A review of the seismotectonics and some considerations on the seismic hazard of the KrŢko NPP area (SE Slovenia)

This paper gives short comments on the seismotectonic, seismological, and seismic hazard characteristics of the area of the nuclear power plant (NPP) of KrŢko (Slovenia). The plant was designed in the second half of the 1970s with no related probabilistic seismic hazard studies. The design was based on Regulatory Guides of 1973 with response spectra anchored to the peak ground acceleration of 0.30 g on the free field. The construction of a new NPP in the area is being taken into consideration by international investors. The KrŢko NPP is located inside a seismically active area, in which low-to-medium magnitude earthquakes have occurred in the last centuries. The greatest intensity felt near the NPP site was VIII for a local MW 5.7 - 6.2 earthquake in 1917, and the strongest event in the region was the M ≈ 6.3 Zagreb, 1880 event, whose epicentre was about 65 km from the NPP. The plant is close to some faults with unresolved neotectonic activity in the post-glacial epoch (18,000 years). Some stress test results from the 1990s would have suggested that 0.3 g were adopted at -20 m depth, corresponding to an effective value of 0.6 g at zero depth; this aspect is also commented. Finally, the difficult problem of estimating the maximum credible magnitude, Mmax, for very long return periods is addressed, based on the geological evidence provided by Geomatrix Consultants Inc. in 2004. From an apparent rupture length of 40 km of the Orlica fault, an Mmax of at least 7.2 could be hypothesized (1 st. dev. included). This hypothesis has anyway a low reliability, because the extent of the spatial continuity of the Orlica fault is still debated.