Probabilistic seismic hazard assessment of Switzerland: best estimates and uncertainties

We present the results of a new genera tion of probabilistic seismic hazard assessment for Switzerland. This study replaces the previous intensity-based generation of national hazard maps of 1978. Based on a revised moment-magnitude earthquake catalog for Switzerland and the surrounding regions, covering the period 1300-2003, sets of recurrence parameters (a and b values, M (max) ) are estimated. Information on active faulting in Switzerland is too sparse to be used as source model. We develop instead two models of areal sources. The first oriented towards capturing historical and instrumental seismicity, the second guided largely by tectonic principles and express ing the alterative view that seismicity is less stationary and thus future activity may occur in previously quiet regions. To estimate three alterna tive a and b value sets and their relative weighting, we introduce a novel approach based on the modified Akaike information criterion, which allows us to decide when the data in a zone deserves to be fitted with a zone-specific b value. From these input parameters, we simulate synthetic earthquake catalogs of one-million-year duration down to magnitude 4.0, which also reflect the difference in depth distribution between the Alpine Foreland and the Alps. Using a specific predictive spectral ground motion model for Switzerland, we estimate expected ground motions in units of the 5% damped acceleration response spectrum at frequencies of 0.5-10 Hz for all of Switzerland, referenced to rock sites with an estimated shear wave velocity of 1,500 m/s(2) in the upper 30 m. The highest hazard is found in the Wallis, in the Basel region, in Graubunden and along the Alpine front, with maximum spectral accelerations at 5 Hz frequency reaching 150 cm/s(2) for a return period of 475 years and 720 cm/s(2) for 10,000 years.