Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial II, 697-700 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X Searching for landslides in the stratigraphic record of the Tagus river delta as evidences for tsunami and earthquakes, using 3D seismic reflection Em busca de deslizamentos submarinos no registro estratigráfico do delta do rio Tejo como evidências de sismos e tsunami, utilizando sísmica de reflexão 3D J. Noiva1*, H. Duarte2, P. Terrinha1, P. Brito3, V. Magalhães1, P. Alves4, M. Rosa1, M. Oliveira2, F. Curado5 Artigo Curto Short Article . © 2014 LNEG – Laboratório Nacional de Geologia e Energia IP Abstract: The deltas of rivers with high sedimentation rate contain a were proposed, and in 1992 the first attempt to find this great potential for the preservation of evidence of mass-transport source was made using multi-channel seismic reflection deposits related to tsunami and earthquakes. The Tagus river delta is a (Zitellini et al., 1999). A number of marine surveys mapped sedimentary body in the Lisbon area with proven potential for the preservation of evidence of this kind of mass-transport deposits. The the area that encompasses the Gorringe Bank and the preliminary analysis of 2D/3D high resolution seismic data acquired Gibraltar Straits, where the Africa and the Eurasia lithospheric during TAGUSDELTA 2013 cruise shows exceptional high quality plates meet, and where the source of the 1755 earthquake imaging of the past Last Glacial Maximum (LGM) transgressive most probably lies. Different sources for this earthquake have surfaces and of various landslides events in the sedimentary record of been proposed, a single source, a subduction zone (Gutscher, Tagus River delta. 2004), multiple sources based on active thrust faults (Gràcia Keywords: Tagus river delta, 3D seismics, Submarine landslides, et al., 2003; Terrinha et al., 2003, 2009) and a new plate Tsunami, Earthquakes. boundary (Zitellini et al., 2009). Resumo: Os deltas dos rios com alta taxa de sedimentação têm um Regarding the tsunami wave propagation, dimensions and grande potencial para a preservação depósitos transportados em times of arrival, various sources were tested; single and massa relacionados com sismos e tsunami. O delta do rio Tejo é um complex sources (Baptista et al., 1998, 2003; Gutscher et al., corpo sedimentar na região de Lisboa com potencial comprovado 2006). Vilanova et al. (2003) and Matias et al. (2005) para a preservação da prova deste tipo de depósitos. A análise documented replicas of the main quake event in the Tagus preliminar dos dados de sísmica de alta resolução 2D/3D adquiridos durante o cruzeiro TAGUSDELTA 2013 mostram imagens de river and discussed their origin in this area. Abrantes et al. qualidade excecional das superfícies transgressivas do Último (2005, 2008) found the evidence of a landslide, correlated Máximo Glacial (UMG) e de vários episódios de deslizamentos with the 1755 tsunami, in a piston core of the Tagus pro-delta. submarinos preservados no registo sedimentar do delta do rio Tejo. Besides that, Neves et al. (2009) and Terrinha et al. (2009) Palavras-chave: Delta do rio Tejo, Sísmica 3D, Deslizamentos showed the northward propagation of Quaternary thrust submarinos, Tsunami, Sismos. faulting along the south margin of Portugal at latitudes near Lisbon, that defines a favorable structural framework for the 1Instituto Português do Mar e da Atmosfera, Divisão de Geologia Marinha e development/amplifications of major regional earthquakes in Georecursos Marinhos. 2 the area near Lisbon. Geosurveys - Consultores em Geofísica, Lda. The ebb delta of the Tagus river is a major shallow water, 3Laboratório Nacional de Energia e Geologia, Unidade de Geologia Hidrogeologia e Geologia Costeira. Late Pleistocene-Holocene body located in the high energy, 4Instituto Português do Mar e da Atmosfera, Divisão de Geofísica. sediment starved, west Iberian shelf (Lantzsch et al., 2009). 5Centro de Estudos do Ambiente e de Mar - Universidade de Aveiro. The high sedimentation rates allowed for the preservation of *Corresponding author / Autor correspondente: [email protected] the 1969 AD and 1755 AD tsunami evidences found in gravity cores in the Tagus pro-delta (Abrantes et al., 2005, 2008), suggesting that the Tagus delta is a sedimentary body 1. State of the art in the Lisbon area with high potential for the preservation of evidence of mass-transport deposits related to tsunami and It is a well-documented fact that Lisbon, one of the most earthquakes. important cities in the XVIII century, was destroyed on the Nevertheless, the architecture of the Tagus delta 1st November 1755 by the Great Lisbon earthquake (M~8.5) remains unknown and a great wealth of information for and subsequent tsunami. Various sources for the earthquake tsunami and earthquake hazard assessment remains hidden. 