Izvestiya, Physics of the Solid Earth最新文献

In this article, the properties of the Earth’s crust of the Western Orenburg region are discussed. The data of the broadband seismic station ORR (Orenburg), which is part of the Neftegaz–Seismika network, were analyzed using the receiver function method. The Zhu and Kanamori method was used to estimate the average parameters of the crust—the thickness and ratio of the longitudinal and transverse seismic wave velocities—using a single-layer model. A one-dimensional velocity section of seismic waves under the station was obtained by inversion of the waveforms of the receiver function. It is shown that the upper part of the constructed section is characterized by low transverse wave velocities. This is consistent with the presence of terrigenous and oil-and gas-containing rocks under the seismic station ORR. The resulting section in general does not contradict the results of the interpretation of the two profiles obtained by the DSS method performed in this region. However, the thickness of the crust calculated by the receiver function method, approximately 35–37 km, is 7–10 km less, compared with the results of the DSS method. To draw a final conclusion about the magnitude of the thickness of the crust, additional research is required.

Abstract—A systematic analysis of seismic moment tensors is conducted for earthquakes in the subduction zones of the Aleutian, Kuril-Kamchatka, and Japan island arcs. Earthquakes whose sources are not fit by the double-couple model (non-double-couple (NDC-type) earthquakes) are identified by estimating the angle between the slip vector and the rupture plane in the source based on seismic moment tensor eigenvalues. By analyzing error distribution of moment tensor component estimates, we revealed earthquakes with a significant NDC component in their source. Some features of the distribution of such earthquakes in the studied region are discussed. We compared the results of identifying sources containing a significant NDC component based on data from various seismological agencies. The comparison has shown that the catalogs of the U.S. Geological Survey (NEIC) and the Japan National Research Institute for Earth Science and Disaster Resilience (NIED) slightly underestimate the magnitude of the angle between the slip vector and the rupture plane compared to the Global Centroid Moment Catalog (GCMT). The applied method and the obtained estimates of NDC source slip component can be used in the numerical modeling of slip motion in the sources of strong earthquakes based on the combination of ground-based and satellite strain data.

Abstract—A number of specialized software packages have now been developed for calculating seismic hazard curves. Their algorithmic basis involves the application of numerical methods. A key feature of such calculations is the discretization of the identified earthquake focal zones (EFZ): each zone is divided into finite elements, with each element having seismic activity proportional to its area. In this case, in calculations, the magnitudes of earthquakes also take a discrete set of values. Numerical methods definitely make it possible to solve problems that cannot be described analytically, but they also introduce certain errors. Too coarse a division may not take into account local features of the seismic regime. The accuracy of the final results depends significantly on two key parameters: the selected magnitude discretization step and the principles of dividing EFZs. In this study, we consider a localized seismic source whose event flow strictly follows the Gutenberg–Richter law. To determine the intensity of seismic vibrations, the Shebalin macroseismic field equation is used.

This study uses open data from an international experiment in Central Mongolia, in particular, data from temporary stations deployed in the area of Lake Khuvsgul. This network consisted of 26 stations and operated in 2014–2016. This time interval includes an interesting for consideration, fairly large event with an aftershock process that occurred directly under Lake Khuvsgul on December 5, 2014, with a magnitude M_L = 5.2. Event detection and location is performed automatically using a publicly available AI-based detector, EQTransformer, designed to simultaneously detect earthquake signals and determine the P and S phases from single station data records, a phase associator using grid search, and a Hypoinverse event locator, which is also open source. The following results were obtained: the focal zone of the Khuvsgul earthquake of 2014 has no contact with the focal zone of the Khuvsgul earthquake of 2021 and is not related to the known faults in this area; at the level of weak earthquakes M_L < 2, increased activity is noted in the northern part of the Darkhad depression, as well as along the western side of the Khuvsgul depression; from the east, in the seismicity structure, an increased number of earthquakes is noted not only in the Busingol and Belin depressions, but also in the internal regions of the Shishkhid Highlands; During the experiment, there are no earthquakes in the area of the focal zone of the Khuvsgul earthquake of 2021 and in the area of development of the Darkhad earthquake swarm in 2022–2023.

