Seismic Zonation for Portuguese National Annex of Eurocode 8

Abstract eng:
The development of the Portuguese National Annexes of Eurocode 8 required the definition of a seismic zonation for Portugal. The goal of this paper is to present the applied methodology to perform the Probabilistic Seismic Hazard Analysis (PSHA) and to obtain seismic zonation for Portugal Mainland, Azores and Madeira Archipelagos. Similarly to the Portuguese code presently enforce (RSA, 1983), two scenarios were considered in PSHA computation: (i) a scenario labeled seismic action 1, characterizing earthquakes with their epicenters mainly offshore and (ii) a scenario labeled seismic action 2, referring to events with their epicenters mainly inland. The model of gross-source zones and the parameters defining seismic occurrence process in each source zone, like the Poissonian process and the exponential distribution of magnitudes, was adapted from Sousa (1996). Two different spectral attenuation laws were applied, consistent with the two above mentioned seismic scenarios, namely: (i) a recent regional attenuation law derived for Portuguese seismotectonic environment, based on a finite fault non-stationary stochastic seismological model calibrated for the region (Carvalho et al., 2008), valid for seismic action 1 and (ii) the Ambraseys et al. (1996) model, valid for seismic action 2. The PSHA was performed for all Portuguese counties and for the 475 years return period. The correspondent hazard map was used as the basis for seismic zonation. Portuguese counties were sorted according to their Housner intensity, which was used as a criterion to classify each county in a pre-defined number of seismic zones.

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