Two-step Seismic Noise Reduction caused by COVID-19 Induced Reduction in Social Activity in metropolitan Tokyo, Japan
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.
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Figure S1. Average seismic noise level at the MeSO-net stations other than the four stations shown in Figure 4. Part 1
Figure S1. Average seismic noise level at the MeSO-net stations other than the four stations shown in Figure 4. Part 2
Figure S2. Noise ratio of UD components and polarization azimuth at the MeSO-net stations other than the four stations shown in Figure 7. Part 1
Figure S2. Noise ratio of UD components and polarization azimuth at the MeSO-net stations other than the four stations shown in Figure 7. Part 2
Figure S2. Noise ratio of UD components and polarization azimuth at the MeSO-net stations other than the four stations shown in Figure 7. Part 3
Figure S2. Noise ratio of UD components and polarization azimuth at the MeSO-net stations other than the four stations shown in Figure 7. Part 4
Figure S3. Median noise ratio during the day in two different frequency bands at the MeSO-net stations other than the four stations shown in Figure 8. Part 1
Figure S3. Median noise ratio during the day in two different frequency bands at the MeSO-net stations other than the four stations shown in Figure 8. Part 2
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Received 20 Aug, 2020
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Two-step Seismic Noise Reduction caused by COVID-19 Induced Reduction in Social Activity in metropolitan Tokyo, Japan
Posted 12 Jan, 2021
On 04 Nov, 2020
On 16 Oct, 2020
On 16 Oct, 2020
On 15 Oct, 2020
On 15 Oct, 2020
Received 07 Oct, 2020
Invitations sent on 25 Sep, 2020
On 25 Sep, 2020
On 24 Sep, 2020
On 23 Sep, 2020
On 23 Sep, 2020
Posted 24 Sep, 2020
On 14 Oct, 2020
On 13 Oct, 2020
On 13 Oct, 2020
On 31 Aug, 2020
Received 30 Aug, 2020
Received 20 Aug, 2020
On 08 Aug, 2020
On 30 Jul, 2020
Invitations sent on 29 Jul, 2020
On 28 Jul, 2020
On 27 Jul, 2020
On 25 Jul, 2020
On 23 Jul, 2020
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.
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