WAVE-FORM ANALYSIS OF SEISMOACOUSTIC SIGNALS RADIATED DURING THE FALL1996 ERUPTION OF PAVLOF VOLCANO, ALASKA

Theoretical modeling of acoustic and seismic signals associated with t he 1996 strombolian eruption of Pavlof volcano suggests that volcanic tremor at Pavlof originates in the deeper part of the magma conduit, a nd is generated by random fluid oscillations in the magma flow. Explos ions are believed to occur in the shallower part of the magma conduit, and to be caused by the rapid and violent expansion of metastable mag ma-gas mixtures. The effect of increasing the exsolved quantities of H 2O and CO2 gas with reduced pressure in the melt is to decrease the so und speed and density of the magma-gas mixture. This causes an acousti c decoupling of the upper and lower parts of the magma conduit. The re duced sound speed and density of the melt at shallow depths present a sharp impedance contrast, which strongly reflects acoustic energy orig inating at depth and traps it in the lower part of the magma conduit. Alternatively, acoustic energy originating from the upper part of the conduit remains trapped in the low-velocity region formed by the exsol ved gas in the melt, and hence shallow explosions may preferentially c ouple into the atmosphere. Explosion signals may be triggered by an in creased flow of melt at depth, and may be preceded and accompanied by vigorous mass flux transients.