Zones Of Strong Earthquakes Research Articles

Determine attenuation coefficients of saturated sand from standard penetration test, applied to Nhon Hoi economic zone

Vietnam is located in a strong earthquake zone with many areas having a ground acceleration (a<sub>g</sub>) of greater than 0.08 g (TCVN 9386:2012). According to Vietnamese standard TCVN 9386:2012, the calculation and design of construction grade III or higher in areas where have ground acceleration a<sub>g</sub> ≥ 0.08 g must include seismic design. In calculations of earthquake-resistant pile foundation, the bearing capacity of the pile including the tip resistance strength (q<sub>b</sub>) and endurance strength (f<sub>i</sub>) need to be multiplied by attenuation coefficients (g<sub>eq1</sub> and g<sub>eq2</sub>). They are the most important parameters and depend on the soil types, saturation conditions, and earthquake intensity. The article introduces a method to determine the attenuation coefficients according to the standard TCVN 10304:2014. In addition, analyzing the theoretical basis to give the expression to determine attenuation coefficients based on the durability factor (I<sub>red</sub>) and pore water pressure ratio (R<sub>u</sub>) of saturated sand under the earthquakes. Furthermore, the paper presents a method to determine R<sub>u</sub> from the results of standard penetration test (SPT) by combining the method of Seed and Alba (1986) with the method of Marcuson and Hynes (1990). In which, Seed and Alba's method was used to determine factor of safety against liquefaction (F<sub>SL</sub>), and then Marcuson and Hynes' method was used to determine R<sub>u</sub> from F<sub>SL</sub>. The application in Nhon Hoi Economic Zone shows that: The silty fine grained sand, which is medium dense and saturated, has a attenuation coefficient of tip resistance g<sub>eq1</sub> = 0.74÷0.76 and attenuation coefficient of friction g<sub>eq2</sub> = 0.90; The fine grained sand, which is dense and saturated, has g<sub>eq1</sub> = 0.79÷0.82 and g<sub>eq2</sub> = 0.94. 

Soil nailing as a ground reinforcement method of a storey building constructed on weathered siltstone: Analytical and numerical evaluation

Vietnam is located in a strong earthquake zone with many areas having a ground acceleration (a<sub>g</sub>) of greater than 0.08 g (TCVN 9386:2012). According to Vietnamese standard TCVN 9386:2012, the calculation and design of construction grade III or higher in areas where have ground acceleration a<sub>g</sub> ≥ 0.08 g must include seismic design. In calculations of earthquake-resistant pile foundation, the bearing capacity of the pile including the tip resistance strength (q<sub>b</sub>) and endurance strength (f<sub>i</sub>) need to be multiplied by attenuation coefficients (g<sub>eq1</sub> and g<sub>eq2</sub>). They are the most important parameters and depend on the soil types, saturation conditions, and earthquake intensity. The article introduces a method to determine the attenuation coefficients according to the standard TCVN 10304:2014. In addition, analyzing the theoretical basis to give the expression to determine attenuation coefficients based on the durability factor (I<sub>red</sub>) and pore water pressure ratio (R<sub>u</sub>) of saturated sand under the earthquakes. Furthermore, the paper presents a method to determine R<sub>u</sub> from the results of standard penetration test (SPT) by combining the method of Seed and Alba (1986) with the method of Marcuson and Hynes (1990). In which, Seed and Alba's method was used to determine factor of safety against liquefaction (F<sub>SL</sub>), and then Marcuson and Hynes' method was used to determine R<sub>u</sub> from F<sub>SL</sub>. The application in Nhon Hoi Economic Zone shows that: The silty fine grained sand, which is medium dense and saturated, has a attenuation coefficient of tip resistance g<sub>eq1</sub> = 0.74÷0.76 and attenuation coefficient of friction g<sub>eq2</sub> = 0.90; The fine grained sand, which is dense and saturated, has g<sub>eq1</sub> = 0.79÷0.82 and g<sub>eq2</sub> = 0.94. 

