Linear sequences of earthquake epicenters (“chains”) related in space and time are studied. A new approach to understanding earthquake chains as a special kind of group (clustered) events is proposed. It is believed that clusters of group earthquakes with pronounced spatial anisotropy potentially represent the desired chains. Such chains are given the physical meaning of markers of activated tectonic faults. A formalized algorithm for the allocation of linear sequences of earthquake epicenters based on the proposed approach has been developed. The search for chains is conducted in the catalog of group earthquakes. Previously, single earthquakes (i.e., not included in clusters) are removed from seismicity. For this, a previously developed algorithm was used, focused on the selection of any interrelated events, and not only (mainly) aftershock and/or foreshock series (Descherevsky et al., 2016a). The proposed method of isolating earthquake chains has been successfully tested on earthquake catalogs of Garm, Iran and central Turkey. Chain maps are provided, and summary statistics of the chain field are discussed. As a rule, these chains can be compared with various tectonic disturbances, but a significant part of them are not tied to known structures. For the Garm district, the continuity of the results obtained with previously performed studies is shown. Like almost any method of analyzing seismic data, the earthquake chain algorithm has a significant number of configurable parameters. Within certain limits, you can vary the criteria for allocating group events, the minimum number of events in the chain and its minimum length, as well as the required level of straightness of the chain. However, all these settings primarily affect the total number of chains found in the catalog, and their location and orientation (azimuths) they almost do not depend on the algorithm settings. This allows us to consider the proposed analysis method as a fundamentally new way of extracting and visualizing information about the spatial and temporal organization of seismicity. A more detailed study of both the structure of earthquake chains and its changes over time in various seismically active regions of the world can contribute to a better understanding of the dynamics of the seismotectonic process.

The analysis of the tectonic situation (according to the literature data) and seismicity in the vicinity of the Mid-Atlantic Ridge (MAR) is carried out. It is confirmed that the spreading of the axial part of the MAR plays a decisive role in the seismotectonics of the region under consideration. The result of spreading is a pronounced rift valley, no more than several tens of kilometers wide, torn into separate segments by transform faults transverse to its strike. Displacements along these faults range from several tens to several hundred kilometers. The southern half of the Atlantic Ocean is characterized by significantly greater tectonic fragmentation (greater frequency of closely parallel faults following each other) as compared to the northern half. Perhaps this is why its significantly lower seismic activity is associated with it: not a single earthquake with M = 8 has been recorded here for more than 100 years, while three such events took place in the Central Atlantic. The vast majority of earthquake epicenters closely mark the MAR axial part occupied by the rift valley, clearly following its stepped appearance caused by displacements along transform faults. This suggests that the main potentially seismically active elements of the MAR are spreading areas characterized by a discharge type of seismotectonic deformation, according to the constructed solutions of the composite focal mechanisms. The composite focal mechanisms have been reconstructed based on fairly large aggregates of known individual solutions in various spatial samples in the immediate vicinity of the MAR. The predominance of normal fault seismotectonic deformation has been established in almost all spatial samples within the MAR, which is in good agreement with spreading processes in the axial part of the MAR. The exception is the largest shear zone separating the southern and central parts of the Atlantic Ocean near the equator. Pronounced shear deformations are observed within this zone, mainly due to right-sided shear movements along the system of faults transverse to the strike of the axial part of the MAR. Their number is more than 80

—Over the whole area of the Eurasian lithospheric plate considered in the paper—from the Atlantic to the Pacific Ocean—the velocity vectors of the horizontal eastward displacements of GPS sites form a gentle, smooth, huge arc, convex towards the north. Three relatively narrow arc-shaped bands can be distinguished within the arc. The southernmost band, most provided with measurement data (referred to as a arc-shaped band A in this paper) is discussed in detail in (Shevchenko et al., 2021). The direct, real results of the published geodetic measurements have shown that the length of this band increases by 5 to 10 mm annually. The arc is lengthening. In the cited paper, we considered and rejected five interpretations of this lengthening, which involve the idea of external action on the rocks of the Earth’s crust/lithosphere of this arc-shaped band. It is most likely that the elongation is caused by the internal process of the increase in the volume of rocks within the band due to the supply of additional mineral material by deep fluids and its subsequent crystallization. In this paper, we present the results of similar geodetic measurements in other two arc-shaped bands, B and C . It turns out that the arc-shaped band C located to the north of the other bands is lengthening (and at a similar rate), as is band A located to the south of the other bands. At the same time, the arc-shaped band B , which is located between bands A and C , is moving eastward without lengthening. Geodetic measurements in the axial part of the Mid-Atlantic Ridge on the Iceland island suggest that the general expansion of the Atlantic Ocean and the corresponding eastward motion of the Eurasian lithospheric plate are associated with the injection of plastic magmatic wedges (of basic composition) in the axial, rift zone of the ridge. Against the background of this general eastward motion of the lithospheric plate, its constituent parts ( whose markers are the arc-shaped bands A and C ) are moving in the same eastward direction with a velocity increasing from west to east. We attribute this increase in the velocity and the corresponding lengthening of the two bands to the above inflow of the additional mineral material and its subsequent crystallization.

