No 2 (2025)
Articles
Application of the seismic interferometry method for studying the earth’s inner core
Abstract
For a more detailed description of the dynamic processes and determination of the properties of the inner core regions, inaccessible to study by traditional methods, the seismic interferometry method, which is based on the technique of cross-correlation analysis of time series for different types of data, was applied in this work. Cross-correlation analyses of the seismic coda window with a start three hours after a strong event and the end 10 hours later are performed for all possible pairs of more than 300 stations and 6 large earthquakes (for each separately) occurring between 2013 and 2024. Synthetic cross-correlograms are calculated for models with different attenuation and an additional boundary in the inner core. Four different types of inner core studies by seismic interferometry were carried out: global, regional, station latitude-dependent, and calendar time-dependent. The stability of the PKIKPPPKIKP wave on global correlograms, the possibility of its observation in areas with high and low density of seismic stations, the dependence of the wave travel time on the angle between the wave direction and the Earth rotation axis, and the stationarity of the wave for the time period from 2013 to 2024 were demonstrated.
3-18
Peculiarities of localization zones of the strongest earthquakes in the Kuril-Kamchatka arc
Abstract
The processes occurring during the interaction of crustal blocks in the subduction zone are reflected both in the features of the seismic regime and in the distribution of the strongest earthquake sources in the study area. In this paper we analyze the peculiarities of localization zones of the strongest earthquakes of the Kuril-Kamchatka arc on the basis of data of two catalogs: regional catalog of Kamchatka branch of FRC GS RAS and global international catalog of USGS NIEC of US Geological Service for 1990–2024. Combination of catalogs based on a modified nearest neighbor method allowed us to remove duplicates — repeated earthquakes in initial catalogs — and to obtain a new unified catalog consisting of 52574 earthquakes. The nearest neighbor method was applied to identify two subsets: independent and connected earthquakes in space and time, which were further used to analyze the density of seismic events. The obtained regularities were compared with the focal characteristics of the strongest earthquakes of the subduction zone. The proposed approach allowed us to identify zones of localization of potential mega-earthquake sources along the Kuril-Kamchatka arc.
19-35
Features of the Sakhalin mantle phase transition zone based on converted wave data
Abstract
The research presents the phase transition zone boundaries’ topography estimates at the depths of about 410 and 660 km on the basis of data set obtained by Sakhalin island seismic station network using receiver function method. A representative data set consisting of 2500 individual PRFs was analyzed. We revealed a depression in the 660 km boundary in the central and northern parts of the island. The 410 km boundary is significantly elevated in the south Sakhalin, while within the rest of the island it is depressed in comparison to the expected standard depth. It has been suggested that the subsidence of the 410 boundary is connected with the presence of hot lower mantle matter in the phase transition zone in the northern part of the island.
36-42
Instanton representation of foreshoсk—aftershock sequences
Abstract
Descriptions of the processes of foreshock and aftershock activations are of great significance in seismology, both for practical and theoretical reasons. An analogy of mathematical expressions describing the origin of the direct and inverse Omori–Utsu laws has been established empirically. Investigations of the generalized vicinity of a large earthquake (GVLE) have revealed an even closer analogy between the properties of foreshocks and aftershocks. This similarity also applies to the characteristics of the activation process, in particular, anomalous changes in the slope of the repeatability plot. It is proposed in this paper to use kinetic equations for the unifying model of the entire foreshock-aftershock process, the equations having solutions in the form of dependencies with explicit maxima, localized in time and called instantons (by analogy with solitons–localized waves). A clear pattern of an instanton solution is a plot of the time derivative of the logistic dependence describing the transition process. The speed of such a process first increases significantly, reaches a maximum, and then asymptotically decreases to zero.
