Seismological Characteristics Research Articles

Updated Predictive Models for Permanent Seismic Displacement of Slopes for Greece and Their Effect on Probabilistic Landslide Hazard Assessment

Earthquake-triggered landslides have been widely recognized as a catastrophic hazard in mountainous regions. They may lead to direct consequences, such as property losses and casualties, as well as indirect consequences, such as disruption of the operation of lifeline infrastructures and delays in emergency response actions after earthquakes. Regional landslide hazard assessment is a useful tool to identify areas that are vulnerable to earthquake-induced slope instabilities and design prioritization schemes towards more detailed site-specific slope stability analyses. A widely used method to assess the seismic performance of slopes is by calculating the permanent downslope sliding displacement that is expected during ground shaking. Nathan M. Newmark was the first to propose a method to estimate the permanent displacement of a rigid body sliding on an inclined plane in 1965. The expected permanent displacement for a slope using the sliding block method is implemented by either selecting a suite of representative earthquake ground motions and computing the mean and standard deviation of the displacement or by using analytical equations that correlate the permanent displacement with ground motion intensity measures, the slope’s yield acceleration and seismological characteristics. Increased interest has been observed in the development of such empirical models using strong motion databases over the last decades. It has been almost a decade since the development of the latest empirical model for the prediction of permanent ground displacement for Greece. Since then, a significant amount of strong motion data have been collected. In the present study, several nonlinear regression-based empirical models are developed for the prediction of the permanent seismic displacements of slopes, including various ground motion intensity measures. Moreover, single-hidden layer Artificial Neural Network (ANN) models are developed to demonstrate their capability of simplifying the construction of empirical models. Finally, implementation of the produced modes based on Probabilistic Landslide Hazard Assessment is undertaken, and their effect on the resulting hazard curves is demonstrated and discussed.

Trade space for time for inspecting an earthquake cycle by modern seismological observation: The central-southern part of the Sichuan-Yunnan rhombic block

The central-southern part of the eastern border of the Sichuan-Yunnan rhombic block provides the research strategy of ‘trade space for time’ with an interesting fault system, where the segments have similar focal mechanisms and cover almost continuous spectra of elapse rates. We experiment to study the seismological characteristics of different segments with different elapse rates. We employed the de-clustered earthquake catalog for the calculation of b values for each segment. The analysis revealed that different segments have similar b values, which implies that, although different segments have different periods of earthquake recurrence, the 'natural time' for the whole fault system elapses with a homogeneous pace. We extended the earthquake potential score (EPS) for nowcasting earthquakes to a quasi-EPS (qEPS). It is found that qEPS increases with the increase of elapse rates, albeit for those fault segments whose elapse rates have exceeded 1, qEPS may better reflect the seismic hazard.

Inelastic Analysis and Damage Assessment of Hydraulic Arched Tunnels in Different Near-Fault Regions

The seismic natural hazard event is a threat of unpredictable events, which can easily cause incalculable damage to hydraulic tunnels. Traditionally, it is necessary to introduce as-recorded ground motions (GMs) appropriately for hydraulic arched tunnels during seismic design to avoid the unfavorable situation of large-scale collapse. However, the lack of adequate current selected methodology considering the accurate engineering site relative to the fault line in this type can result in more considerable deformations and severe damage to structures. Actually, the detailed explanation of the seismic design code of hydraulic tunnels cannot support the specific consideration of the spatial region effects since the considerable uncertainty of engineering sites in water resources and hydropower engineering. To address this issue, the present study proposes a novel selected methodology of as-recorded GMs that integrates the geographic location distribution of the strong earthquake monitoring station instead of the actual engineering site relative to the fault line. Subsequently, three-dimensional finite element models of a hydraulic arched tunnel reveal the possible seismic behavior of the tunnel induced by as-recorded GMs in different near-fault regions. Furthermore, the optimal IM between different near-fault regions and structural responses is also discussed through correlation analysis because it can describe the seismological features of as-recorded GMs, which has a crucial role in seismic design and performance assessment. The results reveal that the as-recorded GMs in the hanging wall region can induce more significant deformations and severe damage to the structure than other near-fault regions of as-recorded GMs. Meanwhile, the seismological characteristics and selected optimal IM for as-recorded GMs in different near-fault regions significantly differ. In summary, data and observations from these results highlight that considering the near-fault region effect is critical in optimizing the seismic design code.

