Avaliação de risco de tsunami na cidade de Lisboa por deslizamentos no delta do Tejo.
- Universidade de Évora(líder)
- Instituto Português do Mar e da Atmosfera (IPMA)(parceiro)
Os grandes tsunamis, como os ocorridos em 2011 no Japão e 2004 em Samatra mudaram o rumo de algumas preocupações societais e económicas a nível mundial. A costa portuguesa foi atingida por eventos de tsunami de envergadura semelhante, por ocasião do Grande Sismo de Lisboa de 1 de novembro de 1755 e por outro em 1531. Outros menores ocorreram em 1969 AD, 1941 AD, 382 AD e 60 BC. Para além de sismos, há outras fontes reconhecidas de tsunamis, como tempestades ou deslizamentos de terras.
O projeto TAGUSDELTA mostrou que metade da parte frontal do delta do rio Tejo colapsou no passado geológico recente, entre os 8 e os 13 mil anos BP. Este deslizamento com 10km de comprimento, 4,5 km de largura e com espessura até 20 m ocupa uma área de 45 km2, e um volume de 0,9km3. Foi ainda mapeada uma área com 33km2 de gás aprisionado em sedimentos do delta, ocorrendo desde o fundo do mar até profundidades mínimas de 10 m abaixo do fundo. Nalguns sectores, este gás está selado e consequentemente sobrepressurizado, criando as condições de diminuição do coeficiente de fricção na frente do delta, aumentando a possibilidade de ocorrência de deslizamentos de massa. O mecanismo causador do deslizamento mapeado do delta do Tejo é ainda desconhecido, podendo ter sido um grande sismo proximal, uma libertação de gás, migração de gás intersticial ou uma grande tempestade.
A ocorrência de gás intersticial levanta as seguintes questões: a) Qual a natureza e origem do gás? Microbiano superficial ou termogénico profundo com origem em formações da Bacia Lusitânica? b) Qual a probabilidade de ocorrência de um deslizamento ou colapso do delta, associado à presença de gás nos sedimentos e que estratégias de mitigação podem ser tomadas? A modelação preliminar da onda de tsunami gerada por movimento súbito de um deslizamento como o cartografado no Tejo indica valores até 3m de altura de onda, nas zonas costeiras adjacentes. Saber se este tsunami afetará a cidade de Lisboa ou não carece de técnicas de modelação numérica avançadas devido à circunstância geográfica particular do estuário do Tejo e das pequenas profundidades de água. Determinar a origem e natureza do gás intersticial nos sedimentos do delta do Tejo, mapear as estruturas de escape de fluidos e as de instabilidade gravítica incipientes são as condições iniciais que carecemos para enfrentar uma estratégia de mitigação do risco de deslizamento em massa no delta do Tejo e seus efeitos não apenas em Lisboa como no litoral vizinho.
A equipa junta investigadores com experiência na aquisição de dados submarinos acústicos e amostragem de fundo (IPMA), biogeoquímica, sedimentologia ambiental, diagénese, mudanças climáticas e dinâmica estuarina (UÉvora e IPMA). A Univ. de Évora tem os laboratórios de análises geoquímicas e biogeoquímicas, o IPMA tem os navios para as missões, amostragem de fundo e o laboratório de interpretação de dados.
Objetivos, atividades e resultados esperados/atingidos
Knowing that half of the front of the Tagus delta was removed by a single landslide event in the recent geological past (~8-13ky BP), that various lesser slides preceded the latter and that half of the delta front contains pervasive shallow gas (Terrinha et al., submitted), we consider that the possibility of large failure of the Tagus River delta is a serious source for tsunami hazard that jeopardizes the coastal population and facilities and submarine infrastructures (namely communication cables, harbours and berths, sanitary sewers). Since the origin and chemical composition of the observed shallow gas is still unknown we also consider important to determine its nature and to assess the volume of gas involved since the degassing of sedimentary basins in continental margins has long been identified as a potential forcer of the climate change (Judd et al, 2002).
Considering that gas and fluids are the most effective way of attenuating friction and thus assist sliding in any rock or sediment formation our departing questions are the following: i) what is the nature and origin of the shallow gas? ii) where and how much overpressured are the sedimentary layers? iii) what are the geotechnical properties of weak layers and at what depth are they?, iv) is slumping or creep going on at present in the delta front? After having mapped the two anomalous areas in the Tagus delta, the TAGUS LANDSLIDE and the SHALLOW GAS AREA, the next necessary steps to take in order to understand the hazard of large landslides of the delta and how this can be mitigated are:
FIRSTLY, scrutinize the seafloor looking for evidences for ongoing down slope instability processes and gas escape. This will be achieved by surveying the delta front between 20m and 105m below sea level using high resolution side scan sonar looking for backscatter contrasts that may arise from gas escape structures, ejected sediment flow and creep or slump instabilities. The area will be surveyed by high resolution multibeam echo-sounder. This second method is necessary because the delta is swept by currents that are liable to homogenize the sedimentary cover and small reliefs attenuating the backscatter response.
SECONDLY, investigate de origin of the gas and the nature of the gas bearing sediments. This information will be achieved by evaluating high resolution depth profiles of early diagenetic electron acceptors/donors in the pore water and in the solid fraction. The chemical/isotopic characterization of the gas coupled with the characterisation of the vertical distribution of organic substances that can originate the gas will also give insights on its origin. The understanding of the sediment nature, structure and chemical status in the vicinity of gas bubbles will also provide vital information about its origin.
THIRDLY, characterize the geotechnical properties of the stratigraphy of both landslide and shallow gas areas in order to assess the (in)stability conditions of the Tagus delta at present and determine the factor of safety. Integration of these data with the existent 2D and 3D seismic will allow the production of depth maps (conversion from TWT) and of a Mechanical Earth Model.
