Locating the sources of volcanic explosions and volcanic tremor at Stromboli volcano (Italy) using beam-forming on diffraction hyperboloids

Abstract

The wave-field of volcano-induced seismic signals at Stromboli volcano (Italy) was observed using a network of four broadband seismometers. In addition to the standard spectral analysis of the recorded signals, new spectral techniques such as wavelet transform and wavelet coherence analysis were also used in order to discriminate between signals related to volcanic tremor and volcanic explosions. The common features of these two types of signals are characterised by the maximum of the power spectral density values located in the frequency band > 2.0 Hz, and the main orientation of the principal axes of the particle motion ellipsoids in the frequency bands analysed. The seismic signals of the volcanic explosions at Stromboli cover the total frequency band, whereas the spectral content of the volcanic tremor is restricted to narrow frequency bands ranging from 0.3–5.0 Hz. In order to localize both types of signals, a modified inversion technique was used. The result of this beam-forming on diffraction hyperboloids of both types of signals reveal a surprisingly shallow source location ranging from 0.2 km–0.7 km above sea level, suggesting that the sources of the volcanic explosions, as well as of the volcanic tremor, are restricted to the uppermost parts of the volcano. This is in agreement with several nonseismological observations of this volcano. The combination of spectral analysis of volcanic tremor and volcanic explosions with the analysis of ground displacement implies a mechanism which can be described by an ascending bubble of volcanic gas in a nearly closed volcanic system, and finally the forced ejection of the material through a narrow channel at the top of the magma column.