Epicentral Zone Research Articles

Geological position, structural manifestations of the Elbistan earthquake and tectonic comparison of two strongest seismic events 06.02.2023 in Eastern Türkiye

The Elbistan (Ҫardak) earthquake with magnitude Mw = 7.5 or 7.6 happened in Eastern Anatolia on 06.02.2023 at 10:24 UTC, following the strongest in the region of East Anatolian (Pazarçik) earthquake with Mw = 7.8 which occurred on the same day at 1:17 UTC to the south of the region. The Elbistan earthquake activated adjacent segments of the Ҫardak and Uluova faults with Quaternary left-lateral strike-slip displacements. The resulting seismic ruptures have a total length of 190 km, of which 148 km are represented by sinistral lateral slip. Their maximum amplitude of 7.84 m was recorded 8 km east of the epicenter. The strike-slip seismic ruptures of the Elbistan and East Anatolian earthquakes represent exposure of their focal zones on the land surface. Both earthquakes exceed average values of these parameters for continental earthquakes of strike-slip type in terms of focal zone sizes and amplitudes of seismic displacements. At the same time, both sources do not propagate deeper than the upper part of the crust (16–20 km). Ophiolite complexes covering the same depths are widely spread in the area of focal zones of both earthquakes. Two maxima were found in the distribution of seismic strike-slip displacement along the epicentral zone of the Elbistan earthquake (i) amplitudes of 5.7–7.84 m in the Ҫardak fault zone and (ii) amplitudes of 3.5–5.1 m in the Uluova fault zone. Both maxima coincide to the areas of ophiolites or their contacts with basement rocks. In crystalline basement rocks, the sinistral strike-slip amplitudes are significantly reduced. We attribute the increased values of focal zone sizes and displacement amplitudes of both earthquakes to the rheological features of ophiolites, which increase a possibility of slip of rocks during seismic movements. We explain the fact that the sources of both earthquakes cover only the upper part of the crust, by the uplift of the top of rocks with reduced P-wave velocities, including the upper mantle and the lower part of the crust and interpreted as heated rocks with reduced strength.

Seismogenic Quasi-Stationary Electric Fields and Currents from Large-Scale Sources on the Earth’s Surface: Comparison of Model Representations

A comparative analysis of various model representations of seismogenic quasi-stationary electric fields/currents from large-scale sources on the Earth’s surface was carried out. It has been established that previously proposed analytical models of seismogenic quasi-stationary sources of electric field/current with field/current amplitudes continuously decreasing to zero at infinity are consistent with extreme values of electric field/current experimentally observed in epicentral zones before earthquakes. It is also shown that sharply spatially limited models of seismogenic sources of quasi-stationary electric fields/currents in the epicentral zones of future earthquakes on the Earth’s surface lead to their values being an order of magnitude or more greater than those actually observed.

Appraisal of Active Tectonics: An Insight from the Morphotectonic Study of Drainage Basins and OSL Dating in the Kangra Area, Himachal Pradesh

ABSTRACT A morphotectonic investigation of the drainage basins and an estimation of the rate of upliftment have been conducted in the vicinity of Palampur, located in the Kangra district of Himachal Pradesh. The assessment utilized the geomorphic index of active tectonics (IAT) and the Optically Stimulated Luminescence (OSL) dating technique. The study area is situated within the epicentral zone of the 1905 Kangra earthquake and is drained by three significant perennial tributaries of the Beas river, namely Baner, Neogal, and Awa. To evaluate the tectonic influence on these drainage basins, Digital Elevation Model (DEM) data with a resolution of 30m, Survey of India toposheets, and Google Earth images were processed using ArcGIS and Global Mapper software. The findings of this investigation indicate that the drainage basins have been impacted by tectonic activity, resulting in the formation of asymmetrical, elongated basins with deep V-shaped valleys and active mountain fronts. The hypsometric integral values derived from the study suggest that these basins have reached an equilibrium stage. Originating from elevations exceeding 4500m in the Dhauladhar range, these rivers, like other Himalayan rivers, partake in active downcutting and erosional processes, as observed in the presence of terraces. By employing the OSL dating technique, the terraces of the Neogal river were dated, enabling the calculation of an upliftment rate ranging from 0.6 ± 0.03 mm/year to 1.7 ± 0.1 mm/year for the area.

