A statistical study of the Stromboli volcano explosion quakes before and during 2002–2003 eruptive crisis

We study the seismic wavefield and the statistical properties of the Stromboli volcano explosions preceding and during the 2002-2003 crisis. We analyze the recordings of a three-component seismometer operating since 23 May 2002 to 30 January 2003, including the first 34 days of the crisis. Before the crisis, we recognize three bell-shaped classes of spectra with maxima falling in the range 1-5 Hz. Spectral content has two main changes, the most prominent one occurring at the crisis onset when the frequency peak at similar to 0.3 Hz increases in amplitude. Independent component analysis extracts three time-stable independent oscillations that peaked at 1.1, 1.8, and 2.5 Hz, with radial and shallow polarization indicating a stable source mechanism. Energy of the explosions is lognormally distributed, except during a 2 month time interval before the crisis when it also shows a higher mean value. The interoccurrence time distributions display an homogeneous Poissonian behavior with a mean intertime of 250 s, without changes at the crisis onset. Only swarms of explosions are not ruled by a Poisson process and display higher occurrence rates and higher energies. Finally, we depict a scheme of the crisis. A modification of the equilibrium is induced by rising magma that produces a change in the boundary conditions of the plumbing system. The escape from the equilibrium produces, at first, variations in the usual statistics of the explosions, then it leads to the lava effusion and to a pressure drop in the plumbing system that induces a deep gas slug nucleation and the excitation of low frequencies.