Defining and resolving seismic complexity through a unified source–path–site framework: the Vrancea intermediate-depth ground motions

Ground motions generated by the Vrancea intermediate-depth seismic source exhibit long-recognized intricate characteristics that challenge standard seismological interpretations. As a result, Vrancea earthquakes are frequently excluded from global comparative studies, being treated as exceptions for which conventional source, path, and site models fail to provide consistency. This study addresses this long-standing problem by defining and resolving the seismic complexity of Vrancea ground motions through a unified source–path–site framework. The proposed methodology systematically decomposes recorded ground-motion variability into source-, propagation-, and site-related contributions while explicitly accounting for the specific constraints imposed by limited recordings of major events and strong structural heterogeneities. Key seismic parameters that have remained debated or poorly constrained in the literature are re-evaluated, yielding physically consistent behaviors that satisfy general seismological requirements. Applied to the Vrancea seismic source, the framework demonstrates that the apparent anomalies commonly reported in previous studies do not reflect fundamentally different physical processes but instead arise from the pronounced complexity and strong interdependence of source, path, and site characteristics. Once treated coherently, Vrancea ground motions conform to general rules of seismic behavior. Beyond the regional case study, the proposed approach provides a transferable methodology for the analysis of complex seismic sources worldwide, particularly in tectonic settings characterized by sparse strong-motion data and non-standard ground-motion features.