INTERACTION OF SEISMIC AND AIR WAVES RECORDED AT STROMBOLI VOLCANO

Explosion-quake seismograms recorded at Stromboli show that seismic phases with a high-amplitude and high-frequency content propagate with a velocity of approximately 330 m/s - the sound speed. The analysis of seismograms, recorded at a distance of 500 m from one of the three active vents, shows for the first onset a low-frequency and particle motion characteristic of a p-wave, which loses its longitudinal polarization with the onset of the air-wave. Recording the explosion-quakes simultaneously with a microphone we could ascertain that the high-frequency onset coincided with the air-wave's. In order to better understand the seismic wavefield generated by the atmospheric pressure, we performed a controlled source experiment at Stromboli using a seismic gun. Seismograms with the same two phases and particle motions comparable with the volcano-seismic data were obtained. A second experiment demonstrated, that the air-wave propagates at least in the uppermost 1 m of the ground. We suggest that the seismic source of the corresponding seismograms is an explosion at the top of the magma column and conclude that the p- and air-waves are both generated in the same point and at the same time.