Lake sediment evidence of paleoseismicity: Timing and spatial occurrence of late- and postglacial earthquakes in Finland

Abstract

Sediment profiles in six lake basins located near the late- and postglacial fault surface ruptures in northern Finland were investigated for turbidites and soft-sediment deformation structures that could reveal information on the timing and spatial occurrence of paleoearthquakes in the area. We used geophysical sub-bottom profiling and lithological logging of sediment cores combined with radiocarbon and paleomagnetic dating. In three lakes, Immeljärvi, Aakenus and Laukkujärvi, sediment records provided evidence of postglacial earthquakes or landslides, whereas in three others (lakes Pyhäjärvi, Kallojärvi and Syväjärvi) no disturbance structures were observed in their sequences that cover roughly the Holocene period. Combined with previous data on paleoseismicity in Finland, the results enhance the non-stationary hypothesis of seismicity throughout the Late Weichselian and the Holocene with three general maxima that cluster around the deglaciation (12.0–10.0 kyr BP), the mid-Holocene (5.0–7.0 kyr BP) and the Late Holocene (1.5–3.0 kyr BP). A strong asymmetry of the offset profiles of the Finnish postglacial fault complexes indicate that the general rupture propagation was from SW to NE. The present data also indicates that the oldest and highest magnitude events (MW ≈ 7 or higher) generally took place in the SW sector of the Finnish PGF complexes during or soon after the retreat of the continental ice sheet. The ruptures then propagated towards the NE sector of PGF complexes in a cascading fashion and were generally characterized by lower magnitude events (MW ≈ 6) during the Holocene.