HIGH PALEOSEISMIC ACTIVITY DURING DEGLACIATION DATING AND RECORDING OF 50 EVENTS IN SWEDEN

Sweden, today an area of low to moderately low seismic activity, was, in Late Glacial time, characterised by a very high paleoseismic activity; both in seismic magnitude and in seismic frequency. This period, or mode, of exceptionally high seismic activity is recorded by multiple criteria; primary faults, bedrock deformation, sedimentary deformations, rock and sediment slides, liquefaction, sorting by shaking, tsunami waves, a number of geomorphic expressions, paleo-magnetic disordering as well as ordering. Most of the events are dated by means of varve chronology providing a time resolution as to a single year. The sharp dating control allows reconstruction of the spatial distribution of individual liquefaction events, by this providing new means of estimating magnitudes. By applying multiple criteria, it was possible to establish a paleoseismic catalogue for Sweden which, for the moment, includes 50 separate events. When plotted against time, more than 50% of the events fall within the period 9000–11,000 BP when the rate of uplift was in the order of some to several 10s of cm per year or 0.4–1.4 mm per day (quite remarkable rates, recently confirmed by observational data). A causal correlation between high seismicity and peak rates of uplift is therefore advocated. The same seems to apply for the entire Fennoscandian region. Within the Mid to Late Holocene, there are a number of significant events, too. In three areas, it has been possible to establish the recurrence time and pattern of paleoseismic events; viz Hudiksvall (7 events), Mälardalen (14 events) and the Swedish West Coast (12 events). The practical implications of a high deglacial seismic mode, is that all efforts of making long-term safety estimates based solely on available instrumental records will totally fail to provide any meaningful information. We are in Fennoscandia facing a major paradigm shift; from a concept of general stability to 21st century concept of an exceptionally high seismic activity during the phase of deglaciation.