Susana P. Vilanova

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 cat- alog with a uniform magnitude scale, the instrumental magnitudes were converted to M through empirical relations. Two seismic zonations were considered, each in- cluding 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 correspond- ing 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 pre- vious studies using intensities highlighted the southwest offshore contribution. Fur- ther work on ground-motion attenuation in western Iberia is necessary to improve the seismic-hazard assessment.

Lisbon 1755: A Case of Triggered Onshore Rupture?

It has been widely recognized, both in classical and in modern studies, that the Lisbon earthquake of 1755 was a multiple event, composed of three shocks separated by a few minutes (see, e.g., Reid, 1914). Attempts to constrain the location of the source have led to a diversity of proposals, reflecting apparent contradictions in the data. The tsunami and damage along the south and southwest Iberian coast and in Morocco favor an offshore source, whereas the presence of an additional zone of strong shaking in the Lower Tagus Valley (LTV), near Lisbon, favors a more northerly location. By combining the contemporary accounts with intensity data from other earthquakes, we favor a compound source with a large distance between the faults. We propose that, although the mainshock was offshore, the resulting stress changes induced the rupture of the LTV fault, at a distance on the order of 350 km (but subject to large uncertainty in the offshore location), a few minutes after the mainshock. We favor this model, rather than site effects causing high intensities in the Lisbon area, because the highest intensities show a negative correlation to soft soil. Several other phenomena described in the eyewitness accounts can also be explained by the local rupture now proposed, such as a tsunamilike wave in the Tagus River, ground deformation affecting the course of the Tagus River, and the spatial pattern of damaging aftershocks. Recognition of this “missing” episode of rupture on the LTV fault significantly changes the hazard estimate for the Lisbon area.

Palaeoseismology of the Vilariça Segment of the Manteigas-Bragança Fault in northeastern Portugal

Abstract The Manteigas-Bragança fault is a major, 250-km-long, NNE-striking, sinistral strike-slip structure in northern Portugal. This fault has no historical seismicity for large earthquakes, although it may have generated moderate (M5+) earthquakes in 1751 and 1858. Evidence of continued left horizontal displacement is shown by the presence of Cenozoic pull-apart basins as well as late Quaternary stream deflections. To investigate its recent slip history, a number of trenches were excavated at three sites along the Vilariça segment, north and south of the Douro River. At one site at Vale Meão winery, the occurrence of at least two and probably three events in the past 14.5 ka was determined, suggesting an average return period of about 5–7 ka. All three events appear to have occurred as a cluster in the interval between 14.5 and 11 ka, or shortly thereafter, suggesting a return period of less than 2 ka between events within the cluster. In the same area, a small offset rill suggests 2–2.5 m of slip in the most recent event and about 6.1 m after incision below a c. 16 ka alluvial fill event along the Douro River. At another site along the Vilariça River alluvial plain, NE of the Vale Meão site, several trenches were excavated in late Pleistocene and Holocene alluvium, and exposed the fault displacing channel deposits dated to between 18 and 23 ka. In a succession of closely spaced parallel cuts and trenches, the channel riser was traced into and across the fault to resolve c. 6.5 m of displacement after 18 ka and c. 9 m of slip after c. 23 ka. These observations yield a slip rate of 0.3–0.5 mm/a, which is consistent with earlier estimates. Combining the information on timing at Vale Meão winery and displacement at Vilariça argues for earthquakes in the M7+ range, with coseismic displacements of 2–3 m. This demonstrates that there are potential seismic sources in Portugal that are not associated with the 1755 Lisbon earthquake or the Tagus Valley, and, although rare, large events on the Vilariça fault could be quite destructive for the region. This work provides an analogue for the study of active faulting in intracontinental settings and supports the view that earthquakes within intracontinental settings tend to cluster in time. In addition, this study highlights the usefulness and application of multiple field, remote sensing and geochronological techniques for seismic hazard mitigation.

Investigations unveil Holocene thrusting for onshore Portugal

A current paleoseismological investigation of the Lower Tagus Valley (LTV), near Lisbon, Portugal, brought to light unprecedented information about the seismotectonics of this area of important onshore intraplate seismicity Through a combination of geomorphic, geological, and geophysical tools, a site with evidence for recent deformation was tracked down and selected for detailed study. Upon trenching, a very clear exposure of a thrust fault can be observed. Its last episode of deformation was constrained with carbon 14 to postdate A.D. 365.

Incorporating Descriptive Metadata into Seismic Source Zone Models for Seismic-Hazard Assessment: A Case Study of the Azores-West Iberian Region

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 docu- ment 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 delin- eate 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 character- istics 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. Online Material: Tables describing characteristics and boundaries of the seismic source zones.

Reply [to “Comment on ‘Investigations unveil Holocene thrusting for onshore Portugal’”]

In Fonseca et al. [2000]we put forward the first results of the paleoseismological investigations underway in the Lower Tagus Valley (LTV), near the Portuguese capital; a region of well-documented historical intraplate earthquakes of magnitude M6.5–M7. Our study focused on the right bank of the Tagus River from Lisbon to Santarem, where a persistent NNE-SSW scarp can be followed in the landscape morphology. We identified deformation of recent deposits in trenches on that scarp, and interpreted it as a thrust geometry with significant left-lateral component. We dated the deformed layers as Holocene using archeological criteria, as well as C14, and measured a minimum displacement of ∼3 m over the last 4000 years. These observations were correlated with the intensity distribution of the destructive 1531 Vila Franca earthquake [Justo and Salwa, 1998], and we concluded that the structure now discovered was the surface rupture due to that earthquake.