Verification of regulatory formulas for assessing the level of accelerations in SP 286.1325800.2016 "Construction facilities of increased responsibility. Rules of detailed seismic zonation" based on real data

The normative formulas of SP 286.1325800.2016 for estimation of peak ground accelerations in earthquakes were checked on records obtained in the near-fault zones of strong earthquakes (MS ~6.9÷7.15) of Japan made by K-NET and KiK-net stations. It was found that the recorded values of peak accelerations have a fairly large scattering even in the selected narrow range of magnitudes, which is obviously due to local site effects (soil response, topographic effects, etc.), as well as to differences in the characteristics of the earthquake sources and the paths of seismic wave propagation. Calculations by the normative formulas produced estimates that are near the lower limit of the recorded peak accelerations, and in the near-source zones, they noticeably underestimate the recorded peak accelerations. In general, the formulas of SP 286.1325800.2016 give us only approximate estimates, and therefore, it is necessary to develop more reliable methods for estimating peak accelerations and other parameters of seismic motion, taking into account soil conditions and regional characteristics, with components of probabilistic analysis.

Tectonics of source zones of strong earthquakes in the southeastern part of the Greater Caucasus

The features of the distribution of earthquake sources in the territory of Azerbaijan have been studied. It has been identified that the sources of strong earthquakes are confined to zones of concentration of weak seismic shocks, which are inherent in active faults, characterized by a contrast of tectonic movements. This shows that by identifying the zones of concentration of sources of weak seismic shocks, it is possible to determine the source zones of strong earthquakes. The relationship between the length (L) of source zones and observed magnitudes (M) of earthquakes has been studied. The result obtained makes it possible to determine the maximum magnitude in potential source zones of strong earthquakes. One of the main tasks of modern seismology is to determine the source zones of strong earthquakes and their maximum magnitudes in these zones. This is necessary to assess the seismic hazard of the territories. Актуальность работы: Одной из основных задач современной сейсмологии является определение очаговых зон сильных землетрясений и их максимальных магнитуд в этих зонах. Это необходимо для оценки сейсмической опасности территорий. Цель исследования: Изучить особенности распределения очагов землетрясений на территории Азербайджана. Результаты исследования: Установлено, что очаги сильных землетрясений приурочены к зонам концентрации слабых сейсмических толчков, присущим активным разломам, характеризующимся контрастностью тектонических движений. Это показывает, что путем выявления зон концентрации источников слабых сейсмических толчков можно определить очаговые зоны сильных землетрясений. Методы исследования: Использование связи между длиной (L) очаговых зон и наблюдаемыми магнитудами (M) землетрясений. Полученный результат позволяет определить максимальную магнитуду в зонах потенциальных очагов сильных землетрясений.

Analysis of Seismic Collapse-Resistant Influencing Factors of RC Frame Structure

The RC frame structure is widely applied in China due to its flexible layout and good anti-collapse capacity during an earthquake. However, a lot of RC frame structures collapsed in the strong earthquake zone during the Wenchuan earthquake. The collapse resistance of the RC frame structures during an actual earthquake is inconsistent to the designed seismic collapse resistance in a project. So the collapse resistance of reinforced concrete (RC) frame structures during an earthquake and its influencing factors are a hot research topic in the field of earthquake engineering. Based on the research results of scholars in recent years, this paper briefly discusses the evaluation criteria of structural collapse, the structural collapse mechanism during a real earthquake and the influence of nonstructural components. Finally, the influencing factors are summarized in terms of design methods, plastic hinges and nonstructural members, which have certain reference significance for the seismic collapse resistance design.

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.

