The Arctic region, including vast shelf zones, has enormous resource and transport potential and is currently key to Russia’s strategic development. This region is promising and attractive for the intensification of global economic activity. When developing this region, it is very important to avoid emergency situations that could result in numerous negative environmental and socio-economic consequences. Therefore, when designing and constructing critical infrastructure facilities in the Arctic, it is necessary to conduct high-quality studies of potential geohazards. This paper reviews and summarizes the scattered information on the main geohazards in the Russian sector of the Arctic Ocean, such as earthquakes, underwater landslides, tsunamis, and focused fluid discharges (gas seeps), and discusses patterns of their spatial distribution and possible relationships with the geodynamic setting of the Arctic region. The study revealed that the main patterns of the mutual distribution of the main geohazards of the Russian sector of the Arctic seas are determined by both the modern geodynamic situation in the region and the history of the geodynamic evolution of the Arctic, namely the formation of the spreading axis and deep-sea basins of the Arctic Ocean. The high probability of the influence of seismotectonic activity on the state of subsea permafrost and massive methane release is emphasized. This review contributes toward better understanding and progress in the zoning of seismic and other geological hazards in the vast Arctic seas of Russia.

This article examines aspects of the application of geomorphometric methods to the identification of regional-scale lineaments using a digital relief model and a gravity field model in marine areas using the Laptev Sea as an example. The results of lineament analysis showed their applicability for further use in developing a model of earthquake source zones and seismic hazard assessment. The study showed the effectiveness of methods of shadow analysis and identification of keel forms by calculating the curvature of the relief for identifying large seismic lineaments, provided that the results of geomorphometric analysis are used together with data on the distribution of earthquake epicenters. The presented approach to identifying seismic lineaments can be very promising for the development of a lineament-domain-focal model of zones of possible earthquake sources in the vast shelf zones of Russia, for which there is no necessary volume of geological, geophysical and paleoseismological data.

Seismotectonic and seismoacoustic studies carried out during an expedition onboard the R/V Akademik Boris Petrov in the Pechora Sea revealed a paleoseismic dislocation confined to a fault zone tracing the North Ural seismic lineament. The amplitude of this dislocation (the relative displacement of its sides) makes it possible to estimate the magnitude of the paleoearthquake that occurred within the North Ural lineament. This magnitude was used to estimate the maximum seismic impacts on offshore oil and gas facilities already in operation and those being designed for construction in the northeastern Pechora Sea. These estimates differ significantly from estimates made previously. The method of searching for paleoseismic dislocations using seismoacoustic methods, as well as lineament analysis, can be used for initial assessment seismic impacts in water areas, including the Laptev Sea, as the most seismically active region of the Arctic.

The parameters of strong ground motions necessary for seismic hazard assessing of construction sites have pronounced regional differences. In inaccessible and poorly studied areas, the development of a ground motion prediction equation (GMPE) requires determination of regional characteristics of the radiation and propagation of seismic waves, in particular, the quality factor of the lithosphere. This work is devoted to determination of the quality factor and its frequency dependence for the Lena Delta, one of the key areas of the Laptev Sea region, the most seismically active of all Arctic marine regions of Russia. For this purpose, the coda normalization method was used in relation to signals from 5 microearthquakes with Ml > 2, recorded by a temporary local network of seismographs operating in the Lena Delta in 2016-2017. Regional values of quality factor Q = 486�42 were obtained. The scatter of values is probably due to the anisotropy of the environment in the study area. The dependence of the quality factor on frequency is described by the found function Q(f) = 84.328f0.8128. The exponent in this relationship is comparable to the values typical for regions of Mexico, Australia and Canada.

