João Cabral

Stress pattern in Portugal mainland and the adjacent Atlantic region, West Iberia

The Portuguese mainland territory is located close to the Azores-Gibraltar plate boundary, in a tectonic setting responsible for significant neotectonic and seismic activities. However, few data concerning the present regional lithospheric stress field were available, as testified by recently published maps of stress indicators for the Europe and Mediterranean regions. One of the authors already presented a synthesis on this subject [Cabral, 1993], where geological and geophysical stress indicators were considered. In this paper we introduce new information, mainly a considerable amount of borehole breakout data. The updated data set comprises 32 reliable stress indicators showing a mean azimuth of 145° (standard deviation 21°) for the maximum horizontal stress direction (SHmax). On the average, the geological data are rotated clockwise and the focal mechanism data deviated anticlockwise to that azimuth, while the borehole elongation results are consistent with the mean SHmax trend. These differences in stress trend suggest a regional progressive rotation of the SHmax direction from NNW-SSE to WNW-ESE since the upper Pliocene. To estimate stress trajectories, new and published stress indicators in the adjacent Atlantic area and northern Africa were also investigated, showing a very uniform NW-SE SHmax trend in west Iberia. A high level of horizontal compressive stress acting oblique to the western Portuguese continental margin is inferred and interpreted in view of a proposed regional geodynamical model, of activation of this passive margin, with the nucleation of a subduction zone in the Atlantic SW of Iberia, at the Gorringe submarine bank, which is propagating northward along the base of the continental slope, at the transition between thinned and normal continental crust.

The Azambuja fault: An active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal)

The Azambuja fault is a NNE trending structure located 50 km NE of Lisbon, in an area of important historical seismicity. It is sited in the Lower Tagus Basin, a compressive foredeep basin related to tectonic inversion of the Mesozoic Lusitanian Basin in the Miocene. The fault is evident in commercial seismic reflection data, where it shows steep thrust geometry downthrowing the Cenozoic sediments to the east. It has also a clear morphological signature, presenting a NNE-SSW trending, east facing, 15 km long scarp, reaching a maximum height of 80 m. The fault scarp is the geomorphic appearance of a flexure expressed as a zone of distributed deformation, where Miocene and Pliocene sediments are tilted eastwards and are cut by steeply dipping meso-scale faults presenting reverse and normal offsets, with a net downthrow to the east. This pattern at the surface is compatible with a steep fault in the basement that tilts and branches through the overlying Cenozoic sedimentary cover. In order to constrain the neotectonic activity of this structure, detailed geological studies were conducted. Morphotectonics was studied through aerial photo interpretation, analysis of topographic maps and digital mapping. Those studies indicate Quaternary slip on the fault in the ranges of 0.05–0.06 mm per year. Seismogenic behaviour was assumed for the Azambuja fault based on the evidence of Quaternary tectonic activity and its location in an area of significant historical seismicity. Mw 6.4–6.7 maximum earthquakes, with recurrence intervals of 10000–25000 years, were estimated based upon the displaced morphological references, cumulative offsets and fault length.

Interpretation of recent structures in an area of cryptokarst evolution—neotectonic versus subsidence genesis

Abstract The study area (Algarve) is located near the Eurasia–Africa plate boundary, experiencing significant tectonic and seismic activities. Regional geology is characterised by the presence of Mesozoic and Miocene carbonate rocks which are affected by karst phenomena. This karst is covered by terrigenous sediments of Upper Miocene and Pliocene–Pleistocene age. In the study area, the Pliocene–Quaternary cover deposits are affected by a large number of mesoscopic structures, including joints, faults, and a few folds, which indicate neotectonic activity. However, these sediments also present similar structures that result from underground karst evolution, raising the need to differentiate the neotectonic structures from those of non-tectonic origin. In fact, a variety of ductile, semi-brittle and brittle structures develop in the sediments that fill up the karst wells, controlled by different rheological behaviour of the cover deposits, various strain rates associated with sudden collapse or progressive sinking, and the variable shape of the karst pits walls. The structure’s geometry, geographical dispersion and directional scattering were used as criteria to infer a non-tectonic genesis. It is discussed whether some karst related structures may be controlled by the contemporary tectonic stress field and consequently are interpreted in the regional geodynamical framework.

