Seismic hazard impact of the Lower Tagus Valley Fault Zone (SW Iberia)

Los terremotos del Bajo Valle del Tajo (Portugal): Lisboa 26 de Enero de 1531 y Benavente 23 de Abril de 1909

“…Search for surface faulting evidence and paleoseismic data has been performed by several teams on the Lower Tagus fault system, though the results are still scarce and controversial (Fonseca et al 2000, Vilanova 2003, Vilanova and Fonseca 2004, Cabral and Marques 2001, Cabral et al 2011, Besana-Ostman et al 2012, Canora et al 2015. Based on detailed geomorphic studies, Besana-Ostman et al (2012) argue to have recognized a major active structure extending NNE-SSW for 85 km along the western bank of the Tagus River, which they call the "Lower Tagus Valley fault zone".…”

Looking for Earthquake Sources in the Lisbon Area

“…Studies of part of these data have been realised with paper migrated stacked sections (Rasmussen, 1998;Kullberg, 2000;Cabral et al, 2003). Reprocessing and reinterpretation of several thousand kilometres of these profiles in digital format has been carried out (Carvalho et al, 2006a;b (Fig. 1) to tie the seismic horizons to the geological formations while aeromagnetic and gravimetric data were used for picking the basement (top of Paleozoic) and locate salt and igneous structures (Fig.…”

“…Besides offshore sources connected to the Eurasia-Africa plate boundary such as in the 1755 Lisbon earthquake and in 1969, local seismicity has been responsible for moderate earthquakes that caused important damage such as in 1531 and 1909 (Moreira, id.). Local seismicity has been recently considered of great importance (Vilanova and Fonseca, 2004;Peláez et al, 2002). However, the sources of local historical earthquakes are still under debate (e.g.…”

“…For the above reasons, available geophysical data and geological data have been used to locate possible earthquake sources. This information has been (Carvalho et al, 2006a;b) and will continue to be complemented with near surface data to confirm if faults are active and to estimate source parameters. Considering that most of the earthquakes are generated at crustal depths, the study of those structures at depth is of great importance.…”

“…These are the Porto Alto fault (Carvalho et al, 2006a), the Pinhal Novo fault (Rasmussen et al, 1998;Cabral et al, 2003;Kullberg, 2000) and Azambuja fault (Rasmussen et al, 1998;Cabral et al, 2004). Except for the papers from Carvalho et al (2006a;b), the previous work was based on paper stacked sections. Here, we present recently reprocessed and reinterpreted digital seismic reflection data, well, geological outcrop, reinterpreted magnetic and gravimetric data, together with relocated hypocenter data to present new enlightenments in the study area major tectonic structures.…”