Search

There were investigated seismic mode and stress state of the Earth’s crust in the South Fergana seismic activity zone. In the central part of the zone, there was identified an area of a long-term seismic quiescence during which no earthquakes with energy class K≥12 (M≥4.5) were recorded. Along the entire length of the seismic quiescence area, there occur an activation of earthquakes of lower energy classes, as well as anomalous fluctuations of the recurrence curve slope. To localize the place of seismic activation probability, the current stress of earth’s crust in the South Fergana seismic activity zone was reconstructed according to focal earthquake mechanisms by the methods of cataclastic analysis of displacements. There were identified the areas with low values of effective confining pressure and maximum shear stresses where strong earthquakes usually occur. Taking into account the stressed state of the Earth’s crust, on the basis of the prognostic parameters of the seismic regime within the South Fergana seismic activity zone there were identified two areas in which M≥6.0 earthquakes may occur in the coming years.

The current stress of Earth's crust in the territory of Uzbekistan has been studied using a focal earthquake mechanisms catalogue that includes the data provided by many authors. Stress reconstructions are based on the cataclastic analysis of displacements along fractures. For reconstructing the stress state at different depths of the crust in several seismically active regions of the study area, we consider a minimum number of earthquakes in a homogeneous sample equal to 6 and an averaging radius of 10 to 30 km within a single domain. The azimuths and dip angles of the principal stress axes, Lode – Nadai coefficients, geodynamic types of stress modes, relative (normalized to rock strength) values of maximum shear stresses, and effective pressure values are determined. Maps showing the spatial distribution of the studied parameters are constructed for both the entire seismically active layer and the depth layers. Stress fields are reconstructed and compared at two hierarchical levels based on the parameters of focal mechanisms of weak and moderate earthquakes (М≤4.5) and those of strong (М≥5.0) earthquakes. "Tectonic Stresses of Eurasia", the Internet resource created by IPE RAS, is used to visualize the stress field reconstructed from the data on strong (М≥5.0) earthquakes.

Seismic intensity assessment in points of a macroseismic scale plays an important role for researching the seismic history of areas characterized by active seismicity, as well as for construction (and updating) of seismic zoning maps in various scales. Macroseismic scale points are generally referred to in construction standards applied in the majority of post-Soviet states. In our study aimed to model the macroseismic field of earthquakes, a large volume of macroseismic data on Central Asia was analyzed, and coefficients used in Blake–Shebalin and Covesligeti equations were aligned. This article presents a generalized dependence model of macroseismic intensity attenuation with distance. The model takes into account seismic load features determined by various depths of earthquakes. The ratios of small and big axes of the ellipse, that approximates real isoseists, are estimated with respect to seismic scale points, earthquake depths and magnitudes. The East Uzbekistan area is studied as an example to investigate whether seismic hazard assessment values may differ depending on a chosen law of seismic influence intensity attenuation with distance.

In the territory of Uzbekistan, seismic activity is high, and ensuring seismic safety of the population is among the most important problems for the country. Our study was focused on discovering relationships between the attenuation of the ground oscillation rates, their spectral amplitudes and the distances to the epicentres of the earthquakes of different energy levels, which took place in the region under study. An objective was to quantitatively assess the seismic hazard of the study area. We analyzed the velocity graphs of M 3.8–6.2 earthquakes that occurred in Uzbekistan and the neighbouring territories, and the recorded earthquake spectrum data. For the region under study, it is established that the attenuation of the ground oscillation rates and their spectral amplitudes depend on the distances. Taking into account the sizes of crustal earthquake foci, the spectra of ground oscillation rates were calculated for the near zone. Based on the established regional dependencies, the maximum ground oscillation rates and spectral velocity amplitudes were estimated, and seismic hazard probability for the territory of Uzbekistan was assessed. For several towns and cities of Uzbekistan, the highest predictable spectral amplitudes were determined, which, under given probability P, will not be exceeded within the coming 50 years. The quantitative characteristics of seismic hazard can be converted to the indicators of seismic activity impacts, as required for calculating the seismic loads in earthquake-resistant engineering and construction projects.

An approach to evaluate the current seismological situation in the Uzbekistan territory is presented. This approach is based on the regularities of seismic processes in strong-earthquake focal areas and the manifestation peculiarities of strong earthquakes in seismically active zones. At the first stage, within seismically active zones, areas with a high seismic activity matching the strong earthquake level were identified during the historical and instrumental observation periods. Considering the low variability in the direction of seismotectonic processes over tens and hundreds of years, which determines the modern stress state of seismically active structures, these areas were considered the most likely areas to experience strong earthquakes over the next few decades. Tectonophysical validation of the division of seismically active zones into areas with different potential hazards of strong earthquakes was carried out within the framework of cataclastic analysis method of rupture dislocations (CAM). At the second stage, temporal fluctuations in seismic regime parameters within the selected areas were studied. Based on the number of current anomalous features identified, the areas were ranked according to the occurrence probability of strong earthquakes over the next few years.

Based on macroseismic survey data for strong earthquakes in Central Asia, the coefficients of attenuation of seismic intensities with distance in the Blake-Shebalin- and Kovesligethy -type equations were refined. A new generalized dependence of macroseismic intensity attenuation on distance, taking into account the depth of the earthquake hypocentre, were obtained. Relations between the minor and major axes of the ellipse approximating real isoseists depending on the shaking strength, source depth and earthquake magnitude were found. With the example of the territory of eastern Uzbekistan, the influence of the choice of the law of seismic intensity attenuation with distance on the obtained seismic hazard assessments is investigated.