L.Mendes Victor

CONSTRAINS ON THE SOURCE OF THE 1755 LISBON TSUNAMI INFERRED FROM NUMERICAL MODELLING OF HISTORICAL DATA ON THE SOURCE OF THE 1755 LISBON TSUNAMI

Abstract The 1755 Lisbon tsunami was felt all over the North Atlantic, being one of the first major events of this kind relatively well documented by historical sources. However, in spite of the extensive research work on the historical reports by a considerable number of authors, the epicentre location of this event is still uncertain and its focal mechanism is still not well understood, implying a great uncertainty in the tsunami generating mechanism. The generally assumed epicentre, inferred from isoseismal maps, is located slightly north of the Gorringe Bank (SW Iberia) and the rupture mechanism has been assumed in the past to be similar to the well studied 1969.02.28 event. While all previous studies have used a seismic-based approach, this paper uses all that is known about the tsunami parameters at the coast — presented in a companion paper — to define the location and geometry of the tsunami source. For that purpose some backward ray-tracing techniques were developed and their results were used to define the initial fields in a number of shallow water simulations of the water height at the coastal locations where the most reliable historical data are available. The source parameters also took into account the estimated seismic energy released. The results obtained here suggest that the 1755 tsunami probably originated on the continental shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coast, in a geodynamic context quite different from the one implied in the 1969.02.28 event. The amplitude of the initial movement in the source region, required by the shallow water simulations to account for the reported magnitudes, suggests an elongated but shallow rupture area, extending along the shelf. It is suggested that this location of the rupture would have significant implications in the geology of the region.

The 1755 Lisbon tsunami; evaluation of the tsunami parameters

Abstract The tsunami generated by the 1755.01.11 earthquake affected mainly the coasts of the Iberian Peninsula and Northwest Morocco and was observed all over the North Atlantic coasts. The catastrophic dimensions of that phenomenon had a tremendous impact on the city of Lisbon and on several villages along the south coast of Portugal. The earthquake was felt all over Europe and the seismic intensity was estimated as X–XI (Mercalli Intensity Scale) at Lisbon and Southwest Portugal (Cape S. Vicente). The most destructive waves were observed along the coast of Portugal, specially in Lisbon, in the area of the S. Vicente Cape, along the Gulf of Cadiz and Northwest Morocco. Throughout historic times, earthquakes have periodically affected the city of Lisbon causing severe damage and casualties. In spite of that, the city kept growing, so the extension of damage and the loss of human lives in 1755, was quite impressive. The down town of Lisbon was flooded by the rising of the waters of the river Tagus and most historical documents reported waves of 6 m height. At Cape S. Vicente (Southwest Portugal) the run-up height, evaluated from historical data, is greater than 15 m. The eye witness accounts from Spain and Morocco reported wave heights greater than 10 m and large flooded areas along the Gulf of Cadiz and in several harbours in Morocco, e.g. Safi and Agadir. In the city of Lisbon, the number of casualties due exclusively to the tsunami, is estimate around 900, and the penetration of the waters is evaluated to be 250 m. Most of the available literature concerning the 1755 earthquake is based on the compilation of Pereira de Sousa (1919) and, sometimes, incorporates both well established historical records and non reliable information. As the 1755 event evaluation is crucial to a quantitative approach of the tsunami hazard and risk assessment in Portugal, a new examination of the historical records was needed before the establishment of reliable tsunami parameters that can be used both in numerical models of tsunami propagation and in geodynamic studies. In this paper, we present a new compilation of almost all the available historical data from the countries affected by the tsunami. In the analysis of these records, the following tsunami parameters are inferred: travel time, polarity of the first movement, maximum run-up height, period, number of waves, duration of the sea disturbance and extent of flooding.

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Abstract In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction. These models were mainly based on morphological analysis of sea bottom topography, sparse magnetic profiling, sidescan sonar surveying over the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detailed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge between 37°N and 40°30′N the magnetic anomalies up to anomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a coherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fourier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more continuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the ridge at 39°30′N, 29°40′W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38°23′N, 30°15′W) and by the Princess Alice Fracture Zone (at 38°00′N, 30°50′W). Anomaly identifications using a two-dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters. The results thus obtained reveal that, at least between anomalies 5 and 3 ( ∼ 10–3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved attached to the Eurasian plate. The triple junction (Azores-North America-Africa, or Eurasia-North America-Africa) moved northward from ∼ 38°00′N, 30°50′W to ∼ 38°20′N, 30°15′W (between 4 and 3A) and probably to 38°50′N, 30°00′W at anomaly 2A time. A detailed reconstruction model is presented.

