Modern Geodynamics and Focal Mechanisms of Earthquakes near the Bushehr Nuclear Power Plant

Izvestiya, Atmospheric and Oceanic Physics - A method for determining the focal mechanisms of earthquakes using the S-wave polarization direction for weak close seismic events is developed based on...     

Oscillatory regime of aftershocks of the 1984 Ddzhirgatal earthquake: Implications for the internal dynamics of an unstable geological system

On the basis of seismological and geological evidence, the aftereffect region of the strong Dzhirgatal earthquake is localized as a real physical object. The time dependence of the number of aftershocks of this earthquake is best described by a simple hyperbolic law (p = 1, or the Omori law), which is well consistent with the ideas of self-similarity of the material fracture and the development of the seismotectonic process. Features characteristic of the evolution of self-similar systems are recognizable in the dynamics of the aftershock flow and parameters of seismotectonic deformation (STD) calculated from focal mechanisms of aftershocks. The evidence for this is the Feigenbaum inverse cascade in predominant frequencies of variations in the numbers of aftershocks and “catastrophes” in the form of an increase in the variability of STD parameters at the time moments when the corresponding predominant periods abruptly change. In this way, the term “catastrophe,” speculative with respect to the geophysical medium, acquires real meaning as a set of observable changes in the dynamics of STD parameters and the flow of aftershocks that occur over a fairly short time compared to the entire duration of the aftershock series.     

Autonomous (Non-Plate-Tectonic) Geodynamics of the Pyrenees

Izvestiya, Atmospheric and Oceanic Physics - The tectonic structure, seismicity, and modern geodynamics of the roughly EW-trending Pyrenees are considered based on the literature data. Geological...     

A layer of an unstable seismotectonic strain as an analog of a waveguide at depths of 12–20 km in the Earth’s crust of the Tajik depression

The existence of a layer of unstable seismotectonic strain, which spatially coincides with the waveguide previously detected here at depths of 12–20 km, is established in the central part of the Tajik Depression [Lukk et al., 1970; Nersesov and Chepkunas, 1970; 1971]. This crustal layer is assumed to be weakened and saturated by fluid. The latter is supposedly achieved due to the supply of the liquid component from the bottom crust or upper mantle in the cracks and pores of the waveguide material. A near vertical pillar-like seismogenic body revealed in the work [Shevchenko et al., 2011], which penetrates the waveguide in the depth interval 0–40 km, is considered as one of the possible channels of such inflow. The detected loosened layer (the waveguide) is characterized by a signficant reduction in seismic activity. However, no such reduction is observed within the pillar-like seismogenic body at these depths. Moreover, compared to the nearest ambient environment, the upper 10–15 km of this body feature considerably increased seismic activity that manifested itself in a series of 13 strong earthquakes (with M ≥ 4.7) that occurred within the past approximately 100 years.     

Peculiarity of the Relationship between the Seismicity and Tectonic Structure of the Pyrenees

The geotectonic position of the Pyrenees mountain massif in the Alpine–Indonesian mobile belt is considered. The geological data testify to the formation of the structure of the Pyrenees in the setting of a subhorizontal compression perpendicular to the ridge. The commonly accepted interpretation considers this compression in the context of plate tectonic notions related to the collision between the Iberian and Eurasian lithospheric plates resulting from the convergence of the Eurasian and African plates. However, this interpretation is challenged by the the geodetic and seismological measurements. The GPS measurements suggest a certain cross-strike spreading rather than shortening of the Earth’s crust; the focal mechanisms of the earthquakes indicate the predominance of a subhorizontal extension perpendicular to the strike of the Pyrenees mountain range. The processes of the gravitational collapse of the mountain chain during the isostatic upwelling of the orogenic crust are considered as the most probable cause of this spreading by a number of the authors.     

Clarification of Tectonic and Geodynamic Models of the Alpine–Himalayan–Indonesian Mobile Belt Extremities Based on Matching Views about the Wide and Narrow Tethys Paleocean

Izvestiya, Atmospheric and Oceanic Physics - Based on the available geological and geodetic data, it has been established that the structures of the Alpine–Himalayan–Indonesian mobile...     

