GNSS and Seismic Strain Reveal Predominantly Aseismic Deformation at Somma–Vesuvius.

The Somma–Vesuvius area represents an ideal setting to investigate the crustal deformation budget in volcanic environments characterized by low-magnitude seismicity and predominantly aseismic processes. In this study, we compare surface strain derived from GNSS geodetic observations with seismic strain estimated from the local earthquake catalogue, with the aim of quantifying the fraction of deformation released aseismically. The objective is to assess the degree of coupling between observed deformation and seismic release, and to provide constraints on the physical processes controlling the deformational dynamics of the volcanic edifice. The GNSS velocity field, obtained from the permanent stations operating in the Vesuvian area over the last twenty years and forming part of the NeVoCGPS network for monitoring the Neapolitan volcanic area, reveals a clear signal of subsidence around the Gran Cono, with vertical rates on the order of a few mm/yr and smaller horizontal components. From these velocities, the average surface strain tensor was computed, highlighting a predominantly compressive regime and the corresponding strain rate. By integrating the strain rate over the observation period, a total accumulated strain is obtained that is significantly higher than the value typically observed in low-deformation tectonic settings. Alongside this, seismic strain was estimated using the local seismicity catalogue, which is characterized by low-magnitude events (M_D < 3) and an overall limited energy release. The cumulative Benioff strain, calculated as the sum of the square roots of the seismic energy of individual events, is found to be markedly lower than the total strain derived from GNSS data. The comparison between total strain and seismic strain indicates that the large part of the deformation observed in the Somma–Vesuvius area is released aseismically. This discrepancy is not interpreted as a strain deficit, but rather as clear evidence that deformation is dominated by non-brittle processes, such as compaction, sliding, including flank instability, and gravitational stress. These results highlight the importance of integrating geodetic and seismological data to improve the interpretation of deformation in volcanic contexts and demonstrate how the quantification of the aseismic fraction represents a key tool for distinguishing between tectonic and volcano-controlled deformation, with important implications for the hazard monitoring.