Earthquake Reflection Imaging and Migration of the Campi Flegrei Caldera from Passive Seismic Data

Seismic imaging of active volcanic areas is crucial for characterizing subsurface structures that govern fluid circulation, deformation processes, and seismic hazard. In this study, we apply earthquake reflection imaging techniques to passive seismic data recorded at the Campi Flegrei caldera (southern Italy), one of the most active and densely populated volcanic areas in Europe. Building upon a methodology previously validated on synthetic datasets, we assess the capability of passive seismic migration to image crustal-scale reflectors from natural earthquake data.

The proposed approach adapts pre-stack depth Kirchhoff migration (Schneider, 1978) to passive seismic data by exploiting multiple earthquake-generated seismic phases (PP, SS, SP, and PS). To assess the effects of irregular source–receiver geometry, focal mechanism variability, and mixed P–S wavefields, the workflow was first tested on a synthetic dataset generated according to the Campi Flegrei source–station configuration and subsurface model. These tests demonstrate that, despite the lack of controlled acquisition geometry, coherent reflectors can be reliably recovered under realistic noise conditions and velocity uncertainties.

We then applied the validated procedure to real earthquake data recorded during the most recent phase of unrest at Campi Flegrei. From a catalogue of 3,900 high-precision relocated earthquakes (NLL-SSST-WC; Lomax et al., 2022) that occurred between January and September 2025, we constructed five vertical seismic profiles, each 1.5–3.5 km in length and extending to depths of approximately 13 km, by selecting subsets of well-aligned events and stations. Migration was performed using a velocity model derived from available seismic constraints (Zollo et al., 2008) and recent tomographic results (De Landro et al., 2025).

From the migrated sections obtained for the four seismic phases (PP, SP, SS, and PS) along five profiles with different orientations, we applied least-squares (LS) migration (Tarantola, 1984), combined with Shifted Total Variation (SVT) regularization (Zand et al., 2023) and Principal Component Analysis (PCA), to enhance and identify laterally continuous, high-amplitude features interpretable as subsurface interfaces. The results consistently reveal reflectors at depth of approximately 2.7 km, 5.0 km, 6.8 km, 9.5 km, and 11 km.

Our results demonstrate that earthquake-based reflection imaging is a powerful and promising approach for resolving the internal structure of active volcanic systems, even under highly irregular acquisition conditions. This study represents a first step toward the systematic application of passive seismic migration at Campi Flegrei, providing a new framework for imaging subsurface structures that are critical to understanding volcanic dynamics and hazard assessment.