Association of earthquake hypocenters with fault zones appears more pronounced in cases with more accurately determined positions of the earthquakes. For complex, branched structures of major fault zones, it is assumed that some of the earthquakes occur at feathering fractures of smaller scale. It is thus possible to develop a «seismological» criterion for definition of a zone of dynamic influence of faults, i.e. the zone containing the majority of earthquakes associated with the fault zone under consideration.In this publication, seismogenic structures of several fault zones located in the San-Andreas fault system are reviewed. Based on the data from a very dense network of digital seismic stations installed in this region and with application of modern data processing methods, differential coordinates of microearthquakes can be determined with errors of about first dozens of meters. It is thus possible to precisely detect boundaries of the areas wherein active deformation processes occur and to reveal spatial patterns of seismic event localization.In our analyses, data from the most comprehensive seismic catalog were used. The catalogue includes information on events which occurred and were registered in North California in the period between January 1984 and May 2003. In this publication, the seismic data processing results and regularities revealed during the analyses are compared with the data obtained from studies of fault structures, modeling and numerical simulation results. Results of quantitative research of regularities of localization of seismic sources inside fault zones are presented.It is demonstrated by 3D models that seismic events are localized in the vicinity of an almost plain surface with a nearly constant angle of dip, the majority of events being concentrated at that conventional surface.Detection of typical scopes of seismicity localization may prove critical for solution of problems of technogenic impact on fault zones for the purpose of partial stress release. The obtained results suggest that the region, wherein active deformation takes place during preparation of medium earthquakes (M≤6.5÷7.0), includes a number of local «strips», each about 100 m in width. The latter size is comparable to a scope of technogenic capabilities of producing an impact on geo-environment. It is hoped that studies of both fine spatial and temporal patterns of seismicity in the vicinity of fault zones will allow to find reliable pinpoints for definition of both place and time for implementation of technogenic impacts.In our opinion, the implemented study demonstrates the burning need to establish test sites with dense and wellequipped local seismic networks in Russia.
Abstract:This study aims to analyze the internal structure of earthquake sources under the modern concepts in physical mesomechanics, which consider the multilevel process of faulting in the geological medium, taking into account specific features of the subsurface and deep levels of the crust. This article includes two parts that present the study results and discuss the interdisciplinary information on the subject of this study. The first part describes the subsurface crustal level, wherein seismogenic faulting takes place. We present the seismogeological data on the structure and development of the sources of three catastrophic earthquakes (М≥8.0-8.5) that occurred in Mongolia in the last century. We discuss the deep drilling (1.0-3.5 km) data on the seismodislocations formed after the recent strong and catastrophic earthquakes in the United States, Taiwan and Japan, including the zone of co-seismic fractures caused by the Tohoku-Oki earthquake (M=9.0, November 11, 2011). In the second part, jointly with specialists in petrology and geochemistry A.V. Travin and V.B. Savelieva, we will analyze the field data on the ages and the physical and chemical characteristics of geomechanical processes that took place at large depths in the fault zones, which are outcropped by the long-term denudation of the upper crustal layer in the study area. In the summary, we will describe our concepts of geomechanical and tribochemical processes taking place in the fault zones during the formation of the earthquake sources. The results of this comprehensive study give grounds to conclude that a multidisciplinary approach is needed to investigate the deep geological and geophysical processes of 'stick-slip' on the fault planes with diverse relief features in the zones wherein seismicity is generated. This conclusion is of paramount importance: it concerns the potentials of applying new approaches to forecasting, management and mitigation of seismic engineering risks arising from the hazardous effects of strong earthquakes.Key words: fault zone; earthquake source; seismodislocation; dislocation parameters; stick-slip; seismic safety GEODYNAMICS & TECTONOPHYSICS P U B L I S H E D B Y T H E I N S T I T U T E O F T H E E A R T H ' S C R U S T S I B E R I A N B R A N C H O F R U S S I A N A C A D E M Y O F S C I E N C E S T e c t o n o p h y s i c s RESEARCH ARTICLEReceived: March 22, 2017 Revised: September 8, 2017 Recommended by K.Zh. Seminsky Accepted: October 18, 2017For citation: Ruzhich V.V., Kocharyan G.G., 2017. On the structure and formation of earthquake sources in the faults located in the subsurface and deep levels of the crust.
