Evidence of multiple strain fields beneath the eastern flank of Mt. Etna volcano (Sicily, Italy) deduced from seismic and geodetic data during 2003–2004

We carried out a study of the seismicity and ground deformation occurring on Mount Etna volcano after the end of 2002-2003 eruption and before the onset of 2004-2005 eruption. Data were recorded by the permanent local seismic network run by Istituto Nazionale di Geofisica e Vulcanologia –Sezione di Catania and by the geodetic surveys carried out in July 2003 and July 2004 on the GPS network. Most of the earthquakes are grouped in two main clusters located in the northeastern and southeastern sectors of the volcano. Furthermore, the areal distribution of seismic energy associated with the recorded earthquakes allowed us to highlight the main seismogenic areas of Mt. Etna. In order to better understand the kinematic processes of the volcano, 3D seismic locations were used to compute fault plane solutions and a selected dataset was inverted to determine stress and strain tensors. The focal mechanisms, in the northeastern sector, show a clear left-lateral kinematic along an E-W fault plane, in good agreement with the Pernicana Fault system. The fault plane solutions, in the southeastern sector, show a main right-lateral kinematics along a NE-SW fault plane suggesting a roughly E-W oriented compression. Surface ground deformation affecting Mt. Etna and measured by GPS surveys highlighted a marked inflation during the same period and exceptionally strong seawards motion of its eastern flank. The 2D geodetic strain tensor distribution was calculated and the results evidenced a main ENE-WSW extension coupled with a WNW-ESE contraction, indicating a right-lateral shear along a NW-SE oriented fault plane. The different deformation of the eastern sector of the volcano, as measured by seismicity and ground deformation, must be interpreted by considering the different depths of the two signals. Seismic activity along the NW-SE alignment is, in fact, located between 3 and 8 km b.s.l. and it is then affected by the very strong additional E-W compression induced by the pressurizing source located by inverting GPS data just westwards and at the same depth. Ground deformation measured by GPS at the surface, on the contrary, is mainly affected by the shallower dynamics of the eastern flank, fast moving towards East, that produces an opposite (extension) shallower E-W strain. The entire dataset shows that two different processes affect the eastern flank at the same time but at different depths; the boundary is clearly located at a depth of 3 km and could represent the decollement surface of the mobile flank