Izvestiya, Physics of the Solid Earth最新文献

Abstract—A source of a strong earthquake, as a rule, consists of subsources which are identified by waveform modeling. This modeling does not yield an unambiguous result. In this paper, we show an example when two significantly different focal mechanism solutions are presented for the same earthquake. In one solution, the subsources are characterized by similar faulting type, while in the other solution, the last subsource has an opposite mechanism. In (Vakarchuk et al., 2013), this discrepancy was interpreted as the realization of compensatory motion. The compensatory movements are detected not only in the subsources but also at the scale level of the source zone overall, where these movements manifest themselves in a certain regularity of the aftershock mechanisms discovered in the study of the 1970 Dagestan earthquake by Kuznetsova et al. (1976). In this paper, perhaps for the first time, compensatory movements are detected in a high-magnitude earthquake swarm lacking a pronounced main shock, which occurred in 2023 in Herat Province, Afghanistan. The results are supported by the set of the seismological and satellite interferometric data.

Abstract—The paper discusses anomalies that are observed in the electric fields above hearth deposits. Hearths are important objects for archaeological studies and are traditionally detected by the positive anomaly in magnetic induction. However, magnetic measurements are not available on every archaeological site, and burning sites are not the only sources of positive magnetic anomalies. In the 1960s, it was established that the hearths can be detected using the induced polarization method (IP), but the technique has not been developed because of the imperfection of equipment at that time. Currently, multi-electrode multichannel electromagnetic prospecting systems allow prompt assessment of the distribution of electrical properties of the object and reconstructruction of its three-dimensional geoelectric structure. In this study, we examined geoelectric anomalies from a hearth, elucidated their nature, and evaluated the possibility of using electrical resistivity tomography (ERT) method to search for such objects. By the example of the fieldwork and laboratory experiments it is shown that hearths create intense anomaly of induced polarization and are distinguished by a very low electrical resistivities. It is established that the boundaries of the burning sites are clearly expressed in the maps of normalized chargeability. To study the nature of the observed geoelectrical anomalies, we carried out X-ray phase analysis and measurements of induced polarization, electrical resistivity, and magnetic susceptibility. The results have shown that the anomaly of low electrical resistivity and the polarizability anomaly are caused by the presence in hearth material of X-ray amorphous carbon in the form of soot, but not magnetite. Application of electrical resistivity tomography combind with induced polarization measurements is recommended in the search for fireplaces in archaeological prospecting.

Abstract—Postseismic movements in the source region of the July 29, 2021 Chignik M8.2 earthquake, Alaska, are analyzed using observations from GPS sites. It is shown that the displacements are have a nature of a postseismic sliding (afterslip). The velocity of postseismic displacements of the points on the Earth’s surface (GPS sites) decreases with time by the power law close to 1/t, where t is time after the earthquake. Over an interval of two years, the magnitudes of displacements increase in time by the law of logarithmic creep. Based on the regional catalog of the Alaska earthquakes, we have analyzed the aftershocks of the Chignik earthquake. The analysis has shown that the postseismic displacements at various GPS sites are proportional to the displacements in the seismotectonic flow caused by residual displacements in the sources of aftershocks. This allows us to consider total displacements in the aftershock sources as a universal mechanism of the postseismic afterslip deformation in the source region of the Chignik earthquake.

Abstract—The hydrogeology of fault zones, especially at considerable depth, is perhaps a most underdeveloped field in earthquake source mechanics. This is due to both the lack of the data on the filtration characteristics of the geomaterial at large depths and to the complexity of the processes of mass transfer, fracture formation and healing under high temperatures and pressures. In this case, the strong influence of fluid on both the friction characteristics and the stress conditions in the vicinity of the slip zone is obvious. Fluids are carriers of dissolved matter and thermal energy, an effective catalyst for various types of metamorphic transformations. According to some models, fluid flows can be triggers for the initiation and arrest of seismogenic ruptures. The current trend in world seismology to constructing a complex computational model that adequately describes the processes of preparation, initiation, and arrest of various fault slip modes requires developing the ideas about fluid dynamics of seismogenic faults. This review considers the information on hydrogeology of fault zones obtained in recent years. Models and ideas involving field data, laboratory and field experiments and numerical calculations regarding the role of fluids at different stages of the seismic cycle are analyzed.

Abstract—Numerous publications have appeared in the literature describing a new seismo-electromagnetic effect—the appearance of magnetic pulses with a duration of 1–40 s, which are detected at distances up to ten thousand km a few minutes before earthquakes of even low magnitude. An assumption was made about the universality of the processes responsible for generation of pulsed precursors and the fundamental possibility of a short-term (a few minutes) warning of an approaching earthquake. In this article, the possibility of the occurrence of ultra-low-frequency pulses that precede seismic events with magnitudes M > 5.0 is studied based on the data from a network of induction magnetometers in the Far East. Pulsed disturbances occur regularly in the records by these highly sensitive magnetometers. Some pulses are observed synchronously at several stations, which excludes their origin from local noise. The spectral maximum of about 7–8 Hz of the oscillatory structure, which appears in many pulses, corresponds to the fundamental frequency of the Schumann resonator. A comparison of magnetic observations with data from the World Wide Lightning Location Network (WWLLN) showed that some of the pulses were caused by lightning discharges in the 650 km vicinity of magnetic station. Although some of the impulses are observed immediately before earthquakes, their connection with seismic activity cannot be claimed with certainty. The calculation of the number of pulses within the 5-min interval before and after the seismic shock shows that they are randomly distributed around the time of the earthquake. Apparently, the pulses are mainly due to the ionospheric response to a distant lightning discharge.

