Heterogeneities of the Field of S-Wave Attenuation in the Lithosphere of the Caucasus and Their Relationship with Seismicity

Аннотация

We have mapped a short-period S-wave attenuation field of in the lithosphere of the Caucasus. We have used a method based on the analysis of the ratio of the maximum amplitudes for S_n- and P_n-waves. We have processed more than 200 earthquakes records, recorded by the Kislovodsk station (KIV) at distances of ~200–750 km. It is shown that, in general, the highest attenuation of shear waves is observed in the western part of the region. It is established that increased and intermediate attenuation takes place for paths simultaneously crossing rupture zones of the 1988 Spitak (M_w = 6.8) and 1991 Racha (M_w = 7.0) earthquakes. At the same time, the source zone of the 1970 Dagestan earthquake with M_w = 6.8 is characterized by mainly lower attenuation. These data are consistent with an earlier conclusion that deep-seated fluids have been ascending into the Earth’s crust from the uppermost mantle for a few decades after large seismic events, and this process leads to a decrease in attenuation of S_n-waves. A high attenuation zone has been identified in the western part of the region, where, according to instrumental and historical data, no seismic events with M ≥ 6.0 have been recorded. It is assumed that processes of a large earthquake preparation may be taking place here.