Spatial and temporal variation of the ambient noise environment of the Sikkim Himalaya

Ambient noise characteristics are perused to assess the station performance of 27 newly constructed broadband seismic stations across Sikkim Himalaya and adjoining Himalayan foreland basin, installed to study the seismogenesis and subsurface structure of the region. Power spectral densities obtained at each station, compared against the global noise limits, reveal that observed vertical component noise levels are within the defined global limits. However, the horizontal components marginally overshoot the limits due to the tilt effect. Ambient noise conditions significantly vary with different installation techniques, analysis revealing that seismic sensors buried directly in the ground have reduced long-period noise in comparison to pier installations. Tectonic settings and anthropogenic activities are also noted to cause a significant rise across short-period and microseism noise spectrum, varying spatially and temporally across the region. Day-time records higher cultural noise than night-time, while the microseism noise dominates during the monsoonal season. An assessment of the effect of the nationwide lockdown imposed due to COVID-19 pandemic revealed a significant decrease in the short-period noise levels at stations installed across the foreland basin marked with higher anthropogenic activity. Our study summarizes the overall ambient noise patterns, validating the stability and performance of the seismic stations across the Sikkim Himalayas.

GEODYNAMICS

The aim of the work is to determine the possibility of using local seismological networks of nuclear power plants as elements of the seismological monitoring system of the territory of Ukraine. Estimation of local seismicity and specification of quantitative parameters of seismological influences is carried out on the basis of materials of seismological observations. Operational processing and analysis of seismic signals registered on the elements of local seismological networks of NPPs is carried out by the Main Center for Special Control of the State Space Agency of Ukraine (MCSC SSA of Ukraine). In the process of implementation of the “Seismic Hazard Assessment and Seismic Resistance Action Plan” of existing NPPs, seismic monitoring networks were deployed at Ukrainian NPPs. To date, the MCSC receives data from the local seismological networks of Rivne, Khmelnytsky and Zaporizhzhya NPPs in a continuous mode. The processing of geophysical information coming from the NPP to the FSC is carried out by the operational next shift of the center with the help of FSC hardware and software and provides reliable data on the parameters of seismic sources registered by stations, their location and energy characteristics. In total, in the period from 2017 to 2021, 36 local earthquakes were registered by NPP seismic networks in Ukraine. The epicenters of the vast majority of them are located within the Ivano-Frankivsk, Ternopil and Lviv regions. The experience of conducting instrumental observations at NPP seismic stations testifies to their high efficiency and possibility of use as full-fledged elements of the seismological monitoring system of the territory of Ukraine. According to the results of primary processing of seismic data of 2017-2021, a catalog of seismic events registered by NPP seismic stations was created. The system of interpretation of the obtained results was improved, which allowed to determine equally well local, regional and teleseismic events of different nature and energy level. The practical significance of the obtained results lies in their direct focus on solving a number of practical problems of processing and interpretation of seismological data. The use of NPP seismic stations as elements of the general system of seismological monitoring of Ukraine will increase the reliability of detection and localization of sources and the probability of correct identification of the nature of seismic phenomena, which in turn will improve the assessment of tectonic structures in Ukraine.

Are Synthetic Accelerograms Suitable for Local Seismic Response Analyses at Near-Field Sites?

ABSTRACT Local seismic response (LSR) studies are considerably conditioned by the seismic input features due to the nonlinear soil behavior under dynamic loading and the subsurface site conditions (e.g., mechanical properties of soils and rocks and geological setting). The selection of the most suitable seismic input is a key point in LSR. Unfortunately, few recordings data are available at seismic stations in near-field areas. Then, synthetic accelerograms can be helpful in LSR analysis in urbanized near-field territories. Synthetic accelerograms are generated by simulation procedures that consider adequately supported hypotheses about the source mechanism at the seismotectonic region and the wave propagation path toward the surface. Hereafter, mainshocks recorded accelerograms at near-field seismic stations during the 2016–2017 Central Italy seismic sequence have been compared with synthetic accelerograms calculated by an extended finite-fault ground-motion simulation algorithm code. The outcomes show that synthetic seismograms can reproduce the high-frequency content of seismic waves at near-field areas. Then, in urbanized near-field areas, synthetic accelerograms can be fruitfully used in microzonation studies.

Seismic activity on the territory of Slovakia in 2020

The National Network of Seismic Stations of Slovakia (NNSS) consists of eight short period and six broadband permanent seismic stations and a data centre located at the Earth Science Institute of the Slovak Academy of Sciences (ESI SAS). The NNSS recorded and detected 11229 seismic events from all epicentral distances in 2020. Totally 96 earthquakes originated in the territory of Slovakia in 2020. This paper provides basic information on the configuration of the NNSS, routine data processing, seismic activity on the territory of Slovakia in 2020 as well as macroseismic observations collected in 2020.

Features of time variations of microseismic noise at seismic stations in Tajikistan

