Topographic effects on the hill of Nocera Umbra, central Italy

Abstract

S U M M A R YDuring the M W 5.7 and 6.0 Umbria-Marche earthquakes of 1997 September 26, the historical centre of Nocera Umbra suffered MCS intensity VII-VIII. The zone is located on the top of a hill, a condition potentially favourable to ground motion amplification. However, also vulnerability is higher on the hill because of the ancient age of buildings. A temporary array of eight seismological stations was installed across the hill to quantify the amplification effect due to topography. Waveforms of 14 aftershocks (2.6 < M L < 4.1) are selected for the analysis. During each earthquake the largest amplitudes are observed on the hilltop, 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 2-4 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 Hz. High-frequency polarization varies for different sites and frequencies suggesting that smaller-scale complexities control the high frequency response. Synthetic seismograms of 2-D models confirm the occurrence of amplification, although not all details are fit by numerical simulations and the agreement between observations and models is significant only in terms of the fundamental resonance frequency, around 3 Hz. In the models, amplifications are much smaller than the observed ones. 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.

Frequency‐Scaled Curvature as a Proxy for Topographic Site‐Effect Amplification and Ground‐Motion Variability

“…The above procedure follows the work by Zevenbergen and Thorne (1987) and Moore et al (1991) and allows topographic curvature estimates at any point of the DEM. Topographic site effects for a given frequency are controlled by topographic features with characteristic sizes comparable with the associated wavelength (Geli et al, 1988;Sánchez-Sesma and Campillo, 1991;Durand et al, 1999;Buech et al, 2010;Pischiutta et al, 2010). The higher the frequency, the smaller are those features.…”

“…The topographic site effect is a complex frequencydependent 3D phenomenon (Sánchez-Sesma, 1983;Geli et al, 1988;Sánchez-Sesma and Campillo, 1991;Bouchon and Barker, 1996;Buech et al, 2010;Maufroy et al, 2012;Massa et al, 2014) that produces large ground-motion amplifications in the vicinity of summits (Gaffet et al, 2000;Buech et al, 2010;Massa et al, 2010) and may cause damages during large earthquakes (Çelebi, 1987;Kawase and Aki, 1990;Bouchon and Barker, 1996;Spudich et al, 1996;Hough et al, 2010;Pischiutta et al, 2010). The seismic waves propagating in areas with topographies are generally disrupted at frequencies above 1 Hz (Boore, 1973;Sánchez-Sesma and Campillo, 1991;Durand et al, 1999;Pischiutta et al, 2010).…”

“…The seismic waves propagating in areas with topographies are generally disrupted at frequencies above 1 Hz (Boore, 1973;Sánchez-Sesma and Campillo, 1991;Durand et al, 1999;Pischiutta et al, 2010).…”

Spectral-Element Simulations of Seismic Waves Generated by the 2009 L'Aquila Earthquake

“…In particular, ground motions amplify on hilltops and along ridges (e.g., Çelebi, 1987;Kawase and Aki, 1990;Bouchon and Barker, 1996;Komatitsch and Vilotte, 1998;Lee, Chan, et al, 2009), with multiple reflections prolonging seismic shaking (e.g., Lee et al, 2008). Remarkable variations in PGV and acceleration values have been attributed to topography, highlighting its role for seismic-hazard assessment (e.g., Komatitsch and Vilotte, 1998;Lee, Chan, et al, 2009; Pischiutta et al (2010), and our simulation volume features relevant topographic relief. As a consequence, inclusion of topography in our models turns out to be of fundamental importance for properly simulating observed ground motions.…”

Site Characterization and Site Response in Port-au-Prince, Haiti

“…Using a boundary element method to investigation topographic amplification in Taiwan, Lee et al (2009) conclude that amplifications on ridges can be as high as a factor of 2. Pischiutta et al (2010) found amplification factors approaching a factor of 4 over the frequency range 2-4 Hz along a hill in central Italy with roughly comparable geometry. The amplitude, frequency, and polarization of the…”