Field Investigation on Reinforced Concrete and Masonry Buildings Damages after November 23, 2022 Gölyaka (Düzce) Earthquake

A strong earthquake with a moment magnitude of [Formula: see text].9 according to the Disaster and Emergency Management Authority (AFAD) occurred at 04:08 local time on November 23, 2022, in Gölyaka district of Düzce province that is located in the Western Black Sea region of Turkey. The earthquake with a shallow focus is 0.82 km away from Kemeryani village of Gölyaka district, which is the nearest settlement. According to observation data, the highest acceleration value of the earthquake was measured as 407.76 gal (0.410 g) in the East-West component measured from the 8102 coded Düzce Central accelerometer station, which is one of the five accelerometer stations closest to the earthquake zone. There was no loss of life in the earthquake, in which 93 people were injured. However, damages occurred due to the earthquake in structures that were built before the year 2000 and did not receive good engineering and supervision services. It has been determined that the structures built after the year 2000 received good engineering and supervision services performed better. The purpose of this study is to ensure that the experience gained as a result of the investigations about the earthquake that occurred will be made permanent and usable. For this purpose, damage and collapses that occurred on the buildings due to earthquake are examined based on field investigation. The types and causes of damage that occurred on both reinforced concrete and masonry buildings are evaluated in detail with considering the conditions of the existing Turkey Building Earthquake Design Code. The seismological characteristics of the earthquake are also evaluated. In addition, the earthquake hazard of Düzce Province is evaluated together with the earthquakes experienced in the past years, which caused significant loss of life and property in the region.

Fragility and comfortability curves development and seismic risk assessment of a masonry building under earthquakes induced by geothermal power plants operation

This research is an attempt to estimate the seismic risk of geothermal power plants (GPPs) operation on the masonry buildings located in the vicinity of these infrastructures through a probabilistic framework. In this regard, a typical masonry building in Germany is considered. In the first part, the fragility and comfortability curves of this building are established. For this purpose, a set of 30 ground motions recorded due to GPPs activity are used for performing elaborated three-dimensional nonlinear incremental dynamic analysis to construct the fragility and comfortability curves. In the second part of the research work, by using the Monte-Carlo simulation technique and adopting the hazard model capable of simulating both seismological characteristics and ground motion intensities, the required number of probable hazard scenarios are generated for different ranges of GPPs injection flow rates. Afterward, the seismic risk imposed on the masonry building is evaluated by considering the developed fragility and comfortability curves. The outcomes highlight that the risk of facing structural damage is not significant; however, the disturbance of buildings’ residents is highly probable and can lead to considerable seismic risk. In addition, in the close vicinity of the earthquake epicenter (less than 5 km), the estimated risk increases tangibly in comparison with the larger distances (more than 15 km).

Seismic damage assessment based on site observation following the Düzce (Gölyaka) earthquake (Mw = 5.9, November 23, 2022)

On November 23, 2022, at 04:08 local time, the Gölyaka district of Düzce, which is located in the west of the Black Sea Region of Turkey, struck an earthquake with a magnitude of Mw = 5.9. A total of 181 reinforced and masonry buildings were severely damaged due to the earthquake and it is reported that need to be demolished urgently throughout the province of Düzce. In this study, the seismological characteristics and recorded ground motion accelerograms of the earthquake are evaluated. The case studies of damaged reinforced concrete and masonry buildings because of the earthquake are also investigated. The factors that cause the buildings to be damaged during the earthquake are evaluated based on site observations.

