Dynamic rupture complexity explains observed azimuthal variability in earthquake source radiation

Abstract Theoretical studies suggest that earthquake source spectra vary systematically with azimuth in terms of corner frequency and strength of the high-frequency radiation. However, extracting such features from observations and their modeling by physics-based models remain unresolved, particularly for weak earthquakes. Here we analyze station-dependent (apparent) source spectra of 49 weak (Mw 3-5) earthquakes in Central Italy, which exhibit such azimuthal variability up to 25 Hz. Moreover, we reveal various station-averaged spectral decays, directivity effects, and positive correlations between corner frequency and spectral fall-off rates for all events. To explain the observations, we employ physics-based dynamic rupture models incorporating stochastic spatial heterogeneity in fault stress and friction. Broadband numerical simulations considering various strengths of heterogeneity replicate the observed spectral features across the full frequency range. This approach provides a framework for understanding azimuthal ground motion patterns, bridging observational seismology with physics-based modeling and offering improved constraints for seismic hazard assessment.