Abstract. Numerical simulation of seismoacoustic emission (SAE) associated with fracturing in zones of shear stress concentration shows that SAE signals are polarized along the stress direction. The proposed polarization methodology for monitoring of slope stability makes use of three-component recording of the microseismic field on a slope in order to pick the signals of slope processes by filtering and polarization analysis. Slope activity is indicated by rather strong roughly horizontal polarization of the respective portion of the field in the direction of slope dip. The methodology was tested in microseismic observations on a landslide slope in the Northern Tien-Shan (Kyrgyzstan).
Seismic strain estimated from about 900 earthquake mechanisms in the Altai-Sayan area indicates that the area evolves mainly under N-S compression caused by the India-Eurasia collision. Plastic flow in the mountains of the western and central Altai-Sayan area is controlled by convergence of aseismic rigid blocks, including the Dzungarian microplate, the Minusa basin, the Tuva basin, Uvs Nuur and other basins in the region of Great Lakes in Mongolia. Earthquake rupture follows the existing fault pattern. The pattern of rigid blocks and mountain ranges correlates well with upper mantle thermal heterogeneity as imaged by seismic tomography.
Earthquake mechanisms in the presence of plate convergence depend on different factors. Slip geometry depends on focal depth at a given geodynamic regime, and the latter, in terms of the seismic process, is controlled by lateral (W-E) constraint. Reverse-slip earthquakes most often originate at shallow depths in regions of constrained compression, such as the Tien Shan, and strike slip is the dominant mechanism under non-constrained and moderately constrained compression. The direction of slip is governed by the parallel component of convergence and by the fault pattern. In regions of strain shadow, slip occurs mostly on normal planes, and shallow earthquakes have strike-slip mechanisms. The crust in the collisional region is divided into systems of rigid and plastic blocks (rheological structure) and of fault blocks (fault-block structure). The two types of systems coexist and determine the generally similar but specifically different features of local mass transfer.
A B S T R A C TAccording to the classical approach to physical interpretation of multiple roots of the Christoffel equation in the theory of elastic wave propagation in anisotropic media, a sum of isonormal plane waves propagating in the direction of acoustic axes can have either arbitrary or circular polarization. The main question posed in this paper is as follows: can one apply conclusions drawn for plane waves to other phenomena, in particular, to waves generated by a point source? The paper proposes a new principle of physical interpretation of degeneracies stating that any assessment of the polarizations (and the group velocities) of waves propagating in anisotropic media is reasonable if there exists an experiment with a point source in which the assessment agrees with the general symmetry of the experiment. From the viewpoint of this interpretation, all degeneracies are considered on the wavefront. The inferences drawn from the performed analysis might appear surprising: in all considered cases of degeneracies (such as conic axes, tangent degeneracy on the symmetry axis of infinite order in transversely isotropic media, intersections of the slowness surfaces), ambiguity in determination of the polarization vectors either does not exist for any experiment or can be removed based on the symmetry of an experiment. A condition for convexity of the slowness surface of the fastest wave is formulated in this context.
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