Seismic Structure Research Articles

A two-phase pure slurry model for planetary cores: one-dimensional solutions and implications for Earth's F-layer

We develop and analyse a continuum model of two-phase slurry dynamics for planetary cores. Mixed solid–liquid slurry regions may be ubiquitous in the upper cores of small terrestrial bodies and have also been invoked to explain anomalous seismic structure in the F-layer at the base of Earth's liquid iron core. These layers are expected to influence the dynamics and evolution of planetary cores, including their capacity to generate global magnetic fields; however, to date, models of two-phase regions in planetary cores have largely ignored the complex fluid dynamics that arises from interactions between phases. As an initial application of our model, and to focus on fundamental fluid dynamical processes, we consider a non-rotating and non-magnetic slurry comprised of a single chemical component with a temperature that is tied to the liquidus. We study one-dimensional solutions in a configuration set up to mimic Earth's F-layer, varying gravitational strength$R$, the solid/liquid viscosity ratio$\lambda _{\mu }$and the interaction parameter$K$, which measures friction between the phases. We develop scalings describing behaviour in the limit$R \gg 1$and$\lambda _{\mu } \gg 1$, which are in excellent agreement with our numerical results. Application to Earth's core, where$R \sim 10^{28}$and$\lambda _{\mu } \sim 10^{22}$, suggests that a pure iron slurry F-layer would contain a mean solid fraction of at most 5 %.

Multi-scale anisotropy in NE China: Evidence for localized mantle upwelling

It is commonly proposed that the subduction of the Pacific plate has been responsible for widespread Holocene intraplate volcanism across NE China and the Korean Peninsula. Yet, how this process drives volcanism and even if it plays a critical role remains a topic of vigorous debate. In this study, using seismic data from four networks across NE China and northern Democratic People's Republic of Korea (DPRK), we analyze shear wave splitting in converted P to S-waves at the Moho (Pms), S-waves from the subducted slab interface (local S), and SKS phases. The Pms phases show a relatively weak crustal anisotropy (<0.25 s), with fast polarization directions aligned sub-parallel to major tectonic features. For the local S and SKS phases, fast polarization directions show significant lateral variations. We further perform a quantitative inversion to show that the depth of the anisotropy is ∼150 km, thus driven by flow within the asthenosphere associated with Pacific subduction. However, the presence of many null SKS splitting phases, together with scattered local S anisotropy across a wide range of incidence angles suggests a localized region of vertical flow directly beneath Changbaishan volcano. Such patterns correspond well to regional upper-mantle seismic velocity structure, and suggest that a localized upwelling with a relatively deep origin drives volcanism in the Changbaishan region. Furthermore, we infer that this mantle upwelling is deflected to the SW beneath Changbaishan and spreads asymmetrically at the base of the lithosphere, possibly because of the long history of volcanism in the region.

Fiber bundle model applied to slope stability assessment: co-detection multi-threshold analysis for early warning

Forecasting the imminent failure of natural slopes is crucial for effective Disaster Risk Reduction. However, the nonlinear nature of geological material failure makes predictability challenging. Recent advancements in seismic wave monitoring and analysis offer promising solutions. In this study, we investigated the co-detection method, which involves real-time processing of micro-seismic events detected concurrently by multiple sensors, to provide easy access to their initial magnitude and approximate location. By studying the Fiber Bundle Model and considering the attenuation of seismic waves, we demonstrated disparities in the statistical behavior of various rupture types before global catastrophic failure. Comparing avalanches with attenuated seismic wave amplitudes directly measured at sensor locations, we observed differences in their evolution towards catastrophic rupture. Leveraging a network of seismic wave sensors, we showed that the co-detection method was effective in detecting precursory seismic events, even with weak signals, making it a valuable tool for monitoring and predicting unstable slopes. Additionally, we demonstrated that a multi-threshold analysis of co-detection activity allowed for instantaneous capture of the seismic activity structure on unstable slopes. These findings contribute to our understanding of slope stability and offer insights for improved hazard assessment and risk management.

