Integrated Passive and Active Seismic Methods for Subsurface Characterization of the Campi Flegrei area (Southern Italy)

Active volcanic areas located within densely urbanized regions require reliable geophysical methods to characterize the subsurface and support seismic hazard assessment and monitoring strategies. The Campi Flegrei area (Southern Italy) represents a paradigmatic example, where hydrothermal activity, fluid circulation, and strong lateral heterogeneities, combined with urban constraints, make subsurface velocity modeling particularly challenging.

In this framework, and in response to the seismic sequence that began in 2023, an extensive campaign of experimental geophysical investigations was promoted and funded by the Italian Department of Civil Protection in the Campi Flegrei area. We present a multi-method seismic investigation based on 66 Horizontal-to-Vertical Spectral Ratio (HVSR), 11 2D seismic array and 20 Multichannel Analysis of Surface Waves (MASW) measurements at 20 sites. Shear-wave velocity (Vs) profiles were derived at each site through joint inversion of the dispersion curve (retrieved from 2D seismic array data and MASW) with ellipticity of the Rayleigh waves.

All HVSR measurements consistently exhibit a low-frequency peak (f0 ≈ 0.2–0.5 Hz), interpreted as the response of the deep caldera fill sediments.  Further peaks at frequencies above 0.5 Hz may be associated with shallow impedance contrasts. The fundamental frequency (f0) seems reflecting lateral variations in the depth and stiffness of the caldera fill. Significant variability in HVSR amplitude, sharpness and polarization, reflects the interplay between geological heterogeneity and urban noise sources.

The joint inversion approach reduces model non-uniqueness and provides well-constrained Vs profiles, improving the physical interpretation of HVSR features in terms of stratigraphy and velocity contrasts. This study highlights the potential of HVSR-based methods in active volcanic and urbanized settings and emphasizes the importance of combining passive and active methods to address geological complexity and anthropogenic interference, paving the way for further multi-scale studies and their application in urban volcanic contexts worldwide.

The resulting Vs velocity profiles provide further information for interpreting the stratigraphic features and discontinuities of the caldera fill, useful for integration with other type of studies (hydrothermal alteration) and other type of geophysical data

Moreover, the results of this study offer valuable tools for geohazard assessment and constitute a preliminary step towards the seismic microzonation of the area.