Relationship between seismicity and water level in the Enguri high dam area (Georgia) using the singular spectrum analysis

The declustered seismic catalog from 1965 to 2010 around the Enguri high dam reservoir in western Georgia was analyzed using the singular spectrum analysis (SSA) technique in order to investigate the relationship of local seismicity with the reservoir water variations. In particular, the seismic activity was analyzed in two periods: a "reference" period, from 1965 to 1970, before the start of dam building in 1971; and an "active" period, from 1978 to 2010, in which the influence of the reservoir was significantly effective on the seismic activity (since the first flooding of the dam occurred in 1978). The SSA was applied to both the monthly number of earthquakes and the time series of the monthly mean of the water level. The first four reconstructed components explained most of the total variance in both seismicity and water level. Clear signatures of the annual oscillation linked with the loading/unloading operations of the dam are present in the periodogram of the second and the third reconstructed components of the seismic activity during the "active" period. Such annual cycle is absent in the periodogram of the reconstructed components of the seismic activity during the "reference" period. This is a clear indication of the reservoir-induced character of the seismicity around the Enguri dam.