Structural Heterogeneity Controlled Rupture Process of the 2021 Mw 7.1 Fukushima, Japan, Earthquake Revealed by Joint Inversion of Seismic and Geodetic Data

Abstract We determined the rupture model of the 2021 Mw 7.1 Fukushima earthquake near northeastern Japan in this study and adopted this model to investigate the cause of this earthquake and its aftershocks. The rupture model was obtained through joint inversion of teleseismic, strong-motion and geodetic data. It is shown that the slips were predominantly distributed on the southwest side of the earthquake epicenter, indicating a unilateral rupture event. We observed that the seismic moment was released in three time periods, producing four slip patches on the fault plane. Through comparison, we demonstrated that our joint inversion model was more reliable in describing the rupture process of the Fukushima earthquake than the automatic inversion models determined using only strong-motion data. By jointly analyzing the slip distribution and seismic velocity structure, we found a good correlation between the slip patches and VP/VS anomalies, suggesting that structural heterogeneities along the fault zone played a critical role in controlling the rupture process of the Fukushima earthquake. In addition, most aftershocks were located in the region characterized by small slips and high VP/VS, and we demonstrated that they were caused by stress changes due to the presence of fluids and the rupture of the mainshock.

Study on mechanical principle and combined structure of shear wall and swing wall in seismic design of high-rise buildings

This paper studies the mechanical principle and combined application of shear wall and swing wall in the seismic design of high-rise buildings. Shear wall is a longitudinal wall perpendicular to the plane of a building, which resists the shear force generated by an earthquake through shear resistance, flexural stiffness, and contribution stiffness. Swing wall is a kind of seismic wall that can produce large horizontal displacement. Its mechanical principle is to improve the seismic resistance through flexible design, energy absorption and stiffness control. Shear wall and sway wall have advantages respectively, but they also have limitations respectively. By optimizing the layout and combination of shear walls and swing walls, give full play to their advantages and improve the overall seismic performance of the building. The effectiveness of the combined application is verified by the buildings with different seismic grades and structure types. However, this study has some limitations, such as the failure of the references to fully cover all relevant studies. Future research can further explore more new seismic structure systems, and combine with the latest materials and technology, and constantly improve the seismic design theory and practice of high-rise buildings. To sum up, shear wall and swing wall are a common structural system in the seismic design of high-rise buildings. By studying the mechanical principles and combined application, we provide new ideas and methods for the seismic design of high-rise buildings, which helps to improve the seismic performance and ensure the safety of personnel and property.

Seismic velocity structure and seismicity at the Kairei hydrothermal vent field near the Rodriguez Triple Junction in the Indian Ocean

The Kairei hydrothermal vent field (KHF) is on the northeastern side of the first segment of the Central Indian Ridge and discharges H2-rich hydrothermal fluids. To identify the source of these fluids based on the crustal and upper mantle structures and tectonic stress fields, we conducted a seismic refraction survey and observed natural seismicity in and around the KHF. We estimated 3D P-wave and S-wave velocity (VP and VS, respectively) structure models from airgun and natural seismic sources and determined 1861 hypocenters during a period of about two months. Beneath Yokoniwa Rise, VP in a convex zone exceeded 6.0 km/s at ~2 km depth below the seafloor and exceeded 7.0 km/s at 3–4 km depth; we thus interpret the rise to be a non-transform offset massif formed by the uplift of deep rocks. We detected a seismic cluster in a normal fault zone 3–5 km northwest of the KHF. Many events in this cluster occurred at 2–5 km depth below the seafloor in an area of high VP/VS ratios (~2.0) that are consistent with serpentinized peridotite, suggesting that seawater seeping downward through the fault zone has serpentinized peridotite at depth, thus becoming H2-rich hydrothermal fluid. This high-VP/VS area extends laterally at depth, and other such areas are observed at <1 km below the seafloor near the KHF. This structural configuration suggests that the high-VP/VS areas might provide conduits for the hydrothermal fluids that issue from the vents of the KHF. At the first segment of the ridge, we detected distinct shallow and deep seismic clusters. Seismicity in the shallow cluster was intermittent whereas deeper seismicity was nearly continuous, possibly indicating distinct magmatic activities at each depth.

