On the Problem of Accounting for Paleoearthquakes When Evaluating the Seismic Hazard of Fennoscandia

Resumo

Earthquakes of magnitude M ≥ 6.5 have been unknown in Fennoscandia for almost 900 years of historical data, but a sharp outburst of postglacial seismic activity with earthquake magnitude up to M ≥ 8 revealed by paleoseismogeological studies occurred there in 11 000–9000 BP. This outburst sharply decreased in time to the relatively low modern seismic activity almost exponentially. Strong paleoearthquakes were not randomly distributed over the timeline, but formed a group, or a cluster, confined to the maximum phase of deglaciation. This indicates a sharp nonstationary mode of paleoseismicity. On this basis, a conclusion about the fundamental difference between the geodynamic regimes of paleo- and ordinary modern seismicity has been drawn. An analysis of the historical seismicity of Fennoscandia over the past ~900 years has shown that the magnitude of the strongest known earthquake, in contrast to paleoearthquakes, did not exceed 6.5. It is logical to assume that the occurrence of earthquakes of intensity 9–10 on the territory of Fennoscandia is very unlikely now or in the near future. Therefore, when assessing seismic hazard and the environs of critical engineering facilities, including nuclear power plants, it is hardly necessary to take into account paleoearthquakes. One exception may be assessing the seismic hazard for disposal sites of highly toxic and nuclear waste, the biological hazard period of which can reach 100 000 years or more, which is comparable with glaciation periods.