Izvestiya, Physics of the Solid Earth最新文献

Reconstructing the tectonic evolution of Kamchatka is crucial for deciphering the mechanisms of fold belt formation and development of subduction systems. This requires reliable paleomagnetic information for the poorly studied segments of the Koryak–Kamchatka fold region such as southern Kamchatka. This paper presents the first paleomagnetic data on Miocene magmatic bodies of the Pribrezhny complex, which is widespread on the Pacific coast of southern Kamchatka. The paleomagnetic pole calculated for the Miocene of southern Kamchatka from 33 sites is statistically significantly different from all the published Cenozoic poles in the nearby regions. The new data indicate the formation of Miocene volcanics at a paleolatitude close to the present position (52.3°) and support the formation of the Miocene supra-subduction volcanic belt on the more ancient basement of the Olyutor–Kamchatka fold system rather than within a separate exotic block. It is shown that most of the sampled volcanics formed before the main phase of tectonic deformation, but at least part of the studied normal-polarity bodies contain postfolding magnetization and may represent products of younger episodes of magmatism.

The article studies the structure of the Earth’s crust and upper mantle of the Avacha Bay region of the Kamchatka Peninsula. One-dimensional sections of the dependence of seismic velocities on depth, obtained during the study are presented. These sections are constructed according to the data of the Petropavlovsk (PET), Dalniy (DAL), Institut (IVS), and Karymshina (KRM) stations for the period from 2000 to 2019. The stations are part of the permanent observational network of seismic stations of the Kamchatka Branch, Geophysical Survey, Russian Academy of Sciences. The sections are constructed to a depth of 300 km, which makes it possible to characterize the structure of the medium in the bay area, namely, to identify structural layers in the crust, the Moho boundary, and to estimate the degree of deviation of seismic wave velocities in the upper mantle from the corresponding values of the IASP91 global Earth model. The average values of velocities calculated from the obtained sections in the crust and upper mantle were significantly lower compared to the global model. The average deviation of the observed velocities from the model ones is 0.5–1.0 km/s in the crust, then it gradually decreases to a depth of about 180 km. At greater depths, the velocities in the obtained models coincide with the standard values. It should be noted that at the locations of the seismic stations, the lower boundary of the subducting Pacific Plate runs at depths of about 180 km. Therefore, the main reason for the difference in velocities is probably related to significant heating of material and the complex fluid-dynamic situation in the region of the mantle wedge.

Active development of coal deposits is underway in the area of some Northern Sea Route strongholds, which is regularly recorded by the seismic stations Amderma1 (AMDE1) and Kolba (KOLBA) of the Federal Center for Integrated Arctic Research, Ural Branch, Russian Academy of Sciences (FECIAR UrB RAS), located on the Kara Sea coast. The main task of the study is a comprehensive analysis of the work of the AMDE1 and KOLBA stations, which will create conditions for cleaning seismic catalogs from man-made events. The location of events in this case is not a determining criterion, since there is a large error in processing data from a single station. As a result, we have compiled a unique set of criteria for identifying the nature of local seismic events for each seismic station. These criteria have been implemented in the processing when analyzing the waveforms from the AMDE1 and KOLBA stations, which ultimately improves the quality of operation of the FECIAR seismic network of the Ural Branch of the Russian Academy of Sciences.

New possibilities of seismic technology for inspection of railway embankment soils using the impact of a moving train on the medium are considered. Records of vibrations made by a broadband velocimeter installed in the lower part of the embankment are analyzed. It is shown that the main information is contained in the amplitude value of the first vibration maximum on the vertical component A_z after filtering in the 0.01–1.25 Hz band. The value of A_z is determined by the shear modulus of the embankment soil. The results of long-term soil state monitoring of stable and unstable embankments are compared. A significant difference in the time variability of shear modulus changes for different embankments during seasonal thawing of the soil is shown. The possibility of predicting the soil state using thermal engineering calculation of the thawing process is discussed, with the recommendation to use monitoring data to obtain a more reliable result.

