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- Tsunami-induced morphological change – A model-based impact assessment of the 1755 tsunami in NE Atlantic from the Morocco coastPublication . Ramalho, Inês; Omira, Rachid; EL MOUSSAOUI, Said; Baptista, Maria Ana Carvalho Viana; Zaghloul, M. N.Tsunamis can be responsible for coastal geomorphic crises. Extensively inundated coasts can suffer widespread and dramatic changes in their morphology due to sediment erosion, transport, and deposition processes. In this paper, we investigate the tsunami-induced morphological changes on coasts prone to inundation and how sediment transport is controlled by both tsunami and sediment characteristics. We focus on a tsunami-prone coast in the NE Atlantic, Tangier-Morocco, that experienced the 1755 Lisbon tsunami. We use a coupled hydrodynamic and morphodynamic numerical model, together with high-resolution digital elevation and sedimentary models, to simulate the impact of tsunamis on the morphology and sediment distribution of Tangier Bay. Due to the uncertainty on the source and effects of the 1755 event, we consider two 1755-like tsunamigenic scenarios that have the potential to cause different levels of inundation. Results show that the coastal zone of Tangier can undergo substantial changes in morphology and sediment distribution following the tsunami impact. For the selected scenarios, the volume of sediments mobilized, at both the offshore and onshore zones of the Tangier site, ranges from 30,000 to 200,000 m3. Moreover, the presence of mobile sediments and conditions favoring tsunami shoaling lead to an increase in wave impact on the coastal configuration. Conversely, the presence of the breakwaters helps to protect the Tangier coast by limiting the extent of the affected area. The results of this study suggest that the assessment of tsunami-induced coastal impact should include the effect on morphology.
- Tsunami vulnerability assessment of Casablanca-Morocco using numerical modelling and GIS toolsPublication . Omira, Rachid; Baptista, Maria Ana Carvalho Viana; Miranda, Jorge Miguel; Toto, E.; Catita, C.; Catalão, J.Earthquakes and tsunamis along Morocco's coasts have been reported since historical times. The threat posed by tsunamis must be included in coastal risk studies. This study focuses on the tsunami impact and vulnerability assessment of the Casablanca harbour and surrounding area using a combination of tsunami inundation numerical modelling, field survey data and geographic information system. The tsunami scenario used here is compatible with the 1755 Lisbon event that we considered to be the worst case tsunami scenario. Hydrodynamic modelling was performed with an adapted version of the Cornell Multigrid Coupled Tsunami Model from Cornell University. The simulation covers the eastern domain of the Azores-Gibraltar fracture zone corresponding to the largest tsunamigenic area in the North Atlantic. The proposed vulnerability model attempts to provide an insight into the tsunami vulnerability of building stock. Results in the form of a vulnerability map will be useful for decision makers and local authorities in preventing the community resiliency for tsunami hazards.
- Study of the 24 September 2013 Oman Sea tsunami using linear shallow water inversionPublication . Baptista, Maria Ana Carvalho Viana; MIRANDA, JORGE MIGUEL; Omira, Rachid; El-Hussain, IssaIn this study, we present a linear inverse approach for the computation of the initial water displacement of a tsunami. The method uses empirical Green functions and linear shallow water wave theory. We apply this methodology to study the source of the 24 September 2013 tsunami off the Makran coast. We re-analyze the 2013 tsunami data, particularly the tide records in the coast of Oman and the DART buoy record in the Arabian Sea, to infer its source. We use wavelet and Fourier analyses to characterize the frequency content of the tide records and the possible influence of local harbor effects. We compute empirical Green functions for the possible source area, and then use a linear shallow water inversion technique to assess the water disturbance in the early stage of tsunami propagation. The comparison between forward modeling and observations shows a fair agreement with available data, particularly in what concerns the DART record. It also suggests the existence of local amplifications in a number of Omani harbors. The free surface elevation obtained by inversion of the waveforms is coherent with a landslide located close to 61.5 degrees E, 24 degrees N, sliding to the S or SSE direction. Our results show that the inversion technique used here leads to a better characterization of the tsunami source than the simple backward ray tracing which assumes a simple point source. In addition, these results strengthen the hypothesis that landslides might have played an important role in previous tsunami events in the area, and must be taken into account for tsunami hazard assessment on the Omani coast.