Earthquake on January 24, 2024 near Krasnodar city, Mw=4.1, I0=5

The article presents instrumental and macroseismic data on the earthquake on January 24, 2024 at 11:19 (UTC), Mw=4.1, h=8 km. The earthquake parameters were determined using instrumental data from the network of regional seismic stations in the North Caucasus of the GS RAS. The earthquake parameters were determined from instrumental data from the network of regional seismic stations in the North Caucasus of the GS RAS. The source spectra of three regional seismic stations (GOYR, MRNR and GUZR) were calculated, from which the seismic moment М0, the stress drop Δσ and the source radius R were determined. The solution of the focal mechanism was obtained from the polarization in P-waves at 71 seismic stations and the type of source fault with a right-lateral strike-slip component. The epicentral zone of the earthquake on 24.01.2024 is located within the high-magnitude Akhtyrka zone of potential earthquake source zones (ESZ) with Mmax=6.5-6.8 and represents the boundary between the Northwestern part of the structures of the Greater Caucasus and the West Kuban trough. According to macroseismic survey in the regions, 300 respondents were interviewed in 20 settlements. The maximum observed intensity was I=V MSK-64.

DEFORMATION INTERACTION OF STRONG EARTHQUAKES OF 2010–2016 IN THE ZONE OF INFLUENCE OF THE HIKURANGA SUPERPLUME (NEW ZEALAND) ACCORDING TO GPS OBSERVATIONS

Between 2010 and 2016, a series of 11 strong M>6 earthquakes occurred in New Zealand. In the area covering the epicentral zones of these seismic events, the spatiotemporal characteristics of movements and deformations of the Earth’s crust were obtained based on the processing of continuous satellite GPS observations at 64 points of the geodetic network. Using these data, we have studied the evolution of horizontal movements and deformations in order to reveal the possible relationship between the observed deformational and seismic processes. Analysis has been made on the total shear deformation, since the main tectonic structures of the region are faults with a shear mechanism of displacement of their sides. The presence of a giant mantle superplume in the area was the reason for the study of the behavior of horizontal dilatation deformation, and horizontal and vertical crustal motions. Based on the obtained digital deformation models, there were created kinematic visualizations, which are synoptic animations providing direct observations of the seismic deformation process and their heuristic analysis. The study revealed that a series of the strongest earthquakes may be interconnected by a long-term single deformation process, which is caused by the occurrence of an anomalous total shear deformation. The general maximum of shear deformation, dilatation deformation, and horizontal and vertical displacements are concentrated in the center of mantle superplume activity. Prior to strong seismic events, there occur zones of deficit (minimum) displacements of the Earth’s crust in the area of future epicenters, which is of research interest in terms of predicting their locations.

Unravelling seismogenesis and characterizing the unique features of two significant felt earthquakes (M > 4.0) of 2020 in the southwestern parts of the Delhi region

