Spatio-Temporal Changes Recorded by an On-fault Seismological Station During the L’ Aquila Earthquakes.

Abstract:

A broad band station (FAGN) installed on a segment of the fault system that generated the April 2009 L'Aquila earthquakes shows much larger ground motions compared to nearby stations. Calderoni et al. (2010) observed that the strongest amplifications were observed for tightly clustered aftershocks aligned with the fault dip beneath FAGN thus indicating a fault-guided effect. In a second paper, Calderoni et al. (2012) found that the most efficient trapped waves at FAGN were clustered at the northwestern and southeastern tips of the fault plane responsible for the M_W 6.1, L'Aquila earthquake.

In this study we examine temporal variations of the fault zone properties analyzing the fault-trapped waves generated by 19 repeaters located at the northwestern tip. Fault-zone trapped waves are particularly suited for this analysis as their amplitude and frequency content depend on the geometry and elastic and anelastic parameters of the fault zone.

We observe that efficiency of trapped waves (estimated through spectral ratio amplitudes) sudden increased immediately following the M_w 4.1, March 30 largest foreshock. This increase persists until the occurrence of the mainshock, then amplitude decreases to the background after a hundred of days. Material properties are the most obvious candidates of temporal changes but the concomitant role of rupture directivity was also checked, thanks to the precise information on along-strike directivity of that fault inferred by Calderoni et al. (2014).