Characterizing Earthquake Rupture Directivity Using Apparent Source Spectra: A Case Study from Central Italy

This study investigates rupture directivity effects on source spectra of small-magnitude earthquakes in Central Italy, based on a dataset comprising 18,994 waveforms from 656 shallow crustal events recorded between 2008 and 2018. The Generalized Inversion Technique (GIT) is employed to isolate frequency-dependent source characteristics. Apparent Source Spectra (AppSS) exhibit clear azimuthal variations, indicating the presence of directivity effects, particularly in events associated with higher standard deviations. The source spectra are analyzed using multiple empirical models, allowing for the estimation of seismic moment and stress drop for 138 events. Model performance is evaluated through residual analysis across a frequency range of 0.5–25 Hz. Our findings indicate that the ω² source model fitting on the plateau (ωest²) provides a better fit to the observed spectra for the selected events in the dataset. Comparison with previous studies confirms the reliability of the spectral estimates and modeling approach. For the two selected events, spatial maps of ground motion are presented, offering valuable insights into the regional variability of shaking. The study results underscore the importance of incorporating rupture directivity in ground motion models, thereby reinforcing the robustness of empirical predictive approaches and their relevance for improving seismic hazard assessments.