Compound intensity measures for improved seismic performance assessment in cross-fault hydraulic tunnels using partial least-squares methodology

Seismic performance assessment of cross-fault hydraulic tunnels is typically expressed in terms of the probability of reaching or exceeding specific damage states related to earthquake intensity measures (IMs) and engineering demand parameters (EDPs). Obviously, this would require determining the appropriate earthquake IMs to reflect the seismological characteristics of ground motions and improving the predictive ability for the EDPs of cross-fault hydraulic tunnels. It is worth emphasising that an unreasonable IM may increase the ground motion uncertainty, which in turn introduces additional uncertainty to the EDP estimates of cross-fault hydraulic tunnels. Hence, this study presents a compound IM, integrating multiple earthquake IMs, to improve the reliability of the seismic fragility analysis of a cross-fault hydraulic tunnel based on the partial least squares methodology. To this end, seven scalar IMs are first determined from the 20 examined earthquake IMs as the fundamental element of the compound IM using four test criteria. Subsequently, a 1080 complete nonlinear dynamic time history analysis of the cross-fault hydraulic tunnel using the incremental dynamic analysis method is conducted to generate a large amount of data. The findings of this study highlight that the compound IM-based probabilistic seismic demand models of the cross-fault hydraulic tunnel incorporate various sources of uncertainties, which enhances the correlation, efficiency, practicality, and proficiency of probabilistic seismic demand analysis and eventually leads to a more reasonable estimation of probabilistic failure. Moreover, the potential of nonlinear polynomial regression of compound IM can further improve the correlation and efficiency of probabilistic seismic demand analysis. Relative to the classical fragility curves utilising a single optimum-IM peak ground velocity, the probability of damage to the cross-fault hydraulic tunnel under a particular damage state related to the compound IM is significantly larger than the corresponding single optimum-IM peak ground velocity. Similarly, the probability of damage to the cross-fault hydraulic tunnel under the Wenchuan seismic event is underestimated, derived from a single optimum-IM peak ground velocity. The abovementioned phenomena demonstrate that a compound IM based on the partial least squares methodology can be suggested for use in estimating the probability of structural damage in performance-based earthquake engineering.

Seismological characteristics of microearthquake sequence near Suncheon, South Korea, from 2009 to 2020

Seismological characteristics of microearthquake sequence near Suncheon, South Korea, from 2009 to 2020

Seismic zonation and default suites of ground-motion records for time-history analysis in the South Island of New Zealand

The rise of performance-based earthquake engineering, in combination with the complexity associated with selecting records for time-history analysis, demonstrates an expressed need for localized default suites of ground motion records for structural designers to use in the absence of site-specific studies. In the current research investigation, deaggregations of probabilistic seismic hazard models (National Seismic Hazard Model, Canterbury Seismic Hazard Model, and Kaikōura Seismic Hazard Model) and the location-specific seismological characteristics of expected ground motions were used to define eight seismic hazard zonations and accompanying suite profiles for the South Island of New Zealand to satisfy the requirements of the New Zealand structural design standard NZS1170.5 for response-history analyses. Specific records, including 21 from the recent Kaikōura, Darfield, and Christchurch earthquakes, were then selected from publicly-available databases and presented as default suites for use in time-history analyses in the absence of site-specific studies. This investigation encompasses seismic hazards corresponding to 500-year return periods, site classes C (shallow soils) and D (deep soils), and buildings with fundamental periods between 0.4 and 2.0 seconds.

Cultural Heritage and Rockfalls: Analysis of Multi-Scale Processes Nearby the Lucus Angitiae Archaeological Site (Central Italy)

Archaeological areas in the mountain region of central Italy can be seriously threatened by geological hazards, and efforts are required to preserve cultural heritage. The Lucus Angitiae is a pre-Roman site located along the western edge of the Fucino Basin, the largest continental depression of central Apennines. The carbonate slope overhanging the area is affected by active rockfall processes from two main rock escarpments. In this paper, rockfall assessment was pursued through a 3D kinematic modelling, performed by adopting a probabilistic approach. Specific attention was dedicated to the choice and calibration of the input data, based on field evidence and a literature review. Two different sizes of wedge-shaped rock blocks were identified on rock escarpments, and specific stability analyses were performed. Sensitivity analyses accounting for possible triggering factors, such as water pressure increase and seismic action, were also carried out, together with an investigation of the seismological characteristics of the area. The results of the numerical simulations were used to design effective countermeasures in the framework of a mitigation plan for protection of the archaeological site. Finally, clues of gravity-driven slope deformations at the slope scale were documented, framing the rockfall process in a wider geological scenario.