The aim of the work is to demonstrate the efficiency of using the instanton model, which generalizes the model of self–developing processes (SDP), but does not provide for the development of physically unrealistic singularity which is a pattern usually simulating an explosive growth in the number of foreshocks and aftershocks in the vicinity of the main event. A comparison of the new model with empirical data is performed by the example of earthquakes in the southern part of Sakhalin Island in 2003–2023.This zone is the most equipped with facilities for seismic events registration. The satisfactory correspondence between theoretical and empirical temporal dependences is shown both for the GVLE built for the territory within (44.5°–50.5° N., 141.5°–143.5° E.) and for individual strong earthquakes on Sakhalin.
43-57
Ring-shaped seismicity structures in the region of South Kamchatka: possible preparation for great earthquake
Abstract
We have been studying some seismicity characteristics in the region of South Kamchatka. Aftershock characteristics for large earthquake of August 17, 2024 (Mw = 7.0) were investigated. It was shown that ring-shaped seismicity structures were formed in the South Kamchatka within three depth intervals: 0–33, 34–70 and 71–110 km. Similarly to the other subduction zones, the structures are characterized by threshold magnitude values (Mt1, Mt2 and Mt3 correspondingly) and also by big axes lengths (L1, L2 and L3). Epicenters of the large earthquake of August 17, 2024 and its strongest aftershocks lie on the shallow ring-shaped seismicity structure (Mt1 = 5.3). This effect confirms an assumption on a preparation of great earthquake in the region of the South Kamchatka. Earlier correlation dependences of Mt1 and Mt2 parameters on Mw values of major earthquakes for the west of Pacific Ocean were created (within an interval of Mw = 7.0-9.0). Using these dependences we estimated magnitude of possible great earthquake in this area: Mw = 8.6 ± 0.2. The reasons for ring-shaped structures formation in different depth ranges of the subduction zones are discussed.
58-65
Emergence of a zone of disturbed rock in the vicinity of dynamic slip along a tectonic fault
Abstract
The article presents results of 2D simulations of formation of a zone of disturbed rock during the development of a dynamic shear along a horizontal tectonic fault. Different sliding regimes are studied: the sub-Rayleigh one (the rupture propagation velocity Vr does not exceed the velocity of Rayleigh wave in the medium) and the supershear one (the Vr value is higher than the velocity of transverse waves). Contributions of tear and shear mechanisms to the process of emerging zone of disturbed rock in the vicinity of a fault at different depths is considered. The degree of alteration in physical and mechanical properties of rock at different distances from the fault is assessed. It is shown that at large depths lithostatic stresses completely suppress rupture, and rock destruction occurs solely due to shear deformation. At shallow depths, the tear mechanism becomes predominant. The stress release associated with emerging of tensile cracks leads to an abrupt decrease of the zone of shear fracture. This zone is localized only in the immediate vicinity of the rupture plane. An increase in the tearing strength leads to an increase in the size of the shear fracture zone. In supershear ruptures, the fracture zone can have a complex, non-simple character. A change in the propagation velocity of longitudinal waves Cp by more than 15–20% occurs only in the immediate vicinity of the slip plane at a distance of 10–20 m. At large distances, the degree of change in the value does not exceed 10%. At shallow depths, there may be tensile cracks that propagate over significant distances from the slip plane.
66-79
Nature of anisotropic response of fluid saturated medium to surface seismic wave propagation
Abstract
Monitoring of pore pressure or water level changes in observation wells shows significant variations both during the passage of P and Rayleigh waves and during the passage of S and Love waves. Recent borehole measurements have shown an azimuthal dependence of pore pressure variations on the stress orientation and strike direction of the fault zone. In the active fault zone, the fracture-induced anisotropy corresponds to the preferred orientation of microcracks and other discontinuities in the medium. This paper is devoted to the development of a modified Skempton equation for a quantitative description of surface wave induced pore pressure variations in a reservoir, related to the orientation and principal values of the stress tensor and rock damage (fracturing). The developed relationships allow the azimuthal dependence of the pore pressure response to be described by a dimensionless parameter defined as the ratio of the amplitudes of the pressure variations caused by the shear component and the volumetric strain. According to the proposed theoretical model, the maximum poroelastic response of the reservoir to the passage of a seismic wave is manifested in the case of subparallelism of the directions of predominant rock fracturing and maximum horizontal stress.
