Regional variation of coda Q in Northwest Caucasus

Abstract

Coda Q has been studied in the North-West Caucasus region using 267 earthquakes and 17 stations from the network of the Geophysical Survey, Russian Academy of Sciences. This is the first Qc study for the region using such a large dataset. The average frequency-dependent coda Q relationship is Qc=90±21∙f1.02±0.11. This value compares closely to other similar tectonic areas in the world where the same processing parameter had been used. Three specific zones, two in the Greater Caucasus tectonic area and one east of the Ciscaucasian trough area, were studied separately. There was a clear difference in Qc between the zones, particularly for the Ciscaucasian trough area which shows a higher Qc than the 2 other zones. However, the difference between the zones became smaller when using long lapse times tending to give a constant Qc as a function of lapse time. For longer lapse time it is assumed that a large part of the coda waves passes the mantle. The 2 zones in Greater Caucasus now had a similar Qc while the East zone in the Ciscaucasian trough still gave the highest values. We contribute this difference to differences in attenuation in the mantle under the two tectonic areas. In our area, there is then a clear difference in Qc for the 3 study areas in the crust but in the mantle the difference is mainly between the two tectonic zones.