The present study is devoted to the analysis of extreme sea level oscillations of the Laptev Sea using the ADCIRC model. The numerical modeling is performed on a high-resolution grid and verified for sea level observations from three tide gauges. We have revealed regional characteristics of extreme sea level oscillations for different parts of the Laptev Sea coast. The maximum total sea level range was 544 cm in Ebelyakh Bay, while the minimum was 267 cm in Khatanga Bay, where maximum tidal ranges were obtained. Some areas in Khatanga Bay and Anabar Bay had maximum tidal ranges exceeding 200 cm. The study provided an estimation of the possible magnitude of coastal flooding by calculating the extreme total and residual sea levels for different return periods: 1, 2, 5, 10, 20, 50, and 100 years. The amplitude of extreme surges calculated for the 100-year return period can exceed 300 cm for several sections of the Laptev Sea coast, with the maximum sea level range being about 680 cm for Anabar and Ebelyakh Bays.

This paper reports the obtained analysis data on the focal mechanisms and general distribution of earthquake epicenters in the Laptev Sea region. The principal stress axes directions were calculated by the formal stress inversion method for four groups of event clusters with known focal mechanisms. The distributions of the earthquake epicenters and crust thickness were compared. According to the seismological data, the extension axis of the Gakkel Ridge on the Laptev Sea shelf currently continues in the vicinity of the group of extension detachments located along the eastern boundary of the Anisin, Zarya, and Belkovskii–Svyatonosskii rift chains. The more ancient extension axis located along the group of detachments marking the eastern boundary of the Ust’-Lena and Omoloi rift systems and continuing the Gakkel Ridge axis is currently much less active. It carries residual stresses near its intersection with the Khatanga–Lomonosov fault zone in the northwestern parts of the shelf area and with the Lena–Taimyr zone of boundary uplifts, in the southwestern part. The axes oriented along the Olenyok and Bykovskii channels and the Siberian Platform boundary form extension conditions in the eastern part and strike–slip conditions in the western part of the Lena delta area.

This paper is devoted to the features of seismological observations in the Arctic seas, which are complicated by harsh climatic conditions, the presence of ice cover, stamukhi and icebergs, and limited navigation. Despite the high risk of losing expensive equipment, the deployment of local networks of bottom seismographs or stations installed on ice is still necessary for studying the seismotectonic characteristics and geodynamic processes of the region under consideration, the deep structure of the crust and upper mantle, seismic hazards, and other marine geohazards. Various types of seismic stations used for long-term and short-term deployments in the Russian sector of the Arctic Ocean, as well as various schemes and workflows for their deployment/recovery, are described. The characteristics of seafloor seismic noise and their features are also considered. The results of deployments demonstrate that the characteristics of the stations make it possible to reliably record earthquake signals and seismic noise. Based on the experience gained, it was concluded that the preferred schemes for deploying ocean-bottom seismographs are those in which their subsequent recovery does not depend on their power resources. Usually, such schemes allow for the possibility of dismantling stations via trawling and are suitable for the shelf depths of the sea. The advantages of such schemes include the possibility of installing additional hydrophysical and hydrobiological equipment. When using pop-up ocean-bottom seismographs, special attention should be paid to the careful planning of the recovery because its success depends on the possibility of a passage to the deployment site, which is not always possible due to changing meteorological and ice conditions. Seismic records obtained on the seafloor are characterized by a high noise level, especially during periods of time when there is no ice cover. Therefore, it is recommended to install bottom stations for periods of time when ice cover is present. The frequency range of the prevailing noise significantly overlaps with the frequency range of earthquake signals that must be taken into account when processing bottom seismic records.

The article presents the results of DSS seismic surveys in the Persian Gulf. Bottom analog-type seismographs and seismic airgun sources were used in the studies. The bottom seismographs were moored and shooting was carried out according to three regional profiles with a length from 100 to 250 km. The main result of these studies was a velocity–depth model of the sedimentary cover and Earth’s crust up to the Moho boundary, which was revealed at a depth of about 43 km in the water area. Given that the thickness of the upper crust is only 4–5 km and based on the velocity characteristics of the remaining layers, the type of crust can be attributed to the subcontinental Archean type. This situation (complete absence or drowning of the upper layer of the Earth’s crust) is typical of the waters closest to the Persian Gulf: the Black, Caspian, Mediterranean, and Red seas. A structure was found in the Earth’s crust of the studied area, which may be a brachyanticline with an isometric dome-shaped shape, which corresponds to the platform-type folding in the areas of salt dome tectonics. No faults have been found in the crust of the water area of the Persian Gulf adjacent to the Bushehr Peninsula.

