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

The Lower Tagus Valley is located in the Lower Tagus Sedimentary Basin. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are still deficiently known due to a thick Cenozoic sedimentary cover. It is usually assumed that regional M = 6 - 7 earthquakes are generated by the Lower Tagus Valley fault. However, this structure has never been characterised, and there is evidence for the presence of other faults that have the potential of generating large seismic events. The hidden character of many of the structures in the Lower Tagus Basin necessitates the use of indirect methodologies for their study. Available seismic reflection lines acquired for oil exploration in the Lower Tagus Valley region were thus examined, with emphasis on the recognition of faults displacing the Cenozoic and particularly the younger geological units. The foreseen complexity of the basin structure was confirmed. Evidence for a continuous 'Lower Tagus Valley fault' is absent. Instead, N-S to NNE-SSW trending, en echelon, sub-vertical and reverse faults showing significant post-Miocene offsets, linked by transverse (NW-SE) faults that accommodate large displacements, were recognised in the seismic sections that cross the Tagus valley. These faults range in length from 10 km to a maximum of ca. 40 km, leading to an estimation of 6.2-6.9 maximum credible earthquake magnitudes, which fall in the magnitude range of the regional historical seismicity.