Spatial and Temporal Variations in the Nucleation Crustal Volume of the 2009 L'Aquila Earthquake

The M_w 6.3 L'Aquila earthquake was preceded by a long suite of foreshocks, the largest one (M_L 4.1) occurred one week before the mainshock. This event marked the beginning of an abrupt spatial and temporal change in different parameters.

After the M_L 4.1 event, rate and magnitude of foreshocks increased and seismicity migrated toward the main shock nucleation zone. These features of seismicity make difficult to assess to what extent the observed changes in seismic parameters are in time or in space. The aim of our studies was to provide new contributions to separate temporal and spatial features. We constrain the temporal changes in the fault zone trough the analysis of Fault Zone Trapped Waves (FZTWs) while the analysis of the source parameters and rupture directivity characterize the spatial variations. We selected the waveforms of repeating earthquakes located in a small volume of the nucleation zone.

The FZTWs show a sudden, up to 100% increase of spectral amplitudes about 10 h after the M_L 4.1 largest foreshock. Changes in the FZTWs indicate that the change of shear wave velocity in the fault zone was very large, likely as effect of an abrupt increase of fluid pressure providing a precise timing of the fault zone change.

The velocity contrast across the fault could be the origin of the preferred along strike rupture directions. Both foreshocks, mainshock and aftershocks show the same tendency of unilateral ruptures. There is no indication of temporal change of directivity, suggesting that rupture directivity is associated to fault properties that do not change in the investigated period.

Source parameters (Brune stress drop, apparent stress and radiation efficiency) provide further evidence of spatial variations. In the locked portion of the fault that originated the main shock, a consistent anti-correlation is found between Brune stress drop values and b-values imaged by other authors in the rupture nucleation volume.