High-resolution seismic velocity structure using induced inter-event interferometry at the geothermal sites, Geysers, Eastern California

The possibility of retrieving Rayleigh wave empirical Green's functions (EGFs) using virtual seismometers provides an opportunity to calculate high-resolution velocity models without a set of dense stations and/or expensive seismic imaging/monitoring. The high-resolution tomographic map can give an overview of the fracturing within the saturated rock. Theoretically, the cross-correlations of earthquakes contain significant information about the EGFs between event-pairs. In this work, we developed an application of virtual seismometers on a very small scale in superficial layers where anthropogenic seismicity occurred. To test this method's capabilities, we applied it to investigate the NW-part of The Geysers (TG) geothermal field. We used 1276 micro-earthquakes, occurred in a cuboid with the horizontal side of ∼1 × ∼2 km and the vertical edge at a depth from 0.9 to 2.2 km, to obtain the high-resolution (100 × 100 m) velocity structure. A group of strict criteria was imposed separately for single event waveform and event pairs to stabilize the tomographic results. After retrieving the inter-event EGF signal and calculating the Rayleigh wave group velocity dispersion curves, we obtained the group velocity maps for each horizontal layer. The tomographic maps as a function of depth indicated a low-velocity anomaly related to existing inactive faults, topography variation of the top of the steam zone, and low-velocity anomalies possibly stimulated by fluid injection. The topography of the top of the steam varies from ∼1.0 km to 1.3 km of maps, and also anomalies connected with fluid migration appear at the depth of injection wells (1.5–1.9 km).

Structural Interpretations for Zubair Formation in Kumait oil field, Using 3D Seismic Reflection Data, Southern-East Iraq

This study deals with the interpretation of structural 3D seismic reflection of the Kumait oil field in southern Iraq within the administrative boundaries of the Maysan Governorate. Synthetic seismograms are prepared by using available data of the Kt-1 oil field by using Petrel software to define and pick the reflector on the seismic section of the Zubair Formation, Which represents the Cretaceous Age. The study shows that the Kumait structure is an anticline fold. It is thought to be a structure trap caused by the collision of the Arabian and Iranian plates and trending in the same direction as driving factors in the area, which are from the northwest to the southeast, and the overall trend of strata is north and northeast. Seismic structure interpretation results show the top of the Zubair Formation and several structure closures. The stratification Phenomenon of the study area was clear evidence of the existence of closures which are probable to be hydrocarbon traps.

Frequency adaptive fault detection by feature pyramid network with wavelet transform

Fault detection is a key step in seismic structure interpretation. Current research has achieved good results in fault detection by using synthetic training data to train deep-learning models. However, there is an inevitable difference in frequency bandwidth between synthetic training data and real seismic data, which makes it difficult for deep-learning models to obtain ideal fault detection results on real seismic data. To solve this problem, the feature pyramid network (FPN) is introduced to obtain multiscale deep-learning features, which can reduce the impact of seismic data frequency bandwidth differences on fault detection. Then, we apply the multiscale wavelet transform to extract multiscale frequency spectral features of the seismic data and combine them with the multiscale deep-learning features through concatenation operation. Furthermore, the seismic data is decomposed into signals with different frequency bands through the wavelet transform, and we use the energy of these signals as the network weights of multiscale mixed features to further improve the frequency adaptability of our method. Based on these works, we not only improve the fault detection effect in a specific work area but also improve the generalization ability of the deep-learning model in different work areas, thus further promoting the application of deep learning in actual production. Compared with the fault detection results by the traditional deep-learning model U-Net and the traditional FPN on multiple real seismic data and synthetic seismic data, experimental results demonstrate the effectiveness of our method.

