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This paper reports results of mathematical modeling applied to the stress and strain in epicentral zones before and after the large earthquakes that occurred on June 22, 2002 in the Qazvin Province, northwestern Iran (Mw = 6.4) and the Gujarat, India earthquake of January 26, 2001 (M = 6.9). The modeling relied on a method for calculating stress and strain in a blocky elastic isotropic heterogeneous medium disturbed by a set of faults that are due to an external tectonic stress field. The boundary conditions were specified based on geological and seismological data. It has been shown that the epicenters of large crustal earthquakes occur in zones of high stress concentration at the ends of tectonic faults. Rupture occurs when the relationship between acting tectonic stresses satisfies the requirement σyy/σxx >3, thus connecting zones of high stress. The evolution of the aftershock process is controlled by the stress drop caused by a new rupture, while the resulting aftershock clusters are in spatial correlation with the stress drop zone. The new rupture propagates in the direction of dominant tectonic fault orientation in the region. We show a relationship to exist between rupture length and the possible retrospectively predicted location and magnitude of the earthquake depending on the elastic energy buildup and its possible release under specified structural tectonic conditions. [ABSTRACT FROM AUTHOR]

The present paper gives a scale of earthquake energy class KF that is to standardize the relevant data based on the existing scales: MS, MW, mb, KS, among many others. The conceptual novelty of the scale we propose consists in shifting the emphasis from seismometric issues that are traditional in such cases, which are concerned with the recording of seismic waves, as well as with issues arising in discussions of their generation and propagation, toward an effective use of basic energy properties of the seismic process (the Gutenberg–Richter law). In the most general case, this approach can significantly simplify the development of regression relations between different earthquake size scales, but the most promising problem consists in developing a single generalized scale. A particular solution of the problem is presented as the development of such a scale for earthquakes of Kamchatka. The KF scale rests on the energy class proposed by S.A. Fedotov (KS at the Kamchatka Branch (KB) of the National Seismological Centre, Geophysical Survey, Russian Academy of Sciences (NSC GS RAS) and on the generally accepted mb magnitude (ISC). [ABSTRACT FROM AUTHOR]

This paper describes the behavior of several seismic precursors prior to the 2016–2019 activation in an area of a complex geodynamic setting at the boundary of the Pacific, North American, and Eurasian plates. We used an integrated approach to combine identified precursors with a view to getting more accurate locations of possible rupture zones of future large earthquakes. The precursors include quiescence based on the RTL parameter and the "Z-function", variations in the slope of the earthquake recurrence graph (γ-value), and the areas of seismic ruptures dS. For the first time in Kamchatka, we analyze anomalies in the parameter dS interpreted as seismic activations to show the presence of a precursory component before the onset of the foreshock activation. Examples include increases in the values of that parameter before events that were the largest for the areas where they occurred: the Near-Islands Aleutian earthquake of July 17, 2017, Mw = 7.8 and the Uglovoe Podnyatie earthquake of December 20, 2018, Mw = 7.3. The overall extent of the anomalous area, which still exists and includes consecutive quiescence and foreshock phases, is ~900 km, which creates conditions for possible occurrence of a new great earthquake in the junction zone of the three plates with a magnitude that can exceed whatever has been recorded in the region so far. [ABSTRACT FROM AUTHOR]

The Chuya earthquake of 2003 exerted an appreciable influence on the seismicity of the entire Altai. This paper presents data on variations in the Altai seismicity from 2003 to 2021. It is shown that earthquakes of the first half-year after the major earthquake of 2003 had their rupture zones in the epicentral zone, while the other Altai structures were aseismic. After 2009 we saw changes both in the near zone that encompasses geological structures that were adjacent to the epicenter (with distances of 60‒80 km from the mainshock epicenter), such as the Aigulak, Kurai, South Chuisky and North Chuisky mountain ranges, and in the far zone at distances of 250‒450 km from the epicenter and in different directions from it. The Aigulak earthquake of 2019 gave rise to an aftershock process that displaced the center of Altai seismicity to the eponymous mountain range. There are no data on large earthquakes during the historical period for many Altai structures which have been activated following the Chuya earthquake, either from seismological or from paleo-geological evidence. One could explain the evolution of seismicity around the rupture zone of the Chuya earthquake in space, and with time delays of some years, invoking the influence of a large earthquake on a medium involving plasticity. We know of nonlinear models that hold promise for future research concerning the influence of large earthquakes on the evolution of seismicity in geological media. [ABSTRACT FROM AUTHOR]