The paper briefly overviews the evolution of ideas concerning causes and mechanisms related to the origin of the Baikal rift zone (BRZ) in the centre of the Eurasian plate, discusses parameters of the recent seismogeodynamic impact on the seismotectonic regime in BRZ due to the Western Pacific subduction and the Indo-Eurasian collision, and attempts at estimating their contributions to the modern geodynamics of rifting processes in Pribaikalie. Seismic migration processes and specific density patterns of the released seismic energy are analyzed for two selected profiles between BRZ and the regions of collision and subduction. A statistical method is proposed to calculate seismic migration from space-time diagrams, and equations are developed to show a decrease in specific density of seismic energy released in the lithosphere at a distance from the interplate boundaries towards the Baikal rift. The modern geodynamic impact on the seismotectonic regime in BRZ due to the Indo-Eurasian collision is reflected in moderate horizontal compression of the lithosphere, mainly in the southwestern BRZ and partly in the central BRZ. In the transition area in this profile, the specific density of released seismic energy is about 1.72×1010 J/km 2. The geodynamic impact on the seismotectonic regime in BRZ from the subduction zone (from the Nankai trough) is shown by a significantly lower specific density of released seismic energy, 1.02×1010 J/km 2. In the lithosphere of the northeastern BRZ, a weaker geodynamic influence is mainly manifested by responses to strong seismic events and earthquake focal mechanisms with a clear strike-slip component in the Chara and Tokka basins located in the Aldan shield of the Siberian platform. We discuss a possible mechanism that drives the propagation of the geodynamic impact on BRZ from the interplate contact areas. In our opinion, the geodynamic influence propagates intermittently in the lithospheric plates due to motions of slow-deformation-wave fronts, which are reflected in the diagrams as seismic activity clusters. The longrange propagation of slow waves is realized through triggering of active faults in the lithosphere. Such faults interacting with slow wave deformations may be manifested as excited sources of dissipation of seismic oscillations resulting from a spontaneous release of the energy accumulated in the Earth interior. This mechanism of endogenous energy supply may explain the observed propagation of recordable slow elastoplastic deformations for many thousands of kilometres. Today, when the new materials are available to show more ancient ages of the early elements of BRZ, and it is established that the tectonic energy is reduced with distance from the interplate boundaries, there is no support for the hypothesis based on the role of the Indo-Eurasian collision in the formation of BRZ. A recordable seismotectonic impact on the seismic regime in BRZ can occasionally occur after a major seismic activity in the regions of collision and subduction. This phenomenon may be used as ...
The ML 6.1 earthquake that occurred on June 18, 2013 in Kuzbass is the strongest seismic event related to mining operations in this region. Opinions about its genesis differ. On the one hand, its hypocenter and most aftershocks occurred directly underneath the Bachat open-pit mine, which suggests that this seismic event was due to anthropogenic impacts. On the other hand, the earthquake focus was located at a depth of several kilometers, which, according to some authors, argues against the anthropogenic factor-the technogenic change in the parameters of the stress field was insignificant against the lithostatic pressure and, especially, the rock strength (e.g. [Lovchikov, 2016]). Our study aims to discover and assess an impact of the mining operations in the near-surface areas of the crust, investigate whether the Bachat earthquake was caused by the mining operations, and clarify which processes in particular were the most probable triggers of dynamic movement in the Bachat earthquake source. The probable geometrical parameters of the fault plane were estimated from the structural and tectonic conditions of the study area and the published locations of the aftershocks [Emanov et al., 2017]. It is established that seimic events of magnitudes similar to that of the Bachat earthquake cannot be caused by the overall anthropogenic load on the area, and it is unlikely that such a strong earthquake may occur due to the direct effects of seismic vibrations resulting from mass explosions during the mining operations. Our analytical models and numerical simulations, as well as the analysis of seismological observation data show that the most probable factor that initiated dynamic movements in the earthquake source was the extraction of the huge rock volume and its transportation from the Bachat open-pit mine. It should be noted that the size of the zone, wherein the geomechanical initiation criteria are met, is considerably larger than the critical size of a nucleation zone for a M 6 earthquake. However, open-pit mining operations can hardly affect the localization of strong earthquake sources. Mining operations can only trigger a seismic event that has been already prepared by the natural evolution of the crust.
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