Abstract
<p> </p><p>The geological and geotechnical characterization of shallow formations is one of the main steps in performing a microzonation study. This paper presents an example of the usefulness of the information compiled in a geological and geotechnical database for the estimation of the seismic response of the shallower formations of the Lisbon downtown area of Baixa. The geotechnical characterization of this area was performed based on the analysis of Standard Penetration Test (SPT) data compiled in the geological and geotechnical database. This database, connected to a geoscientific information system (CGIS), allows, also, the definition of 2D geological profiles used for estimating the thickness of the shallower layers. The shear-wave velocities (V<span><sub>S </sub></span>) for each layer were estimated from empirical correlations using mean SPT values computed from the statistical evaluation of the compiled data. These V<span><sub>S</sub></span> values were further calibrated with ambient vibration recording analysis. The seismic response of Baixa’s superficial deposits was estimated by applying a 1D equivalent linear method to a set of soil profiles, regularly distributed across the area, and using synthetic accelerograms to simulate input motions associated with probable earthquake occurrences in Lisbon. The results are presented in terms of maps of predominant frequencies, with the corresponding amplification level, as well as spectral amplification factors for 1 Hz and 2.5 Hz. The results show that the fundamental frequency of the Baixa area is between 1.2 Hz and 2 Hz, for the whole central valley, reaching 3 Hz near the edges where anthroprogenic and alluvial deposits have less expression. Amplification factors up to 5 were obtained. These results were achieved regardless of the considered input motion. The similarity of the obtained fundamental frequency with the natural frequency of Baixa’s old building stock increases the probability of resonance effects in future earthquakes.</p><p><span style="font-size: medium;"><br /></span></p>
Highlights
It is recognized that the characteristics of seismic ground motion can be locally modified due to the existence of soft surface layers or basin geometry [e.g., Bard and Bouchon 1985, Idriss 1990]
The goal of this paper is to show the applicability of the geological and geotechnical database to microzonation studies, in particular on the geotechnical characterization of the shallower formations, as has been carried out in other countries with similar databases, e.g. in Spain [Cadet et al 2011], Turkey [Hasancebi and LISBON DOWNTOWN SHALLOW FORMATIONS CHARACTERIZATION
The procedure consisted of five steps (Figure 7): (1) computing the horizontal-to-vertical spectral ratio (H/V) curve from ambient vibration records acquired at selected sites close to geotechnical boreholes; (2) identifying the 1D soil profile from the geotechnical information; (3) estimating the shear wave velocity for each layer from NSPT values collected from several geotechnical boreholes, using empirical relationships appropriate to the geological setting; (4) computing transfer functions for these soil profiles using synthetic accelerograms; (5) selecting the most appropriate empirical relationship by fitting the fundamental frequency (F0) of the theoretical transfer functions to the peak frequency of the experimental H/V curve
Summary
It is recognized that the characteristics of seismic ground motion can be locally modified due to the existence of soft surface layers or basin geometry [e.g., Bard and Bouchon 1985, Idriss 1990]. Other examples can be found for more recent earthquakes occurring worldwide (e.g., Izmit, Turkey, 1999; Chi-Chi, Taiwan, 1999; Mula, Spain, 1999; Al Hoceimas, Morocco, 2004; Abruzzo, Italy, 2009), where the upper, normally consolidated soft deposits were responsible for an increase in the ground motion level for some specific periods [e.g., Navarro et al 2000, Maugeri et al 2011]. The estimation of the seismic behavior of soils for a large town exhibiting moderate to high seismic risk, such as Lisbon, is of great importance for the damage assessment for a future earthquake
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