In connection with the 50th anniversary of the destructive Dagestan earthquake on May 14, 1970, information on the parameters, manifestation features, and consequences of this seismic event was collected and summarized. The Dagestan earthquake became a very important event in the seismic history of the entire Caucasus. The reasons for this were determined by several of its features, the most important being that it was the strongest instrumentally recorded earthquake in the Caucasus at that time, and the nearest seismic station, Makhachkala, was at an epicentral distance of less than 30 km and recorded not only the main shock, but also a strong foreshock with its aftershock sequence. After the earthquake, a temporary seismic network was quickly deployed, which made it possible to trace in detail the development of the aftershock process of the main shock. Based on the literature data, the main parameters of the Dagestan earthquake and its most important consequences are discussed. The most realistic model of the source was constructed by S.S. Arefiev et al. (2004) based on data on the inversion of body waves from this earthquake. According to this model, Dagestan earthquake had a multiple source consisting of three subsources. The initial rupture (subsource), with a horizontal extent of 14 km, was located in the center of the focal zone at a depth of 9 km. The second rupture began about 2 s later, 10 km east of the first, at a depth of about 10 km and with a horizontal extent of 20 km. The third subsource was 10 km west of the first, at a depth of 12 km. The Dagestan earthquake made it possible to significantly refine the regional seismic hazard assessment. This was extremely important in regards to construction of the Chirkey hydroelectric power plant (HPP) with a high-altitude dam (232 m) in the 8-point shaking intensity zone for this seismic event. The creation of the reservoir itself during the construction of the high-pressure concrete dam of the HPP led to the impact of its filling and further seasonal fluctuations in the volume of the reservoir (about 3 billion m(3)) on the seismic activity in the upper part of the crust, with an area of at least 1000 km(2). This was the reason for the occurrence of induced technogenic earthquakes in this area. The effect of induced seismicity can be dangerous if completion of the dam and creation of the reservoir coincide in time with the natural rhythm of ordinary seismogenesis. It also turned out that powerful shaking of the upper part of the crust during the Dagestan earthquake disrupted the oil production regime in Dagestan.

A Review of published space geodetic data for Southeast Asia has been used to confirm the assumption that the central part of the region (the Sunda Plate) can be regarded as a rigid stable block with a continental lithosphere. The combined geodetic, geological, and seismological data make it possible to specify the position of the boundaries of this block (Sundaland). These boundaries are largely represented by subduction zones and areas of recent continental tectonic deformation of the Earth’s crust. Here, the overwhelming proportion of deformation is concentrated precisely within the boundaries of Sundaland. Data on the sets of earthquake focal mechanisms have been used to assess the type of the seismotectonic deformation tensor (STD) within 21 spatial samples in the vicinity of various sections of the Sundaland boundary. Compared with existing geological and geodetic data, the results of this assessment showed a satisfactory agreement between them. The STD type has been assessed for the entire observable depth of subduction zones and the revealed variations in the STD type with depth have been interpreted.

A method is proposed for calculating scalar products of pairs of seismic events that are closest in time to isolate sequences of epicenters of crustal earthquakes (chains) connected in space and time. To verify the developed methodology, the model and real catalogs of earthquakes of the Garm region in Tajikistan are compared. The efficiency of the proposed algorithm is shown. Based on the totality of the results of the allocation of chains, the issues that complicate the detection of the mentioned structures in the general case are considered. In the process of applying the created program to the real catalog of earthquakes of the Garm region, the existence of four pairwise orthogonal prevailing directions in their orientation was established. There were also some changes in the time of the predominant role of the selected strike of chains of earthquake epicenters. It is possible that these changes may reflect some elements of the dynamics of the seismic process in the Garm region.