Spatial and temporal analysis of seismicity of the Pütürge Fault, and Gutenberg-Richter parameters before and after Sivrice-Doğanyol earthquake

The seismological characteristics of earthquake activity before and after January 24, 2020 (Mw6.8) Sivrice-Doğanyol earthquake were studied for the period between January 1, 2018 and August 6, 2020. Spatial and temporal distributions of epicenters of this activity, fault mechanism solutions and Gutenberg-Richter parameters (Mc, a- and b-values) show remarkable patterns. Mechanism solutions of Mw≥5.0 magnitude earthquakes prove that the Pütürge Fault is a left-lateral strike-slip fault with a northwest dipping plane. It was observed that the increase in Mc value before January 24, 2020 Sivrice-Doğanyol mainshock started simultaneously with a foreshock of Mw4.9 on December 27, 2019, 28 days before the main earthquake. The decrease trend in b-value started on September 8, 2019, 139 days before the mainshock, and on December 27, 2019, the b-value decreased to the lowest level of 0.6. The reason for the increase in Mc value and the significant decrease of the b-value before the January 24, 2020 Sivrice-Doğanyol earthquake may be the effect of the increase in the effective stress on the Pütürge Fault before the earthquake and the change in physical process. Similar findings can be considered as a pre-earthquake warning mechanism, as previously presented and discussed in many earthquake examples around the world.

Archaeoseismological study of the Cherichira aqueduct bridge, Kairouan, Tunisia

In the past, several destructive earthquakes have occurred in the North African Atlas Mountain ranges located along the Africa–Eurasia plate boundary. Although the region is rich with impressive archaeological sites, including those in modern Tunisia, few comprehensive archaeoseismological studies have been conducted. Historic sources account at least three damaging earthquakes in the Kairouan area in central Tunisia between AD 859 and 1041. Little is known about which faults triggered these earthquakes or the size of these events. The water supply of the city of Kairouan depended on a 32-km-long aqueduct with a large bridge (now partially collapsed) at the confluence of the de Mouta and Cherichira rivers. The original bridge of Roman construction was retrofitted twice during the Aghlabid period (AD 800–903) and probably in AD 995 during the Fatimid period. The ruined section of the bridge shows damage which might be related to the AD 859 earthquake shaking. Here, we present a detailed study of the history, the status and the damage of the Cherichira aqueduct bridge using previous historic accounts and written works, a 3D laser scan model, local geological and seismological characteristics, and include results of radiocarbon dating and a timeline of events. In addition to earthquake ground motions, we consider severe flash floods on the bridge as a potential cause of the damage. We estimate the severity of such flash floods and develop a model with 18 earthquake scenarios on local reverse and strike-slip faults with magnitudes between MW 6.1 and 7.2. While a few damage patterns might be indicative of flooding, most damage can be attributed to earthquakes. It is highly probable that the earthquake in AD 859 caused enough damage to the Aghlabid bridge to render it dysfunctional; however, to resolve the question of whether another earthquake in AD 911 or 1041 caused the complete destruction of the previously retrofitted aqueduct by the Fatimids requires dating of additional sections of the bridge.

Determination of body waves quality factor in the NW Iran, with power spectrum analysis

