Lower Tagus Valley Research Articles

How to mitigate flood events similar to the 1979 catastrophic floods in the lower Tagus

Abstract. The floods that struck the lower Tagus valley in February 1979 correspond to the most intense floods in this river and affected the largest number of people in a river flow event in Portugal during the last 150 years. In fact, the vast area affected significantly impacted circa 10 000 people in the lower Tagus sector (and an additional 7000 in other regions of Portugal), including thousands of people evacuated or made homeless. In this context, the present study focuses on an in-depth analysis of this event from a hydrodynamic perspective by means of the Iber+ numerical model and on developing strategies to mitigate the flood episodes that occur in the lower section of the Tagus River using the exceptional floods of February 1979 as a benchmark. In this sense, dam operating strategies were developed and analyzed for the most important dam along the Tagus River basin in order to propose effective procedures to take advantage of these infrastructures to minimize the effect of floods. Overall, the numerical results indicate a good agreement with watermarks and some descriptions of the 1979 flood event, which demonstrates the model capability to evaluate floods in the area under study. Regarding flood mitigation, results obtained indicate that the frequency of floods can be reduced with the proposed strategies, which were focused on providing optimal dam operating rules to mitigate flooding in the lower Tagus valley. In addition, hydraulic simulations corroborated an important decrease in water depth and velocity for the most extreme flood events, and also a certain reduction in the flood extension was detected. This confirms the effectiveness of the proposed strategies to help in reducing the flood impact in the lower Tagus valley through the efficient functioning of dams.

Fault activity studies in the Lower Tagus valley and Lisbon region using geophysical data

Abstract The Metropolitan Area of Lisbon and the Lower Tagus Valley (LTV) region are located in central Portugal and inhabited by nearly 4 million people. The region has suffered throughout its history the effect of destructive earthquakes caused by hidden faults, possibly related to the plate boundary, which is sited approximately 400 km south of the region (Figure 1). In spite of low slip-rates and big recurrence times that have been estimated for these local, regional faults, they can produce moderate-to-large earthquakes that cause large damage and loss of life, as in 1344, 1531, or 1909 (e.g. Justo and Salwa, 1998; Cabral et al., 2003; 2013). The shorter occurrence time of the earthquakes might be owing to the existence of multiple active faults and/or time clustering owing to stress drop caused by proximal faults (e.g. Carvalho et al., 2006). Therefore, the seismic hazard and risk evaluation of the region has long been a reason of concern. Figure 1 Seismotectonic of the Azores-Iberia region (after Custodio et al., 2016 and references therein), where the study area is included (dashed square). Earthquakes recorded instrumentally since 1996 are shown by small brown dots. Historical earthquakes reported by the SHARE European Earthquake Catalogue – SHEEC – with magnitudes larger than 5.5 are shown by circles with a radius corresponding to the earthquake magnitude (Stucchi et al., 2013). The earthquakes with magnitudes equal to or larger than 7.0 are labelled with year of occurrence and magnitude. The plate boundaries of the global plate tectonics NNR-MORVEL56 model are shown by dark-blue lines (DeMets et al., 2010; Argus et al., 2011). The surface projection of potentially active faults compiled on the SHARE database are shown by red rectangles, the surface trace of the faults is marked by a thick red line (Basili et al., 2013; Vilanova et al., 2014). The SHARE faults in western Iberia are underlain by the original fault traces proposed by Cabral (2012) (green) and Duarte et al. (2013) (blue). The topography in the background is taken from the global SRTM30+ model, obtained from satellite altimetry and ship depth soundings (Smith and Sandwell, 1997; Becker et al., 2009).