Abstract—This paper analyzes the aftershock seismicity observed in the southern Avacha Bay, Kamchatka after the Vilyuchinsk earthquake of April 3, 2023, M_W = 6.6 (φ = 52.58° N, λ = 158.78° E, h = 105 km). The total duration of this activation is estimated at 450 days. The linear dimensions of the study area are limited horizontally by an ellipse with axes a = 34 km (azimuth α = 117°) and b = 22 km in the depth interval h = 95–105 km. The orientation of the ellipse corresponds to one of the two nodal planes of the main event focal mechanism. Two stages are distinguished in the aftershock seismicity decay rate: (1) Omori decay lasting 8.4 days and (2) exponential decay (τ = 140 days) until reaching background seismicity level. The two stages are comparable in terms of the number of earthquakes. The change phase between these two decay modes maps on the time interval between the strongest aftershocks, ML5.5 April 11, 2023 and ML4.9 April 21, 2023. These two earthquakes are shown to be a doublet as suggested by the high correlation of their waveforms and closeness of their epicenters. A sharp postseismic decrease in the Gutenberg–Richter b-value by a factor of 1.5 after the largest ML5.5 aftershock of April 11, 2023 is followed by a subsequent recovery throughout the exponential decay phase of aftershock seismicity. No b-value variations are revealed during the Omori decay phase.

Abstrsct—The effectiveness of TEM-FAST technology using a compact 25 m × 12.5 m single-loop combined receiving-transmitting antenna for measuring transient responses in microsecond period range is demonstrated by the example of transient electromagnetic sounding (TEM) of moraine sediments. The depth of investigation in sandy clay deposits with resistivity 30–300 Ω m is at least 50 m, with a dead zone thickness of at most 6 m. The on-foot soundings used an antenna fixed at four flexible poles at a height of 2.5 m above the ground. The portable measurement system with a total weight of 2.5 kg was moved by a field crew of four people along the profiles at a speed of ~1 m/s. Every 20 seconds after processing 13 000 pulses, the system recorded the measurement results and coordinates in autopilot mode. The repetition rate of current pulses with amplitude I = 3.3 A was f = 3.2 kHz, transient responses were monitored at time gates t = 4–64 µs. Vibration noise generated during motion due to antenna loop deformations in the Earth’s magnetic field reduced the depth of sounding to 50–60 m. The areal sounding data were inverted to design a 3D geoelectric model of the subsurface, which was then used to assess the capabilities of the TEM-FAST acquisition system in flying mode. Transient responses corresponding to different antenna heights above the terrain have been synthesized. Despite the decrease in response amplitudes when raising antennas to a height of 30 m, the depth of the soundings remained within 50 m. Considering the drone-borne survey technology using commercially available lightweight unmanned aerial vehicles (UAVs) with real-time dynamic positioning systems, we propose to tow the TEM acquisition system by a convoy of UAVs flying or hovering above the ground surface in autopilot mode. With decimeter positioning accuracy of antenna corners, the noise floor of measurements of transient responses will not exceed levels obtained in mobile on-foot surveys, and the depth of investigation (DOI) will not drop below 50 m.

Abstract—The accuracy of deep temperature forecasting from seismic velocity data and model temperature logs is studied as a function of distance to the point for which a prediction is made (a forecast point). For this purpose, the velocity sections from seismic tomography of the subsurface along a sublatitudinal profile in the Northern Tien Shan and the temperature model, previously constructed for this profile down to a depth of 27 km, are used. The accuracy assessment of temperature forecast using artificial neural network technology shows that at distances up to 16 km from the forecast point, the residual between the predicted and model temperature is 7.4, 5.7, and 4.6% for forecasts from longitudinal and shear wave velocities and their combination, respectively. With a fourfold increase in the distance to the forecast point, the residuals increase by a factor of 2–3. In general, it can be concluded that neural network forecasting of the temperature of the Earth’s interior based on seismic velocities can be performed with acceptable accuracy at large distances from measurements of input data and can be used as a “seismological geothermometer.”

