On September 13, 2020, an earthquake with a magnitude of Mw = 4.8 occurred in the Uglegorsk district of the Sakhalin region. Residents of the localities nearest to the epicenter felt it with a force of up to 5 points on the MSK-64 scale. A total of 62 aftershocks were recorded. The seismic process lasted for about two days, the major mass of aftershocks was registered during the first 7 hours. The epicenters of the registered earthquakes are confined to the system of the regional West Sakhalin fault. However, the fact of active mining of brown coal at the Solntsevskii quarry and massive blasting in the epicentral zone of the Uglegorsk earthquake does not exclude the relation between the seismic process and technogenic seismicity. The article presents the results of observations of the seismic process in the epicentral zone of the Uglegorsk earthquake in September 2020 and considers the probable causes of its occurrence.

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With the outbreak of the COVID-19 pandemic, many seismologists all over the world have noted a sharp (up to 30–50%) decrease in the daily background seismic noise during the period from March to May, 2020 (Lecocq et al., Science 369(6509):1338–1343, 2020). The authors studied the influence of the self-isolation regime introduced in the Russian Federation from March 30, 2020 and, as a consequence, the restriction of the work of public institutions and the mobility of the population, on the quality of seismological observations at seismic stations in large cities of the Russian Far East for the period from March 23, 2020 to April 12, 2020. The work analyses the records of seismic noise by the seismic stations of Khabarovsk and Vladivostok located in busy parts of the cities and, accordingly, strongly influenced by anthropogenic impact, as well as it analyses the records of the Yuzhno-Sakhalinsk seismic station located in the relatively «calm» part of the city. Power spectra and temporal variations of microseismic noise levels for the listed above seismic stations were constructed based on the data of broadband seismometers records in the range of 1–20 Hz. The analysis of noise level variations with the data on the population mobility was carried out on the basis of self-isolation index by Yandex, which shows the level of town activity over a selected period. The main sources of the increased microseismic noise at seismic stations were identified.

To conduct complex geophysical observations in the area of the Central Sakhalin Fault, an experimental geophysical test site was created, including a broadband molecular electronic type station and a new development of the Molecular Electronics Center - molecular electronic hydrophone. A module for placing a molecular-electronic seismometer at the test site has been developed and arranged. The noise level of the seismometer was analyzed and compared with the reference data. The results of hydroacoustic observations of weak earthquakes in the area of the Central-Sakhalin fault using a molecular-electron hydrophone are presented. The characteristics of the recorded acoustic emission signals from earthquakes perceived by the hydrophone station (frequency spectrum, arrival times of seismic waves) are investigated. The issues of operation and maintenance of molecular-electronic sensors with seismic signal data logger at various stages of seismological observations (setting up equipment, acquisition and storing data) were considered. The analysis of recording capabilities of seismic equipment was performed. Conclusions about the advantages and disadvantages of molecular-electronic sensors and their use on Sakhalin are made.