Tracking the Effect of Human Activity on MeSO-net Noise Using Seismic Data Traffic—Did Seismic Noise in Tokyo Truly Decrease during the COVID-19 State of Emergency?

Abstract

Abstract Human activities cause seismic noise over 1 Hz (cultural noise), and the recent articles have reported that the curtailing of socioeconomic activities during the novel coronavirus (COVID-19) pandemic in 2020 appeared to reduce high-frequency seismic noise amplitudes in cities. The Tokyo metropolitan area in Japan, where seismic stations are densely distributed and various anthropogenic activities have been closely monitored, is an ideal study area to investigate the effect of human activity on high-frequency seismic noise during the pandemic. We demonstrated that the magnitude of seismic data traffic (SDT), indexed by the packet size of continuous seismic data in WIN32 format, is a good indicator for monitoring time-dependent changes in high-frequency noise levels. The SDT of 169 Metropolitan Seismic Observation network (MeSO-net) stations—a continuous accelerometer network that is mostly located at schools in the Tokyo metropolitan area—decreased by approximately 1%–3% from March to June 2020, when a state of emergency in Japan was first declared, compared with that in the previous year. We revealed that the SDT decrease was prominent only at stations near school buildings, and the SDT trend was uncorrelated with the temporal changes in the population and vehicular traffic volume near the seismic stations. We also found strong correlations between the SDT reduction and school size (classified by the number of students enrolled), implying that the noise decrease at the MeSO-net stations during the pandemic was strongly influenced by school-based activities. Thus, the noise reduction observed at MeSO-net stations during the COVID-19 pandemic in 2020 did not provide strong evidence of quieting in the Tokyo metropolitan area.