Is the Vila Franca de Xira Fault Still Active? A Shallow Seismic Reflection Shear-wave Study in an Intraplate Environment

The V. F. Xira fault (VFXF) is considered to be the most probable source of several destructive earthquakes that caused significant damage and loss of lives in Lisbon, Portugal and surrounding cities. The fault outcrops in Miocene sediments but is not seen to affect Quaternary terrains. The combination of low slip-rates in the area with erosion/sedimentation rates prevents the identification of active faults in the region. Our goal is to confirm if this fault has been active and to improve seismic hazard assessment in the LTV area. Previous P-wave seismic reflection data indicated the possibility of two shallow fault segments affecting the Holocene alluvium close to the VFXF outcrop. Due to the lack of resolution of this data to detect a small throw (< 2m) fault, the site was revisited with a high-resolution shallow S-wave reflection profile to confirm the fault segments but results were not completely conclusive. Here, we present reprocessed S-wave data and results of full-elastic seismic modelling to confirm the presence of the suspected faults. Clear indications of faulting were found: change of the shape and the amplitude of the reflection hyperbolae, discontinuities in the reflectors in the stacked section, shear-wave velocity dropping in anticipated the damage zones, and weak fault plane reflections. The results are analyzed further through seismic modeling. We conclude that the shallow fault segments affect the entire thickness of the Holocene alluvium and that the VFXF is active.