MODELING OF VOLCANIC TREMOR AS EXPLOSIVE POINT SOURCES IN A SINGLE-LAYERED, ELASTIC HALF-SPACE

I investigate path effects on the waveform of a volcanic tremor using a model that consist of an impulsive dilatational point source embedded in a single-layered, elastic half-space. Such a source may arise during a variety of volcanic conditions, such as a sequence of rapid explosions at a volcanic vent as well as gas-piston activity observed in basaltic lava lakes. Using acoustic signals recorded during the 1975 Tolbachik eruption, the 1983 Klyuchevskoy eruption, and the 198 9 Tokachi-dake eruption, I propose a general impulsive time function for the tremor source, consisting either of a single impulse or a sequence of identical impulses. Computed seismograms illustrate the influence of a low velocity, surface layer on the shape and spectra of the waveforms. For the same layered model, a single impulse produces a dispersive wave, while a sequence of identical impulses produces a wave of multiple harmonics. For a single impulse, the peak in the seismograms shifts to higher frequencies when the surface layer is very thin. For multiple impulses, spectral peaks shift to higher frequencies as the time between successive impulses increases, indicating an increasing importance of higher-order harmonics.