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Analysis of seismic reflection data as a tool for the seismotectonic assessment of a low activity intraplate basin – the Lower Tagus Valley (Portugal)

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Abstract

The Lower TagusValley is located in the Lower TagusSedimentary Basin. Occurrence ofearthquakes in the area indicates thepresence of seismogenic structures at depththat are still deficiently known due to athick Cenozoic sedimentary cover. It isusually assumed that regional M = 6–7earthquakes are generated by the LowerTagus Valley fault. However, this structurehas never been characterised, and there isevidence for the presence of other faultsthat have the potential of generating largeseismic events. The hidden character ofmany of the structures in the Lower TagusBasin necessitates the use of indirectmethodologies for their study. Availableseismic reflection lines acquired for oilexploration in the Lower Tagus Valleyregion were thus examined, with emphasis onthe recognition of faults displacing theCenozoic and particularly the youngergeological units. The foreseen complexityof the basin structure was confirmed.Evidence for a continuous `Lower TagusValley fault' is absent. Instead, N-S toNNE-SSW trending, en echelon,sub-vertical and reverse faults showingsignificant post-Miocene offsets, linked bytransverse (NW-SE) faults that accommodatelarge displacements, were recognised in theseismic sections that cross the Tagusvalley. These faults range in length from10 km to a maximum of ca. 40 km, leading toan estimation of 6.2–6.9 maximum credibleearthquake magnitudes, which fall in themagnitude range of the regional historicalseismicity.

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Authors and Affiliations

  1. Dep. Geologia, Fac. Ciências de Lisboa, and LATTEX, Ed. C2, 5° Piso, Campo Grande, 1749-016, Lisboa, Portugal

    J. Cabral & P. Ribeiro

  2. Inst. Geológico e Mineiro, Estrada da Portela, 2720, Alfragide, Portugal

    C. Moniz & P. Terrinha

  3. Dep. Física, Fac. Ciências de Lisboa, Ed. C8, Campo Grande, 1749-016, Lisboa, Portugal

    L. Matias

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  1. J. Cabral
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  4. P. Terrinha
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  5. L. Matias
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Cabral, J., Moniz, C., Ribeiro, P. et al. Analysis of seismic reflection data as a tool for the seismotectonic assessment of a low activity intraplate basin – the Lower Tagus Valley (Portugal). Journal of Seismology 7, 431–447 (2003). https://doi.org/10.1023/B:JOSE.0000005722.23106.8d

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