Improved joint constraints on moment tensors, source time functions and uncertainty quantification for moderately large earthquakes

SUMMARY The source characteristics of moderately large earthquakes in the moment magnitude range $M_{\rm w}$ 6.0–6.5 are essential for understanding earthquake source physics and regional structures, yet often remain less constrained than those of larger events. Here, we develop an improved method for characterizing regional moderately large earthquakes by simultaneously resolving the centroid moment tensor (CMT) and source time function (STF). Four earthquakes in regions where the geological structure can be approximated by a 1-D model or a composite of 1-D models for waveform modelling periods down to 10 s are analysed: the 2020 $M_{\rm w}$ 6.5 Idaho (USA), 2020 $M_{\rm w}$ 6.5 Nevada (USA), 2015 $M_{\rm w}$ 6.5 Alor (Indonesia) and 2020 $M_{\rm w}$ 6.4 Petrinja (Croatia) events. Furthermore, by incorporating uncertainty from data noise and Earth model error within a Bayesian inversion framework, we obtain improved CMT and STF solutions compared to those from existing catalogues. We show that while non-double-couple components exist in the Idaho and Nevada earthquakes, their contributions are smaller than previously estimated. The Alor and Petrinja events align with pure double-couple mechanisms. Notably, the Petrinja earthquake exhibits shallower strike-slip faulting (at 5 km depth) and a simple, short rupture, as reported in the SCARDEC catalogue. Its inferred fault geometry diverges from most existing seismogenic fault models. For all four cases, our CMT and STF solutions provide enhanced waveform fits up to 0.1 Hz, offering refined constraints on source characteristics and regional fault geometries. These findings underscore the importance of accounting for data and model uncertainties in source inversions, particularly at higher frequencies, where STFs are most sensitive. The reliable source parameters could have broad implications for seismic tomography, hazard assessment and emergency response.