Two significant earthquakes (M4.6 and 4.2) occurred close to a NE–SW-trending lineament in the southwestern part of the Delhi NCR (National Capital Region) within a short time span of about 5 months in 2020. These events were located to the north of the Alwar district in Rajasthan and generated a significant ground shaking in and around Delhi. In the present study, we tried to understand a causal relationship between the events and a nearby source in the region, geologically demarcated as the lineament. We analyzed the broadband waveform data from 26 seismic stations that recorded the recent events of 03 July 2020 (M4.6) and 17 December 2020 (M4.2). Typically, the epicentral area has been devoid of significant earthquakes since the past six decades; however, a few minor events (M < 4.0) have been recorded till date. Analysis of the earthquake database for two decades (2000–2022) revealed low seismicity (nearly quiescent-like situation) in ∼100 sq km area around the epicentral zone, unlike considerable seismicity along faults/lineaments close to the Delhi region. The full-waveform inversion analyses of the events indicate normal faulting with a minor strike–slip components. The source parameters, viz., source radius, stress drop, and seismic moment, were estimated to be 6 km, 166 bars, and 8.28E+15 Nm, respectively, for the 03 July 2020 event and 4 km, 138 bars, and 2.29E+15 Nm, respectively, for the 17 December 2020 event. The causative source of these events is ascertained based on the stress inversion modeling that indicated a NW–SE tensile stress corroborating well with the NE–SW-trending lineament mapped in the study region. The static Coulomb stress modeling indicated that the event which occurred on 3 July 2020 had advanced the triggering process of the event in the northeast segment of the same source that occurred on 17 December 2020. We further emphasize that the aforementioned lineament probably activated due to the regional tectonics of the study area. The causative source of these events with strike 48°, dip 86°, and rake −60° is found to be in the conformity with the local tectonics and is well-supplemented by a high stress ratio (0.70 ± 0.05) and low friction coefficient (0.5).

SEISMICITY of the CARPATHIANS in 2018–2019

The article describes seismic observations in the Carpathian region in 2018–2019, which were carried out, as before, by two organizations from two states: in Ukraine – the Seismicity department of the Carpathian region of the Institute of Geophysics of the National Academy of Sciences of Ukraine, in Moldova – the seismology laboratory of the Institute of Geology and Seismology of the Academy of Sciences of Moldova. The monitoring of seismicity was carried out 21 stationary digital and 3 temporary stations of Ukraine with a processing centre in Lviv and six stations of Moldova with a processing centre in Chisinau. The consolidated catalog of earthquakes was created in Lviv. The total number of Carpathian earthquakes in 2018–2019 was NΣ=99 in the range KR=4.5–13.6 within the interval of hypocenter depths h=0.4–157.5 km, with total seismic energy ΣE=4.2·1013 J. The maximum earthquake with KR=13.6, MSH=5.3 and hрР=150.6 km was registered on October 28, 2018 in the Vrancea zone. The epicentral zone was subjected to shaking with an intensity up to Ii=V by MM (Mercally) scale. The earthquake was felt by the population of Romania, Bulgaria, Ukraine, Moldova and other European countries. In the Precarpathian and Transcarpathian regions, all earthquakes were weaker. For Transcarpathia, their energy class was in the range of KR=5.5–7.9, and the depths was in the range of h=0.6–8.0 km. The strongest event in Precarpathian was a tangible earthquake that occurred in the Drohobych district of the Lviv region on September 27, 2018 with KR=7.3 at a depth of 1.7±0.9 km. The strongest event in Bukovina was an earthquake of March 24, 2018 with KR=9.4, МSH=2.4 and h=1 km near Novodnestrovsk city, Ukraina, which was felt by the population of the village of Lomachintsy with an intensity of 4 points. In Novodnestrovsk, the earthquake was felt only on the upper floors. In general, in 2018–2019 there was a slight decrease in the level of seismicity in seismically active areas: № 1 (North-Western), № 2 (Vrancea), № 5 (Bukovina) and № 8 (Bacau), compared with that in the previous period 2016–2017. In areas № 3 (South Carpathians), № 4 (Banat) and № 7 (Transylvania) there was no earthquakes, and in area № 6 (Krishana) only there was one earthquake with КР=10.8, MSH=3.2.

Hydrochemical changes in thermal waters related to seismic activity: The case of the 2020–2021 seismic sequence in Granada (S Spain)