Microzonamento de suscetibilidade à liquefação: caso de estudo no Vale Inferior do Tejo

Este trabalho enquadra-se no projeto europeu de investigação LIQUEFACT, do qual a Faculdade de Engenharia da Universidade do Porto é parceira e associada. Durante a realização deste trabalho, foi recolhida uma vasta informação geológico-geotécnica existente, de modo a constituir uma base de dados sólida para a escolha de um sítio-piloto, para realização de ensaios in situ complementares, com vista à elaboração de um microzonamento de suscetibilidade à liquefação. A análise dessa informação geotécnica (sobretudo SPT, CPT e CH) incluiu a avaliação de índices de risco de liquefação, nomeadamente o Fator de Segurança à liquefação (FSliq), Índice Potencial de Liquefação (LPI) e Número de Severidade de Liquefação (LSN), tendose escolhido um sítio piloto na zona da Lezíria Grande de Vila Franca de Xira. A campanha experimental envolveu ensaios SPT, CPTu, SDMT, diversos métodos geofísicos e ainda a recolha de amostras de alta qualidade para caracterização laboratorial. O tratamento dos resultados destes ensaios foi subdividido em três tipos de análises, em termos de Índices de Risco, da classificação de acordo com VS30, e ainda baseada nos assentamentos e deslocamentos laterais esperados. Destas análises, foi possível estabelecer e propor um microzonamento preliminar de suscetibilidade à liquefação induzida por sismos.

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

The Lower Tagus Valley (LTV) has been the source of several local earthquakes that affected the region of Lisbon, in the central part of Portugal. In spite of absence of strong earthquakes during the last 100 years, past events have produced large damage in this area. We present the state of the art concerning the last two major earthquakes, which occurred in 1531 and 1909. The effects of both events are described, based on coeval reports already interpreted by several authors. The source parameters determined by different authors are also presented. The characterization of the seismogenic potential of the LTV is very important to better estimate the seismic hazard and risk of the region of Lisbon and Tagus Valley.

Ground-Motion Simulation in the Lower Tagus Valley Basin

Throughout history, the Lower Tagus Valley (LTV) region has been shaken by several earthquakes, including some with moderate to large magnitudes and with sources located inside the basin, for example the 1344 (M6.0) and 1909 (M6.0) earthquakes. Previous simulations (Bezzeghoud et al. Natural Hazard 69: 1229–1245, 2011) have revealed strong amplification of the amplitude waves in the region, because of the effect of the low-velocity sediments that fill the basin. The model used in those simulations was updated in this work by including new high-resolution geophysical and geotechnical data available for the area (seismic reflection, aeromagnetic, gravimetric, deep wells, standard penetration tests, and geological data). To contribute to improved assessment of seismic hazard in the LTV, we simulated propagation of seismic waves produced by moderate earthquakes in a 3D heterogeneous medium by using elastic finite-difference wave propagation code. The method, successfully used by Grandin et al. (Geophys J Int 171: 1144–1161, 2007), involves evaluation of the seismic potential of known faults in the area studied and three-dimensional seismic ground motion modelling by use of finite difference methods. On the basis of this methodology, we calculated the ground motion for the April 23, 1909, Benavente (Portugal) earthquake (Mw = 6.0) in dense grid points, then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity (MMI) and peak ground velocity (PGV). The synthetic results, in contrast with available macroseismic and instrumental data, enable validation of the source models proposed for the area, identification of the sources of historical earthquakes, and could also indicate which areas are more exposed to seismic ground motion.

V S profile: measured versus empirical correlations—a Lower Tagus river valley example

During the last decades, there has been an increase in the use of geophysical tests for geotechnical characterization. The relatively low cost and the large volume analysed by non-invasive geophysical testing makes them very attractive for a site investigation plan combined with borings and other geotechnical tests. In some countries, for instance Portugal, the use of multichannel analysis of surface waves (MASW) and other surface wave methodologies is still limited. This work shows results obtained with the use of MASW in the geotechnical investigation of the foundation soils of a viaduct and a bridge, part of the IC3 route, with a total length of about 10 km. The study area is located very near the “Lower Tagus valley” active fault and above thick alluvial deposits. Comparisons of MASW with other in situ tests show that the results are in good agreement. The N(SPT)-V S correlations are still in use worldwide for practical purposes, but not always considering all the important parameters as the standard penetration test (SPT) energy measurement. In this paper, we discuss the usability of empirical correlations to obtain the shear wave velocity from SPT results, using the results from the case study presented. The large variability of results obtained proves the importance of measuring the shear wave velocity with geophysical tests.

