Geophysical study of the Ota–VF Xira–Lisbon–Sesimbra fault zone and the lower Tagus Cenozoic basin

Abstract

This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota-Vila Franca de Xira-Lisbon-Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr-1

Tectonic evolution of an intraplate basin: the Lower Tagus Cenozoic Basin, Portugal

“…Gravimetric data are well suited to estimate the depth to the base of Cenozoic due to the large density contrast between these units and underlying Mesozoic or Palaeozoic rocks (e.g. Dobrin & Savitt, ; Carvalho et al ., ,b). 2.5D gravimetric modelling was, therefore, used to estimate the base of the Cenozoic in Area II where no seismic coverage exists.…”

“…Reprocessing and reinterpretation of part of this dataset with the help of potential‐field data has been carried out by Carvalho et al . (, , ). These works also focused more on the location of major seismogenic sources and did not address in particular the evolution of the LTCB and the structure of the base the basin remained unknown in its central sector.…”

Incorporating Descriptive Metadata into Seismic Source Zone Models for Seismic-Hazard Assessment: A Case Study of the Azores-West Iberian Region

“…Likewise, the region affected by the 1858 M w 7.1 Setúbal earthquake, located further southwest within the continental shelf, also displays low instrumental seismicity rates. Several studies have proposed a structural relationship between both regions (e.g., Fonseca and Long, 1991;Vilanova and Fonseca, 2004;Carvalho et al, 2011). A complex system of potentially active faults has been identified within the Lower Tagus Valley using geomorphological analysis, geological outcrop, and geophysical data (e.g., Carvalho et al, 2011;Besana-Ostman et al, 2012).…”

“…Several studies have proposed a structural relationship between both regions (e.g., Fonseca and Long, 1991;Vilanova and Fonseca, 2004;Carvalho et al, 2011). A complex system of potentially active faults has been identified within the Lower Tagus Valley using geomorphological analysis, geological outcrop, and geophysical data (e.g., Carvalho et al, 2011;Besana-Ostman et al, 2012). SSZ 6 was defined taking into account the extent of the onshore region (L0 criterion) formed by extended continental crust (L1 criterion) that comprises the proposed active faults systems (L2 criterion).…”

Earthquake Mitigation in the Lisbon and Lower Tagus Valley area, Portugal

“…Carvalho et al, 2011;Besana-Ostman et al, 2012) but several authors have recognized the importance of the thick Cenozoic cover that covers part of the region (e.g, Carvalho et al, 2006;) and also of local site effects in the fabric of the seismic intensities observed during historical earthquakes (e.g. Teves-Costa and Batló, 2011).…”