Evidence of Low-Frequency Amplification in the City of L'Aquila, Central Italy, through a Multidisciplinary Approach Including Strong- and Weak-Motion Data, Ambient Noise, and Numerical Modeling

Abstract

Recent accelerometric recordings of earthquakes with moderate and intermediate magnitude (4.0 Ͻ M L Ͻ 5.9), at both local and regional distances, show a significant ground-motion amplification effect at low frequencies (0.6 Hz) in the city of L'Aquila (central Italy). The effect involves very long durations characterized by low frequencies in the coda. Starting from these observations, a series of supplementary investigations was performed in the urban area of L'Aquila by collecting and analyzing both weakmotion data from earthquakes with magnitudes ranging from 2.2 to 4.9 at distances from 20 to 105 km and ambient noise data. All the collected weak-motion data share the same characteristics as the strongmotion records and give a better image of the amplification effect in the city. In order to interpret observations in terms of the local geology, we performed 2D numerical modeling of the sedimentary basin underlying the city of L'Aquila using both finite elements and boundary elements based on a geological section derived from gravity measurements. This analysis indicates that the ground-motion amplification in the city of L'Aquila is related to the presence of a sedimentary basin, filled by lacustrine sediments, with a maximum depth of about 250 m. The combined approach to data collection and analysis used here gives useful information for risk assessment in the city of L'Aquila and can be recommended for many other urban areas that share similar characteristics.

Evaluation of Linear and Nonlinear Site Effects for the MW 6.3, 2009 L’Aquila Earthquake

“…The only exception is the station AQK (Fig. 4) that confirms the resonance frequency of 0.6 Hz obtained in the area by previous studies (De Luca et al, 2005). The crucial point is the definition of representative V S profiles in the uppermost hundred metres, significant for the evaluation of site amplification effects.…”

“…7 b , T a b . 1 ) h a v e b e e n a t t r i b u t e d o n available data (De Luca et al, 2005). The geological subsoil beneath the AQV station is similar to that below the receiver R19.…”

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

“…In particular, the epicentral area corresponds to the upper and middle Aterno valley, with Quaternary lacustrine deposits forming its basins. The depth of the deposits in this area varies from about 60 m to more than 200 m from the upper to the middle Aterno valley (Bosi and Bertini, 1970), and much evidence of ground-motion amplification has been found (e.g., De Luca et al, 2005;Akinci et al, 2010). Finally, remarkable topographic ridges characterize the region, including the Gran Sasso and Maiella massifs to the east and the Simbruini Mountains to the west (e.g., Chiarabba et al, 2010;Fig.…”

“…Significant site amplifications have been observed for the 2009 M w 6.3 L'Aquila event in neighboring regions (e.g., Ameri et al, 2009;Bindi et al, 2009;Akinci et al, 2010;Çelebi et al, 2010). At L'Aquila city, for example, based on previous studies (e.g., De Luca et al, 2005), these effects are related to the presence of a basin filled with lacustrine sediments with a maximum depth of 250 m. Central Italy features several other sedimentary basins, and these can have a similar influence on ground motion (e.g., Bindi et al, 2004Bindi et al, , 2009Castro et al, 2004). These effects are of primary importance for seismic-hazard assessment (e.g., Komatitsch et al, 2004;Stupazzini et al, 2009), strengthening our interest in modeling them in detail.…”

Deformation pattern of the 6 and 7 April 2009, <i>M</i><sub>W</sub>=6.3 and <i>M</i><sub>W</sub>=5.6 earthquakes in L'Aquila (Central Italy) revealed by ground and space based observations

“…The Aterno sedimentary basin is characterised by unfavourable site specific conditions. The basin is filled with a few hundred meters of lacustrine sediments that overlie the bedrock (Blumetti et al, 2002), producing significant ground motion amplification effect at low frequencies (De Luca et al, 2005). Most of the secondary ruptures were recorded near the villages of Onna and Fossa.…”