Source mechanism of long period events recorded by a high density seismic network during the 2008 eruption on Mount Etna

129 Long Period (LP) events, divided into two families of similar
events, were recorded by the 50 stations deployed on Mount Etna in the
second half of June 2008. During this period lava was flowing from a
lateral fracture after a summit strombolian eruption. In order to
understand the mechanisms of these events, we perform moment tensor
inversions. Inversions are initially kept unconstrained to estimate the
most likely mechanism. Numerical tests show that unconstrained inversion
leads to reliable moment tensor solutions because of the close
proximity of numerous stations to the source positions. However, single
forces cannot be accurately determined as they are very sensitive to
uncertainties in the velocity model. Constrained inversions for a crack,
a pipe or an explosion then allow us to accurately determine the
structural orientations of the source mechanisms. Both numerical tests
and LP event inversions emphasise the importance of using stations
located as close as possible to the source. Inversions for both
families show mechanisms with a strong volumetric component. These
events are most likely generated by cracks striking SW-NE for both
families and dipping 70 degrees SE (Family 1) and 50 degrees NW
(Family 2). For Family 1 events, the crack geometry is nearly orthogonal
to the dike-like structure along which events are located, while for
Family 2 the location gave two pipe-like bodies which belong to the same
plane as the crack mechanism. The orientations of the cracks are
consistent with local tectonics, which shows a SW-NE weakness direction.
The LP events appear to be a response to the lava fountain occurring on
the 10th of May, 2008 as opposed to the flank lava flow.