Vol 54, No 3 (2018)

Requirements for the seismic resistance of nuclear power plants (NPPs) considerably differ from those for of other industrial facilities due to the necessity of protecting not only the NPP itself, but also the environment from radiation hazard. According to special regulations, even if an NPP construction site is located in a region with low seismicity, the NPP design should be based on the maximum design earthquake and peak acceleration typical of the region. Note that seismic warning systems for NPPs differ fundamentally in design and operation principles from conventional seismic monitoring systems. Seismic warning systems are required to reliably detect seismic events threatening the operability of nuclear power plants, to promptly inform operators, and to prevent false alarms and unauthorized access to the system. Atomseismoizyskaniya LLC has developed a seismic warming system (SSZ-1M) and put it into trial operation at the reactor units of the Smolensk NPP in 2014–2015. The system comprises six seismic groups: two groups per each reactor unit. Each group consists of three independent seismic recording channels, the sensors of which are distributed around the reactor unit at a distance of up to 500 m from the latter. Each channel records three components of seismic signals; the acceleration vector is calculated and compared with a preset threshold. Signals exceeding the threshold are fed to a coincidence circuit that generates an alarm if two of the three signals exceed the threshold. This scheme excludes false alarms. The components of the system are characterized by high reliability and stability. The performance of the system channels has been officially certified. The system automatically performs continuous fault diagnosis. The programs responsible for system function and diagnostics are independent; the system units have individual equipment. To exclude the possibility of external impacts, the SSZ-1M warning system has no Internet connection. Special seismic signal simulators were developed to test the operability of the system when deployed.

All digital seismic recorders of the Crimean seismological network are equipped with sensors inherited from galvanometric recording. Long-term channels are equipped with SKD sensors, and shortterm channels are equipped with SM3, SKM, SKH, and S-5-S sensors. The instruments periodically undergo preventive maintenance due to wear and long operation life. There are no equipment specialists at peripheral stations. All necessary installation, preventive maintenance, adjustment, and calibration work is done by specialists from the Institute of Seismology and Geodynamics’ hardware and software support team during business trips. In this regard, an urgent need arose for the real-time control of velocimeter parameters and the entire end-to-end measuring and recording channel. Experience at peripheral seismic stations and temporary seismic observation points not equipped with shaking tables has demonstrates the convenience of calibrating short-term seismic channels using a reference standard calibrated mobile digital seismic station. This station is assembled based on a Baikal-8 digital seismic recorder and SK1-P three-component sensor. The reference standard is precalibrated on the shaking table located at the Simferopol seismic station. The paper describes the calibration of the reference standard on a shaking table and different approaches to calculating the main parameters of a velocimeter. The relationship is obtained between the main parameters of the velocimeter and the extremum times and zeros of the velocimeter response function to a displacement step. It is possible to control the velocimeter parameters in real time the obtained formulas, even in the field using. Of special note is the idea of calibrating seismometric channels with respect to the reference standard, which can be implemented without forced external actions. A new method for calibrating a digital seismic station using a reference standard is described, based on analysis of the ratio of microseismic background spectra. The parameters of the calibrated station are determined using synchronous recording of the microseismic background and the mathematical optimization package in OriginLab software. The formulas for calculating velocimeter parameters using a standard are given, which make it possible to determine the complex frequency response of the digital station. This is a convenient method because it is real-time and calibration does not require laboratory conditions, the use of microseisms, or the ability to determine all parameters of endto- end measuring and recording channels simultaneously.

Earthquakes that occur on the territory of Vietnam and especially in the South China Sea are a major hazard to the Vietnam population and infrastructure because of possible destruction and tsunamis, like those took place there in earlier times according to geological data. Therefore, in the early 2000s, the government of Vietnam decided to modernize and expand the network of seismological observations on the territory of the republic. First, it was planned to modernize the seismic network in Vietnam with 20 broadband seismic stations and then increase that number to 30. This raised the problem of the optimal arrangement of these stations in the country to predict earthquakes and study the structure of the crust and upper mantle of Vietnam. The map of Vietnam clearly shows the impossibility of constructing a single optimal observation network to locate earthquake hypocenters over the entire territory, because it is strongly elongated from north to south. Any seismological observation network for such an entire territory will not be optimal. In this case, we can speak about improving local observation networks for some areas, such as the north, center, and south of the country. This work estimates the efficiency (distribution of the minimum representative magnitudes and the error in determining the coordinates of earthquake hypocenters) for the new network of seismological observations in Vietnam, consisting of 30 stations. To improve the quality of the network in central and southern Vietnam, six more seismic stations are proposed. Such a network will allow more accurate determination of the hypocenter parameters in central and southern Vietnam. During the construction of the optimal network configuration, one of the main problems was the choice of an effective network radius. The formula for determining the optimal radius in the seismic observation network is obtained for the case of a uniform distribution of hypocenters in a certain cylindrical region based on the radius of the base and height of this cylinder. In this work, it follows from the formula that the radius of the optimal network should be no less than the radius of the hypocentral region. In our case, the choice of network radius is confined within the state borders and coastline of Vietnam. These restrictions are taken into account in the calculations to optimize the number of seismic stations and the configuration of their location in the country.

