Large amplitude variations recorded by an on-fault seismological station during the L'Aquila earthquakes: Evidence for a complex fault-induced site effect

Abstract

A station (FAGN) installed on a segment of the fault system that generated the April 2009 L'Aquila earthquakes shows larger ground motions compared to nearby stations. Spectral ratios using 304 earthquakes result in a station amplification significantly varying event by event in the frequency band 1-8 Hz. The resulting pattern of amplitude dependence on causative earthquake location reveals that the strongest (up to a factor of 10) amplifications occur for tightly clustered aftershocks aligned with the fault dip beneath FAGN thus indicating a fault-guided effect. Fault models are investigated in a grid-search approach by varying velocity, Q, width and depth of the fault zone. Although the problem solution is not unique and there are strong trade-offs among the model parameters, constraints from observations yield a deep trapping structure model where the most likely values of velocity reduction, Q and damage zone width are 25%, 20, and 280 m, respectively. © 2010 by the American Geophysical Union.