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.