An acoustic view into Holocene palaeoseismicity offshore southwestern Finland, Baltic Sea

The Baltic Sea basin consists of Palaeoproterozoic crystalline bedrock, which is to the greatest extent covered by younger Proterozoic and Phanerozoic sedimentary bedrock. The basin contains several ancient deep-seated tectonic lineaments and fracture zones that divide the seabed into blocks. High-resolution, low frequency, echo-sounding was used to study the Holocene submarine sedimentary structures and palaeoseismic activity in the northern Baltic Sea with the aim of tidying previous, more site-specific results (at Olkiluoto) into a larger regional context. Echo-sounding profiles of Holocene submarine sediments show slides and slumps, normal faults, debris flows and turbidite-type structures. The profiles show also pockmarks and other structures related to gas or groundwater seepages. Some of the observed structures are suggested to be associated with palaeoseismic activity  10,700 cal yr BP (∼ 9500 radiocarbon years BP) in the northern Baltic Sea. Post-glacial fault activation and possible gas and/or fluid discharges under high hydraulic heads have relevance in the safety assessment of a planned spent nuclear fuel repository in the region. Evidence of post-glacial faulting in the study area is found only along old bedrock fracture zones indicating reactivation of the old fracture zones. Since no younger or repeated traces of possible seismic events were found, it corroborates the suggestion that the major seismic activity occurred within a short time during and after the last deglaciation.