GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 ground motion residual that depends on systematic factors related to the source of the earthquake, such as the stress drop, the slip distribution, the hypocentral position, etc. To this aim, we consider the between-event residuals ( δ Be) computed on the ITACAext flatfile (Brunelli et al., 2022), that contains ground motion parameters and the related metadata extracted by the Italian strong-motion archive (ITACA). The ITA18 and the ITA18v models (Lanzano et al., 2019; Ramadan et al., 2021), valid for active shallow crustal earthquakes in Italy, are adopted as reference for the horizontal and vertical ground motion components, respectively. To assure a robust estimate of the δ Be residuals, only those computed with more than 10 records have been considered, resulting in 680 investigated events. As shown in figure 1, a linear dependence between the between-event residuals, δ Be, at high-frequency, and the hypocentral depth is observed. A possible cause of this trend may be ascribed to the stress drop, that affects the high-frequency ground motion and tends to increase with depth due to the increasing lithostatic load (Bindi and Kotha, 2002; Brunelli et al, 2022). The relation between the high-frequency δ Be residuals and the hypocentral depth has motivated the extension of our investigation on the entire frequency band, analyzing the shapes of the δ Be curves by applying a cluster analysis ( kmeans algorithm; Hartigan and Wong, 1979) on both horizontal and vertical components. This allowed us to identify six clusters reported in Figure 2, similar for both the ground motion components. Plotting δ Be values versus depth for each cluster (Figure 3), we observe that the very shallow events are mainly included in cluster #5 (the same occur for the vertical components), characterized by negative values at short periods and positive values or close to zero at long periods. This observation is supported by individual cases of well-known very shallow events. These results are promising and support further investigations to identify shallow events (< 5km) based on the shape of between-event residuals.