Measuring seismic attenuation in polar firn: method and application to Korff Ice Rise, West Antarctica

Abstract We present seismic measurements of the firn column at Korff Ice Rise, West Antarctica, including measurements of compressional-wave velocity and attenuation. We describe a modified spectral-ratio method of measuring the seismic quality factor (Q) based on analysis of diving waves, which, combined with a stochastic method of error propagation, enables us to characterise the attenuative structure of firn in greater detail than has previously been possible. Q increases from 56 ± 23 in the uppermost 12 m to 570 ± 450 between 55 and 77 m depth. We corroborate our method with consistent measurements obtained via primary reflection, multiple, source ghost, and critically refracted waves. Using the primary reflection and its ghost, we find Q = 53 ± 20 in the uppermost 20 m of firn. From the critical refraction, we find Q = 640 ± 400 at 90 m depth. Our method aids the understanding of the seismic structure of firn and benefits characterisation of deeper glaciological targets, providing an alternative means of correcting seismic reflection amplitudes in cases where conventional methods of Q correction may be impossible.

Solidified magma reservoir derived from active source seismic experiments in the Aira caldera, southern Kyushu, Japan

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km.Graphical

Seismic structure beneath the Avacha and Koryaksky volcanoes in Kamchatka based on the data of permanent and temporary networks

Avacha and Koryaksky are active volcanoes located in the vicinity of Petropavlovsk-Kamchatsky, the main city of Kamchatka, and they represent a serious hazard to the surrounding population and infrastructure. Here we investigate the upper-crustal structure beneath these volcanoes by implementing local earthquake tomography based on data from permanent seismic stations and from a temporary network installed in 2018–2019. Although the total amount of seismic rays recorded by the temporary network dataset is much lower, adding this data and attributing an appropriate weight lead to considerable increase of the resolution compared to the case of using merely the permanent network data. The resulting distributions of the P and S wave velocities, and especially the Vp/Vs ratio, reveal two magma reservoirs located below Avacha and Koryaksky volcanoes. Different depths of their upper limits explain differences in the eruption activity styles of these volcanoes. Below the Avacha Pass, we observe a thick layer of low-velocity soft volcaniclastic sediments formed due to the activity of both volcanoes. A conservative numerical estimate of a steady-state conductive temperature field around the magma source below Avacha demonstrates that the high temperature zone can be reached by drilling a well to a reasonable depth, thus suggesting that the area might be exploited as a source of geothermal energy.

Continental subduction of Adria in the Apennines and relation with seismicity and hazard

The subduction of continental lithosphere is a complex process because the buoyancy of the crust is higher than the oceanic and should resist sinking into the mantle. Anyway, studies on the Alpine-Himalayan collision system indicate that a large portion of the continental crust is subducted, while some material is accreted in the orogens. The Apennine is a perfect case for studying how such processes evolve, thanks to high quality seismic images that illuminate a critical depth range not commonly resolved in many collisional settings. In this paper, we show the structure of the Apennines orogen, as jointly revealed by seismicity and deep structure from regional and teleseismic tomography and receiver function profiles. The westward subducting Adria lithosphere is well defined along the orogen showing a mid-crustal delamination. Seismicity within the underthrusting lower crust and velocity anomalies in the mantle wedge highlight how the subduction evolution is entangled with the liberation of fluids. The eclogitization of subducted material enhances the fluid release into the wedge, the delamination and retreat of the Adria plate. This delamination/subduction generates a coupled compression and extension system that migrates eastward following the retreat of the lithosphere, with broad sets of normal faults that invert or interfere with pre-existing compressional structures all over the roof plate. The sparseness and non-ubiquity of intermediate depth earthquakes along the subduction panel suggest that the brittle response of the subducting crust is governed by its different composition and fluid content. Therefore, the lower crust composition appears essential in conditioning the evolution of continental subduction.