Between the end of December 2020 and August 2021, 3110 earthquakes were recorded in the Granada Depression in southern Spain, with 353 of them being felt by the population. The highest concentration of earthquakes occurred between January 23rd and February 23rd, 2021, during which five out of six of the highest magnitude earthquakes (greater than 4.1 Mw) were recorded. This shallow seismicity is associated with NW-SE normal active faults, specifically the Santa Fe fault. Hydrochemical sampling was carried out at two deep, upwelling, and thermal boreholes located near the epicentral zone between January 27th, 2021 and July 18th, 2022, to analyze the response of a set of hydrochemical parameters during and after the earthquakes. The methodology employed different statistical tools, including Principal Components Analysis (PCA), PHREEQC modeling as well as Piper and Langelier-Ludwig diagrams, to determine the hydrochemical evolution during the seismic sequence and the parameter variations in relation to the number and magnitude of the earthquakes. The results showed significant variations in almost all of the parameters analyzed, with greater variations in SO4, Cl, Fe, Ca, Zn, and Al. During the days with the highest number of earthquakes of the highest magnitude, there was a rapid increase in Cl, SO4, SiO2, and Mg, while F, T, EC, and pH decreased. Peaks in SO4 seemed to anticipate earthquakes of greater magnitude that could be related to the preseismic dilatation which allows for an inflow of deep sulphated source water into the aquifer system. PCA and hydrochemical diagrams showed appreciable differences between the two boreholes, which could be related to the sampling distance from the epicentral zone. The results suggest the need for a hydrochemical monitoring network in the region, which has the highest seismic hazard in Spain.

From the Editorial Board of the <i>Izvestiya, Physics of the Solid Earth</i> Journal

On February 6, 2023, catastrophic earthquakes struck Turkey, causing extensive destruction and numerous casualties. These seismic events immediately captured the attention of the global geophysical community, leading to the emergence of preprints and articles analyzing various aspects of the earthquakes on the Internet and in print. The Izvestiya, Physics of the Solid Earth journal also received submissions from Russian scientists containing the results of their geophysical investigations related to the Turkish earthquakes. Consequently, the editorial board of the journal decided to dedicate a special issue to this topic. This issue features articles by Russian scientists from academic institutions and universities: Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (IFZ RAN); Institute of Earthquake Prediction Theory of the Russian Academy of Sciences (ITPZ RAN); Institute of Geosphere Dynamics of the Russian Academy of Sciences (IDG RAN); Geological Institute of the Russian Academy of Sciences (GIN RAN); Kola branch of the Geophysical Service of the Russian Academy of Sciences; Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN); the Faculties of Physics and Geology at Moscow State University; Gubkin Russian State University of Oil and Gas; as well as colleagues from Firat University in Turkey. The articles included in this issue cover a wide range of subjects. The geological situation and tectonic position of the earthquakes are described in the article by H. Çelik et al., along with the seismic rupture parameters obtained through fieldwork conducted in the earthquake’s epicentral zone by researchers from the Geological Institute of the Russian Academy of Sciences in collaboration with Turkish colleagues. D.A. Simonov and V.S. Zakharov present the results of a preliminary seismotectonic analysis based on GNSS observations in their article. Yu.L. Rebetskii’s article presents the tectonophysical zoning of seismogenic faults in Eastern Anatolia based on estimations and analysis of Coulomb stresses. R.E. Tatevosyan et al. provide insights into the historical earthquake of 1888 along the northeastern extension of the East Anatolian faults, including an estimate of its magnitude. A.I. Filippova and A.S. Fomochkina developed models of displacements in earthquake sources based on surface seismic waves, while V.O. Mikhailova et al. utilized satellite interference data on displacements on the Earth’s surface. O.V. Pavlenko and V.A. Pavlenko investigated the effects of radiation directivity of earthquake sources. V.B. Smirnova et al. present the results of a posteriori analysis of seismic regime anomalies preceding the earthquake in Turkey. S.V. Baranova et al. offer an earthquake aftershock hazard assessment using an automated system previously developed by the authors. The articles by V.V. Adushkin et al. and S.A. Ryabova et al. consider the geomagnetic and geoelectric effects induced by the Turkish earthquakes. Special Issue Editors V.B. Smirnov P.N. Shebalin

Altay Seismicity Evolution after the 2003 Chuya Earthquake

The 2003 Chuya earthquake had a noticeable impact on the seismicity of the whole Altay region. The article presents data on changes in the seismic regime of Altay from 2003 to 2021. It is shown that in the first six months after a major earthquake in 2003, earthquakes occur in the epicentral zone, and other structures of Altay are aseismic. After 2009, changes are formed both in the near zone, covering geological structures adjacent to the epicenter, such as the Aigulak, Kuray, South Chuya, North Chuya ridges, and in the far zone at distances of 250‒300 km from the epicenter and in different directions from it. After the Aigulak earthquake of 2019, an aftershock process was formed, which ensured the displacement of the center of Altay seismicity into the ridge of the same name. The explanation of the seismicity development in space and with time delays for years can be given on the basis of the influence of a large earthquake on the multi-layered lithosphere in accordance with models that assume the occurrence of disturbances during a large earthquake not only in the fragile part of the earth’s crust, but also in layers with high plasticity.

