Probabilistic Seismic-Hazard Assessment for Portugal

Abstract

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.

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

“…According to the studies performed until now, the LTV can produce an earthquake with magnitude Mw 6.0 or larger. However, some authors argue that the LTV seismic zone extends to the southern part of the Lisbon Metropolitan Area and that the earthquake of 11 November 1858, with epicenter offshore and close to the town of Setúbal, was also produced in this seismogenic zone (estimated magnitude Mw 7.1, Vilanova and Fonseca 2007). In spite of a probable overestimation of its magnitude (Stucchi et al 2013 present Mw 6.8±0.6) this event thus also contributes to the regional seismic hazard.…”

“…It produced important damages in the town of Lisbon (e.g., the main chapel of the Cathedral collapsed) and many people died (Moreira 1991a). The inferred maximum Modified Mercalli (MM) intensity was VII-VIII (Oliveira 1986) and magnitudes assigned to it range from Mb 6.0 (Martins and Mendes-Victor 1990; to Mw 6.7 (Vilanova and Fonseca 2007). However, this is a controversial event because there are not contemporary references (the existing references date from the XVI century onwards).…”

“…Magnitude Mb 7.1 (Martins and Mendes-Victor 2001), Mw 6.9 (Vilanova and Fonseca 2007) or Mw 6.0-6.6 (Baptista et al 2014) is assigned to it. Moreira (1991a) presented an isoseismal map for the Lower Tagus Valley region and Justo and Salwa (1998) presented a general isoseismal map for Portugal and adjacent Spanish region as far as Trujillo, suggesting a very low seismic attenuation in the eastern direction.…”

Seismic performance assessment of conventional steel and steel-concrete composite moment frames using CFST columns

“…Lagos), was considered in this study, both for design and seismic performance assessment purposes. Probabilistic Seismic Hazard Analysis (PSHA) was performed for the site in question, using the open source software OpenQuake (Pagani et al [13] and the seismic hazard model developed in SHARE (Woessner et al [14]), whilst also including additional hazard sources (Vilanova and Fonseca [15]) and employing the ground motion prediction equations from Atkinson and Boore [16] and Akkar and Bommer [17], with a weight of 70% and 30%, respectively (Silva et al [18]). Disaggregation of the seismic hazard (Bazurro and Cornell [19]) on magnitude, distance and  was performed.…”

Intensity-distance attenuation laws for the Portugal mainland using intensity data points

“…Usually, in regions of moderate tectonic and seismic activity, such as the study area, the resulting seismic hazard is computed using pre-existing attenuation laws, developed for similar areas and for similar ranges of magnitude. In Portugal, Vilanova & Fonseca (2007) computed the probabilistic seismic hazard using three attenuation models (included into a logic tree): Ambraseys et al (1996), Toro et al (1997) and Atkinson & Boore (1997), developed for target areas of Europe and Middle-East in one case and Central and Eastern North America on the other two.…”