- Developing an Event-Tree Probabilistic Tsunami Inundation Model for NE Atlantic Coasts: Application to a Case StudyPublication . Omira, Rachid; Matias, Luís; Baptista, Maria Ana Carvalho VianaThis study constitutes a preliminary assessment of probabilistic tsunami inundation in the NE Atlantic region. We developed an event-tree approach to calculate the likelihood of tsunami flood occurrence and exceedance of a specific near-shore wave height for a given exposure time. Only tsunamis of tectonic origin are considered here, taking into account local, regional, and far-field sources. The approach used here consists of an event-tree method that gathers probability models for seismic sources, tsunami numerical modeling, and statistical methods. It also includes a treatment of aleatoric uncertainties related to source location and tidal stage. Epistemic uncertainties are not addressed in this study. The methodology is applied to the coastal test-site of Sines located in the NE Atlantic coast of Portugal. We derive probabilistic high-resolution maximum wave amplitudes and flood distributions for the study test-site considering 100- and 500-year exposure times. We find that the probability that maximum wave amplitude exceeds 1 m somewhere along the Sines coasts reaches about 60 % for an exposure time of 100 years and is up to 97 % for an exposure time of 500 years. The probability of inundation occurrence (flow depth > 0 m) varies between 10 % and 57 %, and from 20 % up to 95 % for 100- and 500-year exposure times, respectively. No validation has been performed here with historical tsunamis. This paper illustrates a methodology through a case study, which is not an operational assessment.
- Boulder deposition during major tsunami eventsPublication . Costa, Pedro J. M.; Andrade, Cesar; Freitas, Maria C.; Oliveira, Maria A.; da Silva, Carlos M.; Omira, Rachid; Taborda, Rui; Baptista, Maria Ana Carvalho Viana; Dawson, Alastair G.A remarkable accumulation of marine boulders located above the present spring tide level has occurred in two coastal lowlands of the Algarve (Portugal). The size-interval of the particles studied here is seldom reported in the literature in association with extreme events of coastal inundation, thus making this study of relevance to many other coasts worldwide. The spreads of boulders extend several hundred meters inland and well beyond the present landward limit of storm activity. The marine origin of the boulders is demonstrated by well-developed macro-bioerosion sculpturing and in situ skeletal remains of endolithic shallow marine bivalves. The good state preservation of the fossils within the boulders indicates that abrasion duringtransport and redeposition was not significant. We envisage boulder deposition as having taken place during the Lisbon tsunami of ad 1755 through the simultaneous landward entrainment of coarse particles from nearshore followed by rapid shoreward suspended-dominated transport and non-graded redeposition that excluded significant sorting by weight or boulder dimensions. We use numerical hydrodynamic modeling of tsunami (and storm) waves to test the observational data on boulder dimensions (density, size, distribution) on the most likely processes of sediment deposition. This work demonstrates the effectiveness of the study of boulder deposits in tsunami reconstruction. Copyright (C) 2011 John Wiley & Sons, Ltd.
- 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-fieldPublication . Omira, Rachid; Baptista, Maria Ana Carvalho Viana; Lisboa, F.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.
- On the source of the 8 May 1939 Azores earthquake - tsunami observations and numerical modellingPublication . Reis, Cláudia; Omira, Rachid; Matias, Luis; Baptista, Maria Ana Carvalho VianaOn 8 May 1939, an earthquake (Ms7.1) occurred near the Azores archipelago, with an epicentre located close to the western end of the Gloria fault. Previous studies present different epicentre locations spreading over a large area, and two different types of focal mechanisms. Given these uncertainties, the interpretation of the seismological information in a complex tectonic environment between the Gloria Fault and the Terceira Ridge is a matter of debate. The event caused a small tsunami recorded in the Azores Islands. In this study, we use the tsunami observations and tsunami numerical modelling to select the earthquake fault rupture that best fits the tsunami observations. We consider the different focal mechanism solutions, perform tsunami numerical modelling, and compute synthetic tsunami waveforms at the tide gauge locations. We find that an earthquake caused by a low-angle dipping fault with dominant strike-slip movement generates a tsunami that reproduces well the record at Ponta Delgada tide gauge. Finally, in areas where earthquakes are rare, the study of ancient earthquakes must use all information available, namely tsunami observations and mareograph data.
