Broadband earthquake source characterization by dynamic rupture inversion of apparent source spectra for two Mw~4 events

Standard characterization of weak earthquakes based on observed source spectra relies on corner frequency and radiated energy estimates only as general source characteristics. However, the routine analysis cannot provide physical details including the heterogeneity of the rupture process. Contrarily, those can be inferred by dynamic rupture modeling that combines elastodynamics and friction law; however, this approach is almost exclusively utilized for large events. Here, we develop a novel Bayesian dynamic inversion of apparent (station-specific) source spectra up to 25 Hz with removed path and site effects for slip-weakening friction parameters heterogeneous along a finite-extent planar fault. The approach is demonstrated through two real-world applications with distinct source radiation: a directive and a nondirective Mw~4 event in Central Italy. We find that, despite the event’s small size, the heterogeneity in the rupture propagation down to the smallest scales (~100 m) is necessary to accurately model the high-frequency radiation of the observed spectra. The inversion demonstrates the ability to resolve various mean source parameters with reasonable uncertainty and thus reliably characterize weak events. The study also pinpoints that the standard seismological estimates based on the omega-squared spectral model of the earthquake source can lead to inaccurate results when additional earthquake complexity, like directivity, is present. Finally, the dynamic inversion reveals fractal spatial characteristics of the governing dynamic parameters, which are essential for reproducing the observed high-frequency apparent source spectral decay. Such advanced studies promise to unravel so-far elusive small-scale characteristics of earthquake ruptures.