Tectonic dynamics of the Lipari-Vulcano islands revealed by geodetic and seismic data

Ground deformations measured during the last two GPS surveys carried out in March 2006 and September 2007 on the Lipari-Vulcano network are analyzed. Through this period, the monitoring seismic network measured an increase of seismic strain release around the Lipari-Vulcano complex with respect to the previous years. GPS surveys measured a significant N-ward motion of Vulcano island with respect to the ITRF05. This motion abruptly decreases in the N part of Vulcano, suggesting a decoupling of the main Vulcano island from the rest of the volcanic complex. By assuming a local reference frame, we observed that the Lipari island and the northernmost part of the Vulcano island are moving SSE-ward with respect to the central and southernmost part of the Vulcano island. GPS data also highlighted a local uplift of the “La Fossa” cone superimposed on the general subsidence of the island, increasing towards N. The vertical movements measured by GPS are compared with the results of the leveling surveys carried out on the network installed and managed by the INGV - Osservatorio Vesuviano for monitoring vertical ground movements on Volcano island. Height variations are computed with respect to a reference benchmark located on the southern part of the island, which is a relatively stable area compared to the northern part. The comparison between the two most recent surveys (October 2003 and October 2008) indicates a significant subsidence of the centre-northern area of the island (bottom Fossa Crater and Vulcanello). We also analyzed the long-term tilt observations through the same period. The tilt network at Vulcano Island currently has five borehole stations, four of which are installed at depth of 8-10 m, allowing recording stable and highly accurate signals with low noise. Tilt vectors concur well with leveling and GPS data, highlighting a deflection towards the central-northern part of the island. Geodetic strain tensor analysis derived by GPS data was performed over the area covered by the Lipari-Vulcano network. Transpressive kinematics of the Lipari-Vulcano volcanic complex, with a main N-S contraction and a minor E-W extension only on the Southern part of Vulcano is revealed. In addition, data inversion was performed using a simple Okada model, inverting the horizontal strain measured on 258 GPS baselines. We prefer using baselines rather than displacement vectors to avoid bias due to the ambiguity in fixing the reference system. The final structural framework defines a main compression with a dextral component of displacement on a NW-SE striking structure. This kinematics agrees with focal mechanisms of the strongest seismic events recorded in this area over the same time span as the GPS surveys. Furthermore, a preliminary analysis shows that the observed tilt fits well with the expected one from the model.