Abstract—Ground-based magnetometers and vertical ionospheric sounding stations were used to record specific variations in the geomagnetic field, caused by the perturbation in the lower ionospheric current systems, and specific variations in the upper ionospheric electron density after a strong volcanic eruption in Kamchatka, Russia, on April 10, 2023. The analysis of the measurements from two series of explosions has shown that the impact on the lower ionosphere is realized via both seismic Rayleigh waves (which are a source of acoustic waves propagating into the ionosphere), and atmospheric internal gravity waves generated by explosions. At distances up to a thousand km from the source, a repeatability of the pattern of ionospheric perturbations after each of the six volcanic explosions is discovered. At larger distances, signals from acoustic waves caused by the Rayleigh waves are clearly recorded in the ionosphere, whereas separation of the signals from atmospheric internal waves is difficult due to the influence of perturbations from other external sources.

Abstract—It is generally recognized that the formation of the fold-and-thrust tectonic structures of mobile belts on continents is associated with crushing and narrowing of the Earth’s crust due to collision of lithospheric plates. The deformation of the Caucasian lithosphere in the recent time is generally consistent with these ideas. However, the block differentiation of the Caucasian lithosphere brings specificity into the directionality of recent vertical and horizontal movements. In this paper, we analyze vertical movements of the Caucasus estimated by means of high-precision leveling over a period of more than a century and consider their spatial correlation with the tectonics, seismicity, stress-strain state, and geophysical fields. A clear correlation indicating the deep tectonic nature of the long-term uplifts of the Caucasus crust is revealed. The differentiation of the Arabian plate movement causes partitioning of the Caucasus territory into provinces that differ by the pattern of recent movements, orientation of faults, and stress-strain state of the geomaterial. These provinces also have distinctions in seismicity in terms of the number of seismic events and focal mechanisms of the earthquakes. We propose a model of a deformation mechanism of the Greater Caucasus, which accommodates the long-term trend of the Caucasus uplift in the conditions of general shortening of the Earth’s crust. The results of the analysis provide a basis for discussion of a probable mechanism of neotectonic evolution of the Greater Caucasus, which can be used in the seismic hazard assessment of the North Caucasus.

Abstract—The generalized cellular model based on the of Olami‒Feder‒Christensen cellular automaton model and modified by considering the durability of the material based on the kinetic concept of the strength of solids developed by Academician S.N. Zhurkov is used to model and clarify the nature of the statistical law of earthquake productivity. The modified model is named Olami‒Feder‒Christensen‒Zhurkov (OFCZ) model. The OFCZ model captures the main statistical regularities and patterns of seismicity: the Gutenberg‒Richter and Omori‒Utsu laws, the Bath’s law, fractal geometry of seismicity, and the law of earthquake productivity. It is shown that the clustering of model events (analogs of earthquakes), corresponding to the earthquake productivity law, is caused by the kinetic component of the OFCZ model. The dependences of productivity on material strength and temperature of the medium are obtained. The influence of the Zhurkov parameter and the cell transmission (coupling) parameter in the cellular model (dissipativity of the model) on the productivity is considered. It is shown that the revealed dependences of productivity on strength and temperature are consistent with the empirical data.

Abstract—A collection of igneous rocks of the Ukrainian Shield sampled from the Korsun-Novomyrhorod pluton (age interval 1760–1735 Ma, Ingul Domain) and from the Korosten pluton (age 1760–1750 Ma, North-Western Domain) is studied. The magnetic and thermomagnetic properties of samples were studied and X-ray diffraction analyses were carried out to obtain reliable determinations of paleointensity (B_anc), It is shown that the carriers of the characteristic component of natural remanent magnetization are single- and small pseudo-single-domain magnetite grains. B_anc was determined by two methods: through the Thellier‒Coe procedure with pTRM checks and by Wilson method. Paleointensity determinations are obtained from five sites and shown to meet quality criteria. For all sites, B_anc and the virtual dipole moment (VDM) values are extremely low, varying within the ranges 3.6–9.76 μT and (0.92–2.43) × 10²² Am², respectively. The analysis of the data from the paleointensity world database (WDB) indicates that the geodynamo operation mode in the Proterozoic can be characterized by an alternation of strong and weak dipole regimes, but the validity of this conclusion entirely depends on the reliability of data reported in the literature and presented in the WDB.

Abstract—We analyze the postseismic processes in the region of the M_w 8.2 Chignik earthquake, Alaska, which occurred on July 29, 2021. Using the model of the seismic rupture surface constructed in our previous work (Konvisar et al., 2023), we have modeled the viscoelastic relaxation process. The results indicate that by reducing the viscosity of the asthenosphere to 10¹⁸ Pa s we an achieve the displacement velocities that are close to those recorded at the coastal GPS sites. However, the displacements on the islands near the source zone of the earthquake differ significantly not only in magnitude but also in direction. At the same time, the constructed postseismic creep (afterslip) model agrees well with displacement data at the GPS sites and with the line-of-sight (LOS) displacement map derived from radar images taken from the descending orbit of the Sentinel-1A satellite. We also analyze temporal variations of gravity field in the earthquake region. The obtained coseismic anomaly is consistent with the anomaly calculated from the rupture surface model. Isolating the postseismic anomaly it is not yet possible due to the insufficiently long time series of gravity models for the time period after the earthquake. The analysis of the postseismic processes is continued in the second part of this work (Smirnov et al., 2024), where we compare development over time of the postseismic displacements of various GPS sites with the aftershock activity, which allows us to draw conclusions about the creep nature of the postseismic processes in the source region of the Chignik earthquake.