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Authors: Lovati, S.* 
Bakavoli, M. K. H.* 
Massa, M.* 
Ferretti, G.* 
Pacor, F.* 
Paolucci, R.* 
Haghshenas, E.* 
Kamalian, M.* 
Title: Estimation of topographical effects at Narni ridge (Central Italy): comparisons between experimental results and numerical modelling
Issue Date: 2010
Keywords: topographical effects, spectral techniques, directional analyses, vertical amplification, numerical modelling
Subject Classification04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring 
Abstract: In the present work the seismic site response of Narni ridge (central Italy) is evaluated by comparing experimental results and numerical simulations. The inhabited village of Narni is located in the central Italian Apennines at the top of a steep massive limestone ridge. From March to September 2009 the site was instrumented with 10 weak-motion stations, 3 of which located at the base of the ridge and 7 at the top. The velocimetric network recorded 642 events of ML up to 5.3 and hypocentral distance up to about 100 km. The great amount of data are related to the April 2009 L’Aquila sequence. The site response was analyzed using both reference (SSR, Standard Spectral Ratio) and non reference spectral techniques (HVSR, Horizontal to Vertical Spectral Ratio). Moreover directional analyses were performed in order to evaluate the influence of the ridge orientation with respect to the selected source-site paths. In general the experimental results show amplification factors for frequencies between 4 and 5 Hz for almost all stations installed along the crest. The SSR technique provides amplification factors up to 4.5 detected considering directions perpendicular to the main elongation of the ridge. The results obtained from the monitoring activity were used as a target for bidimensional and tridimensional numerical simulations, performed using a hybrid finite-boundary element method for 2D and a boundary element method for 3D analyses respectively. In general, the results obtained through numerical simulation fit well the experimental data in terms of range of amplified frequencies, but they underestimate by a factor of about 2 the related amplification factors with respect to the observations.
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