Abstract. After the World Health Organization declared COVID-19 a pandemic
in March 2020, Romania followed the example of many other countries and
imposed a series of restrictive measures, including restricting people's
mobility and closing social, cultural, and industrial activities to prevent
the spread of the disease. In this study, we analyze continuous vertical
component recordings from the stations of the Romanian Seismic Network – one
of the largest networks in Europe, consisting of 148 stations – to explore the seismic noise variation associated with the reduced human
mobility and activity due to the Romanian measures against COVID-19 in
detail. We
focused our investigation on four frequency bands – 2–8, 4–14, 15–25 and 25–40 Hz – and found that the largest reductions in seismic noise
associated with the lockdown correspond to the high-frequency range of 15–40 Hz. We found that all the stations with large reductions in seismic noise
(>∼ 40 %) are located inside and near schools or
in buildings, indicating that at these frequencies the drop is related to
the drastic reduction of human activity in these edifices. In the lower-frequency range (2–8 and 4–14 Hz) the variability of the noise reduction
among the stations is lower than in the high-frequency range, corresponding
to about 35 % on average. This drop is due to reduced traffic during the
lockdown, as most of the stations showing such changes in seismic noise in
these bands are located within cities and near main or side streets. In
addition to the noise reduction observed at stations located in populated
areas, we also found seismic noise lockdown-related changes at several
stations located far from urban areas, with movement of people in the
vicinity of the station explaining the noise reductions.
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