FOURTHLY, determine the age of the Tagus delta landslide (and smaller earlier ones) and describe the sedimentology and chrono-stratigraphy of the delta. This will allow us to calibrate the previously acquired 2D seismic data and give us further insight into the delta geological history including clues on eustatic variations and climate changes that might have been associated with the mass transport deposits.
FIFTHLY, modelling of the landslide body movement and the subsequent tsunami that might be generated considering different scenarios (various landslide dimensions, different sediment rheologies, and changes at the present-day water depths) and of the tsunami propagation and run-up. This part of the work will benefit from the previously described ones, mainly from the geotechnical data that helps to constrain the landslide physical characteristics. Various scenarios will be tested in order to investigate the tsunamigenic potential of the submarine mass-failures in the TAGUS-Delta as well as their possible tsunami impact in the target surrounding coasts.
SIXTHLY, promote the awareness for the hazard of landslides of the Tagus delta and measures to mitigate them. Besides the dedicated website of the project, the IPMA website will also be used for dissemination. More actions will be developed such as creating a set of lectures aiming at different public, such as high-schools and local and civil protection authorities.
In order to ensure the good execution of the activities special care was put on the selection of the team. Besides selecting researchers with proven capabilities and know-how to carry out the activities from a scientific and technical point of view we also engaged on the project the researchers that in the past dedicated an important part of their research to i) tsunami hazard and modelling, ii) geology of mass transport deposits, ii) acoustic imaging of shallow seafloor, iii) diagenesis and geochemistry of estuarine and shallow seafloor; iv) biogeochemistry, and v) mechanics of landslides. All the activity leaders have worked in the respective field for various years after concluding their PhDs and all of them have coordinated projects or been responsible for large amounts of data acquisition. Some of us have dedicated their career to research in natural hazards (tsunami hazard: Pedro Terrinha, Maria Ana Baptista; chemical hazard: Miguel Caetano). Maria Ana Baptista is a tsunami modeller for more than 20 years and led national and European projects on tsunami hazard. Carlos Ribeiro (IR) has been working since his PhD on geochemistry and isotope geology specially focused on the paleoclimatic analysis and in water-rock interaction, Pedro Terrinha (CO-IR) has led national projects and work packages of European projects on tsunami hazards and neotectonics; he is a specialist in structural geology and tectonics of sedimentary basins and of the geology of the study area. Pedro Brito has a PhD in Geology on the study of ebb-tidal delta systems. He has been engaged and responsible for the acquisition of thousands of kilometres of single and multi channel seismic reflection, side scan sonar and multibeam data. Pedro Terrinha coordinated various seismic reflection, side scan sonar and multibeam campaigns on the Portuguese continental shelf, including the TAGUSDELTA in 2013. Miguel Caetano is the Head of the Division of Environmental Oceanography and Bioprospection. His research focus in marine science and chemical oceanography. He has been involved in several research projects and contracts concerning the biogeochemical processes in the water column, interactions of pollutants between sediments and aquatic organisms and diagenetic processes in sediments. Fatima Abrantes has published on the topmost journals and participated in various international projects on paleoceanography and climate change based on deep and shallow marine sediments.
The participating institutions also have the laboratorial facilities to perform most of the tasks, as follows. Some equipment will be rented. The acoustic surveys will be carried out using the IPMA vessel (Diplodus or Noruega research vessels). The side scan sonar will be rented and the multibeam echosounder belongs to IPMA. The processing will be made immediately after the cruises in order to allow for selection of sites for sea floor sampling. Integration of the sonar and multibeam data with the seismic reflection data will be made at IPMA (SEISLAB). For the gravity core operation the vessel Mar Portugal (IPMA) will be used. Gas, mineral, organic compounds and sediment granulometry analysis will be made at: i) the University of Évora HERCULES laboratory and, ii) the IPMA laboratories of environmental oceanography, sedimentology and micropaleontology; 14C analysis will be a service to be acquired at an international recognized lab. At MARUM, U. Bremen a number of geotechnical experiments can be set up to account for a wide range of pressure, temperature and dynamic conditions. Apart from basic standard tests (vane shear, fall cone penetration, oedometer), the lab provides some sophisticated setups for specific settings, e.g. ring shear, dynamic triaxial tests, heated oedometers and heated direct shear apparatus. D. Voelker, with vast experience on studies on instability of continental margins and lab geomechanical testing ensure the connection and tests of the collected samples.
Atividade: 1 - Seabed morphology and sediment sampling
Atividade: 2 - Gas origin and sediment characterization
Atividade: 3 - Geotechnical characterization
Atividade: 4 - Age model and sedimentology
Atividade: 5 - Tsunami modelling
Atividade: 6 - Coordination and data dissemination
A1 - i) Map of backscatter data draped over MBES bathymetry; ii) Geomorphologic map, highlighting backscatter patterns, fluid seepage and sediment
instabilities structures; iii) Sediment sampling.
A2 - i) Geochemical characterization of the sediments; ii) chemical characterization of the gas; iii) determination of the origin of the gas; iv) insight on the pathway of biogeochemical reactions that produce methane.
A3 - i) improved understanding of past slope failure events documented in the seismic record; ii) identification of weak layers within the frontal delta, as well as identification of possible scenarios in terms of static and dynamic loading that are critical for the delta front stability.
A4 - i) age model for the Tagus Delta main stratigraphic units; ii) age of the landslide; iii) description of the lithology and physical properties (from MSCL) of the delta units. :
A5 - i) Test the tsunami potential of the submarine landslides in the Tagus delta through the modeling of the possible tsunami generation following the landslide movement; ii) Estimate the coastal impact along the surrounding zones that may be affected by a tsunami triggered by a submarine landslide in the Tagus delta.