Cluster Analysis of Fourier Amplitude Spectra Residuals for Ground Motion Characterization in Southern Italy.

The main objective of this study is to identify clusters of seismic records with similar Fourier Amplitude Spectrum (FAS) shapes that can be associated with different tectonic domains, path attenuation properties, and site effects in Southern Italy. The analyzed dataset consists of FAS of S-wave windows, computed in the 0.5–25 Hz frequency range from accelerometric and velocimetric records available from EIDA and ITACA for 1349 events and 502 stations, with focal depths up to about 40 km.

We analyzed residuals between empirical FAS-based ground-motion models (GMMs), using ITA18 as reference, and observed spectral amplitudes through a mixed-effects regression framework. This allows us to decompose the total residuals into systematic contributions due to source (between-events term, δBe), path (systematic differences in attenuation, δWes), and site (site-to-site term, δS2S) effects, which are then grouped into clusters.

For the source terms δBe, four clusters are identified. Two of them are particularly interesting: one shows systematic amplification with increasing frequency, while the other shows systematic deamplification at high frequencies. The spatial distribution of the corresponding events highlights the Gargano and southeastern Sicily as regions characterized by amplified spectral amplitudes, whereas northeastern Sicily and the Aeolian area exhibit deamplified amplitudes. Additional insights are obtained by examining the dependence of these clusters on magnitude and focal depth; this analysis reveals that one of the source-related clusters is composed exclusively of shallow events (depth ≤ 10 km), which display distinctive spectral behaviors in specific crustal and volcanic domains.

For the path residuals δWes, four clusters are also recognized, revealing systematic differences in wave propagation across distinct crustal structures. The systematic site terms δS2S are grouped into three clusters: one identifies stations largely unaffected by significant soil amplification, while the other two show, respectively, systematic amplification and deamplification across the whole frequency band, with the clearest separation at intermediate frequencies (about 3–8 Hz).

These results provide a regional framework for ground-motion characterization in Southern Italy, supporting the identification of reference stations and of areas with distinct source and attenuation properties. This work is preparatory to future large-scale and local-scale Generalized Inversion Technique (GIT) studies aimed at the characterization of ground motion for shallow-crustal events and at the definition of key input parameters for earthquake simulations. In particular, the source-related clusters associated with volcanic areas reveal spectral features that deviate from classical ω² source models, pointing to processes likely controlled by complex fluid–rock interactions.