Geodynamics of source zones of strong earthquakes in Armenia

The purpose of the presented work is to determine and evaluate changes in the geodynamic characteristics of the strong earthquake source zone in the Javakheti highlands and central Armenia. The migration of source zones of strong earthquakes is given. A certain pattern has been revealed in the occurrence of destructive earthquakes in the territory of the Armenian Highlands. Active faults and fault zones of the territory of Armenia have been identified. The sources of strong earthquakes are studied according to seismicity data: seismogeodynamics of source zones and their relationship with the occurrence of strong earthquakes. The geodynamic conditions of source zones of strong earthquakes are estimated from the seismicity data of these zones in connection with the occurrence of strong tectonic movements and taking into account stress and strain fields. The periods of seismic calm, the features of changes in the level of groundwater and chemical composition in connection with the preparation of earthquakes are considered. Based on the block structure of the earth’s crust of the territory of Armenia, the dynamic parameters of the blocks of the earth’s crust (deformation, stress state) are determined. The geodynamics of strong earthquake source zones in the north and in the middle parts of Armenia, where an increase in seismic activity of these parts of the region has been observed in recent years, has been identified and studied. The seismic geodynamics is substantiated on the basis of data from the study of modern movements of the earth’s crust and in the historical past, which gives an idea of the dynamics of physical processes occurring in the source zones of earthquakes. A group of mineral water deposits, which is particularly sensitive to seismic events, has been identified. An estimate of the information content of the observational hydrogeodynamic network of Armenia is given. The geodynamic activity of the zone of sources of strong earthquakes in the Javakheti Highlands of the territory of Armenia has been revealed. A seismological section was built along the strike of rocks in order to determine the direction of migration of earthquake sources in the northern part of the region of Armenia.

Results of Geomagnetic Studies on the Problem of Forecasting Strong Earthquakes in Uzbekistan

The article is devoted to the problem of forecasting strong earthquakes by the geomagnetic method. The geomagnetic method is widely used on this problem in seismically active regions of the world as one of the promising, informative and operational geophysical methods. The results of long-term geomagnetic studies on the problem of forecasting strong earthquakes in Uzbekistan are presented. Geomagnetic studies were carried out on the territories of the Tashkent, Ferghana, and Kyzylkum geodynamic polygons in the epicentral zones of strong earthquakes that occurred. Long-term, medium- and short-term precursors of earthquakes have been identified. Anomalous changes in the geomagnetic field associated with the decline in aftershock activity were also revealed. The dependence between the duration of the manifestation of long-term magnetic precursors and the magnitude of earthquakes is determined. Absolute proton magnetometers MMP-1, MV-01 (Russia), and G-856 (USA) were used to measure the geomagnetic field.

Simulation of the grace space system observation data of the focal areas of the strong simushir and okhotsk sea earthquakes

Over the past decades studies of the gravitational field and its anomalies using space technologies have significantly intensified. Satellite data allow at the global and local level with sufficient accuracy to study the geodynamics of source zones of strong earthquakes. In this work for the analysis of seismically active regions of the Kuril-Kamchatka arc and the geodynamic processes occurring here measurements made by the GRACE are used. Additionally, to analyze seismic activity in the study areas we used geophysical information contained in the USGS catalog for the period 2006–2013. Digital maps of the distribution of the EWH parameter relative to the source regions of strong earthquakes with magnitude Mw>8 for background and disturbed conditions have been constructed. The modified anomaly index was calculated and on its basis digital maps of observed gravitational anomalies were built.

Damage Mechanisms of Soft Rock Tunnels in the Western China: A Case Study on the Dujiashan Tunnel