A standard complex of geomorphologic methods, including identification of aerial photos and space images, topographic and structural–geomorphologic area survey, trenching colluvial sediments and their mapping, and sampling of paleosoils and their dating by the radiocarbon method, was used for identification, parametrization, and dating of seismic dislocations of the Karelian coast of the White Sea. A set of kinematic indicators of paleoearthquakes (mass displacements and systematic rotations of fragments of rock ledges), which make it possible to interpret the directions of maximum seismic impact on detailed areas, is elaborated and tested. In the relief of the rock massifs of the Kindo Peninsula, these methods revealed a halo (10 × 6 km) of secondary seismic dislocations with a radiocarbon age of no more than 5.5 ka, which is a zone (4 × 2 km) of extension fractures and numerous displacements of stones surrounded by a belt of seismic gravitation faults. It is shown that some ledges and stepwise faults in the relief of these stones probably resulted from glacial denudation and further erosion of structural heterogeneities. At the same time, displacements of chipped stones versus inclination and their systematic rotation in rock ledges of different strike suggest intense seismic impacts after the formation of the stepwise surfaces and termination of their abrasion after glacial rebound of the territory. It is found that high-frequency seismic oscillations with high values of peak accelerations (0.4–0.8 g) and velocities (100–300 cm/s) are necessary for the formation of stone displacements. Kinematic indicators are used to reconstruct the directions of maximum seismic impact and determine the position of the epicenter of a paleoearthquake at several points. The zones of intensity of 7 and 8 are contoured to estimate the depth of the focus (H = 1.9 ± 0.2 km) and magnitude (M = 4.4 ± 0.2) of a seismic event using the macroseismic field equation. Typical WNW elongation of the first isoseist along the northern coast of the Kindo Peninsula is indicative of a seismogenic fault at the southern end of a micrograben of the Velikaya Salma Strait, which feathers the southeastern wall of the Kandalaksha Graben. The Holocene activity of this fault is confirmed by normal fault displacements of young sediments, which have been revealed in a series of transverse seismoacoustic profiles. These results quantitatively showed for the first time that the zone of the Kandalaksha Graben could provide conditions for low-magnitude "shallow-focused" earthquakes with high seismic intensity.

The upper crustal location of the foci of the Spitak earthquake of December 7, 1988, and its aftershocks is proved. It is demonstrated that the seismological network in Armenia and Georgia allowed unequivocal focal depth identification based on the kinematic data. The waveforms of all the Spitak earthquakes are typical of sources in the upper crust. It is noted that up to now, mantle earthquakes in the Caucasus have only been revealed in the Terek–Sunzha depression.

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.

It is well known that the results of determining earthquake parameters depend to a large extent on data processing algorithms and velocity models of the seismic wave propagation medium used in solving hypocenter problems. In 1992, V.Yu. Burmin developed a hypocentric algorithm that minimizes the functional of distances between the points corresponding to the theoretical and observed travel times of seismic waves from an earthquake source to recording stations. The determination of the coordinates of earthquake hypocenters in this case is much more stable than for the commonly used minimization of the functional of discrepancies in the seismic wave arrival times at a station. Using this algorithm and the refined velocity model of the medium, V.Yu. Burmin and L.A. Shumlyanskaya reinterpreted the earthquake parameters for the Crimea–Black Sea region. The most important result of this reinterpretation was the conclusion about the occurrence of deep earthquakes with a source depth of more than 60 km in the region. This result contradicts the conventional beliefs about the seismicity of the region and therefore aroused strong criticism from experts directly involved in compiling the existing catalogs of regional earthquakes. These comments and criticisms are presented by V.E. Kulchitsky with coauthors in a work published in this issue of the journal. In the present paper, we analyze the comments in detail and respond. In particular, we show that the previously used methods of seismic data processing made it highly unlikely by default that deep earthquakes would appear in the results. As an example, we refer to the use of travel-time curves for depths down to 35 km. It is clear that deep earthquakes could not have been found with this approach.