Stress Constraints From Shear‐Wave Analysis in Shallow Sediments at an Actively Seeping Pockmark on the W‐Svalbard Margin

AbstractMechanisms related to sub‐seabed fluid flow processes are complex and inadequately understood. Petrophysical properties, availability of gases, topography, stress directions, and various geological parameters determine the location and intensity of leakage which change over time. From tens of seafloor pockmarks mapped along Vestnesa Ridge on the west‐Svalbard margin, only six show persistent present‐day seepage activity in sonar data. To investigate the causes of such restricted gas seepage, we conducted a study of anisotropy within the conduit feeding one of these active pockmarks (i.e., Lunde Pockmark). Lunde is ∼400–500 m in diameter, and atop a ∼300–400 m wide seismic chimney structure. We study seismic anisotropy using converted S‐wave data from 22 ocean‐bottom seismometers (OBSs) located in and around the pockmark. We investigate differences in symmetry plane directions in anisotropic media using null energy symmetries in transverse components. Subsurface stress distribution affects fault/fracture orientations and seismic anisotropy, and we use S‐wave and high‐resolution 3D seismic data to infer stress regimes in and around the active seep site and study the effect of stresses on seepage. We observe the occurrence of changes in dominant fault/fracture and horizontal stress orientations in and around Lunde Pockmark and conclude minimum (NE‐SW) and maximum (SE‐NW) horizontal stress directions. Our analysis indicates a potential correlation between hydrofractures and horizontal stresses, with up to a ∼32% higher probability of alignment of hydrofractures and faults perpendicular to the inferred minimum horizontal stress direction beneath the Lunde Pockmark area.

The Khuvsgul Earthquake of January 12, 2021, ML = 6.9, in the Seismicity Structure of the Tuva–Mongolian Block

The Khuvsgul Earthquake of January 12, 2021, ML = 6.9, in the Seismicity Structure of the Tuva–Mongolian Block

Seismic structure of the Eastern European crust and upper mantle from probabilistic ambient noise tomography

The Eastern European lithosphere is a natural laboratory to study continental formation and evolution through time, comprising Archean continental remnants, Proterozoic rifts and belts, and younger accreted terranes. We investigate the seismic structure of the East European Craton (EEC) crust and uppermost mantle, and the transition from Precambrian to Phanerozoic Europe across the Trans European Suture Zone (TESZ) using probabilistic transdimensional ambient noise tomography. We cross-correlate noise recorded at broadband seismic stations from Eastern, Northern, and Central Europe, remove earthquake signals using continuous wavelet transform, and extract Rayleigh wave phase velocity dispersion curves. We invert these for the highest resolution shear wave velocity model of the Eastern European lithosphere to date, using Markov chain Monte Carlo Bayesian inversion. Our shear wave velocity model exhibits spatial correlation with major tectonic units and bears similarities with active seismic survey profiles in terms of seismic velocity patterns and main discontinuities. The crust thickens across the TESZ boundary and the mantle is seismically faster than beneath younger terranes, consistent with a less dense Precambrian lithosphere in the EEC. The crust and lithosphere beneath the Pannonian region is hyper-extended but the adjacent Transylvanian basin crust shows significant heterogeneity. The Precambrian building blocks of the EEC exhibit contrasting seismic fabrics. The Baltic orogens of Fennoscandia are underlain by uniform crust with a flat Moho, while Sarmatia shows alternating high and low velocity layers and a regional south-dipping crustal boundary from beneath the Ukrainian Shield towards the Crimean Peninsula. The last observation supports a geodynamic style driven by horizontal rather than vertical tectonics, with fundamental implications for the formation and evolution of early continents.