In the Western China, soft rocks, in particular, phyllites are very widespread. With the increase of soft rock tunnels in recent years, the secondary lining cracking of newly-built tunnels have frequently occurred. In this work, a case study on the Dujiashan Tunnel in the Western China was conducted in order to get insight into the damage mechanisms of such soft rock tunnels in the strong earthquake zone. Furthermore, for the engineering background, the cracking characteristics, on-site monitoring data, influencing factors and damage causes for the the second lining of Dujiashan Tunnel were also investigated. It was demonstrated that with the concentration of the bending moment and shear stress, the reduction in axial pressure appeared on the lining structure at the cavities due to local collapses. Moreover, the aftershocks of the Wenchuan earthquakes could further weaken the strength characteristics. It was revealed that the secondary lining measures were useful for the reinforcement of the Dujiashan Tunnel, and the treatment measures could achieve mainly the backfilling collapse cavities, leading to reinforcing the surrounding rocks, preventing the cracking behavior, strengthening lining, and smoothing drainage. It is therefore suggested that for the newly-built soft-rock tunnels, especially those located in strong earthquake zones, the use of the secondary lining should be at the appropriate time and backfilling the cavities due to local collapses timely, and all the factors are important for the protection and reinforcements of the surrounding rocks to ensure the safety of the tunnel structure.

Visualization of macroseismic parameters during examination of epicentral zones of strong earthquakes

The article discusses the fragile destruction of building materials during strong earthquakes, the features of the macroseismic effect of cracking expressed in the protective coatings of buildings; it is recommended to optimize the composition based on polymer by composition that increases the crack resistance of plaster solutions. In particular, the visualization of macroseismic parameters is given on the example of the survey of the epicentral zones of the Tashkent (2008), Cannes (2011) and Marzhanbulak (2013) strong earthquakes that occurred in Kyrgyzstan and Uzbekistan. Possible directions of crack development and density of their distribution over the macroseismic area are analyzed. It was revealed that the vulnerability to micro-destruction of types of protective materials is proportional to the seismic intensity and is mainly due to the intensity of seismic vibrations.

Seismotectonics of Armenia: main problems

The article describes the main problems in the existing researches on the seismotectonics of the territory of Armenia. A review was conducted on certain issues related to the accuracy of the initial seismological data, the problem of identifying possible focal zones of strong earthquakes, and the probabilistic assessment of the seismic hazard of the territory of Armenia. As the results of numerous studies show, the initial seismological data from catalogs and bulletins are characterized by their heterogeneity. Different observation systems and different methods of seismological data processing have led to the fact that when visualizing the spatial distribution of earthquake epicenters on the territory of Armenia, during the instrumental period of observations, an artificially lattice network of epicenters is obtained, which does not agree in any way with the fault tectonics of the territory. During the stud of distribution of earthquake hypocenters by depth, the discrete nature of their distribution was revealed. There is also a number of disadvantages in the area of allocation of focal zones of strong earthquakes. The main part of the conducted research was mainly based on data on previously recorded strong seismic events, as well as on data on historical earthquakes, as a result, for the most part of the studied territory, in fact, no research was conducted. In addition, there are relevant questions about the tectonic basis used in these studies. The last group of problems concerns the probabilistic assessment of the seismic hazard of the territory of Armenia. It is shown that in the existing works, there are quite a lot of unsubstantiated assumptions.Based on the analysis, it is concluded that in the field of seismotectonic studies of the territory of Armenia, there are a number of urgent tasks that require priority solutions.

Depth Faults Along The Regmatic Network Of The Chatkalo-Kuramin Region And Their Relationship With Seismicity

Lineaments were identified using LANDSAT-8 satellite images and digital elevation models obtained from the ASTER GDEM satellite over the Chatkal-Kuramin region. Taking into account the stock materials and a comprehensive analysis of the results of processing remote sensing data, a map of lineaments of a 1: 100,000 regmatic network was compiled. Based on the automated visual lineament analysis in the Geomatica PCI program, lineaments of the regmatic network were obtained, which are located in the focal zones of strong earthquakes.

