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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

¹ Dip. to Protezione Civile
Uf. Servizio Sismico Nazionale Via Vitorchiano 4
00189 Rome, Italy
 (G.D.L., S.M.)

² Istituto Nazionale di Geofisica e Valcanologia
Via di Vigna Morata 605
00143 Rome, Italy
 (G.M.)

³ Private consultant to Dip. to Protezione Civile
Rome, Italy
 (T.S.)

Recent accelerometric recordings of earthquakes with moderate and intermediate magnitude (4.0 < M_L < 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 weak-motion 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 strong-motion 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.