Vol 55, No 4 (2019)

A method for experimental studies of crustal structure and composition in marine areas during warm and cold seasons is described. The method employs an electromagnetic low-frequency hydroacoustic transmitter, which generates complex phase-shift keyed signals (M-sequences) in water with a center frequency of 33 Hz, and a mobile laser strainmeter with a measuring arm length of 6 m. In an area with a sea depth of 14 m, the transmitter was lowered to a depth of 12 m. Processing of the obtained experimental data revealed five arrivals of transmitted phase-shift keyed signals to the laser strainmeter, which propagated along the layer boundaries of the upper crust of marine areas. The propagation velocities of these signals have been determined. In winter, they are approximately 2600, 2140, 1750, 1550, and 1280 m/s; in spring, 2250, 1950, 1700, 1480, and 1300 m/s. The calculated velocities agree well with the model data. Future results will make it possible to create a technology to remotely study crustal structure and composition in shelf zones, including shelf zones covered by ice, without destroying it.

The paper addresses marine broadband seismic data acquisition technologies, including conventional seismic oil exploration (frequencies no higher than a few tens of hertz) to map the subsurface down to several kilometers and engineering seismic surveying (frequencies from a few hundred to a few thousand hertz) to characterize sediments within the upper several hundred meters below the seabed. Various methods and approaches used in the two kinds of surveying have a lot in common, although they belong to different specializations. For the first time, this problem is discussed from a single point of view. We used published materials by foreign geophysical companies and experimental data obtained with acquisition techniques that we modified for two-tiered observations. It is shown that the modified techniques are efficient in the Arctic, especially for studying permafrost in upper subseabed sediments. This paper also discusses the application of towed recording systems and ocean floor multicomponent streamer cables and stations, including those using fiber optic technologies. Fiber optic receiver systems are most efficient for seismic time-lapse monitoring of oil and gas reservoirs during production (4D seismic). Analysis of the common problems with conventional and high-frequency marine seismic surveys allows the conclusion that it is time to revise the long-standing concept of marine seismic data acquisition. One possible way is to develop an integrated system for the acquisition, processing, and interpretation of seismic data in all frequency ranges. Our findings can aid in developing a methodology for geophysical surveying in hard-to-reach water areas.

Based on data from regional seismic networks and experiments employing temporary networks, the degree of seismic impact of industrial blasts in southwestern Siberia is studied. Since the blasts at all mining enterprises were ripple-fired shots, magnitudes are studied proceeding from the total explosive charge and the dependence of magnitudes on the delay stage. On average, there is a slight increase in magnitudes with increase in total charge and the charge of a stage, but strong scatter of data from the averaged straight line is observed. It is quite common that large magnitudes are recorded for blasts with smaller charges of stages. The noted experimental features are explained by the coincidence of triggering time for wells in different rows: this intensifies the interference of seismic impact of incorrectly detonated blasts (the technology of ripple-fired shots is not rigorously implemented). The monitoring system made it possible to study the seismic impact of mining enterprises recovering mineral resources in the regional subsurface. Comparison with the recorded induced seismicity shows that open pit mines with the strongest industrial blasts do not demonstrate the strongest induced seismicity. The absence of technogenic earthquakes near excavations with high explosive energy has been established and, on the contrary, the existence of induced seismicity has been revealed in places where there is no direct blast impact on the activated zone.

The study examines the preparation of the recent Ural earthquake of September 4, 2018, using the seismic entropy method. Based on the hierarchy of seismic systems, specific features of the occurrence of weak but perceptible earthquakes in intraplate zones are analyzed, and a classification of such seismic systems is given at an energetically lower level. The technology makes it possible to interpret unusual seismicity in the past, to identify the dynamics of slow accumulation and release of stresses over tens and hundreds of years, and to assess the possibility of a stronger earthquake in the future. The conditions for the occurrence of a perceptible earthquake in the area of the Beloyarsk nuclear power plant have been quantitatively studied. It is shown that the seismotectonic features of structures in the Urals cannot lead to preparation of a strong destructive earthquake, but monitoring the occurrence of weak but perceptible earthquakes is important for preventing possible man-made and environmental disasters at industrial facilities and nuclear power plants.

Assessing the seismic hazard of stable continental areas, where modern seismic activity is insignificant, is a complex task that needs to be solved in order to ensure the safety of people and buildings in these territories. Traditional methods, including seismotectonic research, seismological monitoring, identification of seismic source zones (SSZs), and subsequent hazard assessment, are ineffective in such areas due to the lack of representative earthquake statistics and low informativeness of the data. In the seismic zoning of such areas (especially for solving seismic safety problems for critical objects, such as nuclear power plants), one cannot limit oneself to modern seismicity data: it is necessary to take into account rare seismic events, i.e., earthquakes with a large recurrence period. The joint study of active faults, earthquakes of historical record, and paleoearthquakes can significantly extend the time range of a strong-earthquake catalog and make it possible to identify new, previously unidentified SSZs. To assess the seismic hazard of territories in stable continental regions, the authors of this article propose using paleoseismological data together with the results of seismological and geophysical studies, which makes it possible to combine paleoearthquake and modern seismicity data. seismic observations are analyzed to obtain additional information on the stress–strain state of the study area and determine the earthquake focal mechanisms of identified SSZs. The results of geophysical studies make it possible to judge the existence of ancient but still “nonconsolidated” faults in SSZs, which may, in tandem with other research methods, indirectly confirm their correct identification as active. In the article, this approach is used to assess the seismic hazard of Karelia and the Kola Peninsula. The seismic impacts parameters of events with a long recurrence period are evaluated for individual objects and populated areas of this region.

In relation to the 30th anniversary of the Spitak earthquake that occurred on December 7, 1988, the article provides a brief overview of information on the parameters of this earthquake, its consequences, and some lessons. Attention is focused mainly on the seismological aspects of this event and certain problems of engineering seismology that have arisen. The possible influence of the Spitak earthquake and its consequences on the future development of seismology in general and engineering seismology in particular is discussed. Specific issues of earthquake-resistant design are also touched upon.