Characterization of an Intraplate Seismogenic Zone Using Geophysical and Borehole Data: The Vila Franca de Xira Fault, Portugal

Abstract

The Vila Franca de Xira (VFX) fault is a regional fault zone located about 25 km northeastof Lisbon, affecting Neogene sediments. Recent shear-wave seismic studies show thatthis complex fault zone is buried beneath Holocene sediments and is deforming thealluvial cover, in agreement with a previous work that proposes the fault as the sourceof the 1531 Lower Tagus Valley earthquake. In this work, we corroborate these resultsusingS-wave,P-wave, geoelectric, ground-penetrating radar and borehole data, con-firming that the sediments deformed by several fault branches are of Upper Pleistoceneto Holocene. Accumulated fault vertical offsets of about 3 m are estimated from theintegrated interpretation of geophysical and borehole data, including 2D elastic seismicmodeling, with an estimated resolution of about 0.5 m. The deformations affecting theTagus alluvial sediments probably resulted from surface or near-surface rupture of theVFX fault during M∼7 earthquakes, reinforcing the fault as the seismogenic source ofregional historical events, as in 1531, and highlighting the need for preparedness for thenext event