Maria Ana Baptista

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.

Modeling tsunamis from earthquake sources near Gorringe Bank southwest of Portugal

The Azores-Gibraltar fracture zone with the huge bathymetric reliefs in the area southwest of Portugal is believed to have been the source of large historic tsunami events. This report describes simulations of tsunami generation and propagation from sources near the Gorringe Bank. The well-documented 1969 tsunami event is examined both with a ray-tracing technique and with finite difference models based on various shallow water equations. Both methods show that the most likely source location is southeast of the Gorringe Bank near the epicenter location determined from seismic data. The tsunami source is calculated by formulas given by Okada [1985] for surface deformation of an elastic half-space caused by faulting. Observed wave amplitude and travel time and values computed from an initial wave field according to Okada [1985] formulas show acceptable agreement for most stations along the coast of Portugal and Spain. However, in order to explain a large primary wave with downward displacement observed on the coast of Morocco, an alternative source model with a larger area of downward displacement has been introduced. This also leads to a better overall fit with observed travel time. Implications for disastrous events, as the one in 1755, are also discussed. Linear hydrostatic shallow water models are used for most of the simulations, but the importance of nonlinearity and dispersion is examined with the Boussinesq equations. The sensitivity of the solution to changes in the location and the strength of the source is discussed, and a series of grid refinement studies are performed in order to assess the accuracy of the simulations.

In Search of the 31 March 1761 Earthquake and Tsunami Source

Earthquake catalogs for the Iberian Peninsula report three strong- magnitude events in the eighteenth century: 27 December 1722, 1 November 1755, and 31 March 1761. These events have magnitudes greater than 7 and generated tsunamis that damaged the Portuguese coasts. However, their source areas are controversial because of the lack of detailed and coherent historical descriptions. The 31 March 1761 earthquake was felt in Lisbon at noon, alarming the inhabitants and throwing down ruins of the past 1 November 1755 earthquake. According to several sources the earthquake was followed by a tsunami that was observed as far as Cornwall (United Kingdom), Cork (Ireland), and Barbados (Caribbean). The Portuguese catalogs locate this event on the Horseshoe Abyssal Plain, south of Gorringe Bank, and attribute a magnitude of 7.5. The Caribbean tsunami catalog (Lander et al. , 2002) locates the event further north 37° N 10° W and estimates of its epicenter intensity as IX. In this study we present a reappraisal of the available historical reports concerning the 1761 event, a revision of the macroseismic intensities along Iberia, and the tsunami observations along the western Portuguese and Galicia coasts, England, Ireland, and the West Indies. With this dataset we use backward raytracing techniques to discuss the location of the event and its integration with one of the major tsunami generation areas in the western Portuguese margin. We conclude that the 31 March 1761 earthquake took place at 12:01 a.m. (Lisbon time). Its epicenter was located about 34.5° N 13° W and had a tsunami magnitude close to 8.5. Online material: Felt reports from the 31 March 1761 earthquake.

The 1755 Lisbon Tsunami in Guadeloupe Archipelago: Source Sensitivity and Investigation of Resonance Effects

On the 1 st of November 1755, a major earthquake of estimated Mw=8.5/9.0 destroyed Lisbon (Portugal) and was felt in whole Western Europe. It generated a huge tsunami which reached coastlines from Morocco to Southwestern England with local run-up heights up to 15 m in some places as Cape St Vincent (Portugal). Important waves were re- ported in Madeira Islands and as far as in the West Indies where heights of 3 m and damages are reported. The present knowledge of the seismic source(s), presented by numerous studies, was not able to reproduce such wave heights on the other side of the Atlantic Ocean whatever the tested source. This could be due to the signal dispersion during the propaga- tion or simply to the lack of simulations with high resolution grids. Here we present simulations using high resolution grids for Guadeloupe Archipelago for two different sources. Our results highlight important wave heights of the range of 1 m to more than 2 m whatever the source mechanism used, and whatever the strike angle in some particular coastal places. A preliminary investigation of the resonance phenomenon in Guadeloupe is also presented. In fact, the studies of long wave impact in harbours as rissaga phenomenon in the Mediterranean Sea leads us to propose the hypothesis that the 1755 waves in the West Indies could have been amplified by resonance phenomenon. Most of the places where amplification takes place are nowadays important touristic destinations.

