Seismic noise and blasting activity change in Japan related to COVID-19 pandemic

Abstract:

On April 7, 2020, the Government of Japan declared a state of emergency to the urban area including Tokyo where the number of patients with COVID-19 was increasing. After expanded it nationwide on April 16, it was lifted on May 25. During this period, the Government just requested to the people to stay their home. There was no legal prohibition to go out, and certain levels of social activity continued. As already reported in many countries (Lecocq et al., 2020), seismic noise reduction related to COVID-19 pandemic has been observed in Tokyo, Japan (Yabe et al., 2020). In this study, we checked the nationwide seismograph networks' data and confirmed the regionality of the impact of COVID-19 to our life.

First, we checked the RMS of the NIED Hi-net and F-net data. Hi-net and F-net stations are located where seismic noise by human activities is as low as possible to observe weak seismic signals. However, we can see daily and weekly noise-level change related to the human activities. Although seasonal noise-level change is large at many Hi-net and F-net stations, we confirmed the RMS reduction during the daytime at the stations near the city after April 2020. On the other hand, at some stations in suburb or mountain area especially in Tohoku and southern Kyushu, the noise-level related to factory operations and/or traffic activities showed no change as before February 2020. Number of infected people was relatively small in these areas.

Next, we confirmed the change of blasting activity since 2018. By the automatic hypocenter determination system operated by NIED, both natural and artificial events can be detected if the seismic signal has enough amplitude. Basically, the natural earthquakes do not select the time of occurrence, but the blasting are operated in the daytime. As the ratio of day and night events should be almost the same if the catalog contains natural earthquakes only, we can say the ratio of daytime events is related to the blasting activity. In central Japan, the ratio was kept about 60 % before February 2020, but it reduced to about 50 % in April 2020. This feature was similar to the RMS change at stations in the urban area. The ratio was almost constant to 60-65% and 60% in northern and western Japan, respectively, and no significant change was detected.