As one of the ways to identify seismological characteristics in the region, determining the quality factor of seismic mapping can provide valuable information about inside the earth. This study investigates local site effects as a function of frequency and presents a new relationship for determining the quality factor in northwestern Iran with regard to local site effects. These maps are selected so that their signal-to-noise ratio (SNR) is greater than 5. This study uses the Short-Time Fourier Transform (STFT) method in which a fixed time window and its multiplication by a given signal are used. The coefficients resulting from this transformation are considered as wave amplitudes at any frequency by performing a short-time Fourier transform. The amount of power spectrum decay is used instead of the ground displacement amplitude decay. Since the number of samples will be different at different intervals and this makes it difficult to perform our analysis, the sample mean, presented as the power spectrum, was used. Local site effects and kappa, a function of the path and site effects, were investigated and became the basis of spectral decay calculations. The results of this study were compared with those of the previous work based on conventional and classical methods and the accuracy of the methods was evaluated using standard deviation (SD) values. Finally, the quality factor equations were obtained for the North-South component (N-S) as Q(f)=(78±2)f^((1.37±0.02)), for the East-West component (E-W) as Q(f)=(62±2)f^((1.5±0.03)), and for the vertical component (Z) as Q(f)=(87±2)f^((1.29±0.03)).

Seismic damage assessment of masonry buildings in Elazığ and Malatya following the 2020 Elazığ-Sivrice earthquake, Turkey

On January 24 at 8.55p.m, a major earthquake of moment magnitude Mw = 6.8 struck the Sivrice district of Elazig, located in the southwest of the Eastern Anatolia region of Turkey. A total of 7429 buildings were severely damaged or collapsed. Unreinforced masonry buildings still represent a significant part of the residential building stock in rural surroundings of the Eastern Anatolia region. Unfortunately, most of the masonry buildings near to epicenter were found to have collapsed or suffered severe damage. This study provides an evolution of seismological characteristics of the earthquake, including recorded accelerograms and acceleration response spectra. Case studies of damaged masonry buildings and failure or collapse mechanisms are also examined. It is concluded that almost all the damaged buildings had not been designed or constructed in accordance with contemporary Turkish earthquake codes.

Regional characteristics of the lithospheric structure in the southeast of North China revealed by explosion seismology

Based on the blasting seismic detection data obtained in the southeast of North China in recent years, this paper comprehensively analyzes and studies the crust-mantle lithospheric structure and seismological characteristics of different tectonic regions, such as offshore basins, west Shandong uplift, Tanlu fault zone and Jiangsu-Shandong orogenic belt. The low-velocity Pg waves in Dongying depression and Northern Jiangsu basin reveal the unstable basement structure with extremely thick sediments. The travel time of Pg wave is characterized by relatively low propagation velocity and small crustal thickness of offshore continental margin; the first break time and high apparent velocity of Pg wave in west Shandong uplift indicate that the sedimentary basement is relatively thin. The Pm wave shows the characteristic of dominant wave in the first-order velocity discontinuity of the crust-mantle interface, which reflects the high crustal velocity and stable structure in west Shandong uplift. The Pm and Pl wave are obviously complicated, which can reflect the crust-mantle lithospheric structure of the transitional zone between Tanlu fault zone and Jiangsu-Shandong orogenic belt. The small time difference between Pn and PL waves can be regarded as the highly destructive seismological manifestation of Tanlu fault zone on the crust-lithosphere scale. Based on many geophysical phenomena such as electrical structure, density structure and terrestrial heat flow, it is believed that the lithospheric destruction degree of Tanlu fault zone and Jiangsu-Shandong orogenic belt was high during the destruction of the North China Craton.

Inelastic dynamic analysis and damage assessment of a hydraulic arched tunnel under near-fault SV waves with arbitrary incoming angles