The existence of consistent, significantly extended (up to 4–7 years) intervals of positive (up to +0.8) and negative (up to –0.8) correlations between the time series of the numbers of weak earthquakes (with M ≥ 3) in spatial samples far enough apart from each other within the Zagros (Iran) and the North Anatolian (Turkey) fault systems is established. This result can be considered a manifestation of the remote interaction of seismicity within the territory under consideration. From observations of phase shifts of about 1.5 years between neighboring pairs of Iranian and Turkish samples, the centers of which are located at a distance of about 250–300 km, the rate of propagation of this interaction in the direction from east to west can be presumably estimated as about 200 km/yr. This value is in satisfactory agreement with the migration rate of earthquake sources in the range of 5–150 km/yr according to numerous estimates in the literature.

The article presents a brief overview of the currently existing ideas about the seismotectonic situation in the Earth’s crust of Iran, which is experiencing intense compression in the northeastern direction as a result of collision of the Arabian and Eurasian lithospheric plates. The survey also involved geodetic data in the form of modern GPS measurements of horizontal surface displacements. The stress-strain state of the Earth’s crust of Iran (construction of the average focal mechanism) was assessed based on data on the total set of 945 focal mechanisms of earthquakes of average strength (4.4 ≤ MW  ≤ 6.5) according to the ISC catalog, which occurred from 1975 to 2020, within 12 spatial samplings. The focal mechanisms of the strongest earthquakes in the last 50 years (MW = 6.0–7.4) for one event in each of these samplings are also considered. The calculated parameters of the average mechanisms and focal mechanisms of strong earthquakes are compared both with each other and with the surrounding tectonic situation and the distribution of deformation velocity vectors according to GPS observations. A satisfactory correspondence has been established between all the comparable values. The differences in the type of formation of the seismogenic layer of the Earth’s crust of Iran in different regions are demonstrated. These differences are manifested in different ratios of shear and thrust components in the reconstructed mean mechanism in different spatial samplings. A similar difference is noted in individual focal mechanisms of the strongest earthquakes. However, it is possible to describe the observed nature of deformation of the crust of Iran within a single concept of collisional tectonics, caused by collision of the Arabian and Eurasian plates in the last 5 Ma.

Possible industrial explosion effects on weak earthquake seismicity in Turkey are considered. The spatial and temporal relationships between the studied events within short periods of time and over short distances, as well as their probable long-term effects, are investigated. Potential correlation between normalized distributions of daily and weekly earthquake and explosion numbers is estimated. Possible changes in the fractal dimension of the spatial seismicity distribution in connection with industrial explosions are studied. To search for spatially compact regions of explosions and local earthquake groups, the method of identifying clusters was applied. Clusters of explosions and earthquakes that are closely spaced have been found. In these clusters, the indicated time series behavior was compared. The significance of the observed temporal and spatial changes is assessed by the method of creating artificial catalogs, as well as by constructing inverse statistics of the earthquake-explosion.

As a result of a review of methods for the identification of group events, it is argued that to date there is no universal algorithm for detecting formations of a physical nature in the structure of seismicity. Therefore, the method of catalog declustering should be selected both for the existing structure of the source data and for a specific task. It is proposed to use the declustering of earthquake catalogs with great caution, since such a procedure can significantly distort the seismic hazard assessment. Information on earthquakes in the Yano-Indigirskaya lowland in northern Yakutia, was collected which seismicity is relatively poorly studied. An analysis of the data of 43 seismological agencies resulted in a catalog of 3161 earthquakes from 1920 to 2020 that has been compiled with a representative magnitude of M-W = 3.3 to distinguish the grouped events. The catalog was declustered using different methods, and the seismicity has also been divided into concentrated (group) and scattered (background) components. It is shown that these components differ significantly in recurrence plots and M-max estimates, which significantly affects the seismic hazard assessment.

