremor amplitudes and the surrounding environment (facilities, land use, distance to railroad tracks and roads) at 169 stations of the Metropolitan Seismic Observation Network (MeSO-net). Our results indicate that the microtremor amplitude reduction in 2020 strongly reflected changes in the usage of the facilities where the seismic sensors are installed, rather than human activities in a wide area.
This study investigated microtremor amplitude increases/decreases by comparing the spectral amplitudes recorded in 2020-2021 with those in 2019. Since most of the MeSO-net stations are deployed in schools, we classified the station environment as "next to school buildings", "in schoolyards", and "non-schools", and investigated how the observed microtremor amplitudes after 2020 changed during the pandemic according to the station environment. The results showed that microtremor amplitudes decreased at stations where seismometers are installed near school buildings. We also found that microtremor amplitudes decreased significantly on weekdays at schools with a large number of students. The same tendency was observed when we applied the microtremor monitoring method with the traffic of WIN32 format data (Hayashida et al., 2021SSJ Meeting). Our results reveal the importance of considering the environment around seismic stations when using microtremors as an indicator for monitoring socioeconomic activities. We will report the results of these analyses and introduce a new approach to monitor microtremor amplitude more easily without downloading MeSO-net waveform data.

Acknowledgments:
We used MeSO-net seismic data provided by the National Research Institute for Earth Science and Disaster Resilience (NIED). This study was conducted as part of a joint research project by members of the "Microtremor Research Camp (MRC)", which consists of researchers and engineers in Japan. We express our gratitude to Dr. Hiroyuki Fujiwara (NIED), the chairman of MRC, for his helpful advice.

Reference:
Hayashida et al. (2022), Amplitude reductions of microtremors at MeSO-net stations in 2020: Observations and interpretations, Journal of Japan Society of Civil Engineers, Ser. A1, Vol. 78 (in press)