Portal de Periódicos da CAPES

Seismic deformation of varved sediments in southern Fennoscandia at 7400 cal BP

2018; Elsevier BV; Volume: 744; Linguagem: Inglês

10.1016/j.tecto.2018.06.015

1879-3266

Antti Ojala, Jussi Mattila, Joonas J. Virtasalo, Jukka Kuva, Tomi P. Luoto,

Geological formations and processes

Soft-sediment deformation structures are preserved in clastic-biogenic varves of the Lake Nurmijärvi sequence from central-southern Finland. Deformation structures include centimeter-scale folding and faulting structures and larger convolute bedding, which are separated from the overlying undisturbed varved section by a turbidite dated at 7400 cal BP. The soft-sediment deformation provides evidence for postglacial seismic activity ca. 3500 years after the deglaciation of the study area. By screening the sizes of potential source faults for the event, the maximum earthquake moment magnitudes associated with the deformation structures were constrained to MW ≈ 5.1–6.9 with realistic 40% rupture scenarios, or up to MW ≈ 5.1–7.7 if more conservative scenarios are considered. The results of the study thus indicate a relatively high-magnitude postglacial seismic event in southern Finland, which adds to the evidence that the postglacial seismic activity was not just restricted to northern Fennoscandia, but was a more widespread phenomenon. The age of the Lake Nurmijärvi seismic event further demonstrates that high-magnitude seismic activity was not restricted to a relatively short period after deglaciation, but the period of high-magnitude postglacial seismic activity was much longer than previously thought. By assessing the orientations of faults that are presently active and using available focal mechanism data, our results suggest that strike-slip faulting may be a more important mechanism for the generation of both present-day and postglacial earthquakes in southern Finland than reverse faulting, which should further be accounted for in the assessment of long-term seismic hazards.