Intrusive mechanism of the 2008–2009 Mt. Etna eruption: Constraints by tomographic images and stress tensor analysis

Since January 2008, several geophysical parameters have evidenced a recharging phase at Mt. Etna volcano
culminating with an effusive eruption that began on May 13, 2008. Seismic activity recorded at Mt. Etna
from January 2007 to May 2008 was analyzed in order to provide seismological constraints to the volcano
dynamics leading to the eruption. A total of 336 selected earthquakes, withML≥1.5, were used as data source
for this study. Specifically, we calculated 3D velocity and attenuation tomography, including a 3D relocation
of the events, and we computed 53 selected fault plane solutions (FPSs) that were used for stress tensor inversion.
The most important result obtained from the joint analysis of VP, VP/VS and P-wave attenuation is an
anomalous zone with normal to high VP (values between 3.5 and 4.5 km/s) and low VP/VS (values≤1.64),
which partially overlaps with a low QP (values≤50) volume located along a NS trending channel beneath
the central crater. This can be interpreted as a shallow volume characterized by high temperature where
the magma is located with the presence of supercritical fluids. The analysis of seismic stress tensor evidenced
an extensional regime in the depth range 3–13 km with a vertically oriented σ1. This finding may suggest an
extensional stress regime, probably related to the kinematic response of the volcanic edifice to both a deep
magmatic intrusion and a condition of decreased regional compressive stress facilitated by sliding processes
of the eastern flank of the volcano.