Tsunami Characteristics Along the Peru–Chile Trench: Analysis of the 2015 Mw8.3 Illapel, the 2014 Mw8.2 Iquique and the 2010 Mw8.8 Maule Tsunamis in the Near-field

Tsunamis occur quite frequently following large magnitude earthquakes along the Chilean coast. Most of these earthquakes occur along the Peru–Chile Trench, one of the most seismically active subduction zones of the world. This study aims to understand better the characteristics of the tsunamis triggered along the Peru–Chile Trench. We investigate the tsunamis induced by the Mw8.3 Illapel, the Mw8.2 Iquique and the Mw8.8 Maule Chilean earthquakes that happened on September 16th, 2015, April 1st, 2014 and February 27th, 2010, respectively. The study involves the relation between the co-seismic deformation and the tsunami generation, the near-field tsunami propagation, and the spectral analysis of the recorded tsunami signals in the near-field. We compare the tsunami characteristics to highlight the possible similarities between the three events and, therefore, attempt to distinguish the specific characteristics of the tsunamis occurring along the Peru–Chile Trench. We find that these three earthquakes present faults with important extensions beneath the continent which result in the generation of tsunamis with short wavelengths, relative to the fault widths involved, and with reduced initial potential energy. In addition, the presence of the Chilean continental margin, that includes the shelf of shallow bathymetry and the continental slope, constrains the tsunami propagation and the coastal impact. All these factors contribute to a concentrated local impact but can, on the other hand, reduce the far-field tsunami effects from earthquakes along Peru–Chile Trench.

Can hydrodynamic modelling of tsunami contribute to seismic risk assessment

Abstract In recent years a great effort has been made to quantitatively model major historical events in Europe, searching for models that are compatible with both seismic, hydrodynamic and sedimentological evidences. The possibility to do so, with the available modelling techniques, has been checked against instrumental tsunami and seismological data. Most of this effort has been made within the projects GITEC and GITEC TWO. The quantitavive data that can be gathered from historical sources are: extension of the source area, wave heights, first wave polarity and, only rarely, travel times and time intervals between main waves. With this data the accuracy of source studies is a function of both the model uncertainty (can tsunami waves be accurately modelled ? are seismic parameters compatible with tsunami source parameters ?) and the observation errors. These are large when we deal with historical documents hundreds of years old. Here we present the conclusions obtained from the study on the 1755 Lisbon earthquake and tsunami. The available historical data are reliable enough to allow a good source determination and, surprisingly, the results obtained differ from those obtained from previous macroseismic studies. The analysis of the results obtained in the tsunami modelling, checked against the macroseismic data, allow the identification of an important seismo and tsunamigenic area, close to the SW Portuguese coast, that must also be taken into consideration for the evaluation of seismic hazard.

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

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 …

Tsunami impact and vulnerability in the harbour area of Tangier, Morocco

In this study, we assess tsunami impact and building vulnerability in the harbour area of Tangier – Morocco. Tsunami impact is evaluated through performing high-resolution inundation modelling. To assess buildings tsunami vulnerability, we use a geographic information system (GIS) multi-criteria approach based upon weight and classification factors. The methodology includes various steps: (i) identification of the most hazardous earthquake tsunamigenic sources, (ii) computation of high-resolution digital elevation model, (iii) simulation of inundation, (iv) field survey to classify buildings and defence structures and (v) application of the GIS-based model to produce final vulnerability map. Results show the potential tsunami impact and vulnerability that Tangier coast might face due to the occurrence of a large tsunami event in the region. Inundation map indicates that a coastal area of over 4.5 km2 is prone to tsunami flood with flow depths ranging from 0.5 to more than 6 m. Vulnerability map highlights different levels of expected buildings vulnerability to tsunami impact, which vary from “very high” for single-storey structures, located in the city harbour and along the sandy beach, to “low” for multi-storeys RC structures. Both inundation and vulnerability maps have important implications for decision makers and land use planning aiming to mitigate tsunami hazard in the North East Atlantic region.