The contribution of seismic data in microzonation studies for downtown L’Aquila

Abstract

After the 2009 April 6th Mw 6.3 L'Aquila earthquake (Central Italy) the Italian Civil Defense Department promoted the microzoning study in the ten zones in the epicentral area that suffered major damage. In this paper we present the activities and the results concerning a temporary seismic network installed in the historical L'Aquila city center indicated as "macroarea 1" in the microzoning project. Seismic data were collected to investigate the amplification effects in the city and to support the microzoning activities in verifying both geological profiles and 1D numerical modeling of the seismic response of the city. The conventional spectral approaches using both microtremor and earthquake data allowed to determine the fundamental resonance frequencies and the amplification factors within the city respectively. The spatial variability of these quantities can be related to the geological and geomorphologic characteristics of the investigated area. A comparison between the network data and the data recorded by the two strong motion instruments installed in the city was also made. This allows verifying the relative response of the accelerometric stations that recorded in the city the major events of the sequence.

Application of the microtremor measurements to a site effect study

“…Many theoretical and experimental studies show that, this method has the capability of estimation of two important site response parameters (amplification factor and peak frequency) (Nakamura 1989;Milana et al 2011;Choobbasti et al 2014;Purnachandra Rao et al 2011). …”

Seismic response of L’Aquila downtown from comparison between 2D synthetics spectral ratios of SH, P-SV and Rayleigh waves and observations of the 2009 earthquake sequence

“…2) . These two streets bound where downtown l'Aquila is located, approximately between 1,250 and 2,250 m, and where many seismological measurements for microzoning have been performed (Milana et al 2011). The bedrock topography also shows an asymmetrical valley with the deepest river incision also on the left hand side.…”

“…et al (2004). The impedance operator code is formulated in the frequency-wavenumber domain and has many features in common with the reflectivity technique (Kennett 1983), including its inherent accuracy. Unlike the reflectivity technique, which is restricted to consideration of 1D media, the impedance operator code handles general lateral variations in properties, including the reflection, refraction and interchanges between P and S waves at sloping boundaries and the diffraction of energy where boundaries intersect.…”

“…The reason for this choice is twofold: (1) it is located on the same valley side as the site of reference AQ12 used in the experimental approach (Milana et al 2011); (2) a reference site at the southwestern end of the model is better suited for incoming wave-fronts from that direction, which is consistent with most . A reference site located on the opposite side of the valley with respect to the incoming wave fronts introduces changes in the frequency content during propagation through the sediment filled valley resulting in Fourier amplitude spectra amplified or de-amplified at the opposite bedrock site (AQ18 in the L'Aquila setting), as described by the modeling result of Bordoni et al (2010).…”

“…Non-vertical incidence angles in the range 0-90 • are also modeled. The vertical components of P-SV synthetics 123 enable us to investigate a remarkable amplification effect seen in the vertical component of the recorded ground motion (Milana et al 2011).…”