- Evaluating tsunami impact on the Gulf of Cadiz Coast (Northeast Atlantic)Publication . Omira, Rachid; Baptista, Maria Ana Carvalho Viana; Miranda, Jorge MiguelThe Gulf of Cadiz coasts are exposed to tsunamis. Emergency planning tools are now taking into account this fact, especially because a series of historical occurrences were strikingly significant, having left strong evidence behind, in the mareographic records, the geological evidence or simply the memory of the populations. The study area is a strip along the Algarve coast, south Portugal, an area known to have been heavily impacted by the 1 November 1755 event. In this study we use two different tsunami scenarios generated by the rupture of two thrust faults identified in the area, corresponding to 8.1-8.3 magnitude earthquakes. Tsunami propagation and inundation computation is performed using a non-linear shallow water code with bottom friction. Numerical modeling results are presented in terms of flow depth and current velocity with maximum values of 7 m and 8 m/s for inundation depth and flow speed, respectively. These results constitute a valuable tool for local authorities, emergency and decision planners to define the priority zones where tsunami mitigation measures must be implemented and to develop tsunami-resilient communities.
- Probabilistic and deterministic estimates of near-field tsunami hazards in northeast OmanPublication . El-Hussain, Issa; Omira, Rachid; Al-Habsi, Z.; Baptista, Maria Ana Carvalho Viana; Deif, A.; Mohamed, A. M. E.Tsunamis generated along the Makran subduction zone (MSZ) threaten the Sur coast of Oman, according to deterministic and probabilistic analyses presented here. A validated shallow water numerical code simulates the source-to-coast propagation and quantifies the coastal hazard in terms of maximum water level, flow depth, and inundation distance. The worst-case source assumed for the eastern MSZ is a thrust earthquake of Mw 8.8. This deterministic scenario produces simulated wave heights reaching 2.5m on the Sur coast leading to limited coastal inundation extent. Because Oman adjoins the western MSZ, the probabilistic analysis includes the effect of this segment also. The probabilistic analysis shows onshore inundations exceeding 0.4km northwest of Sur where flow depths are likely to exceed 1m in 500years. Probability analysis shows lesser inundation areas with probability of exceeding 1m flow depth up to 80% in 500-year exposure time. Teletsunamis are excluded from these analyses because far-field waves of the 2004 Indian Ocean tsunami did not impact the Sur coast. Also excluded for simplicity are tsunamis generated by submarine slides within or near MSZ rupture areas. The results of this research provide essential information for coastal planning, engineering and management in terms of tsunami hazard and an essential step toward tsunami risk reductions in the northwest Indian Ocean.
- The transoceanic 1755 Lisbon tsunami in MartiniquePublication . Roger, J.; Baptista, Maria Ana; Sahal, A.; Accary, F.; Allgeyer, S.; Hébert, H.On 1 November 1755, a major earthquake of estimated M (w)=8.5/9.0 destroyed Lisbon (Portugal) and was felt in the whole of western Europe. It generated a huge transoceanic tsunami that ravaged the coasts of Morocco, Portugal and Spain. Local extreme run-up heights were reported in some places such as Cape St Vincent (Portugal). Great waves were reported in the Madeira Islands, the Azores and as far as the Antilles (Caribbean Islands). An accurate search for historical data allowed us to find new (unpublished) information concerning the tsunami arrival and its consequences in several islands of the Lesser Antilles Arc. In some places, especially Martinique and the Guadeloupe islands, 3 m wave heights, inundation of low lands, and destruction of buildings and boats were reported (in some specific locations probably more enclined to wave amplification). In this study, we present the results of tsunami modeling for the 1755 event on the French island of Martinique, located in the Lesser Antilles Arc. High resolution bathymetric grids were prepared, including topographic data for the first tens of meters from the coastline, in order to model inundations on several sites of Martinique Island. In order to reproduce as well as possible the wave coastal propagation and amplification, the final grid was prepared taking into account the main coastal features and harbour structures. Model results are checked against historical data in terms of wave arrival, polarity, amplitude and period and they correlate well for Martinique. This study is a contribution to the evaluation of the tele-tsunami impact in the Caribbean Islands due to a source located offshore of Iberia and shows that an 8.5 magnitude earthquake located in the northeastern Atlantic is able to generate a tsunami that could impact the Caribbean Islands. This fact must be taken into account in hazard and risk studies for this area.