Tsunami hazard assessment of the Lisbon city associated to the Tagus delta landslide.

Cofinanciado por:
Acronym | Tagusgas
Project title | Tsunami hazard assessment of the Lisbon city associated to the Tagus delta landslide.
Project Code | PTDC/CTA-GEO/031885/2017
Main objective | Reforçar a Investigação, o desenvolvimento tecnológico e a inovação

Region of intervention |

Beneficiary entity |
  • Universidade de Évora(líder)
  • Instituto Português do Mar e da Atmosfera (IPMA)(parceiro)

Approval date | 06-07-2018
Start date | 01-10-2018
Date of the conclusion | 30-09-2021
Date of extension | 30-09-2022

Total eligible cost | 239496 €
European Union financial support |
National/regional public financial support | República Portuguesa - 239496 €

Summary

Tsunamis have in very recent times changed the course of societal concerns and economics, such as the 2011 tsunami that hit the coast of Japan or the 2004 of Samatra. The Portuguese coast was struck by large tsunamis: the most well known for their tragic effects are the 1755 and the 1531 earthquake triggered tsunamis. Less known historical seismic tsunamis occurred in 1969 AD, 1941 AD, 1909 AD, 1344 AD, 382 AD and 60 BC. Apart from large earthquakes, other sources for tsunamis are known like meteo-tsunamis and landslide triggered tsunamis.

The previous project TAGUSDELTA showed that half of the frontal part of the Tagus River delta collapsed, between 8 and 13 ky BP. A 10km long, 4.5km wide landslide (TDL) has an area of ~45km2 and a volume of ~0.9km3. 33km2 of shallow gas bearing sediments at the delta front were also mapped. The shallow gas and the TDL areas are almost mutually exclusive neighbours. Most of the shallow gas is known to occur from virtually seafloor to 10m depth below sea floor. The gas trapped in depth has to be overpressured, reducing dramatically the friction conditions in the delta front and creating the ideal conditions for landsliding. The trigger mechanism of the TDL is not known. It could be related with a seismic event or with a non-seismic trigger mechanism related with gas escape, gravity instability of the delta frontal lobe sediments facilitated by pore fluid migration or a meteo-tsunami. The occurrence of shallow gas raises the following problems: i) What is the nature and origin of the gas? Microbial shallow gas or thermogenic gas originated from deep hydrocarbons of the rift basin? ii) If the overpressured area is significant, what is the mitigation strategy to lower the odds of landslide occurrence? The sudden movement of a landslide of ~45km2 and 0.9km3  at the present day water depths of the delta front would generate a tsunami of ~3m maximum wave height on the adjacent coastal areas. Whether this tsunami wave would affect the Lisbon down town is still a matter of discussion because advanced modeling is needed once that the water column is in the same order of magnitude of the thickness of the displaced material. Determining the nature and origin of the trapped shallow gas , mapping the fluid escape structures, search and map ongoing gravity instability structures like slumps, recent landslides or tensile cracks will guide us towards future research directed to mitigate the effects of gas bursts, or pore pressure build up and foster the societal preparedness towards landslide triggered tsunamis.

The team comprehends specialists in acoustic data acquisition, processing and interpretation (IPMA), biogeochemistry, environmental sedimentology, diagenesis, climate change, and dynamics of estuary-delta environments (U. Évora and IPMA). The Univ. of Évora has the laboratorial facilities for the analysis of the sediments, the IPMA has vessels to perform cruises, sea floor physical sampling and a seismic processing lab.


Objectives, activities and expected/achieved results

Objectives

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.

Activities

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

Results

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.