The 1755 Lisbon earthquake and the beginning of closure of the Atlantic

The 1755 Lisbon earthquake and tsunami had one of the highest magnitudes in the history of Europe. The source mechanism requires generation at a subduction zone. Intensity distribution and tsunami modelling excludes the Gorringe Bank as a source area and suggests generation by the incipient convergence of the Atlantic with the Southwest Iberia and Morocco margin rather than at the less active Gulf of Cadiz Accretionary Prism. The comparison with the 2004 Sumatra earthquake and tsunami supports this interpretation. A tsunami warning alert system is urgent for the Atlantic.

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.

Comment on “Lisbon 1755: A Case of Triggered Onshore Rupture?” by Susana P. Vilanova, Catarina F. Nunes, and Joao F. B. D. Fonseca

The work published recently by Vilanova et al. (2003) in the Bulletin of the Seismological Society of America proposes that the accounts of destruction and other effects reported in the Lisbon area as a result of the 1 November 1755 earthquake are best explained by a local rupture on the Lower Tagus Valley fault (ltvf), triggered by the static stress change produced by the main offshore source located in the Gorringe area. Because of the potential impact of this hypothesis on the seismic hazard of the Lisbon area, we discuss and complement the evidence presented by Vilanova and co-workers, concluding that the “local rupture model” should remain, for the moment, as an unsubstantiated speculation. Vilanova et al. (2003) (referred to herein as “the authors” or “the paper”) compare the intensities reported for the 1755 earthquake in Lisbon and Algarve to conclude that Lisbon intensities are abnormally high for the same distance, in particular, when compared with what was observed for the 28 February 1969 earthquake. They interpreted this as the result of local rupture. In Figure 1 we re-plot the macroseismic intensity as a function of the distance to the Gorringe Bank source presumed by the authors. We can see that “Lower Tagus Valley intensities” do not show any abnormal behavior. Similar plots could be made for all source areas proposed by other authors (Baptista, 1998; Baptista et al. , 1998a, 1998b; Zitellini et al. , 1999, 2001; Terrinha et al. , 2003) with minor changes to the conclusion we reached previously. The conclusions reached by the authors were based on their figure 8A, from which intensity values for the Algarve and Portuguese west coast corresponding to distances greater than 350 km are missing. Figure 1. Modified Mercalli (MM) intensities for the 1755 earthquake, plotted as a …

[Comment on “Investigations unveil Holocene thrusting for onshore Portugal”] Paleoseismological studies near Lisbon: Holocene thrusting or landslide activity?

A paper on paleoseismological investigations in the Lower Tagus River Valley (LTV) region, northeast of Lisbon, Portugal, was recently published in Eos Transactions by Fonseca et al. (2000a). The LTV is an area of significant seismicity attested by the occurrence of magnitude 6.5–7 historical earthquakes, and has been the subject of several geological studies. Nevertheless, major uncertainties on the regional tectonic structure have subsisted until now, mainly due to a Cenozoic sedimentary cover that hides most of the active faults rooted in the basement.

