Minor seismic hazard changes in the broader area of Pournari artificial lake after the first filling (W. Greece)

Abstract The broader area of Pournari dam (Western Greece) is investigated regarding the potential seismic hazard change after its impoundment in the early 1980’s. During this impoundment, an increase in local earthquake activity with certain features of induced seismicity was observed. Within the third and fourth month after the first filling, two moderate seismic events occurred, on March 10, 1981 (ML=5.6) and on April 10, 1981 (ML=4.7), with focal depths 13 km and 10 km, respectively, indicating migration of seismicity to shallower depths. The latter is considered as mechanical response change of background from undrained to the drained response (flysch formation). We investigate the potential local seismic hazard and the induced seismicity changes due to the establishment of the reservoir using three time-windows. The first one covers the period up to 1980, before the impoundment, the second starts immediately after the impoundment (1981-2009), while the third is focused on the whole instrumental period 1900-2009. For these time-windows, seismic hazard was initially assessed using the extreme values method. The traditional Cornell-McGuire approach was following applied for the whole instrumental period as a validation test, regarding the reliability of the results obtained by the extreme values method. The results reveal a small decrease of the estimated values of Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV) and maximum expected magnitude (Mmax). The results are compared to the seismic hazard study case of Polyphyto dam (NW Greece), located on a different seismotectonic regime.