Assessing large-scale mantle compositional heterogeneity from machine learning analysis of 28 global P- and S-wave tomography models

SUMMARY Differences between P- and S-wave models have been frequently used as evidence for the presence of large-scale compositional heterogeneity in the Earth's mantle. Our two-step machine learning (ML) analysis of 28 P- and S-wave global tomographic models reveals that, on a global scale, such differences are for the most part not intrinsic and could be reduced by changing the models in their respective null spaces. In other words, P- and S-wave images of mantle structure are not necessarily distinct from each other. Thus, a purely thermal explanation for large-scale seismic structure is sufficient at present; significant mantle compositional heterogeneities do not need to be invoked. We analyse 28 widely used tomographic models based on various theoretical approximations ranging from ray theory (e.g. UU-P07 and MIT-P08), Born scattering (e.g. DETOX) and full-waveform techniques (e.g. CSEM and GLAD). We apply Varimax principal component analysis to reduce tomography model dimensionality by 83 percent, while preserving relevant information (94 percent of the original variance), followed by hierarchical clustering (HC) analysis using Ward's method to quantitatively categorize all models into hierarchical groups based on similarities. We found two main tomography model clusters: Cluster 1, which we called ‘Pure P wave’, is composed of six P-wave models that only use longitudinal body wave phases (e.g. P, PP and Pdiff); and Cluster 2, which we called ‘Mixed’, includes both P- and S-wave models. P-wave models in the ‘Mixed’ cluster use inversion methods that include inputs from other geophysical and geological data sources, and this causes them to be more similar to S-wave models than Pure P-wave models without significant loss of fitness to P-wave data. Given that inclusion of new data classes and seismic phases in more recent tomographic models significantly changes imaged seismic structure, our ML assessment of global tomography model similarity may improve selection of appropriate P- and S-wave models for future global tomography comparative studies.

PREM-like velocity structure in the outermost core from global SKS and ScS waveform modeling

Seismic structure in the Earth's outermost core is important for our understanding of thermochemical stratification in the outer core, and is yet heavily debated. Here we study the compressional velocity structure in the Earth's outermost core based on waveform modeling of a unique untapped SKS and ScS dataset near bifurcation distances, collected from global seismic arrays for earthquakes occurring from 2000 to 2020. Using the SKS-ScS array dataset minimizes the effects of many uncertainties associated with earthquake source parameters and seismic heterogeneities in the mantle, and affords an opportunity to study and assess the seismic structure in the outermost core. We study outer core structure by testing two end-member models: 1) the D″ model that attributes any anomalous seismic observations to the effects of the lowermost mantle structure and is paired with a PREM (the Preliminary Reference Earth's Model) structure in the outermost core and 2) the outer core model that is paired with either a PREM or a tomographic structure in the lowermost mantle and attributes other unexplained seismic signals to an outer core structure. The results of the outer core models present unreasonable large lateral variations of >3.1% in the outermost core, while the inferred D″ models exhibit large-scale seismic anomalies that are consistent with the tomographic models and small-scale anomalies that are confirmed by further analysis of the seismic array data. Our analyses suggest a PREM-like seismic velocity structure and a lack of strong thermochemical anomalies in the topmost ∼200 km of the outer core, placing bounds on possible thermal and compositional conditions in the region of the Earth's outermost core. Our study also identifies the existence of small-scale seismic anomalies and sharp velocity variations in the lowermost mantle beneath the south coast of Alaska, northwestern Atlantic and the middle of Central America.

The Khubsugul Earthquake of January 12, 2021, &lt;i&gt;M&lt;sub&gt;L &lt;/sub&gt;&lt;/i&gt;= 6.9, in the Seismicity Structure of the Tuva-Mongolian Block

Abstract—The paper presents the studies of the Khuvsgul earthquake on January 11, 2021 at 21:32 UTC (January 12, 2021 at 05:32 local time), MW = 6.7, ML = 6.9, and the seismicity structure in the aftershock period for the Altai-Sayan mountain region and the Baikal rift zone, where the epicenter of this earthquake was located. Two faults are seismically activated, diverging from the southern end of the aftershock area at an acute angle: one in the northeast and one in the northwest direction, as well as transverse faults between them. According to the epicenter position and studies of the source area by other authors, the main event corresponds to the northeastern fault, and large aftershocks occurred at the junction of the northwestern fault with transverse faults feathering from the east. The main event was immediately followed by a series of large aftershocks, the strongest of which occurred on March 31, 2021 with ML = 6.2 and on May 3, 2021 with ML = 6.4. Spatial changes in the seismic regime of the aftershock region led to the predominant activity of its southern end. The junction area of the collisional structures of the Altai-Sayan folded zone and the rift structures of the Baikal depressions system is distinguished in seismicity as a block structure with increased seismicity near the block boundaries. First of all, these are the Tuva-Mongolian block and the eastern part of the Sayano-Tuva block. After the Khuvsgul earthquake of 2021, a block structure with the activation of the epicentral zones of the 1991 Busingol earthquake, the 2011–2012 Tuva earthquakes, and other structures seismically active until 2021 has increased seismic activity. It is proved that the 2014 Khubsgul earthquake occurred under the basin of the same name and is associated with other faults than the 2021 earthquake and is not a direct precursor of the 2021–2022 activation.

