Late quaternary dislocations and seismotectonics of the Racha earthquake source, the Greater Caucasus.

The results of studying traces of ancient seismic catastrophes in epicentral zone of the 1991 Racha earthquake are set forth. The source of this earthquake is noteworthy, because it reflects the main tendency in the geological evolution of the region, i.e., thrusting of the southern slope of the Greater Caucasus over the basement of the Transcaucasus Median Mass. It has been ascertained that two large-scale seismic events were related to this source in the Holocene. The approximate duration of the middle Holocene event was 2300 yr (7300-5000 yr ago). The late Holocene event, including the Racha earthquake in 1991, lasted ∼1600 yr. The role of the Racha earthquake in the large-scale seismic cycle is discussed. A surprising circumstance was the absence of datings in the time interval of 2400-9500 yr ago. The lack of these datings could have been caused by the change in climate and absence of paleosoils or by deceleration of seismic activity. The results also allowed us to substantiate seismotectonic segmentation of the southern slope of the Greater Caucasus in the area under consideration. [ABSTRACT FROM AUTHOR]