Abstract—The paper presents the principles of measuring the Earth’s gravitational field (EGF) using a spacecraft (SC) equipped with a high-precision three-axis gravity gradiometer and located in low Earth orbit. Such spacecraft are designed primarily for measuring high-frequency EGF harmonics. Gradiometric measurements are insensitive to low-order EGF harmonics; therefore, to reconstruct the EGF in the entire frequency range, starting from n = 2, high-precision measurements of the spacecraft orbit are required, which are done by an onboard high-precision GNSS receiver. The theoretical aspects of satellite gradiometry are considered, and the problem of reconstructing the EGF harmonics based on model measurements is solved. To calculate the “measured” components of the gravitational potential tensor, the EGM2008 EGF model was used. A program for numerical integration of the spacecraft orbit was also developed based on this model, including additional forces acting on the spacecraft. To reconstruct the EGF, a direct method is used: a matrix of conditional equations is compiled with respect to the Stokes coefficients. The solution of this system by the least squares method makes it possible to obtain corrections to the harmonics of the a priori (initial) EGF model, for which the EGM96 model was used. In this way, the harmonics of the reconstructed field are formed. The criterion for the quality of the solution is agreement between the difference in the amplitudes of the harmonics of the reconstructed model and EGM2008 model. Based on the obtained model solutions, an optimal spacecraft orbit was selected for gradiometric measurements and estimates of the accuracy characteristics of the main key elements of the spacecraft’s onboard scientific instruments were obtained.

Abstract—The paper analyzes the results of shallow geophysical studies and seismicity of the Selenga Delta and adjacent areas of the South Baikal Basin, where the destructive Tsagan earthquake of 1862 with M7.5 occurred. This study area is characterized by the formation of epicentral zones, which overlap each other in a en echelon–like manner, as well as the seismic “quiescence” of the eastern segment of the Delta Fault, to which the dislocations of the Tsagan earthquake are confined. The distribution of seismicity and similarity of coseismic effects during the Tsagan and Middle Baikal (1959, M6.8) earthquakes, allow us to suggest that the 1862 shock was caused by displacement along a fault within the water area, either along the lateral Coastal Fault or along the intradepression Middle Baikal Fault, the length of which allowed an earthquake with M > 7 to occur. The activation of one of these faults in 1862 led to subsidence of the northeastern block of the Delta Trough with the opening of faults along its perimeter, including along the Delta Fault. The consequence of activation of the medium after strong events and clustering of strong shocks in time is a large number of weak shocks, reflected by the increased value of the slope of the recurrence graph (γ = –0.54 ± 0.01). According to geophysical data, the range of vertical movements during the last seismotectonic activation cycle reached 35 m.

Abstract—When studying the distribution of earthquake epicenters in the Northern Tien Shan region, a tendency was discovered for weak seismic events to cluster in relatively small areas. Understanding the conditions under which earthquakes occur and how they are grouped requires analysis of various geophysical features in conjunction with seismic event data. Using the Kohonen neural network, which allows for multidimensional classification of data, clusters were identified based on different sets of characteristics of the geophysical medium directly in the space of geographic coordinates. A comparison was made between cluster maps obtained using one or several geophysical parameters simultaneously and data on seismic events in the region under study. This integrated analysis revealed that there is a spatial correlation between clusters formed by a combination of parameters such as specific electrical resistance (electrical resistivity), temperature, vertical gradient of the ratio of compressional and shear seismic wave velocities, and the distribution of seismic events in the area considered.