This paper is an analysis of the seismic process in the source zones of great (M ~ 8) earthquakes occurring at the Pacific seismic belt and in Southeast Asia. We sought to enhance data homogeneity for different seismic regions by proposing the concept of a generalized energy class of earthquakes. The concept is a promising alternative to the existing procedure for the energy classification of earthquakes. We consider one important characteristic of the source zone of a great earthquake, namely, the energy spectrum of the seismic process; the Gutenberg–Richter law is generally treated as a first approximation to this concept. We have proposed a method for the study of the concept, have identified a significant peaking growth of the slope of the recurrence curve for the period between 5 and 10 years before a great event and a decrease immediately before it. These peaking anomalies are hypothesized to repeat themselves and to be quasi-periodic in character, with the periods of their occurrence prior to a great earthquake being arguably different in different regions. The results of this study can be used both to enhance the reliability of long-term earthquake prediction and for intermediate-term and short-term updating of earthquake hazard. [ABSTRACT FROM AUTHOR]

This paper presents the results of analysis of the spatial distribution of repeated shocks. Data from the world earthquake catalog of USGS/NEIC from 1973 to 2014 were used. Short time intervals, no more than 10 hours after the main shock, and distances from the epicenter of the main shock, no more than 5°, were considered. The dependence of the number of repeated shocks on the distance from the epicenter was studied. Two properties of the spatial distribution of repeated shocks have been identified. The first property is that the curve describing spatial distribution has its maximum at a certain distance (approximately from 10 to 120 km) from the epicenter of the main shock. The logarithm of this distance is directly proportional to the magnitude of the main shock. The second property is that the position of the maximum does not depend on time, i.e., it is a stable spatial characteristic of the earthquake source. Based on these properties, a new way to determine the size of the rupture zone was proposed. [ABSTRACT FROM AUTHOR]

This paper describes a method for calculating the earthquake energy class for the Kamchatka region from coda records in the 0.8–1.8 Hz frequency band. The method is based on an empirical correlative relationship between the coda level and the Fedotov earthquake class. The coda level is found as the mean power of the ground velocity in an interval of 30 s. Five seismic dipping zones have been identified in Kamchatka, each having its own expression for converting the coda level to the energy class. [ABSTRACT FROM AUTHOR]

This paper describes a method for calculating the station corrections to earthquake energy class and the acoustic impedance based on records of earthquake codas in the 0.8-1.8 Hz frequency band. The calculation is based on measurements of the relative coda level at two stations, with one of the two being used as the reference. The data were recorded by digital seismometers: CMG-3TB (GURALP), CMG-5TD (GURALP), CMG-6TD (GURALP), and GMS-AC-73i HHV (GeoSIG). We present estimates of class corrections for the Kamchatka network of seismic stations. [ABSTRACT FROM AUTHOR]

Work was performed in 2008-2014 on the long-term prediction of great earthquakes at the Kuril-Kamchatka arc based on patterns that are observable in the locations of the rupture zones of great earthquakes (seismic gaps) and in the seismic cycle for such earthquakes. The prediction method has been successfully developed and applied to the region since 1965 (the LTEP method). This paper presents a long-term seismic forecast for the September 2013 to August 2018 period for the Kuril-Kamchatka arc earthquake-generating zone. The forecast predicts the following quantities for 20 constituent segments: phases of the seismic cycle, the normalized rate of small earthquakes A, the magnitudes of moderate-size earthquakes to be expected with probabilities 0.8, 0.5, and 0.15, the maximum possible magnitudes and probabilities of occurrence for great (M ≥ 7.7) earthquakes in the most active depth interval, viz., 0-80 km. We also discuss the seismic process in this region for the 2008-2013 period, namely, the patterns that are observable in the locations of earthquake swarms, the values of and changes in several characteristics of the seismic process in the Kuril-Kamchatka seismic zone during the period 2008-2013, which occurred in relation with great shallow earthquakes beneath the Sea of Okhotsk (July 5, 2008, M = 7.7; August 14, 2012, M = 7.7; May 24, 2013, M = 8.3, and the giant Tohoku earthquake of March 11, 2011 with M = 9.0). The results corroborate the high seismic hazard for the area of Petropavlovsk-Kamchatskii and the urgent necessity of continuing and intensifying the ongoing work on seismic retrofitting and seismic safety. [ABSTRACT FROM AUTHOR]