Global Retrofitting Strategies for an Existing Three-storied RC School Building in Mandalay, Myanmar

Low to severe earthquakes occur around the world every year, damaging and causing structural failure in buildings. Consequently, seismic improvements are required for existing buildings that are vulnerable to damage by seismic forces. The objective of this study was to investigate retrofitting strategies in terms of their sustainability. Mandalay, Myanmar, was selected as the study area as it is located near the Sagaing fault, which itself is in a strong earthquake zone (seismic zone 4). A three-storied RC building with a non-seismic design was selected as a case study building. An investigation was carried out into the performance and vulnerability of the building under three earthquake hazard levels. The vulnerability index value was calculated using the Priority Index method. Meanwhile, non-linear static pushover analysis was performed to investigate the performance of the existing building using SAP2000 V14 software. Four different types of retrofitting strategies were considered, namely reinforced concrete shear walls with openings, reinforced concrete shear walls without openings, steel plate shear walls, and finally steel bracing. Among these, it was found that the use of steel plate shear walls was the best retrofitting technique, owing to it having the best performance along with the lowest displacement. Its performance level reached up to the Immediate Occupancy (IO) level even under the conditions of a Maximum Consider Earthquake (MCE).

Correlation of the state of crustal stresses, seismicity and landslide activity (Fergana basin, Tien Shan)

The impacts of seismicity on the landslide activity in Kyrgyzstan have been in the focus of our study since 2010 [Kalmetyeva et al., 2010]. As the study progressed [Kalmetyeva, Moldobekov, 2012, 2013; Kalmetyeva et al., 2013, 2014], the initial problem statement has been revised as follows: do earthquake influence the occurrence of land‐ slides, and, if so, what is the mechanism of this influence. This paper presents the results of detailed analysis of the distribution of earthquakes and landslides in space and time in correlation with focal mechanisms (azimuth and plunge of the principal compression stress axis) of earthquakes occurred in the Fergana basin and its mountainous frame. These are grounds to conclude that the landslide activity is mainly influenced by the response of the internal structure of the upper crust to local stresses. The mechanism of influence of strong earthquakes on the landslide activity is redistribution of local stresses, which results from partial release of regional stresses due to rupturing in the source zones of strong earthquakes. Using this concept of the landslide activity, a methodology of landslide‐hazard mapping is the goal of our future studies aimed at the following: (1) zoning of the study area with respect to the features of the internal structure of the upper crust, (2) geological, geophysical and seismological studies of the state of stresses in the study area, (3) instrumental monitoring of landslides movements in the zones that differ in the structure of the upper crust, and (4) analysis of preparation and consequences of past strong earthquakes that took place in the study area in comparison with the landslide activity.

Dynamics of the Zones of Strong Earthquake Epicenters in the Arctic–Asian Seismic Belt

Our comprehensive study of the Russian Arctic region aims to clarify the features and types of seismotectonic deformation of the crust in the Arctic–Asian Seismic Belt, specifically in the zones of strong earthquakes in the Laptev Sea Segment, the Kharaulakh Segment, and the Chersky Seismotectonic Zone. We have analyzed modern tectonic structures and active fault systems, as well as tectonic stress fields reconstructed by tectonophysical analysis of the Late Cenozoic faults and folds. The investigated neotectonic structures are ranked with respect to the regional classification principles. Changes in the crustal stress–strain state in the lithospheric plate boundaries between the Eurasian, North American, and Okhotsk Sea Plates are analyzed, and regularities of such changes are discovered. A set of models has been constructed for the studied segments of plate boundaries with account of the dynamics of the regional geological structures. The models can give a framework for the assessment of potential seismic risks of seismogenerating structures in the Russian Arctic region.

