Crustal heterogeneity and velocity anisotropy from seismic studies in the USSR

Abstract

Summary. The analysis of data of seismic crustal studies in the USSR, obtained from waves propagating at different azimuths, reveals considerable horizontal and vertical inhomogeneity of the crust. Against this background it is difficult to predict what kind of velocity anisotropy can be expected in the continental crust. The rare cases of disagreement in velocities on intersecting profiles can be attributed both to anisotropy and to horizontal crustal inhomogeneity. There is a definite disagreement in layer velocities measured by reflected waves: fine layers in the crust and upper mantle have been found to have anomalously high velocities. The role of anisotropy in these events is not clear. The frequently observed splitting of S-wave with different polarization, however, positively implies anisotropy in the Earth's crust. Special studies of velocity anisotropy have not been conducted in the crystalline crust in the USSR. The papers by Nevsky (1977) are an exception, but deal only with the upper part of the basement. In the territory of the USSR a large number of deep seismic soundings (DSS) have established a dense network of reversed and overlapping travel-time curves. Continuous profiling, with three-component records and multichannel equipment distinguish the types of waves propagating in the crust in different directions. These data give a first approximation to the kind of horizontal inhomogeneity and velocity anisotropy that can be expected in the continental crust.