This paper presents results from a study of variations in short-period shear-wave attenuation in the lithosphere of the Baikal Rift Zone (BRZ). We used earthquake records made at the Ulan-Bator station (ULN) at epicentral distances of ∼400-1300 km. The ratios of maximum amplitudes in the Sn and Pn waves were considered. We show that these ratios are on the whole considerably higher than those in other areas of Central Asia. It was found that zones of low shear wave attenuation coincide with the rupture zones of large earthquakes that occurred during the 19th and 20th centuries. We identified zones of high attenuation where no large (M∼ > 7.0) seismic events have occurred during at least 180 years. The hypothesis we propose is that precursory processes before future large earthquakes may be occurring in these zones. We discuss the question of whether wave attenuation characteristics may be related to seismicity. [ABSTRACT FROM AUTHOR]

The area of western Transbaikalia is characterized by moderate seismic activity. Nevertheless, there is historical and instrumental evidence to show that rather strong seismic events have occurred in the area and caused considerable material damage to the population centers around their epicenters. Seismological knowledge of the region is scant. The earthquake catalogs for the area and for the historical period of time need to be corrected and supplemented. The present paper considers the earthquake of October 9, 1864, which has not been included in any parametric catalog thus far. New primary data that were found in the regional periodic press were used to determine the epicenter and magnitude (M = 5.1) for the event. The earthquake of October 9, 1864 is a 'forgotten' event, but is a significant addition to the catalog for western Transbaikalia. The materials presented here can be used to assess earthquake hazard for the area, as well as to aid in the search for other unknown or 'forgotten' earthquakes. [ABSTRACT FROM AUTHOR]

This paper is concerned with the variations of apparent resistivity for the period 2000-2005 as obtained by vertical direct-current electrical sounding with an AB separation equal to 300 m. The site was the Dmitrovskoe oil-and-gas field, which is situated on the western coast of the Caspian Sea in a seismic region of Dagestan. Impulsive increases of apparent resistivity at tens of times the measurement error were recorded prior to large (M = 6.8-7.4) earthquakes in the Caspian region. An analysis of the relevant data showed that these impulsive increases in apparent resistivity were unrelated to atmospheric precipitation. Long-lasting atmospheric precipitation can cause decreases in apparent resistivity for periods of several months. The increase of apparent resistivity before large earthquakes is thought to be related to gas emissions from the field along fractured rock zones whose permeability is affected by varying tectonic stresses in the precursory zone of a large earthquake. [ABSTRACT FROM AUTHOR]

We present an analysis of spectral content for three large earthquakes which occurred in the Baikal–Mongolia region, for hard soils in Irkutsk and Ulaanbaatar cities. The earthquakes were recorded from the same seismic source zones (SSZ) in 2020 and in 2021. We used accelerograms, frequency characteristics of upper soil layers, and ground motion spectra to describe and analyze the behavior of the earth at recording sites. We estimated variations in the spectral content of the large earthquakes, and their records were used to set up a common basic signal with due account of the SSZ for soils of the first category in Irkutsk and Ulaanbaatar. The results of this analysis provide the basis for future prediction of seismic effects due to large earthquakes in the Baikal–Mongolia region. [ABSTRACT FROM AUTHOR]

The Vp/Vs kinematic parameter for the Kamchatka seismic subduction zone has been long monitored on a regular basis. This has allowed estimating the behavior of the parameter and revealing its features during the precursory period and actual rupturing of the March 25, 2020 earthquake which occurred in the Pacific plate at the contact between the Pacific plate and the continental slope of Kamchatka. The March 25, 2020 earthquake with its high magnitude (ML = 7.6, MW = 7.5) is a rare enough event to occur in the Pacific plate. Knowledge of earth structure in the source region of the earthquake was updated based on the results of deep seismic sounding and continuous seismic profiling which was carried out in the area in the past. It was shown that the earthquake source area is at an inflection in the descending Pacific plate. The source area lies in the layer at the boundary between the asthenosphere and the crust and mantle. The layer is at depths of 30‒60 km and has lower Vp and Vs velocities. The inflection region along Kamchatka shows lower Vp/Vs values, which is a sign of extension along the plate. The source area is in the contact between the Pacific plate, which is subducting under the southern end of Kamchatka, and the North Kuril Islands, as well as the Sea-of-Okhotsk plate. The region shows a distinct change in the anomalous values of the parameter from lower to higher, with the latter being typical for compression. Our study has allowed estimating possible regions of tension and compression in the geological medium. [ABSTRACT FROM AUTHOR]