Seismotectonic Activation of Modern Structures of the Siberian Craton

Our research of the modern structures of Siberian Craton aims to reveal regional regularities in the seismotectonic destruction of the Earth’s crust and to clarify the dynamics of the formation of the focal zones of strong earthquakes. Analysis of activated structures and marginal suture zones located in the study area was based on structural and geophysical data, the modern structural plan, and the quantitative characteristics of modern and recent tectonic movements, active faults, and tectonic stress fields identified by tectonophysical analysis of deformation and seismological parameters. Unambiguous correlation between the seismic activity level of modern structures and the rates of modern and recent tectonic movements were not determined in our study. The most active structures of the Siberian Craton located in zones of the dynamic influence of marginal sutures are contrasted against the gradient field of modern vertical tectonic movements and characterized by the mosaic field of the mean and low rates of modern movements. The kinematics of seismotectonic deformation and levels of seismic activation of suture zones of the Siberian Craton are governed by global geodynamic processes taking place at the boundaries between the Eurasian, North American and Amur lithospheric plates. The activated structures in the northern regions of Siberian platform, which are characterized by the highest rates of modern movements, are heterogeneous. Both fluid processes and glacioisostatic movements may have influenced the dynamics of the formation of these high-gradient deformation zones. In seismotectonic studies aimed at determining levels of the potential seismic hazard of modern structures, we find it important to take into account errors in geodetic data and ensure more correct reference to the rates of tectonic movements at the neotectonic stage. In order to correctly assess the degree of geodynamic activity of modern structures, special consideration should be given to the fluid-geodynamic factor that controls most geodynamic processes, including tectonic stress accumulation, the formation of earthquake focal zones, and the intensity of seismic events.

Estimation of maximum mass velocities in the focal zones of strong earthquakes based on measured displacements of rock blocks: A case study of Kyrgyzstan

An important task in seismic hazard assessment is estimation of the intensity and frequency of extremely strong earthquake effects, in particular, peak ground velocities (PGV). Earlier, a method was proposed to evaluate PGV values based on the magnitude of displacements of rock blocks (Rodkin et al., 2012). In this study, this method is used to analyze field data on the source zones of the August 19, 1992, MS = 7.3 Susamyr earthquake and the January 3, 1911, Mw = 7.9 Kemin earthquake, and estimate maximum ground shaking at the upper construction site of the Upper Naryn series of hydropower plants, Kyrgyz Republic. It is shown that the resulting estimates are consistent with data obtained through other techniques. Therefore, the new approach can be recommended to estimate earthquake effects.

Permanent Deformation Characteristics of Concrete Face Rockfill Dams on Alluvium Deposit Subjected to Different Strong Seismic Excitations

Using seismic analysis theory of CFRD, the numerical calculation of dynamic response of a high CFRD on alluvium deposit in strong earthquake zone subjected to strong seismic excitations of different transcendental probabilities was carried out, with emphasis on the seismic safety evaluation by permanent deformation. The influence of dynamic mechanical properties of the alluvium deposit on the dam's permanent deformation was also analyzed. The results show that permanent deformation will increase gradually with the increasing of peak value of seismic acceleration. The seismic safety of dam is in a normal range under the earthquake with transcendental probability of 2% in 100 years. The alluvium deposit has significant influence on the dam's dynamic response.

The 1991 Racha earthquake, Caucasus: Multiple source model with compensative type of motion

The source of the 1991 Racha earthquake in the Greater Caucasus generally corresponds to thrusting, which is characteristic of the predominant regional compression stress field. A more adequate view of the rupture process is provided by a complex source model composed of three subsources. This model is reconstructed by the body-wave inversion and consistent with the spatial distribution of the aftershocks. In terms of the suggested model, at the last stage of the rupture process, the opposite slip type (normal faulting) is observed in the source, which seems to be objective. It compensates the rapid (probably short) local redistribution of stresses caused by the thrusts in the first two subsources. The surface deformations observed in the epicentral zones of strong earthquakes are probably the analogs of such a compensative mechanism. For example, in the rear parts of the thrusts associated with the surface ruptures, normal faults trending parallel to the strike of the thrust line occur. Another analog of the compensative motion is probably the peculiarities of the aftershock sources. It has long since been noted (Kuznetsova et al., 1976) that some fault plane solutions in the aftershock sequences of strong earthquakes are close to the main shock solution, while others are different. The explanation of this phenomenon is suggested in (Kuznetsova et al., 1976; Kostrov and Das, 1988). In (Kuznetsova et al., 1976), these events are referred to as the aftershocks due to the fracture growth and aftershocks of relaxation, respectively.