HR: 12:05h
AN: T22C-08    [Abstracts]
TI: The Lisbon-Vila-Franca de Xira Fault System Revisited, Portugal
AU: * Mendes-Victor, L
EM: lavictor@fc.ul.pt
AF: Centro de Geofisica da Universidade de Lisboa, R. Escola Politecnica 58 1250-102 Lisboa, Lisbon, 1250-102 Portugal
AU: Carvalho, J
EM: joao.carvalho@ineti.pt
AF: Instituto Nacional de Engenharia, Tecnologia e Inovacao, Estrada da Portela - Zambujal 2720-461 Amadora, Amadora, 2720-461 Portugal
AU: Goncalves, R
EM: rui.goncalves@ipt.pt
AF: Instituto Politecnico de Tomar, Quinta do Contador 2300-313 Tomar, Tomar, 2300-313 Portugal
AU: Cabral, J
EM: jcabral@fc.ul.pt
AF: Laboratorio de Tectonofisica e Tectonica Experimental, Faculdade de Ciencias da Universidade de Lisboa Ed. C6, 2piso, Campo Grande, Lisbon, 1749-016 Portugal
AB: The study region is located in the Lower Tagus Valley (LTV), central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with historical earthquakes reporting important casualties, serious damages and socio-economic losses. Occurrence of earthquakes in this intraplate area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Holocenic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Basin requires the use of indirect methodologies for their study, including the application of geophysical methods to locate with precision a major tectonic structure that has been recognized previously on deep oil-industry seismic reflection profiles, aeromagnetic data and inferred from the surface geology. This is a NNE-SSW trending reverse fault zone that borders the Lower Tagus Cenozoic Basin (LTCB) at the W (Lisbon-Vila Franca de Xira fault system). The structure, which outcrops 20 km north of Lisbon, was generated in the Mesozoic as a large normal fault zone and was later tectonically inverted, moving with a predominant reverse slip component during the Neogene. This fault zone outcrops as a steeply dipping east-verging reverse fault that thrusts Jurassic rocks of the Lusitanian Basin over tilted Miocene sediments of Tortonian age. The extension of the fault zone northwards of Vila Franca de Xira is expressed at the surface by faults displacing eastwards dipping Jurassic rocks. In spite of poor earthquake locations in Portugal mainland, it is clear that most of the seismicity in the study area is located to the W of this structure. Recently reprocessed and reinterpreted oil-industry seismic reflection have given an improved vision of the fault system, which is composed of several sub-parallel normal faults demonstrate a variable degree of inversion. Furthermore, a short (400m) high-resolution seismic reflection profile was performed to the N of Vila-Franca de Xira in between the oil profiles sparse locations. Two shallow Vertical Electrical Soundings were also performed over the seismic profile for a better near-surface characterization. Several wells with lithologies and SPT data are available a few hundreds of meters from both ends of the profile. According to the interpretation of this collected data, two major faults were detected north of Vila Franca de Xira, extending the length of the Lisbon-Vila Franca de Xira fault system to more than 20 km. One of these faults presents reverse geometry apparently displacing Holocene alluvium. The possible fault offsets of these sediments imply minimum slip rates of 0.15 to 0.30 mm/yr, 3 times larger than previously inferred for active faults in the Lower Tagus Valley, and Maximum Return periods of 1,700 to 7,500 yrs for Magnitudes 6.5-7 co-seismic ruptures concerning this intraplate area.
DE: 0910 Data processing
DE: 0935 Seismic methods (3025, 7294)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8015 Local crustal structure
DE: 8107 Continental neotectonics (8002)
SC: Tectonophysics [T]
MN: Fall Meeting 2005