Catalogs of weak seismic events (M ≤ 4) in Turkey and Iran were analyzed to identify signs of their contamination with blasts. A well-pronounced predominance of the number of seismic events and a decrease in their average magnitude in the period from 10:00 to 17:00 LT were found. The epicenters of seismic events that occurred in this time interval at depths of less than 3 km are located much more compactly than other seismic events. Such indications can be considered as an evidence of contamination of the earthquake catalogs with road, mine, and quarry blasts. The presence of such an anthropogenic effect in the earthquake catalogs is also confirmed by the presence of a well-defined weekly periodicity of seismic events. It manifests in the form of a significant weakening of the discussed daytime extremum on Saturday and Sunday in Turkey and on Friday in Iran. Using the example of Turkey, where the observation service attempted to separate blasts and earthquakes in the seismic catalog, it is shown that in an earthquake catalog, which was purified this way, the prevalence of seismic events in the daytime over their number in the nighttime is still observed. The opposite effect was also revealed, namely, the presence of earthquakes in the blast catalog; i.e., there is a mutual contamination of the catalogs of earthquakes and blasts. However, it cannot be excluded that the relative increase in the number of earthquakes in the daytime is only partially caused by the presence of undetected explosions due to imperfect algorithms for discriminating between blasts and earthquakes. The daytime extremum in the number of earthquakes can be partially generated by the occurrence of additional weak earthquakes triggered by relatively strong blasts (i.e., by the triggering effect of explosions on the natural process of seismogenesis). The obtained results show that even in case of earthquake catalogs that are claimed to be cleared of blasts, it is necessary to carry out preliminary study these catalogs in order to assess their contamination with blasts.

— The comparative characteristics of seismic and deformational effects of three great subduction megaearthquakes in Sumatra in 2004 ( M w = 9.2); in Maule, Chile, in 2010 ( M w = 8.8); and in Tohoku, Japan, in 2011 ( M w = 9.0) are considered. In all cases the main rupture at the time of the earthquake was located in the subduction zone on the surface of the oceanic lithospheric plate plunging gently beneath a continent or an island arc. The process of destruction in each focus is characterized by a megathrust, with a gentle inclination angle of ~8–18º, according to the definitions of its focal mechanism, in almost full compliance with a gentle dipping of the oceanic lithospheric plate. The displacement occurred in a locked area of the subducting plate extended for several kilometers below the ocean bottom to a depth of 30–40 km. In all three cases, the maximum coseismic slip, obtained according to the data of geodetic measurements, occurred in the upper 25 km of the locked area, while its lower part radiated coherent short-period seisms. The trace of the rupture on the surface, marked by the aftershock area, ranged from 400–600 km in the case of Tohoku and Maule earthquakes up to ~1500 km during the Sumatra earthquake. The rupture in the Maule and Tohoku earthquakes was bilateral, with its approximately symmetric propagation from the epicenter, while in the Sumatra earthquake the rupture spread unilaterally relative to the epicenter from southeast to northwest. The propagation time of the rupture also varied. If in the first two cases it was 140–160 s, for the Sumatra earthquake it lasted 500–600 s. The largest tsunami wave, up to 40–60 m in height, was recorded during the Tohoku earthquake, extending for more than 200 km along the coast of Sanriku province. The rupture during the megaearthquakes under discussion is not confined in depth to the locked seismogenic area, marked by the area of the aftershock nearest in time and the zone of maximum coseismic slips determined by the geodetic GPS measurements. The rupture continues aseismically (postseismic slip) in the transition zone from brittle to brittle-plastic slip to depths of ~60–80 km. In addition, there is evidence that the displacements on the megathrust can continue slightly deeper to the region of the brittle–plastic slip. Such episodic events of slow slip (“silent” or “slow” earthquakes) and “seismic tremor” were recorded in southwestern Japan and southern Chile. These differences in seismic and deformation effects can serve as evidence of the change in frictional properties with depth along the surface of the megathrust. It should also be recognized that the strength barriers and asperities on the megathrust surface expressed in certain geological structures or yet unclear nature of segments of high frequency radiation can manifest themselves in the character of distribution of the accompanying seismicity during the great subduction earthquakes.

