Alfonso Brancato

Source Parameters of Microearthquakes in Southeastern Sicily, Italy

We computed the source parameters of 135 local earthquakes that occurred in southeastern Sicily (Italy) between 1994 and 2001. Magnitudes ranged from 0.6 to 2.5. The source parameters were determined from the spectral analysis of P waves, after corrections for attenuation and site effects. P -wave quality ( Qp ) and site ( S ) factors are known in the two frequency ranges 3–9 Hz and 16–27 Hz (Giampiccolo et al. , 2003). Because the correction for Qp and S over the whole spectrum could not be performed, a new approach to estimate spectral parameters was applied. We defined the new notion of “holed spectrum” to specify a spectrum lacking corrected spectral amplitude values within a certain frequency range, whereas we referred to a “whole spectrum” to specify when the entire analyzed frequency band was used. A detailed statistical analysis showed that dealing with the holed spectrum did not lead to significant differences in the estimate of spectral parameters. Displacement holed spectra, corrected by the attenuation and site response, were then used to estimate source parameters. Seismic moments ranged from 1017 to 1020 dyne cm, and source dimensions from 150 to 500 m. In general, the stress-drop values obtained analyzing P -wave spectra were very low, because they mainly span the range of 0.05–5 bars.

Reply to “Comment on Manuella et al. ‘The Hyblean xenolith suite (Sicily): an unexpected legacy of the Ionian–Tethys realm’ by Beccaluva et al. (2015)”

One of the most important pieces of background information left in our pen (Manuella et al. 2015) regards the circumstance that, during the last 25 years, international marine geology expeditions brought crucial advances in understanding the composition and tectonic evolution of present and fossil oceanic lithosphere (e.g., Pearce 2002; Dick et al. 2003; Boschi et al. 2006; Snow and Edmond 2007; Ildefonse et al. 2007; Miranda and Dilek 2010; Silantyev et al. 2011). In particular, we would draw attention to some fault-bounded abyssal highs, called oceanic core complexes (OCCs), located in the crest zone of (ultra) slow-spreading mid-ocean ridges. OCCs mostly consist of serpentinized mantle peridotites and gabbroic rocks exhumed to the ocean floor along systems of detachment faults, related to serpentinite diapirism. Most elevated blocks even reach the ocean surface to form non-volcanic ocean islands, as well as the St. Peter and St. Paul Rocks located near the axial zone of MAR in the equatorial region (e.g., Campos et al. 2010; Sharkov 2012). More in general, magmatic layers of the normal oceanic crust are very thin or even absent at OCCs sites, seismic profiles being compatible with a serpentinite layer overlying almost unaltered mantle ultramafics (e.g., Blackman et al. 2004a, b). In this respect, the concept of a “crust” had to be called into question, and hence, the Moho can be regarded as a serpentinization front (e.g., Minshull et al. 1998). Oxide-rich gabbros with sheared texture are considered obliged components of the gabbroic suite of present and fossil OCCs (e.g., Sharkov 2012). Veins of plagiogranites are also relatively common in these oceanic structures, intruding both gabbros and peridotite bodies. Oxide gabbros and plagiogranites from OCCs typically bear zircon as accessory phase (e.g., Aranovich et al. 2013). OCC basalts, Introduction

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 fre- quency and an upper limiting frequency fmax. Deviations from this behavior are de- scribed 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 seis- mic 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 parame- terized κ in terms of event (κ E ) and path (κ P and κ Diff ) contributions. The term κ P represents the contribution on total κ of both the whole source-to-station path and the near-surface geology, while κ Diff 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 proper- ties 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 × 10 11 and 5:2 × 10 13 N · m, the source radii range between 176 and 669 m, while the stress drop varies from 0.01 to 0.67 MPa. Online Material: Tables of stations in the deployed portable seismic networks, coordinates, and average values of κ.