Index Geophysics

Geotechnical characterization and seismic response of shallow geological formations in downtown Lisbon

<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>_S</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>_S</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>

Paula Teves-Costa

Isabel M. Almeida

Isabel Rodrigues

Rita Matildes

Claudia Pinto

Site effects

Lisbon

Geotechnical database

Theoretical transfer functions

Ground motion amplification

Meteorology. Climatology

QC851-999

Geophysics. Cosmic physics

QC801-809

Istituto Nazionale di Geofisica e Vulcanologia (INGV)

2014-08-01T00:00:00Z

article

1593-5213

2037-416X

10.4401/ag-6390

https://doaj.org/article/546a0456e5474f8db24bfc1fbb71d9e7

Annals of Geophysics, Vol 57, Iss 4 (2014)

EN

http://www.annalsofgeophysics.eu/index.php/annals/article/view/6390

https://doaj.org/toc/1593-5213

https://doaj.org/toc/2037-416X

Journal Licence: CC BY