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Authors: Pischiutta, M.* 
Cultrera, G.* 
Caserta, A.* 
Luzi, L.* 
Rovelli, A.* 
Title: Topographic effects on the hill of Nocera Umbra, central Italy.
Issue Date: 10-Dec-2008
Keywords: topographic effects
Nocera Umbra
Subject Classification04. Solid Earth::04.06. Seismology::04.06.04. Ground motion 
Abstract: During the two MW 5.7 and 6.0 Umbria-Marche earthquakes of September 26, 1997, the historical centre of Nocera Umbra suffered MCS intensity VII-VIII. The zone is located on the top of a hill, a condition potentially favorable to ground motion amplification. However, also the higher vulnerability of ancient buildings could have increased the damage level. A temporary array of eight seismological stations was installed across the hill to quantify the amplification due to topography. Waveforms of 14 aftershocks (2.6< ML <4.1) are selected for the analysis. During each earthquake the largest amplitudes are observed on the hill top, spectral ratios are computed using rotated horizontal components to search for directional effects. Amplifications are found in two separate frequency bands: one in the range 2 - 5 Hz, where the increase of amplitude is moderate (never exceeding a factor of 4) and the polarization is transversal to the hill major axis; the second above 10 Hz, where amplifications are larger and reach values as high as 25. High-frequency polarization varies for different sites and frequencies suggesting that smaller-scale features control the high frequency response. Synthetic seismograms of 2D numerical simulations confirm the occurrence of amplification, although not all the details are fit by modeling and the agreement between observations and models is significant only for the fundamental resonance frequency. We conclude that topography could have been responsible for a small increase of damage in the hill zone but the most significant role on damage was played by the locally higher vulnerability.
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