Intrinsic and scattering seismic wave attenuation in the Central Apennines (Italy)

Recently, an earthquake sequence formed by a series of subsequences characterized by large earthquakes (three events M >= 6.0 and five events M >= 5.0) occurred in the Central Apennines in 6-7 months during 2016-2017. Several studies interpreted this cascading type of earthquake occurrence as associated with fluid movements through an interconnected network of cracks around the active faults that triggered the subsequences in different time and location. In the present study we aim to verify if the presence of fluids is confirmed by the pattern of seismic wave attenuation. To do so we separately estimate the space-averaged parameters intrinsic Q(i)(-1) and scattering Q(s)(-1), proportional to the correspondent attenuation parameters, by applying a modified Multiple Lapse Time Window Analysis (MLTWA), in order to establish reference background values for the study region. MLTWA measures the decay of S-wave energy with distance and lapse time and fit it to the Energy Transport Equation solution for scattering, yielding the seismic albedo, B-0 = Q(s)(-1)/Q(t)(-1) and the extinction Length, L-e,proportional to Q(t)(-1) parameters as a function of frequency. The knowledge of the separated attenuation parameters allows determining which process of seismic energy dissipation dominates in the region. In the present study B-0 results to be around 0.6 (at 1.5 Hz) and decreases with increasing frequency down to 0.2 indicating that the attenuation of the seismic energy carried by S-wave is controlled by intrinsic dissipation rather than by scattering. Moreover, the intrinsic attenuation is frequency dependent (f(-0.9) for the uniform model and f(-0.4) for the crust/Moho model). This behavior well matches with the presence of fluids in the faults and fracture zones in the central Apennines, in agreement with results from other independent studies in the region. Finally, a comparison among the attenuation parameters estimated for this area and those measured in several areas of Italy and other parts of the world is presented together with a discussion on the comparison between the present MLTWA attenuation estimates and homologous results obtained for the same area with different approaches.