Relationship between Observed Seismicity and Water Level Fluctuations in Polyphyto Dam Area (North Greece)

The spatial-temporal seismicity in Aliakmona river area is presented for the period 1974-2010 (NW Greece). This study area, which was classified as low seismicity, presents a particular interest due to the unexpected strong earthquake of Ms=6.6 which occurred between the cities of Kozani–Grevena, at a distance of 18 km from the southern edge of Polyphyto lake in 1995. According to [1], seismic hazard changes have been identified SE of the Polyphyto dam after the impoundment of the lakes. In this study, we examined the possible correlation between the impoundment and the water level fluctuations due to the three artificial lakes which are established in the region and the observed seismicity using data from the catalogues of the National Observatory of Athens (NOA) (see details for seismological data on Mc, RMS, etc. in [2,3]). For the latter purpose, spatial and temporal distribution maps were developed as well as correlation diagrams between water level variations in respect to monthly seismicity for distances 10 Km, 20 Km, 30 Km, 40 Km and 50 Km around the artificial lakes using the ESRI ArcMap 10 software. A remarkable change is observed in seismic activity in the vicinity of reservoirs for the period commencing 10 years prior to the first filling in comparison with the period 10 years after. This increased seismicity is correlated to the daily rate of water level fluctuations (dh/dt) for the period 1984-2010 reveals the presence of mainly shallow seismic activity with focal depths of 0-5 km in a high percentage (67%). This protracted seismicity which is mainly located in the SW area of Polyphyto lake, has characteristics of the second type of induced seismicity according to Talwani [4], and seems to be controlled by the water level fluctuations of Polyphyto reservoir due to the mechanism of pore pressure diffusion.