Volcanic unrest leading to the July-August 2001 lateral eruption at Mt. Etna: seismological constraints

A close relationship between earthquake swarms, volcanic eruptions, and ground deformation at Mt. Etna was
well documented shortly before the beginning of the July–August 2001 eruption. Past experiences at this volcano
suggest how magma/dike intrusion in the shallow crust or in the upper part of the volcanic pile normally occurs
after several years/months of internal recharging. Since seismic investigations provide a means to study the scale
and origin of stress perturbations at active volcanoes, allowing to better investigating the preparation phase of an
eruption, in this paper, we performed a close examination of the seismic activity recorded at Mt. Etna in the
months preceding the 2001 eruption and in particular between November 2000 and July 2001. After integrating
data recorded by the two networks operating during that time and run by the Istituto Internazionale di
Vulcanologia and SISTEMA POSEIDON, we relocated 522 earthquakes by using the tomoDD code in a 3D velocity
model, and then we computed their fault plane solutions. The application of different selection criteria enabled
obtaining a good-quality revised data set consisting of 111 fault plane solutions.
The high-precision locations identified well-defined seismic clusters, in different periods, suggesting a link with
the magma migration from a depth of 8–13 km b.s.l. towards shallower zones. Moreover, the computed maximum
compressive stress axis, as inferred from earthquake focal mechanisms, indicated a roughlyW-E-oriented
σ1. This findings reflect an overpressure of the mid to shallow crust due to the progressive magma uprising in
central portion of the volcano and also highlighted a rotation of the local stress field with respect to the regional
one N-S trending. In addition, P-axis distribution pointed out the presence of a center of pressure located to the
south of the Central Craters.
These results provide particularly compelling evidence for a direct causal link between pressurization of the midlevel
volcanic plumbing system by ascending magma and precursory local stress field reorientations, demonstrating
that seismological analysis can be used to detect subtle local stress changes that herald eruptive activity.