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Costanzo M.R., Nunziata, C., Gambale, V. (2013)

 

Vs crustal models and spectral amplification effects in the L’Aquila basin (Italy)

 
Engineering seismology, geotechnical and structural earthquake engineering, D’Amico S. (Ed), Intech Publisher (www.intechopen.com), 
ISBN 978-953-51-10038-5, cap. 4, 79-99
 
 
Abstract
  
 

nunziata coppitoOn April 6, 2009 a strong earthquake (ML 5.9, MW 6.3), hereafter called main shock, struck the Aterno Valley in the Abruzzo region (central Italy) causing heavy damage in L’Aquila and in several nearby villages and killing more than 300 people. The event had a pure normal faulting mechanism, with a rupturing fault plane NW striking and 45°SW dipping; hypocentral location was at 9.5 km depth and epicenter at a distance of about 2 km WSW from L’Aquila center [1]. Few days later, the aftershock activity involved also the area NE of L’Aquila toward Arischia and Campotosto. The overall distribution of the aftershocks defined a complex, 40 km long and 10-12 km wide, NW trending extensional structure. The largest damage was mainly distributed in a NW-SE direction [2], according to the orientation of the Aterno river valley. The area has a high seismic hazard level in Italy [3] and has experienced in the past destructive earthquakes such as the 1349, I=IX–X; the 1461, l'Aquila, I=X and the 1703, I=X [4]. Many active faults are recognized in the area and several of them are indicated as potential sources for future moderate and large earthquakes by several authors (see for a review [5]).

The Aterno river basin is a complex geological structure with a carbonate basement outcropping along the valley flanks and elsewhere buried below alluvial and lacustrine deposits with variable thickness. The surface geology is even more complicated by the presence at L’Aquila of breccias consisting of limestone clasts in a marly matrix. Such complex geological scenario reflected in a large spatial variability of amplitude and frequency content of the ground motion (e.g. [6]). Among several studies performed on the recorded events, there is not a note dedicated to the modeling of the shear seismic velocities of the crust structures, yet it is useful for the geological reconstruction and fundamental for computing seismograms. Simulation of the ground motion has been performed at L’Aquila based on literature data [7] by using the Neo-Deterministic Seismic Hazard Analysis (NDSHA) [8,9], an innovative modeling technique that takes into account source, propagation and local site effects. In order to estimate realistic ground motion we need physical parameters of rocks from surface to depths greater than the earthquake hypocenter. At engineering scale, microzoning activities promoted by the Italian Civil Defense Department [10] have performed VS measurements at depths around 25 m, in gravelly soils with different degree of cementation, alternating to thin layers of finer deposits (sands and/or silts) that often include carbonate boulders (www.cerfis.it). The investigated depths are too shallow to define the vertical and lateral passage from soft sediments to rock basement (VS at least of 800 m/s) which was sporadically found. At regional scale, a physical model is available extending to depths of about 300 km [11].

Aim of this paper is to retrieve VS models of the shallow crust in the Aterno river valley from the non-linear inversion of the group velocity dispersion curves of the fundamental mode extracted with the FTAN method (e.g. [12,13,14]) from recordings of earthquakes with ML ≥ 2.9 (Table 1) between April 5 and November 10, 2009, in the selected coordinate window of 42.4 ± 0.2 N and 13.4 ± 0.2 E. In addition, VS of the superficial 30 m of Aterno alluvial soils are defined by an active seismic experiment in the Coppito area, and compared with nearby cross-hole measurements. The VS profiles vs. depth are then attributed to lithotypes along a geological cross section from the epicenter to a seismic station at L’Aquila. Simulation of the main shock is performed with the NDSHA approach and the computed response spectra and the H/V spectral ratios are compared with those recorded.