The incoming directions and incoming angles of near-fault (NF) seismic waves may play important roles in potential seismic-induced damage to hydraulic arched tunnels due to the uncertainty and randomness of NF ground motions. Moreover, the correlation between the seismological characteristics of NF ground motions and the seismic behaviour of underground structures, especially hydraulic arched tunnels, is not well understood. However, the current seismic dynamic analysis procedures and seismic designs of hydraulic arched tunnels generally ignore the abovementioned key factors. For this reason, the focus of this paper is to evaluate the seismic behaviour and damage degree of a hydraulic arched tunnel under NF SV waves with arbitrary incoming angles. Based on the elasticity and wave theory, an obliquely incoming method of SV waves that considers the incoming angle and direction is first developed by the equivalent nodal force method and verified through two examples. Ten as-recorded NF seismic events that display ground motion with a high frequency content are selected to reflect the seismological characteristics of NF ground motions. In addition, a hydraulic arched tunnel is built using the commercial software ABAQUS, and it considers fluid-structure-rock interaction systems based on the coupled Eulerian-Lagrangian method to approximate the environmental conditions experienced by hydraulic arched tunnels when a real earthquake event occurs. Subsequently, an inelastic dynamic analysis of hydraulic ached tunnels under NF SV waves with arbitrary incoming angles are conducted. The numerical results indicated that the propagation directions and incoming angles of SV waves have notably different impacts on the inelastic dynamic responses and damage degrees of hydraulic arched tunnels. In addition, by comparison with our team’s previous work, it can be found that obliquely incident SV waves may be more likely to cause severe damage to hydraulic ached tunnels compared to obliquely incident P waves. On the other hand, significant durations and velocity-related intensity measures present strong correlations with the global tensile damage index of the linings of hydraulic arched tunnels. Therefore, both the incoming angles of SV waves and the seismological characteristics should be considered when evaluating the seismic performance and design of hydraulic arched tunnels.

Variable vs. invariable elastic response spectrum shapes: impact on the mean annual frequency of exceedance of limit states

In research studies focusing on the evaluation of the fragility function of buildings, numerical analyses are almost always carried out using a single suite of accelerograms, which is scaled in intensity to simulate earthquakes of different magnitude.Therefore, such numerical analyses usually neglect the effects of the variation of important seismological characteristics, e.g. the shape of the pseudo-acceleration response spectrum, on the structural response.To evaluate the impact of the variation of the elastic response spectrum shape with the selected earthquake return period on the mean annual frequency of exceedance of assigned limit state functions, the response of a large set of systems is determined in this paper by two different methods of analysis. In the first method of analysis, a multiple stripe analysis is carried out with accelerograms characterised by median response spectrum shape and ground motion duration variable with the earthquake intensity measure. The information pertaining the probabilistic description of the acceleration spectra has been obtained from the website of the National Institute of Geophysics and Volcanology for the Italian territory, whereas the one regarding the earthquake duration has been derived from semi-empirical relationships reported in the literature. In the second method of analysis, instead, the accelerograms are characterized by median response spectrum shape and ground motion duration invariable with the intensity measure.Finally, to evaluate the influence of the scattering of the spectral responses about their mean value, a third analysis is carried out by means of a set of artificial accelerograms with response spectra that are characterized by virtually zero standard deviation and spectrum shape and ground motion duration invariable with the magnitude of the earthquake.To increase the relevance of the results, the investigated systems include single-degree-of–freedom systems with degrading or non-degrading inelastic response and reinforced concrete and steel multi-degree of freedom systems.

The MW = 7.8 Earthquake of July 17, 2017 near the Commander Islands: Tectonic Position and Geodynamic Setting

The tectonic position and overall geodynamic setting, as well as the seismological characteristics and peculiarities of the aftershock process, are considered for the source of the strongest Near Island Aleutian earthquake of July 17, 2017, М W = 7.8, on the Commander Islands. It is shown that in contrast to the eastern segments of the Aleutian island arc, the subduction of the Pacific lithospheric plate beneath the Commander block is not observed. The analysis has shown that, according to the distribution of the aftershock epicenters in the form of a linear elongated narrow zone with a length of about 400 km, the seismic source occupied almost entire northern slope of the Commander Island rise and spread in the Bering fault zone. It spanned the whole of this seismogenic zone up to the transverse structure west of the Near Islands (Attu islands). The focal mechanism solutions and the pattern of displacements in the sources of the main shock, as well as the strongest foreshocks and aftershocks, suggest that the slip in the source was an almost pure right-lateral shear. The aftershock process of the earthquake on July 17 developed quite feebly for an earthquake of this magnitude. Besides, it has two specific features distinguishing it from the aftershock processes of most of the Kuril-Kamchatka earthquakes: (1) a low release of the cumulative scalar seismic moment (M 0cum aft ), which, according to different estimates, made up from 0.75 to 1.0% of the main-event seismic moment (M 0me ); and (2) a very slow growth of the deficit in the release of seismic moment (M 0 ). At the same time, the duration of the quasi-stationary phase of the M 0cum release in the aftershocks, which is estimated at approximately half-a-year and which took a considerable span of the total length of the aftershock process from this earthquake, appears to be untypically long. These features of the aftershock process of the Near Island Aleutian earthquake of July 17, 2017 distinguish it from the aftershock processes peculiar to most of the strong Kuril–Kamchatka earthquakes. Overall, its source can be considered as a transform one between the two Benioff zones, the Aleutian and Kuril–Kamchatka ones, rather than as a subduction source characteristic of the last two zones.