The Khubsugul Earthquake of January 12, 2021, <i>M<sub>L </sub></i>= 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.

ELECTROMAGNETIC MONITORING DURING THE AFTERSHOCK PERIOD OF THE 2003 CHUYA EARTHQUAKE IN THE MOUNTAIN (GORNY) ALTAI: MEASUREMENT METHODOLOGY, RESULTS

The article considers the method of observations, interpretation of data and results of electromagnetic monitoring with a controlled source for one of the seismically active regions of Siberia – Mountain (Gorny) Altai. The monitoring is carried out during the aftershock period in the epicentral zone of the destructive Chuya earthquake of 2003 with M=7.3. For regular observations, a measurement technique has been developed with several modifications of the transient electromagnetics method (TEM) to determine variations in electrical resistivity and anisotropy coefficient. The long-term series of these two geoelectric parameters of the section are presented, compared with the characteristics of the ongoing seismic events. The analysis shows that variations in electrical resistance and electrical anisotropy reflect the development and gradual attenuation of aftershock activity of a powerful earthquake. The advantages of the TEM method and the chosen methodology for monitoring in complex areas are reflected.

Weather Events Associated with Strong Earthquakes and Seismic Swarms in Italy

This study discusses the possible relationship between potentially destructive seismic events, earthquake swarms, and intense weather events occurring in the same epicentral zone at time intervals ranging from one day to a few weeks. The objective of the present study is, therefore, to analyze the interaction between the lithosphere, atmosphere, and ionosphere in order to propose, prospectively, a new hydro-climatic model to be applied not only in Italy, where this research was carried out. The study concerns some of the most intense Italian earthquakes starting from 1920, with the destructive event in Lunigiana, in North Western Apennines, until the recent earthquake swarm that hit the Emilia-Romagna region followed, as in the cases analyzed in this research, by strong atmospheric disturbances. The recurrence associating seismic events with atmospheric precipitation allows us to propose some hypotheses about the triggering mechanism. In tectonically stressed areas, during pre-seismic and seismic phases, the release of gases from the ground and electrical charges near active faults is known. It is hypothesized that water condensation nuclei are carried by radon gas on atmospheric gases, also originating from cosmic rays in the upper atmosphere, generated by air ionization.

KHUBSGUL EARTHQUAKE OF JANUARY 12, 2021, <i>M</i>w = 6.7, NORTHERN MONGOLIA: GEOLOGICAL EFFECTS AND TECTONIC POSITION OF THE SOURCE

The geological effects of the Khubsgul earthquake on January 12, 2021 with Mw = 6.7, which became the strongest in the Khubsgul region at the instrumental stage of seismological observations, were studied and mapped. Using the ESI-2007 scale, the epicentral zone was contoured with an intensity VIII. The primary seismic rupture is represented by a dextral normal fault with displacement up to 20 cm, which renewed the Late Holocene tectonic scarp in the western side of the Khubsgul rift basin. The dynamics of rupturing in the Late Holocene testifies to the gradual expansion of the depression due to the destruction of the adjacent mountain range. The data of satellite interferometry and seismological observations are in good agreement with the results of field studies, which allows us to consider the selected region of maximum shaking as a projection of the upper part of the source zone onto the surface.

