A P-wave Seismic Reflection Study of an Intraplate Structure: The Azambuja Fault, Portugal

The Azambuja fault is a NNE oriented fault zone located 50 Km north of Lisbon, the capital of Portugal and has been considered as a possible source of historical earthquakes. Therefore, its study is a priority in seismic hazard evaluation. It has a clear morphological signature and Miocene and Pliocene sediments are tilted eastwards and cut by steeply dipping meso-scale faults presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate Quaternary slip on the fault in the range 0.05 to 0.06 milimetres/year. However, the fault has not been observed to affect Pleistocene or Holocene sediments. Here, we present a P-wave seismic reflection study carried out to image the fault below the Holocene alluvium cover southwards of the fault outcrop. We show that the fault is present below the Holocene cover as suspected and, with the aid of nearby well data, that fault segments are affecting the 14 000 year aged alluvium cover, suggesting a larger slip rate then previously admitted.