Estimation of topographical effects at Narni ridge (Central Italy): comparisons between experimental results and numerical modelling |
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Authors: | S Lovati M K H Bakavoli M Massa G Ferretti F Pacor R Paolucci E Haghshenas M Kamalian |
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Institution: | 1. Istituto Nazionale di Geofisica e Vulcanologia, sezione Milano-Pavia, Milano, Italy 2. International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran 3. Universit?? degli studi di Genova, Dip.Te.Ris., sezione de Geofisica, Genova, Italy 4. Dipartimento di Ingegneria Strutturale, Politecnico di Milano, Milano, Italy
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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 central 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 (standard spectral ratio, SSR) and non reference spectral techniques (horizontal
to vertical spectral ratio, HVSR). 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 in a direction perpendicular to the main elongation of the ridge. The results obtained from the data analyses
were used as a target for bidimensional and tridimensional numerical simulations, performed using a hybrid finite-boundary
element method and a boundary element method for 2D and 3D modelling, 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 observed amplifications. |
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