Earthquake july 17, 2017, Mw = 7.8 near the Komandorsky islands and strong seismic manifestations in the western segment of Aleut island arc

The tectonic position, seismological characteristics and features of the aftershock process of the source of the strongest Near-Aleutian earthquake on July 17, 2017 on the Commander Islands with Мw = 7.8 are considered. The analysis showed that the seismic source according to the distribution of aftershock epicenters in the form of a linearly elongated narrow zone with a length of about 400 km almost completely occupied the northern slope of the Commander island elevation and was located in the Bering fault zone. It covered the whole of this seismic-generating zone up to the transverse structure to the west of the Near Islands (Attu is.). In accordance with the focal mechanisms solution and the nature of the displacements in the foci of the main shock, the strongest foreshocks and aftershocks, the shift in the source was an almost pure right-sided shift. The aftershock process of the July 17 earthquake developed quite enough inertly for an earthquake of such strength. In addition, it has two features in comparison with the aftershock processes of most of the Kuril-Kamchatka earthquakes: 1) low release of the cumulative scalar seismic moment (M0cum aft), which according to various estimates was from 0.75% to 1.0% of the seismic moment of the main shock (M0me); 2) a very slow increase in the deficit in the release of the seismic moment (M0). At the same time, the duration of the quasi-stationary phase of M0cum release in aftershocks, estimated at about ½ year and covering a significant part of the duration of the entire aftershock process of this earthquake, seems unusually long. These features of the aftershock process of the Middle Aleutian earthquake on July 17, 2017 distinguish it from the aftershock processes characteristic of most strong Kuril-Kamchatka earthquakes. In general, its source can be considered as a transform between the two Benioff zones – Aleutian and Kuril-Kamchatka, and not subduction, that is characterise the last two.

Earthquake july 17, 2017, Mw = 7.8 near the Komandorsky islands and strong seismic manifestations in the western segment of Aleut island arc

The tectonic position, seismological characteristics and features of the aftershock process of the source of the strongest Near-Aleutian earthquake on July 17, 2017 on the Commander Islands with Мw = 7.8 are considered. The analysis showed that the seismic source according to the distribution of aftershock epicenters in the form of a linearly elongated narrow zone with a length of about 400 km almost completely occupied the northern slope of the Commander island elevation and was located in the Bering fault zone. It covered the whole of this seismic-generating zone up to the transverse structure to the west of the Near Islands (Attu is.). In accordance with the focal mechanisms solution and the nature of the displacements in the foci of the main shock, the strongest foreshocks and aftershocks, the shift in the source was an almost pure right-sided shift. The aftershock process of the July 17 earthquake developed quite enough inertly for an earthquake of such strength. In addition, it has two features in comparison with the aftershock processes of most of the Kuril-Kamchatka earthquakes: 1) low release of the cumulative scalar seismic moment (M0cum aft), which according to various estimates was from 0.75% to 1.0% of the seismic moment of the main shock (M0me); 2) a very slow increase in the deficit in the release of the seismic moment (M0). At the same time, the duration of the quasi-stationary phase of M0cum release in aftershocks, estimated at about ½ year and covering a significant part of the duration of the entire aftershock process of this earthquake, seems unusually long. These features of the aftershock process of the Middle Aleutian earthquake on July 17, 2017 distinguish it from the aftershock processes characteristic of most strong Kuril-Kamchatka earthquakes. In general, its source can be considered as a transform between the two Benioff zones – Aleutian and Kuril-Kamchatka, and not subduction, that is characterise the last two.