Radial anisotropy of S-waves is observed as a difference between SV- and SH-wave velocities with vertical and horizontal polarization, respectively, which are inverted from Rayleigh and Love wave dispersion curves. In contrast to isotropic models, presently available distributions of S-wave velocities, accounting for the radial anisotropy, significantly contradict each other. One reason for such discrepancies is that, as a rule, different datasets (paths) for Rayleigh and Love waves are used to calculate the radial anisotropy coefficient. This leads to the fact that the inverted velocity patterns of SV- and SH-waves are smoothed over areas with different shapes and sizes. To exclude this effect, we offer an approach in which the initial data contain only Rayleigh and Love wave dispersion curves along the same paths in the same periods. Then, standard procedures of surface wave tomography and inversion of local surface wave velocities to S-wave velocity patterns are implemented. Using such an approach, we obtained the distribution of the radial anisotropy coefficient ((alpha = {{left( {{{V}_{{SH}}} - {{V}_{{SV}}}} right)} mathord{left/ {vphantom {{left( {{{V}_{{SH}}} - {{V}_{{SV}}}} right)} {{{V}_{{{text{av}}}}}}}} right. kern-0em} {{{V}_{{{text{av}}}}}}}), where ({{V}_{{{text{av}}}}} = {{left( {{{V}_{{{text{SH}}}}} + {{V}_{{SV}}}} right)} mathord{left/ {vphantom {{left( {{{V}_{{{text{SH}}}}} + {{V}_{{SV}}}} right)} 2}} right. kern-0em} 2})) in the upper mantle of Southeast Asia to a depth of 300 km within 70°–145° E and 20°–40° N. It has been shown that at depths of 50–70 km, maxima of the α‑coefficient are associated with areas with low SV-wave velocities. Moreover, at a depth of 50 km, the highest α values are confined to territories with the maximum horizontal displacement rates according to GPS data (relative to stable Eurasia). We also have found that the areas in which the radial anisotropy is truly negative (α < –1%), i.e., in which V_SV > V_SH, are confined to lithospheric plate boundaries.

The article presents the methodology and results of electromagnetic soundings of the Meyer thrust zone of the Northern Ladoga region, which separates volcanoterrigenous Paleoproterozoic cover complexes with different metamorphism intensity. Obtained in the course of detailed geological mapping, ideas about the structural and material features of the studied fold-thrust forms of this zone were supplemented by depth characteristics and visualization of the latter in the Paleoproterozoic cover–Archean basement system. Owing to the adequate choice of the audio-magnetotelluric sounding method as the main one, combined with magnetic survey, meaningful geoelectric images of the fold–thrust structures to a depth of 1 km were identified. Elements of both “thick-skinned” and “thin-skinned” tectonics, with joint and autonomous deformations of basement and cover complexes, were traced in the constructed electrical conductivity sections. A new dome-shaped structure of the Archean basement, inscribed in the linear series of small highs of the Archean basement of the Sortavala Group, was distinguished.

Abstract—The problem of induced seismicity has both practical and theoretical aspects. The practical aspect is related to the danger of induced seismicity. In a number of cases, the potential hazard from strong induced seismicity has prompted the cancellation of significant industrial projects. The theoretical aspect is related to the well-known paradox of seismicity that the earthquakes that rupture by the mechanism of ordinary brittle failure cannot occur at depths greater than a few tens of kilometers. This suggests that the physics of induced, typically shallow earthquakes can differ from the physics of most of the deeper events. Examples of a number of areas of induced seismicity both in the vicinity of large reservoirs and in the regions of extensive hydrocarbon and ore extraction are considered. A set of common trends is identified in all considered regions, with varying degrees of certainty. After the buildup of induced seismicity, even under a continuing strong anthropogenic impact, a declining trend is observed in seismicity rate. Furthermore, the analysis using the generalized vicinity of large earthquakes (GVLE) method revealed the closeness of the intensities of the fore- and aftershock process in the zones of induced seismicity. This contrasts with the patterns of ordinary seismicity, where aftershock activity process is typically much higher. It is hypothesized that the decay of induced seismicity is related to the unloading of the initial tectonic stresses, while the closeness of the intensities of the foreshock and aftershock processes suggests that the physical mechanism of induced shallow earthquakes differs from that of ordinary, deeper earthquakes.

