Modelling of the April 5, 2003 Stomboli (Italy), paroxysmal eruption from the inversion of broadband seismic data

On April 5, 2003, one of the largest eruptions in the last decades was observed at Stromboli volcano, Italy. The eruption
occurred in a period of increased volcanic activity, following a first explosion in December 2002, which interrupted the typical
moderate “Strombolian” behaviour. We present an exhaustive analysis of the available broadband seismic data and relate them to
the observed eruption phases. Prominent features of the seismic signals include an ultra long period signal starting a few tens of
seconds prior to the explosive eruption as well as a strong energetic signal a few seconds after the onset of the eruption. Both
signals are not exactly synchronized with the other geophysical observations. We present a detailed study of those signals using
spectral and particle motion techniques. We estimate eruption parameters and seismic source characteristics by different inversion
approaches. Results clearly indicate that the paroxysmal eruption was triggered by a shallow slow thrust-faulting dislocation event
with a moment magnitude of Mw=3.0 and possibly associated with a crack that formed previously by dike extrusion. At least one
blow-out phase during the paroxysmal explosion could be identified from seismic signals with an equivalent moment magnitude of
Mw=3.7 and is represented by a vertical linear vector dipole and two weaker horizontal linear dipoles in opposite direction, plus a
vertical force.