Geophysical study of the Ota–VF Xira–Lisbon–Sesimbra fault zone and the lower Tagus Cenozoic basin

This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota–Vila Franca de Xira–Lisbon–Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr –1 ). A more accurate estimation of the OVLS throw in the Quaternary

The 23 April 23 1909 Benavente (Portugal) M 6.3 Earthquake

Lisbon is forever linked to the history of seismology through the eponymous 1755 earthquake. But a less-known series of moderate to strong earthquakes, some with intraplate origin, has caused major destruction in the Lower Tagus Valley (LTV) region around the Portuguese capital throughout historical times. Although references to earthquakes can be found in coeval documents dating from the 14th century, the earliest descriptions of damage distribution with reasonable detail concern the 1531 Lisbon earthquake (Justo and Salwa 1998), with estimated moment magnitude of 6.9 (Vilanova and Fonseca 2007). In 1858, the town of Setubal, 30 km southeast of Lisbon, was shaken by a severe earthquake of estimated moment magnitude of 7.1 (Vilanova and Fonseca 2007) at what can be regarded as the southern limit of the LTV region. …

Estuarine–fluvial floodplain formation in the Holocene Lower Tagus valley (Central Portugal) and implications for Quaternary fluvial system evolution

We present a brief synthesis of the Quaternary fluvial record in the Lower Tagus Basin (central Portugal), concentrating on factors controlling infill and incision. The Holocene part of the record forms the focus of this paper and guides the questioning of the basic assumptions of the established Quaternary fluvial evolution model, in particular the link between sea-level change and fluvial incision-deposition. We suggest that several incision-aggradation phases may have occurred during glacial periods. Major aggradation events may overlap with cold episodes, while incision appears to concentrate on the warming limb of climate transitions. The complex stratigraphy of the Quaternary record in the Lower Tagus valley is influenced by repeated base-level and climate changes.This paper submits the first chronostratigraphic framework for valley fill deposits in the Lower Tagus area. Sea-level rise forced aggradation and controlled deposition of the fine-grained sedimentary wedge underlying the low-gradient Lower Tagus floodplain. Investigations have focused on the lower Muge tributary, where rapidly aggrading estuarine and fluvial environments were abruptly established (∼8150calBP) as sea level rose. Base level at the valley mouth controlled the upstream extent of the fine-grained backfill. Tidal environments disappeared abruptly (∼5800calBP) when the open estuary at the Muge confluence was infilled by the Tagus River. The decrease and final still stand of sea-level rise led to floodplain stabilisation with peat (∼6400–5200calBP) and soil formation (∼5200–2200calBP). Localised renewed sedimentation (∼2200–200calBP) is linked to human activity.

Holocene palaeoecology and floodplain evolution of the Muge tributary, Lower Tagus Basin, Portugal

The lower Muge valley, a tributary of the lower Tagus River (Portugal), features an important archaeological record of Mesolithic shell midden sites. Archaeological research has long assumed that tidally influenced valley floor environments in the immediate locality of the sites provided a rich food resource, attracting Mesolithic settlement. To date there has been little attempt to use palaeoenvironmental records to reconstruct Holocene floodplain evolution in the Lower Tagus valley. The cultivated freshwater lower Muge floodplain is locally underlain by ∼11 m of homogeneous fine-grained sediments and peat, comprising buried floodplain environments contemporary with Mesolithic occupation (∼6200–4800 cal BC). Pollen and foraminifera analyses demonstrate that fine-grained deposition, forced by sea-level rise, commenced ∼6200 cal BC in an estuarine setting. The lower Muge floodplain experienced maximum tidal influence ∼5800–5500 cal BC. Subsequently, sediment supply rates overtook the decreasing rate of sea-level rise and fluvial environments expanded. The pollen record may suggest regional desiccation from ∼5000 cal BC. Estuarine environments disappeared suddenly ∼3800 cal BC when freshwater wetlands were established. Although the initiation of Mesolithic settlement is shown to coincide with the beginning of tidal influence, site abandonment does not match with any major environmental change. Sea-level still stand (∼2600 cal BC) has been linked to valley floor stabilisation and soil formation. Alder floodplain woodland developed prior to ∼230 cal BC and was cleared, probably during Roman times, for agriculture. Renewed deposition after ∼230 cal BC may relate to internal mechanisms or to human impact upon the catchment vegetation.

