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Studying locations of strong earthquakes (М≥8) in space and time in Central Asia has been among top prob-lems for many years and still remains challenging for international research teams. The authors propose a new ap-proach that requires changing the paradigm of earthquake focus – solid rock relations, while this paradigm is a basis for practically all known physical models of earthquake foci. This paper describes the first step towards developing a new concept of the seismic process, including generation of strong earthquakes, with reference to specific geodynamic features of the part of the study region wherein strong earthquakes were recorded in the past two centuries. Our analysis of the locations of М≥8 earthquakes shows that in the past two centuries such earthquakes took place in areas of the dynamic influence of large deep faults in the western regions of Central Asia. In the continental Asia, there is a clear submeridional structural boundary (95–105°E) between the western and eastern regions, and this is a factor controlling localization of strong seismic events in the western regions. Obviously, the Indostan plate’s pressure from the south is an energy source for such events. The strong earthquakes are located in a relatively small part of the territory of Central Asia (i.e. the western regions), which is significantly different from its neighbouring areas at the north, east and west, as evidenced by its specific geodynamic parameters. (1) The crust is twice as thick in the western regions than in the eastern regions. (2) In the western regions, the block structures re-sulting from the crust destruction, which are mainly represented by lense-shaped forms elongated in the submeridio-nal direction, tend to dominate. (3) Active faults bordering large block structures are characterized by significant slip velocities that reach maximum values in the central part of the Tibetan plateau. Further northward, slip velocities decrease gradually, yet do not disappear. (4) In the western regions of Central Asia, the recurrence time of strong earthquakes is about 25 years. It correlates with the regular activation of the seismic process in Asia which is mani-fested in almost the same time intervals; a recurrence time of a strong earthquake controlled by a specific active fault exceeds seems 100–250 years. (5) Mechanisms of all the strong earthquakes contain a slip component that is often accompanied by a compression component. The slip component corresponds to shearing along the faults revealed by geological methods, i.e. correlates with rock mass displacements in the near-fault medium. (6) GPS geodetic meas-urements show that shearing develops in the NW direction in the Tibet. Further northward, the direction changes to the sublatitudinal one. At the boundary of ~105°E, southward of 30°N, the slip vectors attain the SE direction. Further southward of 20°N, at the eastern edge of the Himalayan thrust, the slip vectors again attain the sublatitudinal direc-tion. High velocities/rates of recent crust movements are typical of the Tibet region. (7) The NW direction is typical of the opposite vectors related to the Pacific subduction zone. The resultant of the NE and NW vectors provides for the right-lateral displacement of the rocks in the submeridional border zone. (8) The geodynamic zones around the cen-tral zone (wherein the strong earthquakes are located) are significantly less geodynamically active and thus facilitate the accumulation of compression stresses in the central zone, providing for the transition of rocks to the quazi-plastic state and even flow. This is the principal feature distinguishing the region, wherein the strong earthquakes are loca-ted, from its neighboring areas. In Central Asia, the structural positions of recent strong earthquakes are determined with respect to the following factors: (1) the western regions separated in the studied territory; (2) the larger thickness of the crust in the western regions; (3) strong submeridional compression of the crust and upper lithosphere in combination with shear stresses; (4) high rates of recent crustal movements; and (5) the rheological characteristics of the crust.

The article presents results of experimental studies using a bi-axial servo-control system to apply load on samples with extensional and compressional en echelon faults. During the experiments, variations of temperature and thermal images were recorded synchronously by a multi-path contact-type thermometric apparatus and a thermal image system, respectively. A digital CCD camera was employed to synchronously collect images of specimens’ surfaces. The digital speckle correlation method (DSCM) was utilized to analyze the images and to define displacements and strain fields. Our experimental results show that temperature fields have clear responses to opposite stress states in the jog areas of both types of the en echelon faults. Prior to failure of the jog area, its temperature is the highest at the compressional en echelon faults and the lowest at the extensional en echelon faults. Records by DSCM give evidence that mean strain of the jog area is the highest at compressional en echelon faults and the lowest at the extensional en echelon faults. It is revealed that deformation of the en echelon faults occurs in two stages, developing from stress build-up and fault propagation in the jog area to unstable sliding along the fault. Correspondingly, the mechanism of heating-up converts from strain heating into friction heating. During the period of transformation of the temperature rising mechanism, three events are observed in the jog area and its vicinity. Analyses of our experimental results demonstrate that variations of temperatures in the jog area can be indicative of fault sliding and suggest sliding directions. Observations and studies of temperature changes during transformation of the temperature rising mechanism at sensitive portions of faults are of great importance for early detection of precursors of unstable slip on active faults.

