Probabilistic seismic-hazard assessment for Portugal

The probabilistic seismic hazard of Portugal is analyzed with a logic tree approach. A critical part of the work was the review of the seismic catalog and the moment magnitude (M) estimation for historical earthquakes. To produce a catalog with a uniform magnitude scale, the instrumental magnitudes were converted to M through empirical relations. Two seismic zonations were considered, each including two broad tectonic zones and a set of smaller seismicity zones. Catalog completeness and the b-value for the truncated exponential recurrence model were calculated for the broad sources defined by tectonic criteria. The smaller seismicity sources were used to calculate the a-value using the fixed b-value of the corresponding tectonic zone. This approach allows for a larger amount of data to estimate the most critical parameters by statistical methods, without compromising the spatial detail of hazard results. Three published attenuation relations were used in the logic tree, with weights that were based on tectonic considerations and on the comparison with macroseismic data converted to horizontal peak ground acceleration (PGA). The Ambraseys et al. (1996) attenuation model for PGA, used by most previous hazard studies of the region, seems to underestimate considerably the ground motion for mainland Portugal. A total of 96 hazard curves were calculated with SEISRISK III for each point of the map. The resulting mean hazard map for 10% exceedence probability in 50 years displays PGA values that range from 0.05g to 0.20g. These mean values are slightly higher than in previous PGA studies. The hazard patterns obtained display a maximum related to intraplate onshore seismicity, whereas previous studies using intensities highlighted the southwest offshore contribution. Further work on ground-motion attenuation in western Iberia is necessary to improve the seismic-hazard assessment.