Pore pressure monitoring data from the Arbuckle wastewater disposal reservoir (Oklahoma, USA) are used to verify the proposed theoretical model. It is shown that the observed diversity of pore pressure response in wells located in the vicinity of a fault zone intersecting the reservoir to the passage of seismic waves from seismic events at different distances is described with high accuracy by the developed model.
80-94
Reconstruction of parameters of shear stresses during the formation of multi-rank faults in the western Baikal region based on tectonophysical interpretation of lineaments
Abstract
The author′s software made it possible to perform a detailed identification of lineaments at various scale levels for the region of the Obruchev fault system of the western Baikal region. The identified lineaments significantly complement the mapped framework of faults and are consistent with the strike of fault structures of the corresponding rank. Based on the analysis of the relative specific density of lineaments of the local scale level, reflecting the feathering megacracks of large faults, heterogeneous zones of dynamic influence of regional structures were established, which were divided into relatively homogeneous segments. For each identified segment and each structure as a whole, using the developed software “Lineament Stress Calculator”, a reconstruction of shear stress parameters was carried out using the P.L. Hancock′s model. It has been previously proven that the main features of the Early Paleozoic stage of the region development during the accretion of the Olkhon terrane to the southern margin of the Siberian craton were accompanied by activation of right-lateral strike-slip displacements along the SW-NE accretionary sutures and active metamorphism processes. The obtained results confirm that the main faults of the SW-NE strike, subparallel to the marginal suture of the Siberian platform, were formed at the early stage of their development as right-lateral strike-slips with the compression axis orientation of ≈90°. Second-order faults of the NW-SE orientation are defined as left-lateral strike-slips and were probably formed at that time as antithetical shears in relation to the main structures, having received their development during further structural rearrangements of the region.
95-113
Reconstruction of the spatial distribution of filtration properties of heterogeneous geologic media based on variations of microseismicity resulting from fluid injection
Abstract
Determining the properties of heterogeneous reservoirs based on microseismic evolution data is an important task in field development. Analyzing the propagation of microseismic events occurring during fluid injection/withdrawal provides valuable information about permeability and stress state of the reservoir. In this paper, we consider the inverse problem of determining reservoir filtration properties from microseismic event propagation data. For this purpose, the influence of various geological factors on the distribution of microseismic event sources is investigated. Machine learning methods were used to identify correlations between geologic model parameters and microseismicity evolution. Due to the insufficient variability of in-situ data, an artificial database of catalogs of microseismic events containing the coordinates of sources and their occurrence times was created to train the model. For this purpose, numerical modeling of fluid injection and generation of microseismic events in synthetic models of permeable media with different geological structure was carried out. Thus, a comprehensive approach to the restoration of filtration properties of heterogeneous reservoirs from microseismicity evolution data using machine learning methods is proposed. The proposed methodology can be applied to optimize field development, improve the efficiency of fluid extraction and reduce the risks associated with the occurrence of undesirable anthropogenic seismic activity.
114-127
Acoustic emission accompanying preparation of dynamic slip on a model heterogeneous fault of meter scale
Abstract
Regularities in distribution of tectonic fault sections with different frictional properties control to a great extent the dynamics of fault sliding. The impossibility of directly studying the structure of fault zones at seismogenic depths makes it especially important to develop diagnostic methods that would provide information on the structural features of earthquake sources formation areas and, thereby, predict the sliding dynamics.
This work presents results of laboratory experiments directed to studying regularities of elastic wave emission during shear deformation of a model fault with a spatially inhomogeneous structure of the sliding 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, the so-called asperities. The relative position of asperities changed in experiments.
The process of dynamic slip formation, caused by asperity disruption, was accompanied by emission of a great number of acoustic pulses that were recorded in the frequency range of 20–80 kHz. During the experiments, the data on spatial distribution of pulses allow 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.