Diurnal periodicity of weak earthquakes in Central Asia

Data on the time moments and numbers of weak earthquakes (M ≥ 0) are analyzed in five regions of Central Asia where long-term extensive instrumental observations of seismicity were conducted. Spectra of time moments of seismic events were calculated in the range of periods from 1 to 48 h with a step of half an hour. The time variations in the numbers of these earthquakes were also calculated in windows of 4 to 48 h wide with a summation step of 1 h. The presence of significant high-quality spectral extrema at periods of 24 and 12 h is established for all catalogs used, and less significant extrema at a period of 8 h are established for some catalogs. These periodicities are observed in the analysis of weak earthquakes with M < 2.2 (K < 8) and are virtually absent for stronger seismic events. It is shown that lunisolar tides have no appreciable gravitational effect on the observed variations in seismic emission. Daily variations in the actual sensitivity of the observation network due to the daytime increase in the noise level as a factor responsible for the observed diurnal periodicity raise doubts.     

Diurnal periodicity of weak earthquakes and high-frequency underground noise in Kamchatka

We have analyzed the data on the times and number of weak earthquakes (M = 0–2.5) included in the up-to-date (final version) Catalog of Kamchatka Earthquakes for 1995–2008, and the intensity of highfrequency underground noise measured in the deep borehole near Petropavlovsk-Kamchatskiy (according to the literature data). We calculated the spectra of the seismic time series within the range of periods from 1 to 48 hours with a step of one minute. It was found that the spectrum of the earthquake data series contains a significant high Q extremum in a period of 24 hours that can be linked with a similar period in the high-frequency underground noise in Kamchatka and in the Russian Platform. There are some grounds to suggest that the weak earthquakes and the underground noise (seismoacoustic emission) have the same nature. In both cases, the shapes of the curve of the diurnal periodicity are found to depend on the duration of light during the day. The probable reasons for the solar impact on the seismic emission processes are discussed.     

Induced seismicity and the possibilities of controlled relaxation of tectonic stresses in the Earth’s crust

Natural and anthropogenic impacts on seismicity are considered. Taking into account the importance of the discussed problem, the authors propose to open the discussion on the questions considered. In this connection a wide circle of known experimental data is considered, which are indicative of the possibility in principle of active impact on the seismogenic medium for the smooth relieving of accumulated tectonic stresses in the Earth’ s crust. The reasoning is presented of one of the promising ways of the smooth controlled relaxation of the accumulated tectonic stresses in the Earth’s crust at the places of the probable onset of strong earthquakes due to a considerable increase in the plastic slips, which facilitate the decrease of the number and energy of earthquakes. The approach proposed is based on the results of the works on the excited seismicity, obtained in different regions of the Earth. Special attention is given to the most detailed long-term investigations of the excited seismicity in the region of the reservoir of the Nurek hydroelectric station in Tadzhikistan and in the neighborhood of the actively mined Romashkinskoe oil deposit in the Republic of Tatarstan. The results of the laboratory investigations of the behavior of samples made of materials of crystalline and amorphous structures under the action of pressure and vibration are invoked for the substantiation of the physical nature of the observed effects. For the reduction of seismic hazard, it is proposed to use vibration actions and water injection in the bore holes at the places of the expected seismic catastrophes in a time mode matched with the tidal motions of the Earth.     

Seismotectonic characteristics of the northern flank and axial part of the Tajik Depression (the Garm Geodynamic Research Area)

Detailed estimates of seismotectonic strain (STS) characteristics are obtained for the upper crust from focal mechanisms of numerous earthquakes (M ≥ 1) in the northern and axial parts of the Tajik Depression (the Garm area, Tajikistan). It is shown that, in the majority of cases, a close interrelation exists between tectonic structures and the STS type. Changes in the STS characteristics in various areas of the depression admit a reasonable geological interpretation. Based on geological, seismological, and geodetic data, a unified scheme of deformation of layered rocks of the Tajik Depression is presented. The main factor controlling the deformation process is the extrusion of the rocks in the axial part of the depression in the NNW and SSE directions (with the formation of imbricate thrusts) and in the WSW direction. The latter case is characterized by left-lateral motions in fault zones in the south of the depression (including the Darvaz fault) and by right-lateral motions in northern zones (the Peter the First Range) located from seismological data. In our opinion, such expulsion is caused mainly by an increase of the volume of layered rocks in the Tajik Depression, apparently due to deep fluids penetrating into these rocks and supplying additional mineral material. However, it is also possible that the region is influenced, to an extent, by the general subhorizontal N-S compression associated with the convergence of the Indian and Eurasian lithospheric plates in accordance with the concept of plate tectonics.