We have conducted paleo- and archaeoseismological surveys in the Durankulak Eneolithic archaeological settlement in northeastern Bulgaria, identifying many earthquake-related deformations of different ages. The ubiquitous presence of counterfort walls added to damaged original (nearly north–south) walls on the eastern side provides evidence of the first significant earthquake in Durankulak that seems to have occurred during Middle to Late Eneolithic time (4650–4100 B.C.). This seismic event made the north–south walls tilt westward, so that the ancient residents had to build counterfort walls to prevent the original walls from collapsing. However, the upper excavations in the archaeological settlement show that the next, later, seismic deformation induced a tilting and collapse of remains of the original walls eastward, producing a gap of a few tens of centimeters between the original walls and the counterfort walls. Afterwards the settlement was abandoned. It may be surmised that the later earthquake was stronger than the earlier one, which left room for reconstruction work. The local seismic intensity of the earlier earthquake seems to have been at least Il ≥ VIII, while the later event had Il ≥ IX on the МSК-64 scale. Considering that the masonry of both walls, the original and the counterfort ones, is about the same age, the time interval between the two seismic events must have been short, one or two hundred years. Nevertheless, the epicenters of both of these earthquakes were to different directions from Durankulak. The seismic motion (compressive waves and shocks) went from the west during the first earthquake and from the east during the second. The rupture zone of the first seismic event was obviously related (1) to an earthquake-generating zone in continental Bulgaria situated west of the archaeological settlement in question, or (2) to the Intramoesian active fault that was responsible for the large earthquake of 1444. The rupture zone of the second seismic event was nearby, in the coastal part of the Black Sea; the responsible feature seems to have been the Shabla–Kaliakra seismogenic zone. Considerable seismic deformations were also identified in structures on the southern slope of the island in a dwelling dating back to the First Bulgarian Kingdom (the 9th century A.D.). This seismic event produced distortions in the walls (in map view), sigmoidal patterns, as well as fractures and rotations in the walls. The local seismic intensity due to this earthquake is supposed to be at least Il ≥ VIII. The rupture zone of a third seismic event was likely to be nearby, in the coastal part of the Black Sea, and the responsible feature must have been the Shabla–Kaliakra seismogenic zone. [ABSTRACT FROM AUTHOR]

We performed a tectonophysical analysis of earthquake frequency–size relationship types for large Central Asian earthquakes in the regions of dynamical influence due to major earthquake-generating faults based on data for the last 100 years. We identified four types of frequency–size curves, depending on the presence/absence of characteristic earthquakes and the presence or absence of a downward bend in the tail of the curve. This classification by the shape of the tail in frequency–size relationships correlates well with the values of the maximum observed magnitude. Thus, faults of the first type (there are characteristic earthquakes, but no downward bend) with Mmax ≥ 8.0 are classified as posing the highest seismic hazard; faults with characteristic earthquakes and a bend, and with Mmax = 7.5–7.9, are treated as rather hazardous; faults of the third type with Mmax = 7.1–7.5 are treated as posing potential hazard; and lastly, faults with a bend, without characteristic earthquakes, and with a typical magnitude Mmax ≤ 7.0, are classified as involving little hazard. The tail types in frequency–size curves are interpreted using the model of a nonlinear multiplicative cascade. The model can be used to treat different tail types as corresponding to the occurrence/nonoccurrence of nonlinear positive and negative feedback in earthquake rupture zones, with this feedback being responsible for the occurrence of earthquakes with different magnitudes. This interpretation and clustering of earthquake-generating faults by the behavior the tail of the relevant frequency–size plot shows raises the question about the physical mechanisms that underlie this behavior. We think that the occurrence of great earthquakes is related to a decrease in effective strength (viscosity) in the interblock space of faults at a scale appropriate to the rupture zone size. [ABSTRACT FROM AUTHOR]

Separate domains of the Mediterranean belt (Zagros, Vrancea, and the Crimea–Caucasus– Black-Sea region) are discussed to argue for a diversity of earthquake-generation mechanisms as an impact of very variable systems of tectonic stresses, which are at the same time characterized by selectiveness in the orientations of the principal axis. We give stereographic models of earthquake-generation mechanisms, as well as the results from parameterization and structural kinematic identification of elements in the internal structure of seismogenic regions. We have identified a discrete character of earthquake-generating stress fields both with respect to the cardinal points (to the Earth’s axis of rotation) and with respect to the ground (horizontal) surface. We show that the diversity of seismogenic settings results from the transformations of stress fields that occur both at the regional and at local levels in the zones affected by the impact of major seismogenic structures in the study regions. [ABSTRACT FROM AUTHOR]