AMPLITUDE REDUCTIONS OF MICROTREMORS AT MeSO-net STATIONS IN 2020: OBSERVATIONS AND INTERPRETATIONS

<p> Due to decreased socio-economic activities in response to the Novel Coronavirus Disease (COVID-19) pandemic in 2020, amplitude reductions of high-frequency (>1 Hz) microtremors (seismic ambient noise) had been reported at some seismic stations in areas around Tokyo by some researchers. This study investigated the existence of amplitude decreases of the microtremor records obtained at each of 169 MeSO-net (Metropolitan Seismic Observation network) stations in and around Tokyo using continuous seismic data from January 2019 to December 2020. We compared the variations of microtremor spectral characteristics with the number of populations, the traffic volume around each station, and the number of students in schools near the location of the seismic stations before and during the pandemic. We found that (1) amplitudes of microtremor between 20 and 90 Hz decreased from early March 2020 at many MeSO-net stations regardless of the number of population, (2) amplitude reductions below 10 Hz were also observed during the first state of emergency (April 7–May 25, 2020), (3) amplitude reductions during the first state of emergency were more significant (60–80 % of 2019 levels) at stations installed in schools with large numbers of students, and (4) there were no significant amplitude reductions at seismic stations in deep boreholes (>1900 m). These observations indicate that the microtremor amplitude reductions in 2020 were mainly affected by local phenomena where seismic sensors were installed.</p>