698 J. Noiva et al. / Comunicações Geológicas (2014) 101, Especial II, 697-700 2. Objectives 1) Decipher the stratigraphy and sedimentary architecture of the frontal part of the Tagus delta; In the framework of the TAGUSDELTA project 2) Image the evidences for mass transport deposits (PTDC/MAR/113888/2009) the TAGUSDELTA 2013 (scouring, sediment transport and landslide deposition); seismic survey was carried out between 4 and 15 3) Correlate these results with the gravity core that December 2013, onboard of research vessel Noruega. The recorded the 1755 and 1969 earthquakes and tsunami main objective of the TAGUSDELTA 2013 cruise (Fig. 1) deposits (M~8.5 and M=7.9, respectively) and scouring was to develop and implement a viable technical solution of sediments (Abrantes et al., 2005, 2008) that will to acquire a high resolution 3D seismic image of the Tagus allow a first trial on the Pleisto-Holocene mapping and river delta and pro-delta sediments. Together with a 3D chronostratigraphy of landslides in the Tagus delta; high resolution seismic survey on a specific targets, a 2D 4) Image the morphologies of landslide structures with Multichannel Seismic (MCS) regular grid and long MCS 3D seismic allowing the quantification and precise lines were acquired on a larger scale to obtain a high location of the events, thus enabling modelers to play resolution 2D and 3D image of the stratigraphy and with quantitative parameters with unprecedented structure of the Tagus delta and of the landslide here realism. present. This data will allow to: Fig. 1. Blue lines represent 2D multichannel seismic lines coverage, black lines represents the 3D seismic coverage, both acquired during the TAGUSDELTA 2013 cruise. Isopachs map show the Tagus landslide area interpreted with previous surveys. The map shows previous work done in the study area, the grey lines are boomer single channel profiles from Lisboa98 cruise and the red lines are sparker single channel profiles from PACEMAKER 2011 cruise. Fig. 1. As linhas azuis representam os perfis sísmicos 2D multicanal, as linhas pretas representam a cobertura da sísmica 3D, ambas adquiridas durante o cruzeiro TAGUSDELTA 2013. O mapa de isópacas mostra a área do deslizamento interpretada com os dados dos cruzeiros anteriores. O mapa mostra a cobertura das linhas adquiridas nesses cruzeiros, as linhas a cinzento são perfis sísmicos boomer monocanal adquiridos no cruzeiro Lisboa98 e as linhas vermelhas são perfis sísmicos sparker monocanal adquiridos durante o cruzeiro PACEMAKER 2011. Search Tagus river delta landslides with 3D seismic 699 3. Data and methods sparker seismic source powered by a 6 kJ Pulsed Power Source (PPS) and 2 streamers with 24 channels each towed The results obtained from seismic data acquired on previous in a triangular shape geometry and positioned by geophysical (Lisboa 98 and Pacemaker 2011) and sediment Automatic Identification System (AIS) antennas. sampling campaigns (Poseidon 287 and Discovery 249), This 3D towing geometry was trialed for the first time shown in figure 1, in the area off the mouth of the Tagus in the Tagusdelta survey and allows the acquisition of river, point to the existence of landslides (Fig. 1) in the seismic volumes with a horizontal resolution of 1 m and a Tagus delta and suggest that these landslides are linked to vertical resolution of 15 cm, and a signal penetration of 50 seismic activity and the generation of tsunamis. to 150 m. This acquisition system is scaled for deployment, also, in small coastal research vessels (12 to 3.1. Previous work 25 m long) allowing for cost-effective coverage of a 10-20 The results obtained from seismic data acquired on km2 area, at 5 to 200 m water depth, in 10-20 survey days. previous geophysical (Lisboa 98 and Pacemaker 2011) and The 3D acquisition concept trialed in the Tagusdelta sediment sampling campaigns (Poseidon 287 and survey shares some conceptual similarities with the 3D P- Discovery 249), shown in figure 1, in the area off the Cable system used by Masson & Berndt (2006) and Perez- mouth of the Tagus river, point to the existence of Garcia et al. (2009), but is significantly different in its landslides (Fig. 1) in the Tagus delta and suggest that these hardware components (i.e. seismic source, receiver arrays landslides are linked to seismic activity and the generation and positioning) and in the specific way the receivers are of tsunamis. towed behind the vessel. 3.2. The TAGUSDELTA 2013 cruise 4. Preliminary Results The objective of this mission was to run a 2D seismic The 3D seismic data processing requires specialized reflection survey to get an accurate velocity model of the software that could not be deployed onboard, and because study area and a 3D seismic reflection survey in order to it is time consuming, it was not possible to obtain obtain a high resolution 3D image of the stratigraphy and preliminary processed data products during the cruise. So structure of the study area. The research vessel Noruega far the 2D seismic reflection acquired data is being from IPMA was used to carry out the TAGUSDELTA processed and the interpretation of the data has started.