The Seismic Response at High Frequency in Central L’Aquila: A Comparison between Spectral Ratios of 2D Modeling and Observations of the 2009 Aftershocks

Abstract

The city of L’Aquila is built on a deep intramontane basin filled by silty clay of lacustrine origin with average S-wave velocity of about 725 m=s, topped by a calcareous breccias unit with a higher speed of about 900 m=s. New geologic and geophysical investigations following the 6 April 2009 Mw 6.3 L’Aquila, Italy, earthquake have allowed us to develop a new velocity model for the basin deposits that, coupled with the inversion of velocity, includes lateral velocity variations in the top breccia layer and a newly found red silts unit, the Limi Rossi del Colle dell’Aquila (LRCA). The city area where the LRCA unit outcrops is correlated with clusters of reinforced concrete buildings that collapsed in the nor- mal faulting earthquake. We simulate the 2D seismic response of this new velocity model along two orthogonal cross sections and compare the synthetic spectral ratios to the experimental ones evaluated using aftershock recordings. As a result, taking into account the full scatter about the mean of the observed spectral ratios, we are able to predict not only the main features of the primary observed low-frequency resonance peak, but also the level of amplification at high frequency at most of the sites investigated.