The Russian sector of the arctic shelf is the longest in the world. Quite a lot of places of massive discharge of bubble methane from the seabed into the water column and further into the atmosphere were found there. This natural phenomenon requires an extensive complex of geological, biological, geophysical, and chemical studies. This article is devoted to aspects of the use of a complex of marine geophysical equipment applied in the Russian sector of the arctic shelf for the detection and study of areas of the water and sedimentary strata with increased saturation with natural gases, as well as a description of some of the results obtained. This complex contains a single-beam scientific high-frequency echo sounder and multibeam system, a sub-bottom profiler, ocean-bottom seismographs, and equipment for continuous seismoacoustic profiling and electrical exploration. The experience of using the above equipment and the examples of the results obtained in the Laptev Sea have shown that these marine geophysical methods are effective and of particular importance for solving most problems related to the detection, mapping, quantification, and monitoring of underwater gas release from the bottom sediments of the shelf zone of the arctic seas, as well as the study of upper and deeper geological roots of gas emission and their relationship with tectonic processes. Geophysical surveys have a significant performance advantage compared to any contact methods. The large-scale application of a wide range of marine geophysical methods is essential for a comprehensive study of the geohazards of vast shelf zones, which have significant potential for economic use.

In 2007–2015, Shirshov Institute of Oceanology of the Russian Academy of Sciences (IO RAS) carried out seismological observations in the water area and on the coast of the Baltic Sea using autonomous seismic stations. Here, in the area of the Sambia Peninsula, previously considered aseismic, a strong perceptible earthquake with a magnitude of about M = 4.6 occurred in 2004. The most interesting data were obtained by IO RAS in 2008–2009 from seismological monitoring using autonomous bottom and coastal seismic stations. The data obtained in 2010–2015 turned out to be unsuitable for full-scope processing due to several causes (losses of bottom seismographs, high noise level at coastal stations, etc.). Seismological monitoring in the west of the Kaliningrad region and in the adjoining area of the Baltic Sea detected weak earthquakes with magnitudes ML = 2.5-3 whose sources are confined to the development are of the Kravtsovskoe offshore oil field. Some of these earthquakes have been recorded by the stations of the Norwegian seismic array NORSAR and by the seismic stations of Sweden. The Kravtsovskoe oil field is located on the shelf northwest of the unique natural object, the Curonian Spit. The detected weak earthquakes are likely to be anthropogenic, induced by reservoir pressure disturbances as a result of intensive hydrocarbon production, and are probably precursors of a strong man-made earthquake.

OBS observations in the Persian Gulf during a short time span have revealed the occurrence of low magnitude (ML = –0.2–2.9) earthquakes with hypocenters in the mantle beneath the Gulf and beneath the Zagros Mountain Massif. A cross-section across the shoreline of the Persian Gulf shows the projections of these hypocenters beneath the Zagros Mountains to make inclined layers that dip northeast at a high angle into the mantle down to depths of 120‒180 km. The 3D distribution of large and moderate magnitude earthquakes based on an improved earthquake catalog as reported by the US Geological Survey and by the International Seismological Centre (ISC) is not at variance with the distribution of microearthquakes and low magnitude earthquakes but seems rather to supplement it, forming a separate seismic dipping layer. According to the data acquired by OBS observations, seismic activity occurs throughout the entire crust and upper mantle of the region rather than in the upper crustal layers only as was asserted in previous publications. It is possible that collision and accompanying phenomena (mantle seismicity and destruction of the granitic layer in the crust) are related to the hypothetical rotation of the Earth around the center of rotation placed at Cyprus Island.