The spectral structure of microseismic noise in the frequency range of 0.01-40 Hz at different times of the day and year, recorded by broadband equipment at eight IRIS group seismic stations in Tajikistan in 2005-2020, was analyzed. Two disjoint frequency ranges are distinguished, which we conditionally call "high-frequency" (2-40 Hz) and "low-frequency" (0.01-0.75 Hz) noise, separated by a natural drop in the noise amplitude to 20-30 Db. It is assumed that the high-frequency range of noise has a local nature, due to exogenous sources of natural origin in the form of wind gusts, concussions from powerful watercourses and fluctuations in the level of large reservoirs, as well as man-made in-terference due to road and quarry explosions, the work of large industrial enterprises and concussions from road traffic. Low-frequency noise is most likely caused by global storm microseisms. High-frequency noise has a well-defined daily frequency, which is completely absent in low-frequency noise. At the same time, in both frequency ranges, the existence of a clearly pronounced seasonal peri-odicity has been established, the amplitude of which reaches 6-7 Db for high-frequency noise and about half as much for low-frequency noise. However, at the same time, the seasonal frequency of high frequency and low-frequency noise turns out to be antiphase, which indicates in favor of the different genesis of these two components of microseismic noise. The amplitude of the diurnal periodicity in variations of the high-frequency noise level is maximal during the daytime, remaining approximately constant for 8-10 hours. At the same time, the decline in the noise amplitude in the evening lasts longer than the steeper morning growth. The time intervals of a sharp increase and decrease in the intensity of the discussed daily extreme are quite well correlated, respectively, with morning and evening twilight at different times of the year. This is reflected in the wider flat part of the maximum noise level in summer compared to winter and the differences in its level up to 6 Db in favor of summer time. This observation can be considered as a manifestation of the deep influence of the Sun on the oscillatory processes that generate high-frequency microseismic noise.

SEISMICITY of the RUSSIAN PART of EAST EUROPEAN PLATFORM and ADJACENT TERRITORIES in 2015

It is reported that 41 stationary seismic stations, 2 arrays, and 7 temporary seismic stations, located in the area of Novovoronezh and Kursk nuclear stations, monitored seismicity of the Russian territory of the East European Platform (EEP) in 2015. The registration capabilities of the seismic network at the EEP as a whole were estimated based on the average station noise level and the equation for the energy decay of seismic phases. Zones with the best capabilities have been allocated. A feature of seismicity in 2015 is the manifestation of earthquakes of moderate magnitudes (ML=2.7–3.9) in the peripheral regions (in the southwest, west, and northwest) and in zones associated with paleorift structures: in the southwest – with the Dnieper Donetsk and in the northeast – with the Kirov-Kazhim and Soligalich (Central Russian) aulacogenes. The results of the macroseismic survey are given for the earthquake in Poltava on February 2, 2015, with M=3.7; focal mechanisms of two earthquakes (03.02.2015 and 12.06.2015) are constructed. According to the data of the Latvian Center, an earthquake was recorded in the region of Lithuania bordering the Kaliningrad region. Weaker natural seismicity with ML≤2.5 was recorded in Karelia and the regions bordering with Finland, near the Kandalaksha Bay, near the Khibiny, and Lovozersky massifs on the Kola Peninsula, and on the territory of the Voronezh crystalline massif.

SEISMISITY of THE VOLCANIC AREAS of KAMCHATKA in 2015

The results of near real-time monitoring of the active Kamchatka volcanoes are described. Continuous monitoring was carried out using three remote methods: 1) seismic monitoring according to automatic telemetric seismic stations; 2) visual and video observation; 3) satellite observation of the thermal anomalies and the ash clouds. Annual results of seismic activity of the Northern (Shiveluch, Kluchevskoy, Bezymianny, Krestovsky, and Ushkovsky), the Avacha (Avachinsky, and Koryaksky), the Mutnovsky-Gorely volcano groups and the Kizimen volcano are presented. 5464 earthquakes with КS=1.8–8.1 were located for the Northern volcano group, 302 earthquakes with КS=1.7–5.7 – for the Avacha volcano group, 295 earthquakes with КS=2.1–6.8 for the Mutnovsky-Gorely volcano group, 462 earthquakes with КS=2.2–8.3 for Kizimen volcano, and 165 earthquakes with КS=2.5–8.4 for Zhupanovsky volcano in 2015. Maps of epicenters, quantities of seismic energy and earthquake distribution by energy classes are given. All periods of activity were fixed and investigated by remote methods in 2015: intensive volcanic activity of the Sheveluch volcano associated with a new cone; the summit explosive-effusive eruption of the Kluchevskoy volcano in January–April; and a continuation of seismic and volcanic activity of the Zhupanovsky volcano after 56-year quite period.

SPECTRAL and FOCAL PARAMETERS of NORTHERN CAUCASUS EARTHQUAKES in 2015

The dynamic sources parameters of 29 earthquakes of the North Caucasus for 2015 with KR=9.5–11.4, determined from 38 S-wave amplitude spectra are analyzed. Records of four regional digital seismic stations, “Anapa”, “Kislovodsk”, “Makhachkala” and “Sochi”, located no more than 300 km from the sources are used. For the environment near these stations, the values of the frequency-dependent Q-factor, necessary for recalculating the station spectra to the focal ones. In 2015, the majority of earthquakes studied were located in the Eastern Caucasus. The dependence between log M0 and KR for this zone was constructed using together with the data for 2010–2014.

SEISMICITY of the ALTAI and SAYAN region in 2015

A review of seismicity of the Altai and Sayan region in 2015 is presented. The stationary network of seismic stations included 52 stations. Compared to 2014, four new seismic stations were added in the region. Additionally, two local temporary networks including 37 temporary seismic stations operated in the region. 6206 earthquakes were recorded with a total seismic energy of 5.97•1012 J. A brief description of the seismicity in the Chui-Kurai zone of the Altai Mountains is given.

SEISMICITY of YAKUTIA in 2015

The results of monitoring the seismicity of the territory of Yakutia, based on observations of 23 digital seismic stations, are presented. A total of 607 earthquakes were recorded. A map of epicentres was compiled and their location was given in seismically active regions. The most active areas are Olekminsk in the south, Verkhoyansk and Chersky Ridge in the northeast and the Arctic part of the region. A significant decrease (19 times) in the amount of released seismic energy continued in comparison to its level in 2014. Instrumental and macroseismic data, focal mechanisms and seismotectonic connections are analyzed. The characteristics of the shaking South-Verkhoyansk earthquake occurring at the junction of the Verkhoyansk and Sette-Daban Ridges are considered.