Temporal variation of <i>S</i>-wave attenuation during the 2009 L'Aquila, Central Italy, seismic sequence

SUMMARY We investigate temporal and spatial variations of the spectral decay parameter kappa (κ) before and after the 6 April 2009, L'Aquila earthquake (Mw 6.1), in Central Italy. We analysed foreshocks 10 days before and aftershocks occurring 10 days and 6 months after this main event. We select earthquakes with magnitudes Mw ≥ 3.2 registered by the seismic network of Central Italy within a radius of 20 km from the epicentre of the L'Aquila main shock and having hypocentre distances of less than 170 km. We separate near-source, along-path and near-site contributions of κ for each group of events and we detected temporal variations of this S-wave attenuation parameter. We find that 10 days before the main shock κ along the path has the lowest values, probably due to high tectonic stress accumulated, in agreement with previous investigations performed with other techniques, then κ increases during the main event and remains constant during the first 10 days of aftershocks. The aftershocks that occurred 6 months after show an increase in the regional attenuation probably due to the tectonic stress released during the main shock and the earlier aftershocks. From the spatial point of view, 10 days before the principal event the foreshocks located to the south show an increase in the near-source attenuation towards the northeast, in the direction of the main shock. These spatial variations of κ may be related to the presence of crustal fluids near the rupture area, as evidenced by other previous studies. The first 10 days of aftershocks that concentrate around the main earthquake have high near-source κ, and those located north of the main rupture have lower values. These observations are consistent with previous investigations that show variations of elastic and anisotropic crustal properties during the L'Aquila earthquake sequence due to dilatancy and fluid diffusion processes within the nucleation zone. We conclude that temporal variations of the spectral decay parameter κ provide important clues for the earthquake cycle in Central Italy.