The application of the horizontal-to-vertical spectral ratio (HVSR) modeling and inversion techniques is becoming more and more widespread for assessing the seismic response and velocity model of soil deposits due to their effectiveness, environmental friendliness, relative simplicity and low cost. Nevertheless, a number of issues related to the use of these techniques in difficult natural conditions, such as in the shelf areas of the Arctic seas, where the critical structures are also designed, remain poorly understood. In this paper, we describe the features of applying the HVSR modeling and inversion techniques to seismic records obtained by ocean-bottom seismographs (OBS) on the outer shelf of the Laptev Sea. This region is characterized by high seismotectonic activity, as well as sparse submarine permafrost distribution and the massive release of bubble methane from bottom sediments. The seismic stations were installed for one year and their period of operation included periods of time when the sea was covered with ice and when the sea was ice-free. The results of processing of the recorded ambient seismic noise, as well as the wave recorder data and ERA5 and EUMETSAT reanalysis data, showed a strong dependence of seafloor seismic noise on the presence of sea ice cover, as well as weather conditions, wind speed in particular. Wind-generated gravity waves, as well as infragravity waves, are responsible for the increase in the level of ambient seismic noise. The high-frequency range of 5 Hz and above is strongly affected by the coupling effect, which in turn also depends on wind-generated gravity waves and infragravity waves. The described seafloor seismic noise features must be taken into account during HVSR modeling and interpretation. The obtained HVSR curves plotted from the records of one of the OBSs revealed a resonant peak corresponding to 3 Hz, while the curves plotted from the records of another OBS did not show clear resonance peaks in the representative frequency range. Since both OBSs were located in the area of sparse distribution of submarine permafrost, the presence of a resonance peak may be an indicator of the presence of a contrasting boundary of the upper permafrost surface under the location of the OBS. The absence of a clear resonant peak in the HVSR curve may indicate that the permafrost boundary is either absent at this site or its depth is beyond the values corresponding to representative seismic sensor frequency band. Thus, HVSR modeling and inversion techniques can be effective for studying the position of submarine permafrost.

The paper is devoted to the problem of numerical modeling of earthquake response of porous saturated soil deposits to seismic waves propagation. Site-specific earthquake response analysis is a necessary and important component of seismic hazard assessment. Accounting for the complex structure of porous saturated soils, i.e., the content in them, in addition to the solid matrix, pore water, gas mixture and ice, is especially important for the water areas in the zones of continuous or sparse permafrost, as well as the massive release of bubble gas from bottom sediments. The purpose of this study is to introduce an algorithm and its Matlab implementation for numerical modeling of the nonlinear response of porous saturated soil deposits to vertical P- and SH-waves propagation. The presented MatNERApor package consists of a set of Matlab scripts and functions. The package was tested and verified using the records of vertical seismic arrays of the Kik-net network. In addition, the records of local earthquakes obtained by ocean bottom seismographs in the Laptev Sea in 2019–2020 were used to demonstrate the effect of the water layer above the seabed sites on the reduction of vertical motions spectra. The results of the calculations showed good agreement with the data obtained from real seismic records, which justifies the correctness of the theoretical basis of the presented algorithm and its software implementation.

The results of new local seismic observations in the Laptev Sea obtained using ocean bottom seismographs were compared with actual data from global and regional earthquake catalogs. Most earthquake epicenters on the Laptev Sea shelf are grouped into several clusters located within the East Laptev province of horsts and grabens and enclosed between two extension detachments. The first one marks the eastern boundary of the Ust–Lena and Omoloi rift systems, and the second one is the eastern boundary of the Anisin, Zarya, and Belkovskii-Svyatoi Nos rift chains. At the same time, the linear zone of epicenters confined to the Gakkel Ridge, when moving to the shelf, shifts along the Khatanga–Lomonosov fault zone to the northeast in the direction of the second extension detachment, which probably remains the only active one. The spatial distribution of earthquakes in the Laptev Sea, taking into account the epicenters of micro- and weak seismic events on the shelf, does not have sufficiently clear boundaries that determine the contours of the previously assumed lithospheric microplate.

