Probabilistic seismic hazard assessment (horizontal PGA) for Sweden, Finland and Denmark using different logic tree approaches

Recent methods of probabilistic seismic hazard assessment allow multiple values to be assigned to the various input parameters. Each combination of values (branch) gives an output hazard estimate. From all combinations (logic tree), percentile hazard estimates can be obtained. In the present application, values of the annual rate of horizontal peak ground acceleration (PGA) are derived for a grid of sites, spaced at 1 degree longitude by 0.5 degree latitude, covering Sweden, Finland and Denmark. The computation is based on two different sets of calculated moment magnitudes, M sub(W). Two different approaches of hazard assessment are used, one based on source regionalisation models selected from seismicity distribution and/or seismotectonic criteria, and the other independent of regionalisation. Each approach applies the same two attenuation functions. In the first approach, three regionalisation models with a total of 87 source regions are introduced, and completeness tables, seismicity rates and beta -values, five focal depths and four maximum expected magnitudes are assigned to each of two gross zones (land and sea areas). Seismicity rates and beta -values are also assigned to each source region. In the second approach, the seismicity rate is derived for spatial windows of the size 5 degree longitude by 2.5 degree latitude. Instead of different regionalisation models, two sets of data completeness criteria are applied to each gross zone. From each approach, a map of median hazard values for 90% probability of non-exceedence in 50 years is plotted. For the sites with the highest median hazard values at this probability level, full fractile hazard curves are given. The highest calculated horizontal peak ground acceleration from each approach and for 90% probability of non-exceedence in 50, 100, 500 and 1000 years, corresponding to return periods of 475, 950, 4745 and 9490 years, are 0.3/0.3 (regionalisation approach at 17.0 degree E, 68.0 degree N/non-regionalisation approach at 14.0 degree E, 64.5 degree N), 0.5/0.5, 1.1/1.3 and 1.6/1.9 m/s super(2), respectively. There are some dissimilarities in the geographical distribution of the hazard obtained from the two approaches.