OBSERVATIONS OF SEISMIC VOLCANIC SIGNALS AT STROMBOLI VOLCANO (ITALY)

Digital recordings of ground motion during a seismic operation organized at Stromboli volcano (Italy) from May 31, 1986 to June 26, 1986 have been studied. The amplitude of background seismic noise is greater at the summit than at the base of the volcano and the spectra calculated on different days are dominated by at least two frequencies: 2.0 and 6.0 Hz with subdominant frequencies at 4.1, 4.8 and 8 Hz. According to their waveform and their spectral content, three types of discrete volcanic signals are distinguishable. The first type is characterized by a dominant frequency at 2 Hz and a subdominant frequency at 4.1 Hz. The second type also has a dominant peak frequency at 2 Hz with two important subdominant frequencies at 5 and 8 Hz. The type 3 has a sharp dominant frequency at 6 Hz and subdominant frequency at 3.2 Hz. The frequencies observed for discrete signals and ground noise suggest that they are produced by the same repeated mechanism which probably has a volcanic origin. The discrete signals are linked to explosions at the active vents within the craters. © 1990.