Tilt offset associated with local seismicity: the Mt. Etna January 9, 2001 seismic swarm

On the 9th of January 2001 a seismic swarm on
the southeastern flank of Mt. Etna at 3.5 km beside sea
level (b.s.l.), caused co-seismic variations on short and
long baseline tiltmeters of the Mt. Etna permanent tilt network.
Taking account of the geometry and mechanism of the active
tectonic structure obtained by seismological studies,
the theoretical tilt linked to the faulting source was calculated
at multiple different recording stations. It was found
that the amount of measured deformation exceeded that
which was generated seismically, indicating that much of
the deformation along the fault was aseismic.
The 9 January 2001 episode represents a shear response to
a local stress caused by a volcanic source that acted in the
period preceding the 2001 eruption. Tilt data also suggest
a marked slip of 70-140 cm along the fault, probably due to
the presence of fluids.