Horizontal inhomogeneities in the upper mantle of the carpathians and caucasus

Abstract

The travel-times of waves coming from distant earthquakes, recorded by seismological stations in the Carpathians and the Caucasus were used to construct a model of horizontal inhomogeneities in the upper layer of the mantle in these regions. In comparison with the adjoining platform, the East Carpathians are characterized by higher velocity, the South Carpathians and the Carpathian foredeep by lower velocity, while the West Carpathians have a velocity similar to that of the platform. The Vrinci earthquakes originate in the high-velocity block of the East Carpathians, at its boundary with the low-velocity block of the South Carpathians. The Caucasian territory can be divided into several different mantle blocks. The western part of the Great Caucasus has a higher velocity. A submeridional belt of low velocities, extending west of the line Piatigorsk—Tiflis, has been determined; the belt passes through the Stavropol part of the Great Caucasus, the Transcaucasian central massif and part of the Little Caucasus. More to the east there extends a parallel belt of greater velocities, which also intersects a series of different structures. In the East Caucasus, a low-velocity block has been established in the Caucasian foredeep and the Great Caucasus regions; the boundary between this block and a high-velocity block lying west of it passes through the Caspian Sea. The mantle earthquakes of 1935 occurring NE of Derbent seem to be connected with this boundary. The low-velocity region of the mantle also exists in the Caspian Sea, in the vicinity of the Apsheron Peninsula. A relationship between the determined velocity variations and other geophysical fields has been discussed; some known gravity anomalies in the Caucasus, interpreted as being connected with the earth's crust, are believed to be due to the vertical inhomogeneities of density in the upper mantle.