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.