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Vallone, Paola
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Vallone, Paola
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- PublicationOpen AccessMICROTREMOR MEASUREMENTS IN PALERMO, ITALY: A COMPARISON WITH MACROSEISMIC INTENSITY AND EARTHQUAKE GROUND MOTION(2004-08-01)
; ; ; ; ; ; ; ; ; ;Cultrera, Giovanna; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Azzara, Riccardo Mario; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cara, Fabrizio; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;D'Anna, Roberto; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Di Giulio, Giuseppe; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Giammarinaro, Maria Stella; Dipartimento di Geologia e Geodesia, Università di Palermo, Italy ;Passafiume, Giuseppe; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Rovelli, Antonio; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Vallone, Paola; Dipartimento di Geologia e Geodesia, Università di Palermo, Italy; ; ; ; ; ; ; ; The city of Palermo is an appropriate test site where the efficiency of microtremors in predicting ground motion properties during earthquakes can be checked. Palermo is a densely populated city with important historical heritage and was object of previous studies. Areas of local amplification of damage were identified in downtown Palermo using historical macroseismic data. Moreover, aftershocks of the September 6, 2002, earthquake (Mw 5.9, 40 km offshore) provided a dataset of seismograms that quantify spatial variations of ground motion. The availability of more than 2000 boreholes in the city allowed a reconstruction of the 3D structure of surface geology, indicating that all the higher damage zones correspond to sediment-filled valleys. The high variability of the surface geology is mostly due to the presence of two filled river-beds of about 150 m width. In the framework of the SESAME project (Seismic EffectS assessment using Ambient Exctations, funded by the European Union), 90 microtremor measurements were performed across several profiles crossing the soft sediment bodies. The measurement points were intensified close to the valley edges (every 20 m), according to our geological reconstruction. H/V spectral ratio on ambient noise (HVSR) show significant variations along each profile: as soon as the transition stiff to soft is crossed, a typical spectral peak exceeding a factor of 3 in amplitude appears in the HVSR. The peak falls between 1 and 2 Hz and, along each profile, the peak disappears as soon as the other edge of the valley is crossed. These results indicate that microtremors are sensitive to the presence of large impedance contrasts of deep soft soil, at least in the Palermo area, with an important implication: the HVSR method seems to be able to recognize conditions potentially favourable to the occurrence of higher damage even when local geological characters are masked by the urban growth. However, we were not able to establish a quantitative correlation between microtremor properties and ground motion (or damage) amplification.300 227 - PublicationOpen AccessEffects of the 6th September 2002 earthquake: damage amplification in the south-eastern sector of Palermo explained by GIS technology(2003)
; ; ; ; ;Giammarinaro, M. S.; Dipartimento di Geologia e Geodesia, Università di Palermo, Italy ;Canzoneri, V.; Laboratorio METRO s.r.l., Palermo, Italy ;Vallone, P.; Dipartimento di Geologia e Geodesia, Università di Palermo, Italy ;Zuccarello, A.; Dipartimento di Geologia e Geodesia, Università di Palermo, Italy; ; ; During the 6th September 2002 earthquake the highest damage level in Palermo was observed in the SE sector. This is a recent urbanization area where reinforced concrete structures predominate. A detailed analysis of soil properties in Palermo was carried out by City-GIS to investigate a possible role of nearsurface geology on earthquake effects. City-GIS is a tool dedicated to natural hazard evaluation in urban areas. The availability of high density of well log data (stratigraphic and geotechnical) allowed a realistic modeling of surface geology and physical-mechanical properties that control the seismic response. In wide zones of the above mentioned sector of Palermo, outcropping terrains are composed of thin calcarenite layers, lying above remarkably thick siltyclayey sands that overlay the Numidian Flysch, commonly considered the bedrock of Quaternary sediments. Since silty-clayey sands feature greater deformability properties (Young's modulus) and smaller resistance properties (undrained cohesion and shear resistance angle) than Numidian Flysch, these zones of the SE sector exhibit high values of the acoustic impedance contrast. Moreover, a quite wide portion of the study area, crossed by the Oreto River, is characterized by very thick alluvial deposits. Here, the significant lateral variations of the lithostratigraphic geometry may be an additional cause of strong site effects.138 384 - PublicationRestrictedEffect of Local Geology on Ground Motion in the City of Palermo, Italy, as Inferred from Aftershocks of the 6 September 2002 Mw 5.9 Earthquake(2005-12)
; ; ; ; ; ; ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Azzara, R. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cultrera, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Giammarinaro, M. S.; Dipartimento di Geologia e Geodesia, Università di Palermo ;Vallone, P.; Dipartimento di Geologia e Geodesia, Università di Palermo ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ; On 6 September 2002 a Mw 5.9 earthquake occurred in the southern Tyrrhenian sea, 40 km off the coast of Palermo (Italy). In the days after the mainshock, eight temporary stations were installed in the city to record aftershocks on different geological formations. Seismograms of about 30 earthquakes with magnitude > 2.8 are analyzed. The data analysis confirms the role of near-surface geology in causing locally significant variations of the ground-shaking level as already inferred by Guidoboni et al. (2003) from historical damage scenario studies. The largest spectral variations estimated through aftershock recordings result in a factor of 10 difference between stiff and soft sites, in frequency bands varying from 1 to 3.5 Hz. The geological structure of the study area is reconstructed by using data from more than 2000 boreholes organized in a Geographic Information System specifically dedicated to the assessment of natural hazards in urban areas. Vertically varying velocity models are used for a comparison with the observed data. In general, 1D transfer functions fit the largest amplification frequency but underestimate amplitudes of observations probably because of 2D and 3D complexity. Because the seismic stations were not installed in free field but at ground or underground level inside buildings, a possible influence of the structure was also investigated. Simultaneous ambient noise measurements were performed on the top, at the base, and outside the buildings where stations were installed. For all but one site, this analysis shows that the estimated ground-motion amplifications do not reflect the building vibration modes and therefore, in these cases, soil–structure interaction does not bias the free-field response of the study sites. Finally, speculations on the effect of the local geology in terms of response spectra of the Mw 5.9 mainshock are discussed in the framework of the Eurocode 8 prescriptions.262 32 - PublicationRestrictedMicrotremor Measurements in the City of Palermo, Italy: Analysis of the Correlation between Local Geology and Damage(2008-06)
; ; ; ; ; ; ; ; ; ;Cara, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cultrera, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Azzara, R. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;De Rubeis, V.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Di Giulio, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Giammarinaro, M. S.; Dipartimento di Geologia e Geodesia, Univ. di Palermo ;Tosi, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Vallone, P.; Dipartimento di Geologia e Geodesia, Univ. di Palermo ;Rovelli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ; ; ; ; 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.307 35