The Azambuja fault: An active structure located in an intraplate basin with significant seismicity (Lower Tagus Valley, Portugal)

Abstract

The Azambuja fault is a NNE trending structure located 50 km NE of Lisbon, in an area of important historical seismicity. It is sited in the Lower Tagus Basin, a compressive foredeep basin related to tectonic inversion of the Mesozoic Lusitanian Basin in the Miocene. The fault is evident in commercial seismic reflection data, where it shows steep thrust geometry downthrowing the Cenozoic sediments to the east. It has also a clear morphological signature, presenting a NNE-SSW trending, east facing, 15 km long scarp, reaching a maximum height of 80 m. The fault scarp is the geomorphic appearance of a flexure expressed as a zone of distributed deformation, where Miocene and Pliocene sediments are tilted eastwards and are cut by steeply dipping meso-scale faults presenting reverse and normal offsets, with a net downthrow to the east. This pattern at the surface is compatible with a steep fault in the basement that tilts and branches through the overlying Cenozoic sedimentary cover. In order to constrain the neotectonic activity of this structure, detailed geological studies were conducted. Morphotectonics was studied through aerial photo interpretation, analysis of topographic maps and digital mapping. Those studies indicate Quaternary slip on the fault in the ranges of 0.05-0.06 mm per year. Seismogenic behaviour was assumed for the Azambuja fault based on the evidence of Quaternary tectonic activity and its location in an area of significant historical seismicity. M w 6.4-6.7 maximum earthquakes, with recurrence intervals of 10000-25000 years, were estimated based upon the displaced morphological references, cumulative offsets and fault length.

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

“…80 m near the village of Azambuja, where it is interrupted by the present Tagus riverbank. The fault continues southwards hidden under the recent alluvium of the Tagus River, probably extending for at least further 15 km where it was detected affecting Tertiary horizons in a seismic section, thus totalizing an overall length of over 30 km (Cabral et al 2004;. Geological and morphotectonic studies indicate Quaternary slip on the fault in the range of 0.05-0.06 mm/yr (Cabral et al 2004).…”

“…The fault continues southwards hidden under the recent alluvium of the Tagus River, probably extending for at least further 15 km where it was detected affecting Tertiary horizons in a seismic section, thus totalizing an overall length of over 30 km (Cabral et al 2004;. Geological and morphotectonic studies indicate Quaternary slip on the fault in the range of 0.05-0.06 mm/yr (Cabral et al 2004). The southern, hidden sector of the Azambuja fault extends to the meizoseismal area of the 1909 earthquake, making this fault a likely source for the seismic event (Cabral et al 2011).…”

Intensity-distance attenuation laws for the Portugal mainland using intensity data points

“…This event is located in the Low Tagus Valley, where severe earthquakes already occurred (1344, 1531), and is associated to the fault system of the Low Tagus Valley (Cabral et al 2000(Cabral et al , 2004. The epicentre location was estimated at 38.9 • N, 8.8 • W, using the available macroseismic information, by Kárník (1969).…”

Looking for Earthquake Sources in the Lisbon Area

“…Data from seismic reflection, potential-field, well, geological outcrop and seismicity are used in this paper to improve our knowledge on several recently known geological structures based on seismic reflection data. 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.…”