Yabe Suguru, Imanishi Kazutoshi, Nishida Kiwamu
Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567 Japan.
Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan.
Earth Planets Space. 2020;72(1):167. doi: 10.1186/s40623-020-01298-9. Epub 2020 Nov 4.
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.
始于2019年末的新冠疫情迫使全球各地的人们减少社会和经济活动；人们认为这可以防止疾病传播。在本文中，我们报告了对日本东京都地区在这种人为导致的社会活动减少期间的地震噪声的分析。利用从都市地震观测网络（MeSO-net）的18个台站获取的地震数据，在东京新冠疫情期间观测到了两步地震噪声降低。第一次噪声降低发生在2020年3月初，频率范围为20 - 40赫兹。这与日本首相要求全国学校停课相对应。尽管此时社会活动尚未显著减少，但在位于学校场地的一些MeSO-net台站记录到了高频段（20 - 40赫兹）地震波激发的局部降低。第二次地震噪声降低发生在2020年3月末至4月初，频率范围更宽，包括1 - 20赫兹的低频段。这个时间点分别对应东京都知事要求市民居家以及政府对东京都发布紧急状态声明的时间。从那时起，火车站的估计人流量急剧下降，这表明社会活动大幅减少。这种大规模的社会活动变化影响了低频段的地震噪声水平。地震噪声水平从5月中旬开始上升，这与火车站人流量的增加相关。这表明社会活动甚至在5月末紧急状态解除之前就已经重新开始。本研究中观测到的两步地震噪声降低在世界其他城市尚未见报道。新冠疫情导致的社会活动意外减少提供了一个难得的机会来研究人类活动引起的地震噪声的特征。