The neomobilist plate tectonic concept is currently at the forefront of geotectonics. This concept is based on the idea of the division of the Earth’s crust/lithosphere into plates, blocks, and massifs of different size that move laterally due to external forces. The greatest importance is attached to their movement due to the viscous coupling of plates with convective flows in the mantle. At the same time, the results available on geodetic (mainly GPS) measurements of real movements within the Eurasian lithospheric plate indicate that the size of its northern part is significantly increasing from west to east. Existing ideas about the geodynamics of the Earth’s crust/lithosphere do not take this phenomenon into account. Based on the materials of systematic repeated GPS measurements and focal mechanisms of earthquake foci, an interpretation of this elongation of the northern part of the Eurasian lithospheric plate to the east is provided in this paper. This elongation develops against the background of the predominance of subhorizontal compression stress in the Earth’s crust of the part of Eurasia under consideration. At the same time, there is a systematic reversal of the orientation of the main compression axis from 147° in the west within the European margin of the plate to 283° at the extreme eastern edge of the plate (Kolyma Highlands). The environs of the Upper Rhine Graben and the Baikal Rift with predominant subhorizontal stretching, corresponding to the known geological data by the orientation, fall out of this pattern. It is assumed that the observed elongation of the Earth’s crust/lithosphere of the considered part of the Eurasian lithospheric plate to the east, while being in a stressed state of subhorizontal near-meridional compression, may occur due to the development of strut stress in the crust/lithosphere. Such stress is most likely caused by an increase in the volume (and area) of layered rocks as a result of the introduction of additional mineral material by deep fluids from the lower crust/upper mantle and its subsequent crystallization.

Based on the available geological and geodetic data, it has been established that the structures of the Alpine–Himalayan–Indonesian mobile belt are oroclinally (horseshoe-shaped) closed at its western and eastern extremities. In the west, from the Atlantic Ocean, this closure is represented by the Bet–RIF arc and, in the east, from the Pacific Ocean, by the Band and Mindanao arcs. The presence of these closures means there is no structural connection between the mobile belt under discussion and the Atlantic and Pacific oceans, respectively. Thus, the existing plate-tectonic reconstructions, according to which the Tethys paleocean was a wide bay of the Pacific Ocean connected in the west with the Atlantic Ocean (wide Tethys), are in opposition to the facts. The Alpine–Himalayan–Indonesian mobile belt has to be considered an epigeosynclinal rather than an epioceanic structure.

Abstract—The leading role in geotectonics is currently played by the neomobilistic plate tectonic concept which is based on the division of the the Earth’s crust/lithosphere into plates, blocks, and massifs of different sizes. These crustal units move laterally, driven by the forces that are external to them. For example, it is assumed that under the action of gravity, a plate slips off the mid-ocean ridge and then moves horizontally following the heavy subducting slab sinking into the mantle. The motion due to viscous coupling between plates and convective flow in the mantle is considered as most important factor. Mechanical action from the neighboring lithospheric plates is not excluded. The data accumulated to date on the geodetic (mainly GPS) measurements of these real movements within the Eurasian continent indicate that the size of the northern part of the latter has significantly increased. This increase is not taken into account in the existing notions of geodynamics of the Earth’s crust/lithosphere. Several possible interpretations of this phenomenon are discussed.

We studied the time series of the total number of weakest earthquakes (microearthquakes) with magnitudes M ≤ 2 recorded by 15 seismic stations of the Garm test area in 1975–1985, as well as the microseismic noise level at different frequencies of about 0.005–50 Hz recorded by the Garm station of the IRIS observation network for the periods 2006–2009 and 2013–2019. The main goal of the research was to find a correspondence and, if possible, cause-and-effect relationships between the quasiperiodic components present in both types of time series. Seismic events recorded by less than four stations and not included in the main earthquake catalog due to the impossibility of reliable determination of their hypocentral parameters were used as microearthquakes. At each station, only the number of such events per day was recorded. To identify stable periodicities, all series were studied using spectral and periodogram analysis methods, adapted to work with nonstationary signals. The initial data series were analyzed, as well as their regularized variants (logarithms and ranks of the number of events). Regularization made it possible to reduce the contribution of nonstationary effects (random bursts of activity, increase in the number of microearthquakes during periods of aftershock series, etc.) and ensured robustness of the results. The analysis revealed that of all possible rhythms common for the two types of time series, only the seasonal one is significant. The seasonal rhythm of microseismic noise at different frequency channels differs in phase and amplitude. High-frequency noise (channels of the IRIS system with a center frequency from 1 to 47 Hz), falling in the frequency range of the measuring and recording channels of the seismic stations of the Garm test area, is in antiphase with the seasonal variation in the number of recorded microearthquakes at all stations, with some specificity for individual stations. The antiphase change in the number of recorded microearthquakes and the level of microseismic noise in the recording band of the seismic stations suggests that the reason for these changes is the noise discrimination of microearthquakes when they are identified on seismograms.