Using tensorial electrical resistivity survey to locate fault systems

This paper deals with the use of the tensorial resistivity method for fault orientation and macroanisotropy characterization. The rotational properties of the apparent resistivity tensor are presented using 3D synthetic models representing structures with a dominant direction of low resistivity and vertical discontinuities. It is demonstrated that polar diagrams of the elements of the tensor are effective in delineating those structures. As the apparent resistivity tensor shows great inefficacy in investigating the depth of the structures, it is advised to accomplish tensorial surveys with the application of other geophysical methods. An experimental example, including tensorial, dipole–dipole and time domain surveys, is presented to illustrate the potentiality of the method. The dipole–dipole model shows high-resistivity contrasts which were interpreted as corresponding to faults crossing the area. The results from the time domain electromagnetic (TEM) sounding show high-resistivity values till depths of 40–60 m at the north part of the area. In the southern part of the survey area the soundings show an upper layer with low-resistivity values (around 30 Ω m) followed by a more resistive bedrock (resistivity >100 Ω m) at a depth ranging from 15 to 30 m. The soundings in the central part of the survey area show more variability. A thin conductive overburden is followed by a more resistive layer with resistivity in the range of 80–1800 Ω m. The north and south limits of the central part of the area as revealed by TEM survey are roughly E–W oriented and coincident with the north fault scarp and the southernmost fault detected by the dipole–dipole survey. The pattern of the polar diagrams calculated from tensorial resistivity data clearly indicates the presence of a contact between two blocks at south of the survey area with the low-resistivity block located southwards. The presence of other two faults is not so clear from the polar diagram patterns, but their location can be afforded combining tensorial, dipole–dipole and TEM results.

The 1755 Lisbon Earthquake: A Review and the Proposal for a Tsunami Early Warning System in the Gulf of Cadiz

The present paper reviews the proposed sources for the 1/11/1755 Lisbon earthquake in the context of the geodynamics of Southwest Iberia, the region where it was generated. We will also refer briefly the implications for seismic hazard both at global and regional scale. Because the 1755 earthquake occurred before the foundation of instrumental seismology, the source location and mechanism can be inferred only on the basis of the Historical Seismicity through interpretation of records of direct witnesses of the earthquake and tsunami (Pereira de Sousa 1919–1932), through the geological record left by the tsunami in the SW coast of Portugal (Andrade 1992) and through the evidences of downslope mass movement triggered by the earthquake as recorded in the sedimentary column of the surrounding Abyssal Plains (Lebreiro et al. 1997). The isoseismal distribution derived by the description of the damages suggested a seismic moment magnitude of 8.7 and source area located SW of Lisbon (Richter 1958, Johnston 1996, Buforn et al. 1988, 2004). The West-Iberia continental margin is more active in terms of instrumental and historical seismicity than usual for passive Atlantic-type margins. Epicentres distribution shows that Iberia is characterized by an inner core with increased rigidity and stability with respect to its margins. Reconstruction of the present day stress field shows deviations of the maximum compressive stress trajectories from the core of Iberia, where they are oriented NNW-SSE, to its western margin, where they become WNW-ESE oriented (Ribeiro et al. 1996, Ribeiro 2002). Along theN-S trendingWest Iberia Margin the orientation of the main compression also changes from approximately NW-SE in the north toWNW-ESE in the south. It was firstly argued by Ribeiro and Cabral (1986, 1987) and Ribeiro et al. (1996) that the West Iberia Margin initiated the process of transition from passive margin to active margin

Genesis of deformation structures affecting Plio-Pleistocene sediments in the western Portuguese mainland (West Iberia). Implication on the regional neotectonics

A careful analysis of the morphology, geologic setting, stratigraphic distribution and sedimentological context of deformation features in sediments can give valuable information for the interpretation of their genesis, namely if they are of sedimentary or tectonic origin. This approach was used for the study area, located in the western region of the Portuguese mainland (West Iberian margin), where significant neotectonic and seismic activities occur, contrasting with the stabler interior of Iberia. Notwithstanding the regional neotectonics (intended as upper Pliocene to Recent), part of the deformation features observed in the studied deposits (Pliocene to Pleistocene in age) may be attributed to other phenomena but seismic shaking and/or active faulting. Slump beds and flame structures in coarse sand deposits are probably related to sedimentary overloading in high supply delta plain setting. Part of the deformation observed in the sediments that overlie Jurassic marls and limestones was probably induced by karst sink. This process is plausible even in Plio-Pleistocene sediments that overlie low carbonate content basement rocks, such as marl dominated successions. Thus, the abundance of deformation features affecting the studied Plio-Pleistocene sedimentary cover may lead to an overestimation of the regional active tectonics. The neotectonic activity in this area is reconsidered from the interpretation of the triggering mechanisms responsible for the observed deformation.