Structure and dynamics of the Somma-Vesuvius volcanic complex

We review recently obtained results about the velocity structure of the Som ma-Vesuvius (Southern Italy) volcanic complex and present an interpretation of structural features, both at local and regional scale, and of the local seismicity. The local structure of Somma-Vesuvius is reviewed, referring t o three depth ranges; i.e. shallow (0-5 km), intermediate (5-15 km) and dee p (from 15 km to the upper mantle). The shallow velocity structure is infer red by the joint inversion of shot and local earthquake arrival time data. The main feature pointed out by this inversion is a high-velocity anomaly a t the crater axis extending down to a depth of about 5 km. This anomaly can be explained with the presence of residual magma crystallised in the shall ow conduits, which accumulated during the last eruptive cycles. The local s eismicity is strongly clustered around this anomaly, due to the focusing ef fect of the rigidity contrast. The space-time seismicity pattern at Somma-V esuvius is the result of the superposition of background seismicity, mainly due to gravitational instability of the volcanic edifice and to small exte rnal stress perturbations, with intense episodic earthquake swarms possibly due to magmatic or hydrothermal activity into the shallow system. The velo city structure in the 10-15 km depth range is characterized by the presence of a low-velocity layer, which has been independently confirmed by multi-c hannel seismic reflection data and P-Sv conversions from teleseismic wavefo rms. The study of the deep structure was performed by regional tomography w ith teleseisms; it confirmed the presence of a low-velocity anomaly underne ath the volcano, which appears to have roots at greater depths. The regiona l structure between the Thyrrenian and the Adriatic sea has been inferred b y tomographic inversion of teleseismic arrival times. The main result from this study which is very important for geodynamic interpretations is the fi rst evidence for a continuous subducting stab under the Apennines, in an ar ea where previous models hypothesized a slab window.