Abstract
The report contains a review of the current state of knowledge on Swedish paleoseismic events, as well as information on the Swedish earthquake catalogue updated to early autumn 2016 and the revised Nordic earthquake catalogue (Fencat). It includes a section on the possibilities to assess spatial and temporal variations in seismicity, given the sparse early catalogues and the low earthquake activity rate in Fennoscandia. The updated earthquake data is compared to the SKI Technical Report 92:3 (SKI 92:3) data and it comments on how this may affect the ground motion spectra and how this work could continue. SKI 92:3 contains envelope ground response spectra especially developed for Sweden in a joint research project between the Swedish nuclear power inspectorate (SKI) and the Swedish licensees. It was not within the scope of this study to perform a quantitative assessment of how the existing spectra are affected by the updated data set.
Background
The number and quality of instrumentally recorded earthquakes in Fennoscandia have significantly increased since the publication of SKI 92:3, mainly depending on the expansion and modernization of the Nordic seismic networks. The Fencat catalogue accordingly contains many more events than when data were extracted for SKI 92:3. In addition, the historic part of the catalogue has been updated with new estimates of location and magnitude of the larger events, and cleared of a number of frost and blast related events. The large number of smaller earthquakes recorded in the last two decades give a good indication of current spatial variability in the earthquake rate and it is also making it possible to outline fault systems in Sweden that are currently seismically active, although with a low rate.
Therefore, the Swedish Radiation Safety Authority (SSM) has commissioned Seismology group at Uppsala University to carry out the present study with the objective as it set out below. SSM especially emphasized the need to include information on paleoseismic events, which are not included in the SKI 92:3 data.
Objectives of the project
The objective was to review the current status of paleoseismology and historical earthquakes in Fennoscandia and to update and evaluate the earthquake data used in SKI 92:3, as well as to qualitatively assess how the new results would affect the envelope ground response spectra in SKI 92:3.
Results
There are twelve confirmed post- or endglacial fault scarps in Sweden where glacial landforms have been displaced. For many of these there is, however, lack of stratigraphic information which would help constrain both the timing of the events and whether they formed as a result of a single rupture or not. It will also provide information critical to magnitude calculations. There are, however, no indications of observations of multiple large ruptures on the endglacial faults.
Swedish paleoseismic events associated with a fault scarp, as well as confirmed events from Norway and Finland are included in the final inventory. The report presents an updated table of events with location, timing and magnitude, as needed by a seismic hazard assessment.
Based on comparisons of different time slices of Fencat, it could be find that on the large scale the spatial variation in seismicity in Sweden has been relatively stable during the last century.
The results in SKI 92:3 are not expected to be affected by the paleoseismic data, if analysed with the SKI 92:3 methodology.
Occurrence rates estimated with only the recent Swedish earthquake data or the Fencat data indicate that the SKI 92:3 estimate may be conservative for earthquakes of magnitude 5 and smaller but that it may underestimate the rate of large events.