LATERAL INHOMOGENEITIES OF THE UPPER MANTLE IN THE KOPETDAG-CASPIAN SEA REGION AND THEIR RELATIONSHIP TO TECTONICS AND SEISMICITY

Abstract

A method allowing detailed mapping of lateral inhomogeneities of the upper mantle in mobile regions of various types has recently been developed. The method is based on the analysis of the absorption patterns of short-period transverse waves, as indicated by the recordings of distant crustal earthquakes at intermediate distances. To the epicenters at such distances, in regions with relatively strong absorption in the crust and upper mantle, the traces prove to contain no short-period Lg group, but one or two blurred wave groups, generated by single and multiple reflections of transverse waves from numerous weak boundaries in the crust and upper mantle, are identifiable. The absorption field is mapped in terms of the ratio of the maximum amplitudes in the first wave group to that of the p-wave. These waves travel similar paths, and their ratio clearly reveals lateral inhomogeneities of the S-wave absorption field in the upper mantle near the epicenter and the station. In the practical applications of the method, a correction is made for the epicentral distance.