THE ROLE OF LATERAL HETEROGENEITIES AND REVERSAL OF VELOCITY AT L’AQUILA (CENTRAL APPENNINES, ITALY) FROM A COMPARISON BETWEEN 2D MODELING AND OBSERVATIONS FROM THE 2009 EARTHQUAKES SEQUENCE

Abstract

The city of L’Aquila is built over a terraced alluvial basin filled by silt and silty-clay of lacustrine origin with average S-wave velocity of about 725 m/s, topped by a breccias unit (BrA) with Vs of about 900 m/s. Coupled with a reversal of velocity, there are marked lateral heterogeneities. In the southern area the stiff BrA unit laterally passages into softer deposits and is topped by red silt, i.e. LRCA, with an average velocity of 350 m/s. Bedrock is limestone and marls and its estimated maximum depth is of about 300-400 m. Recent geological and geophysical surveys performed during microzoning activities constrain this model.

Throughout the city measurements of horizontal ground motion (SSR and HVSNR) show a marked low frequency peak (< 1 Hz), with variable amplitude reaching its maximum in the southern area. However amplification level often exceeds two over the 2-10 Hz frequency range. Vertical ground motion has a remarkable peak at frequency higher than the horizontal one.

Bordoni et al. [2011] in their simulation work have shown that: 1) the velocity reversal filters out the high frequency content and 2) synthetic Rayleigh waves can predict the vertical ground motion amplification. Here we further our investigations on 2D ground motion focusing on the behavior of lateral heterogeneity in the BrA layer coupled to reversal of velocity. Synthetic spectral ratios from 0° and 90° incidence angle from SH waves are compared to observations from sites investigated by the microzoning portable network.