STATISTICAL-ANALYSES AND CHARACTERISTICS OF VOLCANIC TREMOR ON STROMBOLI-VOLCANO (ITALY)

A study of volcanic tremor on Stromboli is carried out on the basis of data recorded daily between 1993 and 1995 by a permanent seismic stat ion (STR) located 1.8 km away from the active craters. We also conside r the signal of a second station (TF1), which operated for a shorter t ime span. Changes in the spectral tremor characteristics can be relate d to modifications in volcanic activity, particularly to lava effusion s and explosive sequences. Statistical analyses were carried out on a set of spectra calculated daily from seismic signals where explosion q uakes were present or excluded. Principal component analysis and clust er analysis were applied to identify different classes of spectra. Thr ee clusters of spectra are associated with two different states of vol canic activity. One cluster corresponds to a state of low to moderate activity, whereas the two other clusters are present during phases wit h a high magma column as inferred from the occurrence of lava fountain s or effusions. We therefore conclude that variations in volcanic acti vity at Stromboli are usually linked to changes in the spectral charac teristics of volcanic tremor. Site effects are evident when comparing the spectra calculated from signals synchronously recorded at STR and TF1. However, some major spectral peaks at both stations may reflect s ource properties. Statistical considerations and polarization analysis are in favor of a prevailing presence of P-waves in the tremor signal along with a position of the source northwest of the craters and at s hallow depth.