Depth structure of the Transcarpathian Depression (Ukrainian part) according to density modeling data

The presented research is devoted to the construction and calculations of the density model along the regional CMRV-DSS profile RP-17 (Chop–Velykyy Bychkiv), running along the Transcarpathian depression. Based on the results of density modeling, the distribution of density in the Earth’s crust was obtained in accordance with its seismic structure and gravity field as well as the density structure of individual layers. A tectonic interpretation of the obtained results was provided. The Mukachevo and Solotvyno parts of Transcarpathian depression have their own structural features, autonomous geological development and are distinguished by Neogene geodynamics. The analysis of density properties showed that in the Neogene sedimentary layer of both depressions there is a change in the density of various rocks in the depth intervals of 200—950; 950—1450; 1450—2050 m. In the Solotvyno depression in the depth intervals of 200—950 and 1450—2050 m, the density is greater than in Mukachevo, due to the presence of sandstones, tuffs, mudstones, and siltstones. In the interval 950—1450 m of Solotvyno depresson, on the contrary, the density is lower than in Mukachevo one, due to the presence of salt and clay. The Earth’s crust of Mukachevo depression is more compacted, as it contains a «basalt» layer. The Solotvyno depression consists of two parts and its average density corresponds to a diorite composition. The north-western block is of higher density and more homogeneous. The south-eastern one is of lower density and composed of a large number blocks of different densities separated by faults. The boundary of the lower density zone (PK 105—110) runs along the south-eastern branch of the Stryi-Latorytsia shear zone. This zone appears fragmentary on the density section, being similar to a mantle fault, with a lateral differentiation of density values, as well as the largest concentration of earthquakes, especially in the upper part of the Earth’s crust. The low-density area is probably associated with the transition from the Solotvyno depression to the structures located to the south-east of it. Thus, the block with the lowest density (2,38 g/cm3) of the Mesozoic-Paleozoic folded basement can be attributed to the Fore-Alkapa suture zone, represented by the Pieniny Klippen Belt and the Monastyrets nappe, which turns in the meridional direction in the zone of junction with the Tisza-Dacia terrane. The block located below with a density of 2,64 g/cm3 can be connected with the Marmarosh massif, or with the Rakhiv nappe. It was established that the crust of the Alkapa terrain along the profile is represented by three large blocks with a smaller block structure inside each one. The two more density blocks with different crust structure correspond to the Mukachevo depression. The lowest density third block belongs to the north-eastern part of the Solotvyno depression, the eastern border of which coincides with the area of clustering of earthquake hypocenters. The south-eastern part of the Solotvyno depression probably represents a transition zone between the Alkapa and Tisza–Dacia terrains. Two lithospheric fault zones are distinguished. The first one separates Mukachevo and Solotvyno depressions. The second zone is located between the Khust and Tyachiv faults in the low-density zone along the entire section of the Earth’s crust. These two zones are assumed to be connected by the Vynogradiv fault. It is characterized by a large cluster of earthquakes and located in the transition from extension and subsidence to compression and uplift. Low velocity (density) zones of the Transcarpathian depression can be associated with lithospheric fault zones. There are the most active horizons of modern geological and geophysical transformations of the mineral environment of the Earth’s crust and can be a potential source of deep oil and gas.