ASSESSMENT OF SEISMIC SITE RESPONSE BASED ON MICROTREMOR MEASUREMENTS

Microtremor measurements is a cost-effective and non-invasive technique based on the ambient vibrations recordings of three components at ground surface. It is used to estimate the fundamental frequency of soils, f0, and
its amplification ratio, A0, based on the spectral ratio between the horizontal (H) and vertical (V) components of the measurements. In the scope of the H2020 EU funded, LIQUEFACT project, which addresses the mitigation of the risks associated with the liquefaction induced due to the seismic action, in situ geotechnical tests were performed, including microtremor measurements, in the Lisbon area in Portugal. Each measurement had an approximate duration of 40 minutes at 26 different sites, using a SYSCOM velocity sensor (MS2003+) connected to an SYSCOM acquisition unit (MR2002), considering an acquisition frequency of 400 Hz. The H/V curves at some points exhibit clear single peaks with large amplitude, which could be associated to sharp discontinuities corresponding to a profile with a single fairly homogeneous layer with a low value of the shear wave velocity contrasting a much higher value at a certain depth (“seismic bedrock”). The studied areas are characterized by peak frequencies ranging from 0.92 to 11.01 Hz and peak amplitudes ranging from 2.58 to 4.73. The linear equivalent model was used to assess seismic site effects, using Cross-Hole data to build the soil profile, along with strain-dependent curves from resonant column and cyclic torsional tests. The peak horizontal acceleration computed through numerical simulation was then compared with the frequency, the amplitude and the shape of HVSR curves to assess HVSR curves reliability in the prediction of seismic siteeffects.