Earthquakes and seismic hazard for Norway and Svalbard

Abstract We document a complete seismic hazard study for mainland Norway and the Svalbard archipelago. The study is based on a Probabilistic Seismic Hazard Analysis (PSHA) method, and for the first time a new earthquake catalogue is presented publicly that covers Norway, Svalbard and the adjacent offshore regions. The catalogue is developed from an extensive analysis of historical earthquakes combined with more recent instrumental data with 33,864 reports between 1497 through 2018, and with magnitudes up to Mw 6.7. With this catalogue seismic hazard is computed for 10% exceedance in 475 years through a logic tree computation with 12 branches: two area-zonations, one zonation free branch and four GMPEs. These 12 branches were defined with the aim to reduce the model bias, i.e., to centre the model, and to capture the epistemic uncertainty of the results. While the conventional Vs30 reference velocity is usually around 800 m/s we have targeted a reference velocity of 1200 m/s, based on extensive documentation of Norwegian rock velocities. This has significant bearing on the calculated hazard and provides for results that better reflect the bedrock conditions in Norway. As a result of this, the predicted shaking intensities are lower than the values previously reported in the (1998) national building code. In the Supplementary Information we have provided a brief overview of the seismotectonic setting, some tests that further demonstrate the uncertainty in our hazard estimates, a model for H/V ground-motion response spectra, examples of the sensitivity to the bedrock reference velocity and a comparison between the present study and the ESHM20 results.