High-frequency spectral decay in P-wave acceleration spectra and source parameters of microearthquakes in Southeastern Sicily, Italy.

In widely used ω−2 source models the characteristics of high-frequency radiation are described as being flat for frequencies between the source corner frequency and an upper limiting frequency fmax. Deviations from this behavior are described in a parameter κ, which is understood as a general measure of the changes the signal undergoes on its way from the source to the receiver. In this study, we calculated κ in southeastern Sicily by using microearthquakes belonging to three different seismic sequences occurring in the area in 1990, from 1999 to 2000, and in 2002. The selected events form four different clusters whose seismic sources are located within a 2-km radius. Although the source-to-station paths are approximately the same inside a given cluster, the values of κ change considerably at the same recording site from one event to another, also in the case of events having the same magnitude. We parameterized κ in terms of event (κE) and path (κP and κDiff ) contributions. The term κP represents the contribution on total κ of both the whole source-to-station path and the near-surface geology, while κDiff models the possible spatial variation in the parameter measured with respect to a reference source-station direction. Results show that the source contribution is not negligible and that a positive correlation with source size exists. Moreover, the hypothesis of a laterally homogeneous crustal structure within the area in question is not appropriate, and significant variation in attenuating properties of the medium may occur in a very small distance range (also in the order of a few tens of meters). Our analysis suggests that the origin of the previously mentioned variability is located near the recording site. Synthetic spectra are also computed in order to verify the actual significance of the parameterization employed and its capacity to separate the source and the path contribution to κ. We describe our spectra as a product of a Brune-type source spectrum and an exponential shaping term accounting for propagation effects. The seismic moments range between 3:8 × 1011 and 5:2 × 1013 N · m, the source radii range between 176 and 669 m, while the stress drop varies from 0.01 to 0.67 MPa.