Microtremor Measurements in the City of Palermo, Italy: Analysis of the Correlation between Local Geology and Damage

Abstract

This study presents the results of 90 seismic ambient noise measurements in Palermo, the main city of Sicily (Italy). The dataset has been processed using the horizontal-to-vertical spectral ratio (HVNSR) technique and interpreted in terms of local geology, which is characterized by the presence of alluvial sediments of two riverbeds masked by urbanization since the seventeenth century. HVNSRs show significant variations in the study area: when the transition stiff to soft is crossed, a typical spectral peak appears in the HVNSRs, mostly in the frequency band 1-2 Hz, and exceeding a factor of 3 in amplitude. Using available information on subsurface geological structure, we compute theoretical 1D and 2D transfer functions. The resonance frequencies of soft soils obtained by HVNSR are well reproduced by the fundamental frequencies from numerical modeling.The distribution of frequency peaks of HVNSR and their amplitudes are also compared with the local damage caused by historical earthquakes. Previous studies demonstrated that damage variations in Palermo were controlled more by near-surface geology than building vulnerability. A uniform vulnerability is an ideal condition to test statistical methods and their capability in seeking correlation between HVNSR and potential damage due to local geological conditions. We apply two well-established multivariate statistical methodologies (factor analysis and canonical correlation) to the HVNSR dataset and macroseismic data (damage grades of the European macroseismic scale). Through these analyses we quantify the significance of the correlation between the HVNSR peak in the low-medium frequency range (0.5-3 Hz) and the occurrence of the highest damage grades. This approach allows us (1) to estimate the threshold value in the resulting linear combination of the HVNSR amplitudes, which separates zones of light damage from zones of significant damage, and therefore (2) to improve the spatial definition of potentially high hazard zones through a denser grid of microtremor measurements.

Correlation between damage distribution and ambient noise H/V spectral ratio: the SESAME project results

“…However, the geometry of the river-beds is well constrained by the high density of boreholes and historical maps. Their infilling is about 150 m wide and the thickness of soft deposits increases moving towards the confluence from 10 to more than 20 m in the Papireto River, and from 15 to more than 30 m in the Kemonia River (Di Giulio et al 2005;Cara et al 2008). The size of the studied area is about 1.2 km × 1.2 km wide.…”

“…In this study, we try to go one step further by quantifying the significance of this correlation between the HVNSR values and damage: this is achieved through the use of standard multivariate statistical analyses, namely, factor analysis and canonical correlation (Davis 2002). These analyses have already been used for the dataset of Palermo (Cara et al 2008) and represent an improvement of the previous studies.…”

Love wave contribution to the ambient vibration H/V amplitude peak observed with array measurements

“…Thus, H/V measurements are extensively used in microzonation to map the variations in resonance frequencies in densely populated, earthquake-prone areas (e.g. Fäh et al, 1997;Konno and Ohmachi, 1998;Alfaro et al, 2001;Duval et al, 2001;LeBrun et al, 2001;Tuladhar et al, 2004;Panou et al, 2005a;Souriau et al, 2007;Cara et al, 2008;Picozzi et al, 2009;Bonnefoy-Claudet et al, 2009). Bragato et al (2007) propose an algorithm that uses H/V data for the automatic generation of zonation maps for urban areas.…”

Historical damage pattern and causes of differential damage: an approach based on combining geophysical prospecting, geological and historical-technical data

“…To identify the causes of the uneven damage in Palermo some studies, examined as a whole, show an integrated use of geological, geotechnical, geophysical, and historical data (Guidoboni et al, 2003;Giammarinaro et al, 2005;Cara et al, 2008).…”