The 2008 Wenchuan MS8.0 earthquake occurred on Longmenshan fault zone (LMSF), which is at the eastern margin of the Tibetan plateau. The epicenter is near the Shuimogou earthquake swarm, which was thought to be triggered by the Zipingpu reservoir after its impounding in 2004. People have speculated that the large earthquake was triggered by the water filling of the reservoir. To figure out the role of the Zipingpu reservoir on the earthquake, the local seismicity recorded by the Zipingpu local seismic network during the period from 31 July 2004 to 11 May 2008 were analyzed in detail. The distribution of hypocenters showed that most earthquakes occurred on Yingxiu-Beichuan fault (YBF) in the reservoir area with hypocenters depth less than 10 km, which is a major source fault of the Wenchuan earthquake. Useful information on fault geometry in the depth was also obtained. The spatial-temporal distribution of hypocenters demonstrated clear migration pattern that indicated pore-pressure diffusion, it also showed a hydraulic diffusivity (D) of 0.7 m2/s. Previous experiments show the existence of the synergism process of the fault under a meta-instability state before fault sliding. It enhances the stress on the stronger portion of the fault and the synergism degree by reducing strength of the weak portions and by increasing the total length of weak portions. According to this view, the pore pressure diffusion by water filling of Zipingpu reservoir increased the total length of weak portions and enhanced the stress at the focal.

The Shanxi tectonic belt is a historically earthquakeabundant area. For the majority of strong earthquakes in this area, the distribution of earthquake foci was controlled by the N–S oriented local structures on the tectonic belt. Studies of the present stress state of the Shanxi tectonic belt can contribute to the understanding of the relationship between strong earthquakes’ occurrence and their structural distribution and also facilitate assessments of regional seismic danger and determination of the regions wherein strong earthquakes may occur in future. Using the Cataclastic Analysis Method (CAM), we performed stress inversion based on the focal mechanism data of earthquakes which took place in the Shanxi tectonic belt from 1967 to 2010. Our results show that orientations of the maximum principal compressive stress axis of the Shanxi tectonic belt might have been variable before and after the 2001 Kunlun MS=8.1 strong earthquake, with two different superior trends of the NW–SE and NE–SW orientation in different periods. When the maximum principal compressive stress axis is oriented in the NE–SW direction, the pattern of the space distribution of the seismic events in the Shanxi tectonic belt shows a trend of their concentration in the N–S oriented tectonic segments. At the same time, the stress state is registered as horizontal shearing and horizontal extension in the N–S and NE–SW oriented local segments in turn. When the maximum principal compressive stress axis is NW–SE oriented, the stress state of the N–S and NE–SW oriented tectonic segments is primarily registered as horizontal shearing. Estimations of plunges of stress axes show that seismicity in the Shanxi belt corresponds primarily to the activity of lowangle faults, and highangle stress sites are located in the NE–SW oriented extensional tectonic segments of the Shanxi belt. This indicates that the stress change of the Shanxi belt is caused by adjustment of the regional stress field, rather than by the diverse seismic activities.

The stick­slip process of bending faults with one angle change of 5° at the connection location between the two line fault segments is investigated in this paper. The dynamic process and physical field evolution were observed in the laboratory, and measurements were recorded with application of fault displacement measurement, strain tensor analysis and acoustic emission (AE) techniques. The experimental results from the study of bending faults give grounds for the following primary conclusions: (1) It is clearly revealed that there is a negative correlation between the logarithms of the stick­slip cycle and loading rate; (2) With different loading rates, most of the instabilities from bending faults occurred as earthquake doublets. It means that an instable event can contains two sub­events which occur with an interval of time from 100 ms to 200 ms. (3) With application of different observation approaches, regardless of the fact that the sampling rates were the same, differences of the co­seismic response were observed. For instance, strain measurements indicated significant strain weakening stage, but fault displacement was not significantly changed before fault instability. (4) Experimental studies of high sampling rates contribute to a better understanding and awareness of earthquake precursors and the seismogenic process; such studies are helpful in analyzing the mechanism of strong earthquake processes and parameters of aftershocks.