Air-wave phases in strombolian explosion quake seismograms: a possible indicator for the magma level?

Abstract

Many explosion-quake seismograms recorded near the active craters at Stromboli are dominated by two distinct onsets: a first low-frequency phase and a second high-frequency one. Particle-motion diagrams show a P-wave characteristic for the low-frequency phase, whereas in the high-frequency onset the longitudinal polarization is lost and the pattern becomes chaotic in the horizontal as well as in the vertical plane. The air-pressure pulse generated by the volcanic explosion is detected by the seismometer as a high-frequency signal, and we assume as source mechanism an explosion at the top of the magma column, generated by rising gas bubbles. The different path lengths and velocities for the seismic wave and the air-wave are the reasons for the difference in arrival times Dt between the low-frequency and the high-frequency onset. We tried to deduce the magma level and gas velocity in the conduit, from observed Dt values. If the pressure pulse inside the consuit is assumed yo be lower (30-70 m/s) than the sound speed, the magma level remains reasonably stable, even for large changes of Dt.