High-frequency (> 1 Hz) ambient noise is usually closely related to anthropogenic activities. During the outbreak and spread of the COVID-19, as various anthropogenic activities are restricted, high-frequency ambient noise level has been observed to be reduced on a worldwide scale. The continuous waveform data at dense broadband seismic stations from ChinArray in eastern North China provides a good opportunity to study the temporal and spatial patterns of the ambient noise level in the region, and to further study the influencing factors, such as the topography and the population density. In this study, we calculated the average power spectral density of ambient noise at each station +/- 90 days around the Spring Festival in 2019 and in 2020, analyzed the noise level at different stations through normal times, Spring Festivals, epidemic control period, and recovery period, and studied the influencing factors of the noise level. We found that normally high-frequency (1-10 Hz) ambient noise correlates well with the surrounding sedimentary thickness: The noise level is higher when the surrounding sedimentary layer is thicker and vice versa. It correlates moderately with local population density and is time-varying due to anthropogenic activities. During the Spring Festival in 2019 and in 2020, and the epidemic control period after the Spring Festival in 2020, the reduction extent of the noise level correlates moderately with both the sedimentary thickness and population density; the ambient noise level reduces more significantly to the south of 40 degrees N than to the north of it in the study region. Considering that the sedimentary thickness beneath each station is not time-varying, the variation in ambient noise level due to anthropogenic activities is clearly amplified by the sedimentary layer.
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