Geophysical measurements for site effects characterisation in the urban area of Girona, Spain

Abstract

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.

Integrated seismic ambient noise, magnetotellurics and gravity data for the 2D interpretation of the Vallès basin structure in the geothermal system of La Garriga-Samalús (NE Spain)

“…In all of them, it has been verified that the soil fundamental frequency is related to the contact between the bottom of the Neogene sediments and bedrock. The expression proposed by Ibs-Von Seht and Wohlenberg (1999) shows the best fit for a broad range of frequencies (Macau et al, 2018). Fig.…”

“…This methodology has been applied in several Neogene basins near the Vallès basin (Benjumea et al, 2014;Gabàs et al, 2016;Macau et al, 2018). In these studies, the HVSR results have been compared with other geophysical exploration methods (seismic noise array technique, magnetotelluric method or even seismic refraction tomography).…”

Estimation of earthquake local site effects using microtremor observations for the Garhwal–Kumaun Himalaya, India

“…Eleven of 14 sites are situated on weathered/phyllite/granite gneiss/granite schist rock type situated on structurally highly dissected hills. In general, the sites exhibit a flat H/V curve without a clear peak revealing that the station is on bedrock (Surve and Mohan, 2010; Mohamed et al ., 2016; Macau et al ., 2018; Fat‐Helbary et al ., 2019).…”

“…Ambient noise observations are feasible to the acquisition, analysis time and an appealing approach for evaluation of ground response in large areas; these make this method ideal for characterizing the dynamic behaviour of shallow subsurface (Singh et al ., 2019). In recent times, this method has been widely used to study site effects to provide critical inputs for seismic microzonation and learn local geology (Surve and Mohan, 2010; Panzera et al ., 2015, 2017b; Kumar and Kumar, 2018; Macau et al ., 2018; Vicêncio et al ., 2018; Singh et al ., 2019). The HVSR method has been used to identify the sediment thickness over the bedrock, glacier thickness and local site effects (Parolai et al ., 2002).…”