Geophysical measurements for site effects characterisation in the urban area of Girona, Spain
A. Macau, B. Benjumea, A. Gabàs, F. Bellmunt and S. Figueras
Journal name: Near Surface Geophysics
Issue: Vol 16, No 3, June 2018 pp. 340 - 355
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Price: € 30
Amplitude level, duration, and spectral content of earthquake ground motions are strongly influenced by local soil conditions. Reliable estimation of site effects is, therefore, crucial in order to avoid damage to infrastructures and mitigate other losses. Traditional geophysical exploration techniques are restricted in urban environments due to the presence of anthropogenic noises causing low seismic signal-to-noise ratio and other logistical issues. These problems are even more critical when the maximum investigation depth extends to hundreds of metres. The city of Girona, located in northeastern Spain, has seismic hazard represented by a peak ground acceleration value of 0.08 g for a return period of 500 years. The city was built at the confluence of four rivers, generating a complex surface geology with bedrock outcrops and the presence of stiff soils, soft soils, and also a volcanic basalt layer. This paper presents the results of the geophysical work, which was carried out in order to characterise the soil layers present in the urban area of Girona. All information obtained in this research will be useful in computing the amplification of ground motion and to perform microzonation studies. We have obtained the shear-wave velocity profile in the study area using a combination of seismic noise array and multichannel analysis of surface waves techniques. Using the horizontal-to-vertical spectral ratio method, we have obtained the soil fundamental frequency. The combination of shear-wave velocity and values of soil fundamental frequency provides a complete map of the bedrock topography. The expected velocity inversion due to the presence of shallow basalt flow has been targeted. Electrical resistivity tomography is found to be suitable to define the volcanic basalt thickness. This thickness value is used to constrain the inversion of surface wave dispersion curves and reduce shear-wave velocity uncertainty. The new methodology overcomes the limitations that are typical to urban conditions and other geological complexities.