Seismic site effects in Setúbal county (Portugal) using REMI technique

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Abstract

Setúbal is a county located in the Lisbon metropolitan area (Portugal) which has been hit by historical earthquakes that were responsible for high social and economic losses. The 1858 earthquake, with an epicentre located offshore just south of the Setúbal region, is a good example of an earthquake that caused high damage (IX-VIII MM) in this county. The main city of the region is Setúbal itself, an important and populated urban agglomeration with several industrial facilities very close by. The old city centre is mostly settled upon shallow geological units and taking into consideration the historical seismicity, it is important to examine the possible site effects that could occur during future earthquakes affecting the Setúbal region.

The objectives of this study are therefore the characterisation of the seismic behaviour of Setubal's shallow geological formations and the evaluation of potential site effects. Shear wave velocity profiles were computed with the Refraction Microtremor (ReMi) method, spatial distribution of VS30 values was estimated and a map with the VS30 classification of soils according to the EC8 was produced. Spatial distributions of different data are presented and discussed:VS30 values, thickness of the shallow formations, top Pliocene surface, and results from Standard Penetration Tests (NSPT values). Numerical correlations between these parameters were established. Geotechnical characterisation was based on the analysis of 307 logs and 2950 Standard Penetration Tests. A total of 266 refraction microtremor measurements were performed obtaining VS profiles and VS30 values for 43 different sites. It was found that in Setúbal the majority of t heHolocene units belong to EC8 class C (180–360 m/s) but, in the areas where the thickness of shallow formations (TSF) is lower, the Holocene units may belong to class B (> 360 m/s). The spatial distribution of VS30 highlighted that the Plio Pleistocene, Miocene and Mesozoic areas show the best soil conditions corresponding to EC8 classes B and A. However, it was possible to identify a Plio Pleistocene area with VS30 varying between 232 and 355 m/s, therefore belonging to class C. This fact highlights that site effects studies should not be performed solely based on geological criteria, but it is essential to estimate S wave velocities (VS) in situ.
Original languageEnglish
Article number100037
Number of pages12
JournalResults in Geophysical Sciences
Volume9
DOIs
Publication statusPublished - Mar 2022

Keywords

  • Site effects
  • VS profiles
  • VS30
  • Setúbal

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