The April-May 2006 volcano-tectonic events at Stromboli volcano (Southern Italy) and their relation with the magmatic system

Between April 10th and May 22th 2006, a small seismic swarm of 5 volcano-tectonic events occurred on the volcanic island of Stromboli (Southern Italy). Two of these, having M > 3 and an intensity of about V-VI MCS, were clearly felt causing concern in the population. They were recorded during a period of increased explosive activity and were followed by two major explosions at the summit craters on May 22th, few hours after the last earthquake and on 16th June. The location of such events has been performed using a probabilistic approach based on the Equal Differential Time tecnique. Using this tecnique, we were able to locate all the events, showing how they cluster below the volcanic edifice at a depth of about 5÷6 km. From observed P wave polarities we determined the focal mechanisms of the 4 major events. Using earthquake scaling laws, we calculated the fault area and the average slip for the two major events. Finally, assuming an homogeneous half-space model we computed the isotropic stress changes below the volcano edifice. The negative stress variation over the central axis of the volcano suggests that the earthquakes were triggered by a pressurization of the magmatic system. 1 The April-May 2006 seismic swarm Stromboli is among the most active volcanoes of the world. Its continuous mildly explosive activity is sometimes interrupted by lava effusions and by major explosions [1]. The occurrence of volcano-tectonic events at Stromboli, is not a usual feature, compared with other volcanoes. The last swarm recorded in the area occurred in June 1999 [5]. It was composed of small magnitude events (M ≤ 3.2) located about 6 km North of the island, none of them was felt. The last earthquake, located close to the island and having a macroseismic intensity higher than V occurred in 1967 (see Table 5). The 5 volcano-tectonic (VT) events recorded between April 10th and May 22th 2006 (see Table 5) show clear P and S phases, indicating a shear mechanism. This contrasts with seismicity usually recorded at Stromboli that consists of explosion-quakes and Very-Long-Period (VLP) events related to the strombolian activity at the summit craters [7]. The Peak Ground Acceleration (PGA)