A geophysical study of the S. Marcos–Quarteira fault, Portugal

Abstract

The Algarve province is located in the southern part of Portugal, a few hundred kilometres North of the crossing E–W Eurasia–Africa plate boundary. It is characterized by moderate seismicity, with some important historical and instrumental earthquakes causing loss of lives and important material damages. It has therefore been the target of several seismic risk assessment projects financed by the Portuguese government and the European community. The CAPSA is one of these projects, in which extensive geological and geophysical studies were carried out. This paper discusses the application of the seismic reflection method as part of the project to confirm a fault location and to determine if it is (or not) an active fault. Seismic reflection profiles were collected to study and confirm the location of the S. Marcos–Quarteira fault in the Quarteira area. This is a regional fault that affects Paleozoic and Mesozoic rocks. A fault is observed in an outcrop near the Boliqueime village, located a few kilometres away from the study area and affects the Pleistocene cover sediments. The aim of this work was to verify if this active fault and the suspected location of the S. Marcos–Quarteira fault in Tertiary and Quaternary terrains was the same structure. Refraction interpretation of the reflection data and vertical electrical soundings were also used to complement the information. An important deformation was found in several seismic profiles that also affects Quaternary formations, in accordance with the observed seismicity. The probable location of the S. Marcos–Quarteira fault in the Quarteira area was confirmed, as well as its Quaternary activity, confirming an increased seismic risk scenario envisioned for the Algarve region of Portugal.