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Historical earthquakes are of major importance in the analysis of seismic hazards, in particular for stable continental regions. In this article, we propose a methodology that uses seismic scenarios to provide constraints on the location of the seismic source of historical earthquakes. Our methodology involves generating seismic scenarios for the proposed seismic sources and comparing the results to the observed intensity field of the earthquake. To avoid the bias related to strongly heterogeneously distributed datasets, we focus on data points that are useful in discriminating between competing ruptures. These data are identified by the spatial patterns of residuals between seismic scenarios produced for each source. We apply this methodology to a test event--the 1999 Athens earthquake--for which both the magnitude and location are constrained by independent data, and to the 1909 Benavente earthquake, for which the magnitude is constrained by seismological studies, but the location is uncertain due to the very poor azimuthal coverage available. Within its application limits, the proposed methodology was capable of identifying the source of the Athens earthquake amongst different ruptures located few kilometers apart. The analysis performed for the 1909 Benavente earthquake suggests that the eastern strand of the lower Tagus Valley fault zone is the most likely seismic source for earthquake, amongst those proposed in the literature. [ABSTRACT FROM AUTHOR]

The inclusion of site-specific conditions is essential to adequately represent the seismic hazard and the seismic risk for a region. We acquired, gathered, and organized a near-surface shear-wave velocity database for Portugal and applied a three-step methodological approach for developing a VS30 site-condition map using extrapolation based on surface geology. The methodology includes (1) defining a preliminary set of geologically defined units, (2) calculating the probability distribution of logVS30 for each unit, and (3) merging the units according to the results of statistical tests. The final model comprises three geologically defined units characterized by logVS30 distributions that are statistically significantly different from each other: F1, igneous, metamorphic, and old sedimentary rocks; F2, Neogene and Pleistocene formations; and F3, Holocene formations. The site conditions for the F3 unit may be further refined using correlations with topographic slope based on the Shuttle Radar Topography Mission at 3 arcsec resolution (SRTM3) dataset. We analyzed the performance site-condition models based on correlations with exogenous data (topographic slope and surface-geology analogs). The results show that the residual distributions between logVS30 values measured and estimated from those proxies are strongly biased for some geological units, emphasizing the need for acquiring regional VS data. [ABSTRACT FROM AUTHOR]

In probabilistic seismic-hazard analysis (PSHA), seismic source zone (SSZ) models are widely used to account for the contribution to the hazard from earth-quakes not directly correlated with geological structures. Notwithstanding the impact of SSZ models in PSHA, the theoretical framework underlying SSZ models and the criteria used to delineate the SSZs are seldom explicitly stated and suitably docu-mented. In this paper, we propose a methodological framework to develop and document SSZ models, which includes (1) an assessment of the appropriate scale and degree of stationarity, (2) an assessment of seismicity catalog completeness-related issues, and (3) an evaluation and credibility ranking of physical criteria used to delineate the boundaries of the SSZs. We also emphasize the need for SSZ models to be supported by a comprehensive set of metadata documenting both the unique characteristics of each SSZ and the criteria used to delineate its boundaries. This procedure ensures that the uncertainties in the model can be properly addressed in the PSHA and that the model can be easily updated whenever new data are available. The pro-posed methodology is illustrated using the SSZ model developed for the Azores-West Iberian region in the context of the Seismic Hazard Harmonization in Europe project (project SHARE) and some of the most relevant SSZs are discussed in detail. [ABSTRACT FROM AUTHOR]

The lack of constraints regarding ground-motion propagation characteristics in western Iberia remains one of the major hindrances to accurate seismic-hazard and seismic-risk assessment in the region. The fact that western Iberia is geographically located near the confluence of oceanic crust, active continental crust, and stable continental crust further complicates the selection of appropriate ground-motion models. To address this question we compare, through visual inspection and residual analysis, the regional ground-motion data for moderate magnitude earthquakes with a set of six ground-motion prediction equations, some of which were developed for active tectonic regions and the others for stable continental regions. Although the available data pertain mostly to offshore moderate magnitude earthquakes (M 4.8-6.0) at large distances (100-400 km), the few records from an onshore M 4.0 earthquake and from the 1969 M 7.8 earthquake display similar trends. The results show that the performance of the models is period-dependent, with shorter spectral periods (0.01 s, 0.1 s, 0.2 s, and 0.5 s) being strongly underpredicted by models developed for shallow active tectonic regions. More interestingly, the performance of the models for stable continental regions seems to be magnitude-dependent, with response spectra amplitudes for magnitudes of engineering interest being accurately estimated by the models. The inspection of the residual distributions of intensity observations leads to similar conclusions for the offshore data, while highlighting the extent of epistemic uncertainty in predicting the ground motions for moderate magnitude earthquakes at short to moderate distances. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This section presents a response by Susana P. Vilanova et al to a commentary on their article "Lisbon 1755: A Case of Triggered Onshore Rupture?" by L. Matias et al that was published in a 2003 issue of the "Bulletin of the Seismological Society of America." It answers a question regarding the suitability of using maps drawn before and after the 1755 earthquake.