The presence of seismic threat multiplies the environmental hazard, especially for oil and gas production and transport facilities in water areas. Currently, there are no normative maps of general seismic zoning of the water areas of the inland and marginal seas of Russia, especially since there are practically no maps of detailed seismic zoning and seismic microzoning of even individual parts of the water areas. Taking into account the fact that intensive development of offshore oil and gas fields and the Northern Sea Route has begun, the development of such maps becomes a very urgent scientific and practical task. The seismic hazard assessment for the submerged crossing was carried out in 2008. The initial seismic effects were calculated using a probabilistic seismic hazard analysis based on five models of seismic zones and three types of models of attenuation of peak and spectral accelerations. The results of the performed calculations, including deaggregation, have shown that the initial seismicity of the area of the gas pipeline crossing route through the Nevelskoy Strait for a return period of 1,000 years is lower than indicated on the OSR-2016-B map, where the eastern end of the crossing route through the strait is characterized by the initial seismicity equal to 9 points. The soil conditions (seismic microzoning) have been taken into account by three different methods: the seismic rigidity method, the calculation method, and the method considering the thickness of Quaternary deposits. The present studies show that seismic effects vary along the pipeline route from the mainland to Sakhalin Island from 8.4 to 8.9 on the MSK-64 scale for the recurrence period of seismic shaking T = 1,000 years and from 9.3 to 9.7 points for T = 5,000 years.

Numerical modeling of seismic response of soil deposits is usually conducted as part of seismic hazard assessment, preceding facility construction in any tectonically active regions, including offshore sites. A significant feature of subsea soils is their porous and water-saturated structure. Thus, the purpose of the present study is to introduce a procedure for modeling nonlinear behavior of porous, moist soils during SH-wave propagation, to verify it and compare response for synthetic soil profiles with porous medium parameters specific for low moisture onshore and high moisture offshore sites with cohesive and non-cohesive soils. The well-known and approved NERA code was used as a basis and improved to incorporate the Biot and Gassman equations for elastic waves propagation in a fluid-saturated porous solid. The applicability of the presented approach was substantiated for integration into other well-known algorithms. Obtained results showed good agreement between the simulated by different methods and observed spectra. The modeling also showed that the response of cohesive and non-cohesive soils with moisture specific both for onshore and offshore sites is explained by effects of resonances and effect of seismic amplitude saturation, which, in turn, depend on the corresponding value of the layer thickness and S-wave impedance for porous saturated soil layer. The proposed scheme could have significant practical usage for studying the effect of porous medium parameters on the seismic response of the moist soil deposits.

Summary To assess the seismic hazard parameters along the gas pipeline route on the shelf and continental slope of the Russian sector of the Black Sea records of bottom seismographs were used. The initial seismicity was clarified and the influence of the bottom soil on the parameters of seismic effects was taken into account.

Summary Currently, there are no normative maps of general seismic zoning of the water areas of the inland and marginal seas of Russia, especially since there are practically no maps of detailed seismic zoning and seismic micro-zoning even for individual parts of the water areas. Taking into account the fact that intensive development of offshore oil and gas fields and the Northern Sea Route has begun, the development of such maps is becoming a very urgent scientific and practical task. This article substantiates the need to use bottom seismological observations when performing work on detailed seismic zoning (DSZ) and seismic microzoning (SMZ) of water areas.

This paper is a review of current concepts concerning the seismotectonics and seismicity in the Laptev Sea region. The chief feature of the region is a rift system extending in the shelf from the continental slope with the adjacent Gakkel Ridge to the mainland coast. There are several models to describe the present-day evolution of the region, but no one of these is preferable because of the lack of local instrumental observations of shelf microseismicity, while such microseismicity is characteristic of rift zones. One alternative approach consists in the installation of ocean bottom seismometers on the shelf itself. During the 73rd cruise of the R/V Akademik Mstislav Keldysh, a temporary network was deployed consisting of 7 broadband bottom seismic stations on the Laptev Sea shelf. Comparative analysis of noise spectra based on records of a hydrophone, a bottom seismograph, and a wavegauge, showed that the noise amplitude strongly depends on wind waves, which is in turn dependent on marine ice cover, so that the recording potential of a local network of bottom seismographs is restricted to the winter period of observation.