An example of seven seismogenic regions of the world with different conditions of tectonic deformation confirms the reliability of confinedness of weak earthquakes to two horizons in the Earth’s continental crust at depths of 5 and 10 km, regardless of the region. The width of extrema at the level of 0.7 from the maximum does not exceed 2–3 km in most cases. This markedly pronounced bimodal depth distribution of the numbers of weak earthquakes is interpreted from the standpoint of increasing strength and decreasing inhomogeneity of the material in the Earth’s crust with depth (Mogi, 1962; Scholz, 1968; Mori and Abercombie, 1997). It is assumed that the concentration of weak earthquakes near a depth of 5 km is determined by the fact that, due to a relatively high degree of inhomogeneity and low brittle fracture strength of the material in the crust, the fracture process stops before it develops into a major seismic event because of an obstacle to its development in the form of a site with increased strength. At greater depths (9–15 km), an initiation of rupture more likely develops into a larger event, since the rock material at these depths becomes more homogeneous and more durable due to the growth of confining pressure and temperature within the brittle–ductile transition zone. This leads to a relative increase in the number of larger earthquakes near a depth of 10 km, providing the second extremum in the depth distribution of the number of earthquakes. Such implications also explain the confinedness of strong earthquakes observed in seismology to the bottom rather than to the roof of the seismogenic layer. At the same time, the almost complete absence of relatively strong earthquakes at depths greater than 15–16 km may indicate a sharp change in elastic properties of the material and in the deformation nature in the lower part of the brittle–ductile transition zone and deeper. Here, brittle seismogenic deformation gives way to a plastic aseismic flow.

Abstract—Statistical analysis of the focal mechanisms of the earthquakes based on the ISC standard catalog for 2001 to 2017 is carried out in the spatiotemporal vicinity of epicentral zone of the catastrophic seismic event of 2011 with Mw = 9.0 in Tohoku, Japan. It is established that during the 10-year period preceding this earthquake, more than 80% of the intermediate (B) axes of the focal mechanisms of the earthquakes in the depth interval from 0 to 65 km have a dip angle of at most 20° and are mainly oriented within the azimuthal sector from 190° to 210° enclosing the strike of the Japan Oceanic Trench. More than 75% of the pressure axes (P) are inclined to the horizon at 5°–35°, and more than 75° of the axes of tension (T) have a dip angle from 50° to 80°. This orientation of the principal axes of focal mechanisms unambiguously suggests the predominance of thrusting or underthrusting regime at these depths. This corresponds to the well-known tectonic hypothesis that the Pacific Ocean lithospheric plate subducts beneath the continental Okhotsk plate in the region of the Islands of Japan in the approximately sublatitudinal direction. The mega-thrust slip in the source of the catastrophic Tohoku earthquake also corresponds to this type of the focal mechanism. A similar type of the focal mechanism was also observed in the aftershock sequence of the earthquakes located in the vicinity of the surface separating the hanging and subducting edges of the lithospheric plates colliding here, exactly at the location of the catastrophic earthquake. The predominant depth range of these earthquakes is 30 to 55 km. At the same time, it is established that the aftershock sequence of this event at shallow depths (less than 20 km) within the coastal accretionary prism has a striking feature in the form of the earthquakes having a normal-fault mechanism, which are untypical of the subduction zones. These earthquakes make up about 70% of the total number of the aftershocks. It is hypothesized that the existence of aftershocks with a normal-fault type of focal mechanism at shallow depths within the accretionary prism is associated with the details in the internal structure of the prism and with the thickness of weak terrigenous sediments sucked into the interplate space by the subduction of the lithospheric plates. These details determine the observed pattern of relaxation of the accumulated tectonic stresses from a wide vicinity of the prism into a compact region of their almost complete release during the intense coseismic slip.