Instability of layered media under gravity stress

Abstract It is proposed to consider data both on the deep density distribution (gravity instability) and the bulk modulus of elasticity on estimation of the stability of layered mountain ranges. It is shown that an increase in bulk elastic rigidity with depth is the stable state of layered media in terms of energy in the field of gravity stress. In the tectonosphere, in which the moduli of elasticity are inverted (decrease) with depth, excess energy of elastic strain of volume change is generated by body forces, and it will determine the instability of the geologic medium.</jats:p

Concerning the theory of LURR based deterministic earthquake prediction

This paper considers theoretical aspects of a trigger effect of earth tides on earthquake initiation under the LURR approach. The growth of Coulomb stress, which appears resulting from this phenomenon, is shown to occur not for all regimes of stress state acting in the studied region. Its greatest increase corresponds to the regime of the horizontal extension and shear associated with the faults with kinematics of the normal and strike-slip faults. The low level of additional Coulomb stress for the horizontal compression regime allows asserting the low probability of the trigger effect for the faults with kinematics of the reverse faults. It is noted, that there is also an indirect factor in the form of additional pressure caused by the sea tides in addition to the main factor of the earth tides effect on deformations in the solid earth for island arcs and coastal areas of the continental crust. This is an additional vertical pressure for the ocean floor, and a lateral pressure for the crust of island arcs and coastal areas of the continents. Indirect factors significantly complicate the effect of earth tides on the Earth’s crust, completely neutralizing the influence of the direct factor in some cases.</jats:p

On some aspects of the article «On the stress drop in North Eurasia earthquakes source-sites versus specific seismic energy»

In the article by N.A. Sycheva and L.M. Bogomolov, the authors proposed to combine the interrelated data on the stress drop in the earthquake sources, ∆σ, and reduced seismic energy, ePR, to analyze the dependence of these parameters on earthquake scale along with expansion of the measurement statistics (assessments). The dependence of these parameters of a source on the seismic moment or on the earthquake magnitude within 2.2 ≤ М ≤ 4.0 magnitude range has been determined using the example of the Northern Tien Shan (Bishkek geodynamic polygon with the KNET network). The author of the letter to the editor notes the article conclusions to be limited, because such relationship is only manifested within the more or less narrow range of the magnitudes. Attention is also drawn to the semantic difference between the ∆σ and ePR parameters. It is the reduced seismic energy that reflects the mean strain in the source area, and its appliance to the analysis of scale dependences of earthquake sources is more informative.</jats:p

Stress state and deformation of the Earth’s crust in the Altai–Sayan mountain region

Abstract We present the results of tectonophysical reconstruction of natural stresses of the Earth’s crust in the Altai–Sayan mountain region using cataclastic analysis of fault slips and seismic data on the focal mechanisms of earthquakes. This method allows one to obtain the parameters of the total stress tensor by invoking additional data: generalized experimental data on the brittle fracture of rocks, seismic data on the released stress of strong earthquakes, and data on the topography and density of rocks. Results of the tectonophysical reconstruction of stresses showed significant inhomogeneity of the stress state, which is manifested not only in the variation of the strike and dip of the principal axes of the stress tensor, determining changes in the geodynamic regime of the Earth’s crust, but also in the close location of the regions of high and low isotropic tectonic pressure in relation to the lithostatic pressure. The variance of the ratio of tectonic pressure to lithostatic pressure is in the range of 0.59–1.31, with an average value for the region close to unity. This paper discusses internal or external mechanisms capable of generating the stress field obtained by the tectonophysical reconstruction.</jats:p

A MODEL OF THE EVOLUTION OF THE LITHOSPHERE OF THE HIMALAYAN-TIBETAN OROGEN

The Himalayan-Tibetan orogen is one of the active orogens on Earth. The processes caused by the collision of two continents have attracted attention of many researchers, and over the past decades, a large amount of geological and geophysical data has accumulated, on which models of the evolution of the region are based. The paper presents a model of the evolution of the Tibet plateau and the adjacent mountain chains, which complies with the modern concepts of the structure of the crust. The reference parameters of this model are the data on the values of stresses and on the patterns of the spatial distribution of principal stresses obtained in our own tectonophysical studies in region, as well as in other intracontinental orogens and in subduction zones between lithospheric plates. The basic assumptions of the model are the factors of the long stage of the Indian plate underthrusting beneath the Eurasian continent, metamorphic processes in the submerged slab (oceanic lithosphere) and in the continental lithosphere above it, combination of absolute horizontal movements of the Eurasian and Indian plates, small-scale convection in the upper mantle and vertical movements of matter, both in the continental lithosphere itself and in the upper mantle.</jats:p

ON THE MECHANISM OF INTERACTION BETWEEN STRONG EARTHQUAKES AND VOLCANISM IN SUBDUCTION ZONES

The relationship between strong earthquakes and powerful volcanism in subduction zones are discussed. It is shown that abnormally strong earthquakes or a group of strong earthquakes and powerful volcanic events or volcanic activation of large areas can be considered as an interconnected geodynamic pair. At high level of horizontal compression, magma-conducting faults are clamped and volcanism in the upper part of the crust is hindered. Mega-earthquakes or a series of strong earthquakes reduce the level of horizontal compression in the crust of the island arc (continent active margin), which creates favorable conditions for the resumption of volcanic activity. For major faults located along the strike of subduction zones, upward movement of magma during volcanic eruptions or intracrustal magmatism leads to an increase in horizontal compression stresses in the surrounding rocks to the pressure level of rising magma. As a result, a horizontal compression stress state is restored in the crust and, thus, the cycle is closed. Once again, a state emerges in which earthquakes are an effective mechanism for reducing stresses.</jats:p