The understanding of shallow intrusive processes during 2002--2004, as well as the causes of the volcano-tectonic seismicity, has been improved at Mt. Etna by comparing the inversion results from GPS data with accurate 3D hypocentral locations. Our findings indicate that short periods of deflation (about six months) were followed by recharging phases after the end of both the 2001 and 2002--2003 flank eruptions. During the last recharging phase (June 2003--August 2004), modeling results and seismic observations suggest a composite mechanism of re-injection of magma into the rift-zones (S and NE), similar to that leading to the 2002--2003 flank eruption, which could have triggered the summit eruption started on September 7, 2004.