Kirchhoff migration of the shallow crustal discontinuities using local earthquake synthetic records in the Campi Flegrei Volcano, South Italy

Understanding the complex subsurface structures of volcanic regions is crucial for hazard assessment and resource management. Traditional active seismic studies are normally limited for the exploration of shallower crustal structures using low-energy sources located at the Earth surface. While, using earthquake data, the deep seismic interfaces of the crustal structure can be imaged with high-energy and in-depth located sources. Here we propose a data simulating-processing-migration workflow for imaging the subsurface shallow crustal structure by earthquake synthetic signals. As a reference test-site for our study we used the Campi Flegrei caldera internal structure. Thanks to accurate earthquake locations now available in this area, we treat passive data like active data and employ advanced seismic imaging methods. For methodological assessment and improvement, we simulate 400 micro-earthquakes randomly distributed between 1 and 3 km depth in a vertically layered velocity structure and recorded by 13 stations. Exploiting the cylindrical symmetry of the vertically layered velocity model, we construct representative 2D seismic sections by sorting synthetic source–receiver records as a function of source–receiver distance. This provides a controlled framework for identifying the different seismic phases and relating them to their corresponding reflecting interfaces prior to migration. Finally, we apply a modified Kirchhoff migration technique combining the Shifted Total Variation regularization (STV) and Principal Component Analysis (PCA) to reconstruct images of subsurface structures. This workflow suppresses migration artifacts, enhances coherent events, and extracts structural features that remain stable under model perturbations and noise contamination. The resulting images reliably recover major subsurface interfaces and provide a coherent and robust characterization of the subsurface structure.