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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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<p>The lockdown measures taken to control and stop the spread of the novel coronavirus (COVID-19) in cities around the globe caused an unprecedented reduction of anthropic activities. The signature of this reduction, different from one place to another, has been captured by the seismic stations installed in the urban areas where lockdown restrictions have been implemented. Bucharest, the capital of Romania, was no exception from this phenomenon.</p><p>In this paper, we investigate the effect of the COVID-19 lockdown measures imposed by the Romanian authorities on the high-frequency ambient seismic noise (ASN) data recorded by the Bucharest Metropolitan Seismic Network (BMSN). BMSN consists of 26 stations of which 19 are equipped with strong motion sensors and 7 have both short-period velocity and accelerometer sensors. All the stations are continuously recording the ground motion and the data is sent in real-time to the data center of the National Institute for Earth Physics.         </p><p>The reduction of ASN was first observed at stations installed in educational units (kindergartens, schools) starting with 11th of March 2020, when the Romanian government decided to close the schools in Romania. For these stations, the largest reduction of ASN, up to 82%, was noticed in the 25-40 Hz frequency band. On 16th of March the state of emergency was imposed in Romania and a few days later, on 25th of March, the stay-at-home order was issued. These new restrictions caused substantial reduction in urban traffic and people’s mobility and reflected in significant reduction of ASN at almost all the other BMSN stations, located either free-field or in buildings. For these stations, we observed a decrease of the noise levels by as much as 66% in the 15-25 Hz frequency band. We also correlated the ambient seismic noise with other types of data that might be affected by human activity, such as the mobility data from Google and Apple, and we found good correlation between ASN in different frequency bands and various mobility data categories. Finally, we showed that the noise reduction due to lockdown measures improved the signal-to-noise ratio of the stations in the Bucharest area, allowing us to record smaller earthquakes which otherwise would not have been recorded.</p>

<p>In September–November 2013 a seismic swarm occurred in Galati region of southeastern Romania. The area was previously known as characterized by low seismic activity along the major crustal faults. During the period of swarm, between September 23rd and November 5<sup>th</sup>, over 1000 events with the magnitudes (Ml) of 0.2–4.0, located at the depth of 5–10 km, have been detected. Despite the relatively small magnitude, events generated ground motions that were well felt by local people, leading to panic in the area. The proximity of active oil fields caused additional annoyance.</p><p>Advanced seismic monitoring in the region started in 2013 with deployment of mobile seismic stations immediately after the beginning of the swarm. Additionally, active seismic measurements were performed in order to characterize the shallow velocity structure at specific sites. Starting from July 2015 new permanents stations were installed in the area marking the beginning of Galati local network development. The routine seismic catalog derived using the acquired data and applying the standard detection and location techniques pointed that area continues to be seismically active, however with low rate of activity and magnitude of events. These made it a perfect study case for development of new advanced schemes for seismic monitoring of the regions with low and complex seismicity aiming on an understanding of the phenomenon underlying the 2013 seismic swarm as well as the current seismic activity in the area.</p><p>We developed and automatic monitoring scheme based on the network-based full waveform detection and location method BackTrackBB (Poiata et al. 2016) that exploits the coherency of signals’ statistical features recorded across the seismic network. Once extracted from the flux of continuous data, seismic events are compared against the database of previously detected events using coherency and allowing to identify potential repeaters or multiplets. The earthquake catalog provided by the system starting from 2017 was compared to the routine ROMPLUS catalog of NIEP showing an increase in the number of detected events by the order of 3. We present the details of the implementation and discuss its advantages and drawbacks.</p>