Source mechanism of explosions at Stromboli volcano, Italy, determined from moment tensor inversion of very-long-period data

Seismic data recorded in the 2-30 s band at Stromboli Volcano, Italy, are analyzed to quantify the source mechanisms of Strombolian explosions during September 1997. To determine the source-centroid location and source mechanism, we minimize the residual error between data and synthetics calculated by the finite difference method for a point source embedded in a homogeneous elastic medium that takes topography into account. Two source centroids are identified, each representative of the distinct event types associated with explosive eruptions from two different vents. The observed waveforms are well reproduced by our inversion, and the two source centroids that best fit the data are offset 220 and 260 m beneath and similar to160 m northwest of the active vents. The source mechanisms include both moment-tensor and single-force components. The principal axes of the moment tensor have amplitude ratios 1: 1: 2, which can be interpreted as representative of a crack, if one assumes the rock matrix at the source to have a Poisson ratio nu = 1/3, a value appropriate for hot rock. Both imaged cracks dip similar to60degrees to the northwest and strike northeast-southwest along a direction parallel to the elongation of the volcanic edifice and a prominent zone of structural weakness, as expressed by lineaments, dikes, and brittle structures. For our data set, the volume changes estimated from the moments are similar to200 m(3) for the largest explosion from each vent. Together with the volumetric source is a dominantly vertical force with a magnitude of 10(8) N, consistent with the inferred movement of the magma column perched above the source centroid in response to the piston-like rise of a slug of gas in the conduit.