Tectonic setting of the Azores Plateau deduced from a OBS survey

The studies of Azores seismicity generally show shocks with either normal faulting or right-lateral strike-slip along the ESE direction, compatible with a eastward relative motion of the Eurasian (EU) relative to the African (AF) plate. However, the 1 January 1980 earthquake was interpreted as a clear left-lateral strike-slip shock along the N150E direction. This pattern is difficult to explain in terms of the relative motion between the EU, AF and North American (NA) plates: all available models for the present day movement of this triple junction fail to explain the regional variability in the stress conditions of the area. Here we present data from a 34-day long Ocean Bottom Seismograph array deployment. We show that the seismicity is distributed along a band aligned with the island chain itself, and is concentrated along several faults with an approximate N150E strike, cutting the Azores plateau in all the area covered by the OBS network. The combination of these new results with other geophysical data permits us to conclude that the tectonic setting of the Azores plateau is characterised by the existence of two sets of faults, in the N120E and N150E directions, defining several crustal blocks, whose relative motion accommodates the interaction of the three megaplates. The deformation of these tectonic blocks is probably driven by the shear between the EU and AF plates. This model explains well the spatial variability of the stress conditions in the Azores domain, the combination of dextral and sinistral strike slip mechanisms and the observed seismotectonics of the Azores islands.

Tectonic framework of the Azores Triple Junction

The lack of accurate and detailed magnetic information has, in the past, limited the development of well constrained models for the plate tectonic evolution of the Azores Triple Junction. An aeromagnetic survey, made possible by the existing airport facilities, has long been desired as it can provide high quality magnetic data, whose homogeneity and coherency is far better than those provided by classic marine surveys. The results presented in this paper concern only a part of the Aeromagnetic Survey conducted by the Portuguese Instituto Nacional de Meteorologia e Geofisica and already allow an improved definition of the basic tectonic boundaries at the central part of the Azores plateau.

Aeromagnetic anomalies in mainland Portugal and their tectonic implications

The aeromagnetic anomaly map for mainland Portugal shows a clear zonation correlated with the main structural units of the Hercynian foldbelt. Four zones can be distinguished from south to north: the South Portuguese Zone (SPZ) marked by small-amplitude negative anomalies of medium to large wavelength, the Ossa Morena Zone (OMZ) with medium positive amplitudes (about 100 nT) of short wavelength, the Centro Iberian Zone (CIZ) with low magnetic anomalies and, finally, the Galicia-Tras-os-Montes Subzone (GTS) corresponding to a negative regional anomaly. The limits of these zones are in agreement with those described by geologists except for the western border of the CIZ which does not have any magnetic signature in this new survey. The good correlation between neotectonics and magnetic anomalies confirms the role of Hercynian tectonics in understanding recent movements. The lack of magnetic response of the supposed great shear along the Odemira-Avila fault is noticeable. Last, the isolated anomalies associated with Cretaceous volcanism underline the southwest limit of the Iberian peninsula.

Rupture extent of the 1755 Lisbon earthquake inferred from numerical modeling of tsunami data

Abstract The occurrence of tsunamis, affecting the Portuguese coasts, has been reported since the year 60 BC and the Gorringe bank region has been assumed as the most prone area for tsunami generation in the southwestern Iberian area. The tsunami generated by the 1755.11.01 earthquake is the largest one known in this area, having deeply affected the coasts of Iberia and Morocco. The earthquake, itself, was felt all over Europe, its estimated magnitude being 8.3/4 and its MSK intensity at the epicenter I 0 =XI-XII. The great similarity between the isoseismal maps of this event and those of the recent 1969.02.28 earthquake lead several authors to locate the epicenter of the 1755 tsunami close to the Gorringe Bank (cf. Fig 1) and to infer the same type of focal mechanism (e.g. Machado, 1966, Martinez Solares et al., 1979, Levret, 1991) as the one deduced for the 1969 event (Fukao, 1973). The correct identification of the tsunamigenic sources is the essential task for the determination of tsunami risk at the Iberian area. The question we try to answer in the present study is whether or not there is only one major tsunami generating area in the western Iberia as it has been assumed until now. The results obtained here suggest that the 1755 tsunami was probably originated on the continental Iberian shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coasts (cf. Fig. 1), closer to the coast. A complex source is proposed in order to justify tsunami observations in Morocco and seismic intensity values along the lberian and north Morocco coasts

A comparison of the spectral characteristics of observed and simulated tsunamis

Abstract The spectra of real tsunamis depend on the source characteristics, propagation effects on the deep ocean, shelf topography and harbour geometry. The separation between all these effects is possible if a large number of instrumental tsunami data is available and, in that case, the source parameters may be computed by cancellation of path and instrumental effects. This is not generally the case in the SW Iberian region, where, in spite of some very large historical events there is only a small number of instrumental tsunamis. A good case study for that methodology is given by the 28.02.1969 tsunami, which has been previously studied with rather encouraging results in terms of the comparison of arrival times and amplitudes between observed and simulated tsunamis. The aim of the present study is the comparison between the spectral characteristics of instrumental tide records and the synthetic wave forms computed with a shallow water model.