Migration of Anomalous Displacements on the Flanks of a Preparing Earthquake Source: A Swing Model

The geomechanical modeling results on the stress-strain state dynamics before the 2019 M = 7.1 Ridgecrest earthquake, Southern California, show the alternating development of maximum displacements at the fault’s ends, which simulates the process of “swinging” of the epicentral zone of the earthquake. These results together with the existing theoretical ideas about the block structure of the Earth’s crust and the corresponding connections between blocks are used to construct a geomechanical “swing” model based on the stick-slip motion of three interconnected blocks. The model parameters are related to the rheological properties of the Earth’s crust in the region of the rupture of the bridging isthmus which determines the source size of the seismic event.

The Khubsugul Earthquake of January 12, 2021, Mw = 6.7, Northern Mongolia: Geological Effects and Tectonic Position of the Source

The geological effects of the Khubsugul earthquake of January 12, 2021, with Mw = 6.7, which became the strongest in the Khubsugul region during the instrumental stage of seismological observations, are studied and mapped. Using the ESI-2007 scale, the epicentral zone with intensity VIII is contoured. The primary seismic rupture is represented by the right-lateral strike–slip with a displacement amplitude up to 20 cm, which renewed the Late Holocene tectonic scarp on the western side of the Khubsugul rift basin. The dynamics of rupturing in the Late Holocene indicates the gradual expansion of the basin due to the destruction of the adjacent mountain range. The data of satellite interferometry and seismological observations are in good agreement with the results of the field studies, which allows considering the delineated region of maximum shaking as a projection of the upper part of the source zone onto the surface.

CHANGES IN GEODYNAMIC SITUATION MANIFESTED IN SOIL RADON AND TEMPERATURE VARIATIONS AND ABNORMAL NATURAL PHENOMENA IN THE ZONE OF KATAV-IVANOVSK EARTHQUAKE (05.09.2018), THE SOUTHERN URALS

The strongest earthquake in the Urals registered during the period of instrumental observations, with a magnitude of ML-5.4 occurred in the area of Katav-Ivanovsk city, Chelyabinsk region, on 05.09.2018. In October 2018, Institute of Geophysics, the Ural Branch RAS organized temperature and radon observations. The conducted studies have shown that the behavior patterns of the volume radon activity (VRA) established for the tectonic event preparation in the subduction zone (Southern Kuriles), are also true for the South Ural region. The natural phenomena caused by the earthquake 2018 preparation are analyzed. During the observation period from 28.11.2018 to 23.05.2019, there was an increase in temperature with a gradient of 0.02°C per month. The observed patterns of VRA abnormal behavior before aftershocks in the Katav-Ivanovsk area are typical for the compression zone in the radon monitoring area. It is noted that the assumed compression deformations were manifested in the hydrogeodynamic field at least 10 years before the event proper. The proximity of earthquake foci to the identified aseismic zone within the boundaries of the East European Platform, the Pre-Ural Regional Trough, and the Bashkir anticlinorium indicates the most likely places of seismic events. The arranged temperature and radon monitoring network in the epicentral zone by observing aftershocks has shown the possibility of tracking the earthquake preparation process.

Weak Seismicity and Strongest Earthquakes Against the Background of the Variations of <i>S</i>-wave Attenuation Field