128-136
Evolution of microcracks in the rock deformation process: X-ray microtomography and discrete element method
Abstract
In this study, we directly observed microcracks formed in a sample of rock under uniaxial compressive load. Detection of defects in the volume was carried out with the help of X-ray computed microtomography. The peculiarity of the experiments is that a tomographic image of the sample was taken directly under mechanical load. Based on the analysis of tomographic slices, the fractal dimension and relative volume of microcracks were calculated at three stages of loading. Three-dimensional models of the defect structure were constructed to illustrate the change in the morphology of the main crack. Numerical experiments on the fracture of samples of heterogeneous materials have been carried out using the discrete element model. The change in the fractal dimension of main cracks in the process of their growth was investigated. A good agreement between the results of computer simulations and laboratory experiments has been established, which indicates the adequacy of the proposed model and allows in further studies to use it to study the behavior of local parameters that cannot be measured experimentally.
137-144
On the regime of the induced seismicity
Abstract
The problem of induced seismicity has both an important practical and theoretical aspects. The practical aspect is related to the danger of induced seismicity. In a number of cases, the danger of strong induced seismicity led to the closure of important industrial projects. The theoretical aspect is related to the well-known paradox of seismicity, which is the impossibility of the occurrence of conventional earthquakes at depths greater than several tens of kilometers. It follows that the physics of induced, usually near-surface earthquakes may differ from the physics of the deeper events. Examples of a number of areas of induced seismicity are considered, representing both the vicinity of large reservoirs and areas of intensive extraction of hydrocarbon and ore raw materials. In the considered areas a few common trends were more or less clearly identified. After the growth of induced seismicity, even with continuing strong technogenic impact, a tendency for seismic activity to decline is observed. Also, by analyzing the generalized vicinity of a large event (GVLE), the proximity of the fore- and aftershock process intensity is revealed for induced seismicity zones; which contrasts with the case of ordinary seismicity, for which the aftershock process activity is usually much stronger. It can be assumed that the decline in the induced seismicity is associated with the release of initial tectonic stresses, and the proximity of the fore- and aftershock process intensity indicates a difference in the physical mechanism of induced near-surface earthquakes from ordinary, deeper earthquakes.
145-157
The 20th anniversary of the installation of the small-aperture “Mikhnevo” array. Monitoring induced seismicity
Abstract
Over 20 years of continuous operation of the small-aperture seismic array (SASA) “Mikhnevo”, a huge experience in recording ultra-weak signals generated by regional and global seismicity has been accumulated. High-resolution data processing methods have been developed and applied, including directional summation and waveform cross-correlation. Within the framework of 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 method of waveform cross-correlation (WCC). The effectiveness of the approaches in relation 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 SASA of the IDG RAS “Rostov-Don”. The location accuracy is comparable to the accuracy provided by 49 three-component (3-C) stations of the FRC UGS RAS and the International Monitoring System (IMS). In the five days after the earthquake, 12 aftershocks were detected and located relative to the mainshock using the WCC method. The group stations of the IMS 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 UGS RAS detected 5 aftershocks, and the IMS did not detect a single one. The location of explosions in the DPRK using the WCC made it possible to determine their relative location with an accuracy of 100–200 m. The sixth explosion could not be accurately located relative 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 WCC method 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 IMS array stations KSRS and USRK. Over a five-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.
158-178
Energy release in the atmosphere induced by the impact of meteoroids 20–200 meters in size
Abstract
The results of calculations of destruction, evaporation and deceleration of stony meteoroids with sizes from 20 to 200 meters in the Earth’s atmosphere are presented. The redistribution of thermal and kinetic energy between the condensed matter of the meteoroid, its vapors, and air is studied in detail. It is shown that when the size of the impactor is several tens of meters, the vaporized matter is not decelerated immediately, but flies along the trajectory for a long time, gradually transferring energy to the air. As a result, the main energy release in the atmosphere occurs at the stage of vapor jet deceleration, after the meteoroid and its fragments have completely vaporized.
179-186