Regional variation of coda Q in Northwest Caucasus

Abstract

Coda Q has been studied in the Northwest Caucasus region using 267 earthquakes and 17 stations from the network of the Geophysical Survey, Russian Academy of Sciences. This is the first Q_c study for the region using such a large dataset. The average frequency-dependent coda Q relationship is \(Q_c\;=\;90\;\pm\;21\cdot f^{1.02\pm0.11}\) . 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 Q_c between the zones, particularly for the Ciscaucasian trough area which shows a higher Q_c than the 2 other zones. However, the difference between the zones became smaller when using long lapse times tending to give a constant Q_c as a function of lapse time. For a 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 Q_c 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 Q_c for the 3 study areas in the crust but in the mantle the difference is mainly between the two tectonic zones.