Moment tensor inversion with single-component historical seismograms: The 1909 Benavente (Portugal) and Lambesc (France) earthquakes

SUMMARY Moment tensor inversion of early XX century mechanical seismograph recordings offers the opportunity to estimate source parameters for a number of important earthquakes. This knowledge is crucial for understanding regional tectonics and estimating seismic hazard; but processing of the historical seismograms is often difficult. Especially, the rotation of horizontal seismograms into radial (P–SV) and transverse (SH) components tends to fail; for example, if one polarity is wrong, if the alignment of the seismograms is not correct, or if one component is lost. To avoid these kind of problems, we use a moment-tensor-inversion scheme that processes the original single-component horizontal traces without rotation, and rotates the theoretical Green's functions instead. We show how this approach simplifies the analysis, and allows a better use of available data. We present moment tensor solutions for two destructive earthquakes in southwestern Europe in 1909. The April 23 earthquake near Benavente in the lower Tagus valley (Portugal) had a moment magnitude MW= 6.0 and an estimated centroid depth of 10 km. Our preferred moment tensor solution indicates reverse faulting (nodal planes have strike/dip/rake of N51°E/52°/83° and N242°E/38°/99°). We propose a blind thrust beneath the Tagus valley sediment basin as the responsible fault. The June 11 earthquake near Lambesc, Provence (France) was found to be slightly smaller (MW= 5.5) and related to oblique reverse faulting at 4-km depth. Nodal planes of the preferred solution have strike/dip/rake of N80°E/53°/53° and N311°E/50°/128°, subparallel to the Trevaresse fold.

Seismic hazard impact of the Lower Tagus Valley Fault Zone (SW Iberia)

The seismic hazard of SW Iberia is composed of two contributions: offshore, large to very large events on the plate boundary between Africa and Eurasia such as the Lisbon earthquake of 1755 or the Gorringe Bank earthquake of 1969; and onshore, moderate to strong intraplate earthquakes on inherited crustal fractures. One of these zones of crustal weakness is the Lower Tagus Valley (LTV) fault zone, which displays the highest level of seismic hazard in Western Iberia. In this paper we review the active tectonics and seismicity of the LTV, integrating previous geophysical data with recent results of paleoseismological investigations, and discuss its impact on the seismic hazard of SW Iberia. We conclude that the seismic zonation for hazard assessment currently in force in the building code is biased towards the scenario of distant offshore rupture, and does not take adequately into account the LTV seismic source.

Analysis of seismic reflection data as a tool for the seismotectonic assessment of a low activity intraplate basin – the Lower Tagus Valley (Portugal)

The Lower TagusValley is located in the Lower TagusSedimentary Basin. Occurrence ofearthquakes in the area indicates thepresence of seismogenic structures at depththat are still deficiently known due to athick Cenozoic sedimentary cover. It isusually assumed that regional M = 6–7earthquakes are generated by the LowerTagus Valley fault. However, this structurehas never been characterised, and there isevidence for the presence of other faultsthat have the potential of generating largeseismic events. The hidden character ofmany of the structures in the Lower TagusBasin necessitates the use of indirectmethodologies for their study. Availableseismic reflection lines acquired for oilexploration in the Lower Tagus Valleyregion were thus examined, with emphasis onthe recognition of faults displacing theCenozoic and particularly the youngergeological units. The foreseen complexityof the basin structure was confirmed.Evidence for a continuous `Lower TagusValley fault' is absent. Instead, N-S toNNE-SSW trending, en echelon,sub-vertical and reverse faults showingsignificant post-Miocene offsets, linked bytransverse (NW-SE) faults that accommodatelarge displacements, were recognised in theseismic sections that cross the Tagusvalley. These faults range in length from10 km to a maximum of ca. 40 km, leading toan estimation of 6.2–6.9 maximum credibleearthquake magnitudes, which fall in themagnitude range of the regional historicalseismicity.

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.

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.

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.