Geophysical Characterization of the V.F. De Xira-Lisbon Fault System, Portugal

The Lower Tagus Valley (LTV) has suffered large historical seismic events which originated important material damages and loss of lives. Besides the plate boundary activity, the study area has been recently considered an important seismic source to the seismic hazard of the LTV area. The V.F. Xira-Lisbon, was selected as a priority target for investigation, based upon its near-surface expression on the oil-industry seismic reflection profiles, its significance in the Cenozoic basin structural pattern and the apparent relationship to the regional seismicity, its closeness to Lisbon and its seismic potential. Using aeromagnetic, seismic reflection and seismicity along with geological data, new insights into this fault system have been established. Its deep rooting into the paleozoic basement, the unknown northward and southward prolongations and its connection to relocated seismic epicentres are a few examples. The expected total length of the fault will be more than 90km, increasing the estimated magnitude of the maximum credible earthquake and the seismic hazard of Lisbon significantly. Acquisition of high resolution seismic data will help confirming estimated vertical offsets of the fault in Quaternary times, allowing an improved assessment of the seismic potential of the V.F Xira-Lisbon fault system.