Application of Wave-Theoretical Seismoacoustic models to the Interpretation of Explosion and Eruption Tremor Signals Radiated by Pavlof Volcano, Alaska

摘要

Tremor and explosion signals recorded on September 29 during the Fall 1996 Pavlof eruption are interpreted using video images, field observations, and seismic data. Waveform analysis of tremor and explosions provided estimates of the melt\u27s volcano-acoustic parameters and the magma conduit dimensions. Initial mass fractions of 0.25% water and 0.025% carbon dioxide in the melt can explain the resonance characteristics of the tremor and explosion pulses inferred from seismic data. The magma conduit is modeled as a three-section rectangular crack. We infer that the tremor-radiating region consists of the lowermost two sections, both with cross-sectional areas of ∼10 m2. The deeper section is 43 m long, with magma sound speed of 230 m/s, density of 2600 kg/m3, and viscosity of 1.0×106 Pa s. The section above it, defined by the water nucleation depth, is 64 m long, with magma sound speed of 91 m/s, density of 2000 kg/m3, and viscosity of 1.4×l06 Pa s. An average magma flow velocity of 1.2 m/s, with superposed random oscillations, acts as the tremor source. Explosions are postulated to occur in the uppermost part of the magma conduit after water comes out of solution. The explosion source region consists of a 15 m long section, with cross-sectional area of 20 m2, sound speed of 51 m/s, density of 1000 kg/m3, and viscosity of 1.5×103 Pa s. A burst pressure of 220 MPa at 14 m depth would generate an acoustic pulse whose amplitude and character match the observed signal. Waveform analysis of the explosion pulses shows that the explosive event may be preceded by a long-period fluid transient which may trigger the metastable magma-gas mixture. The modeling procedure illustrates the synergy of fluid dynamic, seismic, and acoustic models and data with geological and visual observations.