INFRASONIC WAVES AND VOLCANIC TREMOR AT STROMBOLI

The origin of the volcanic tremor is still under debate. Many theories have been proposed in the last years, but none has yet been completel y accepted. In 1993, highly sensitive pressure sensors (2.175 Pa/Volt) used to monitor the explosive activity at Stromboli have revealed une xpected correlation between small spike-shaped pressure signals (1-2 P a) and volcanic tremor. These pressure pulses repeat regularly in time with a recurrent period of ca. 1 s. Video camera images allowed us to correlate the pressure pulses with small gas bursts occurring at one of the active vents. The striking correlation (0.971) between infrason ic and seismic energy fluctuations is particularly meaningful in the f requency domain. Infrasonic and seismic signal share the same spectral content (3 Hz) for every station within a range of 700 m around the c raters. Correlations in time and frequency domain remained unaltered d uring the 1994 field experiments. Moreover, during 1994, the increased degassing activity has been followed by an increase in pressure relea se (7-8 Pa) and by a shift towards higher frequencies (8 Hz) both in t he infrasonic and seismic records. Infrasonic waves and volcanic tremo r show similar energy fluctuations and frequency contents, appearing t herefore to be produced by the same dynamical process. On this basis, we claim that volcanic tremor at Stromboli originates by continuous ou tbursting of small gas bubbles in the upper part of the magmatic colum n.