The COVID-19 pandemic that started at the end of 2019 forced populations around the world to reduce social and economic activities; it is believed that this can prevent the spread of the disease. In this paper, we report an analysis of the seismic noise during such an induced social activity reduction in the Tokyo metropolitan area, Japan. Using seismic data obtained from 18 stations in the Metropolitan Seismic Observation Network (MeSO-net), a two-step seismic noise reduction was observed during the timeline of COVID-19 in Tokyo. The first noise reduction occurred at the beginning of March 2020 in the frequency band of 20–40 Hz. This corresponded with the request by the Prime Minister of Japan for a nationwide shutdown of schools. Although social activity was not reduced significantly at this juncture, local reduction of seismic wave excitation in the high-frequency band, 20–40 Hz, was recorded at some MeSO-net stations located in school properties. The second reduction of seismic noise occurred at the end of March to the beginning of April 2020 in a wider frequency band including lower frequency bands of 1–20 Hz. This timing corresponds to when the Governors of the Tokyo metropolitan area requested citizens to stay home and when the state of emergency was declared for the Tokyo metropolitan area by the government, respectively. Since then, the estimated population at train stations abruptly dropped, which suggests that social activity was severely reduced. Such large-scale changes in social activity affect the seismic noise level in low-frequency bands. The seismic noise level started to increase from the middle of May correlating with increase in population at the train stations. This suggests that social activity restarted even before the state of emergency was lifted at the end of May. The two-step seismic noise reduction observed in this study has not been reported in other cities around the world. Unexpected reduction of social activity due to COVID-19 provided a rare opportunity to investigate the characteristics of seismic noise caused by human activities.

中文翻译：

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由 COVID-19 引起的两步地震降噪导致日本东京都市区社会活动减少

始于 2019 年底的 COVID-19 大流行迫使世界各地的人们减少社会和经济活动；据信，这可以防止疾病的传播。在本文中，我们报告了日本东京都市区在这种诱发社会活动减少期间的地震噪声分析。使用从大都会地震观测网络 (MeSO-net) 的 18 个台站获得的地震数据，在东京 COVID-19 的时间线期间观察到了两步地震降噪。第一次降噪发生在 2020 年 3 月上旬，频率为 20-40 Hz。这与日本首相提出的在全国范围内关闭学校的要求相符。虽然此时社交活动并没有明显减少，在位于学校物业的一些 MeSO-net 站记录了 20-40 Hz 高频段地震波激发的局部减少。地震噪声的第二次减少发生在 2020 年 3 月底至 2020 年 4 月初，覆盖范围更广，包括 1-20 Hz 的较低频段。这个时间分别对应于东京都知事要求市民留在家中的时间和政府宣布东京都区进入紧急状态的时间。从那以后，火车站的估计人口突然下降，这表明社交活动严重减少。社会活动的这种大规模变化会影响低频段的地震噪声水平。地震噪声水平从 5 月中旬开始增加，这与火车站人口的增加有关。这表明，甚至在 5 月底解除紧急状态之前，社会活动就已经重新开始。本研究中观察到的两步地震降噪在世界其他城市尚未见报道。COVID-19 导致的社会活动意外减少为研究人类活动引起的地震噪声特征提供了难得的机会。