The paper considers the role of relatively weak earthquakes as a tool for studying the environment, including of strong earthquake process. The spatial structure of the attenuation field of several seismically active regions (of the Garm prognostic polygon in Tajikistan, Altai, the Caucasus, Eastern Anatolia, the Western Tien Shan), as well as the epicentral regions of a number of strong earthquakes, and the confinement of deepened seismicity to it are considered. It is shown that the attenuation field obtained from the short-period code of weak earthquakes in seismically active regions is inhomogeneous and consists of blocks with a high Q factor and weakened zones of strong attenuation. An uneven distribution of deepened earthquakes is noted. It is associated with the block structure: in weakened zones, their share is greater than in blocks with a high Q factor. Examples of variations in the of deepened seismic activity in weakened zones are demonstrated. It varies over time, increasing before strong earthquakes. Facts are presented that testify to the existence of a relationship between the Earth’s rotation rate and the activity of deepened seismicity. Examples are given of the activation of weak seismicity in the form of seismic swarms (series of weak earthquakes concentrated in space and time) in connection with strong events. A characteristic feature of these swarm series is the isometry of the earthquake localization areas in plan view and vertical elongation. As a rule, they coincide with weakened zones of strong absorption of S-waves. Intense localized seismicity, confined to one-dimensional volumes, is most likely associated with increased conductivity channels through which deep fluids migrate. The activation of swarm series is the result of active migration of deep fluids and an increase in fluid saturation of weakened zones. Fluids, in turn, are a catalyst for processes that lead to a decrease in the strength of rocks and the destruction of blocks in epicentral zones. In this case, the clusters to which the swarm series belong can be considered as local seismogenic zones. The appearance of compact isometric in plan and nearly vertical in section clusters of weak seismicity is often observed outside the epicentral zones of strong earthquakes. Such zones may simply be indicators of the seismotectonic situation in the region as a whole. It is assumed that a sharp change in the dynamics of atmospheric pressure during the preparation of a strong earthquake at hydrometeorological stations located in such areas is a consequence of the intergeospheric interaction of the lithosphere and atmosphere. Deep degassing seems to be one of the main mechanisms of the anomalous behavior of atmospheric pressure during the implementation of strong events. It is most active in weakened zones. The mechanisms of the impact of deep degassing on the outer geospheres remain the subject of discussion.

Seismotectonic Consequences of the Strong Earthquake Mauli (February 27, 2010, <i>M</i><sub>w</sub> = 8.8) in Chile: Digital Modeling of the Crust Stress-State of the Western Margin of the South American Plate

This article presents seismotectonic consequences of the strong Mauli earthquake in Chile, which occurred on the February 27, 2010, Mw = 8.8. The consequences are considered as a manifestation of a large-scale fragment of the general seismotectonic process on the western edge of the South American plate (Chilean sector). Our study shows that manifestations of postseismic processes of the Maule earthquake cover a much larger area compared to the epicentral zone of the aftershocks. Based on the comparison of the results of numerical modeling of the stress-strain state before and after the earthquake, seismological, geodetic, and satellite data, an alternative model of the development of the seismotectonic process in the Chilean sector of the South American plate was proposed. The stress-strain modeling was performed by the finite element method. The source of the Mauli earthquake, at a depth of 33 km, falls into the region of relatively high values of compression stresses and positive maximum shear stresses. It was shown, that other strong earthquakes of the Chilean sector in the interval of depths from 20 to 50 km are caused by high concentration of tectonic stresses in the region of transition from oceanic to continental lithosphere. Within the framework of the proposed model of the seismotectonic process, ruptures weaken the contact between the oceanic and continental lithosphere after strong earthquakes. Abrupt sinking of the continental lithosphere into the mantle causes an increase in viscous melt pressure, promotes penetration into mega-cracks, and rises to the surface, causing subsequent volcanic eruptions. It is shown that the results obtained in comparison with the coseismic consequences of earthquakes do not contradict these results of numerical modeling and give new insights into the structure of the lithosphere in the continent‒ocean transition zone and the development of the seismotectonic process.

Retrospective Prediction of the Location and Intensity of Two Strong Crustal Earthquakes in Iran And India

The paper presents the results of mathematical modeling of the stress-strain state of the epicentral zone before and after the strong earthquakes Mw = 6.4 of June 22, 2002 in the north-west of Iran in the region Qazvin and 26 January 2001 in India with M = 6.9. The method of modeling of the stress-strain state of the block elastic-isotropic heterogeneous medium disturbed by the system of faults under the influence of the external field of tectonic stresses was used, boundary conditions were set according to seismological data. It is shown that at a certain ratio between the main stresses the epicenters of strong cow earthquakes are localized in the area of high concentration of stress intensity. The development of aftershock process is caused by stress release caused by a new break, and the emerging clusters of aftershocks spatially correlate with the zone of released intensity of stresses. It is assumed that the tectonic rupture propagates in the direction of the zones of high intensity of stresses. The actual correlation between the rupture length and the earthquake magnitude from the accumulated elastic energy and its possible realization under given structural and tectonic conditions is shown.