Abstract—The research presents estimates for the topography of the boundaries of the phase transition zone at depths of 410 and 660 km based on the data set obtained by Sakhalin island seismic stations using the receiver function method. The data set we analyzed incorporates a total of 2500 PRF functions. We revealed a depression at the 660 km boundary in the central and northern parts of the island. The 410 km boundary is significantly elevated in the southern Sahalin, while within the rest of the island, it is depressed (especially in the northern part) compared to the expected standard depth. We hypothesize that the depression in the 410 km boundary is related to the presence of the hot lower mantle melts within the mantle transition zone under the northern part of the island.

Abstract—Over the 20 years of continuous operation of the Mikhnevo small-aperture seismic array (SASA), vast experience in recording ultraweak signals generated by regional and global seismicity has been accumulated. High-resolution data processing methods have been developed and applied, including beam-forming and waveform cross correlation. In this review of the results of instrumental observations and processing, two approaches to reducing the detection threshold for seismic events when monitoring induced seismicity are considered: the use of array stations and the waveform cross-correlation method (WCCM). The efficiency of the approaches with respect to the detection, location, and identification of weak seismic sources is illustrated by the aftershock sequence of the earthquake near Mariupol that occurred on August 7, 2016, as well as the aftershocks of the fifth and sixth announced explosions in the DPRK, detected during the period from September 9, 2016, to September 11, 2021. The coordinates of the earthquake were estimated using the data of the Mikhnevo array and the temporary Rostov-on-Don SASA of IDG RAS. The location accuracy is comparable to the accuracy provided by 49 three-component (3-C) stations of FRC GS RAS. In the five days after the earthquake, 12 aftershocks were detected and located with respect to the mainshock using the WCC method. The group stations of the International Monitoring System (IMC) AKASG and BRTR and the 3-C station KBZ also participated in the detection and estimation of the parameters. The network of stations of the FRC GS RAS detected five aftershocks, and the IMC did not detect a single one. The location of explosions in the DPRK using the WCCM made it possible to determine their relative location with an accuracy of 100–200 m. The sixth explosion could not be accurately located with respect to the others due to the finite size of its source, which introduced significant changes in the differential travel time, depending on the direction to the station. The WCCM was also used to detect and identify weak seismic events within the DPRK Punggye-ri test site using template waveforms from explosions and aftershocks of the fifth and sixth tests, recorded at the IMC array stations KSRS and USRK. Over a 5-year observation period, 89 events were detected. Based on estimates of the cross-correlation characteristics of signals at both stations, it was possible to divide the general aftershock sequence into two separate ones associated with processes in the zones of influence of the fifth and sixth explosions.

Abstract—Regularities in the distribution of sections with different frictional properties govern fault slip dynamics to a great extent. Since it is impossible to directly study the structure of a fault at a seismogenic depth, developing fault diagnostics methods that can yield information on structural peculiarities of the zone where an earthquake source nucleates, in addition to a specific fault dynamics, is of primary interest. This article presents results of laboratory experiments directed at studying the regularities of elastic wave emission during shear deformation of a model fault with a spatially inhomogeneous structure of the slip interface. The model fault was a loaded contact of diabase blocks 750×120 mm² in size. Two round zones, each 100 mm in diameter, were made at the interface. Those zones had high strength, showing the property of velocity weakening, so-called asperities. The relative position of asperities varied in experiments. The process of dynamic slip nucleation and emergence, caused by asperity disruption, was accompanied by emission of a large number of acoustic pulses that were recorded in the 20–80 kHz frequency range. The data on spatial distribution of pulses make it possible to detect two separate contact regions only when the distance between these regions exceeded 20 mm. Differences in the statistics of pulses emitted at different asperities were observed.