Contribution for the knowledge of the Alcochete-Pinhal Novo-Setúbal fault (Portugal) using ReMi and H/V Tests

The city of Setúbal is situated 30 km southeast of Lisbon, Portugal, and has experienced significant impacts from various earthquakes, including the one that occurred in 1858. This earthquake serves as an example of a nearby earthquake that caused significant social and structural damage, reaching a intensity of IX-VIII on the Modified Mercalli scale. Despite an average recurrence period of 3000–11000 years, as suggested by several authors, the Alcochete-Pinhal Novo-Setúbal fault (APSF) could potentially cause significant social, structural and economic impacts in future events. Several authors have reported on the APSF with slight variations in its outline, as it traverses multiple urbanized areas. The aim of this study was to locate the APSF in the Setúbal region and, to the best extent possible, determine its orientation and vertical offsets. 24 passive refraction microtremor tests (ReMi) and 145 horizontal-to-vertical (H/V) spectral ratios were employed to compute the variation of transverse wave velocity (V_S) with depth and to compute the fundamental frequency (F₀) and other natural frequency (F₁) of the soils when present. Horizontal sections of V_S and the spatial distribution of peak frequencies reveal alignments that can be related to the presence of the APSF fault zone. Considering all the alignments identified in this paper, two areas with distinct orientations for the APSF zone were proposed: one with a NNE-SSW strike and another with a NNW-SSE orientation. The lower values calculated for the fault plane dip (maximum of 5º) in the APSF area suggest a minimal vertical displacement, potentially corresponding to a strike-slip fault.