Monitoring of Crustal Seismic Velocity Variations in the L’Aquila Fault Zone Inferred from Noise Cross-Correlation

herpinus simanjuntak;

The relative seismic velocity variations possibly associated to large earthquakes can be readily
monitored via cross-correlation of seismic noise. In a recently published study,more than 2 yr of
continuous seismic records have been analysed from three stations surrounding the epicentre of
the 2009 April 6, Mw 6.1 L’Aquila earthquake, observing a clear decrease of seismic velocities
likely corresponding to the co-seismic shaking. Here,we extend the analysis in space, including
seismic stations within a radius of 60 km from themain shock epicentre, and in time, collecting
5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal
damage is visible through this technique, and to detect a potential post-seismic recovery of
velocity variations.We find that the co-seismic drop in velocity variations extends up to 40 km
from the epicentre, with spatial distribution (maximum around the fault and in the north–
east direction from it) in agreement with the horizontal co-seismic displacement detected by
global positioning system (GPS). In the first few months after L’Aquila earthquake, the crust’s
perturbation in terms of velocity variations displays a very unstable behaviour, followed by
a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the
co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the
velocity changes in the first months after the earthquake prevent to detect the fast exponential
recovery seen by GPS data.Atest of differently parametrized fitting curves demonstrate that the
post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting
that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be
acting concurrently.