A Phreatic Explosion Model Inferred from a Very Long Period Seismic Event at Mayon Volcano, Philippines
Monday, 15 December 2014
Mayon is one of the most active volcanoes in the Philippines with 49 known historical eruptions from 1616 to 2010. A phreatic explosion took place at Mayon on 7 May 2013 that killed five climbers. During the explosion, a very long period seismic event with a peak frequency of 0.4 Hz was recorded by three broadband seismometers. Our frequency-domain waveform inversion solution of the event in the frequency range 0.1-0.6 Hz is consistent with a subhorizontal tensile crack and a vertical single force at a shallow location beneath the summit crater. The source time functions obtained by the waveform inversion are band-passed forms (filtered source time functions; FSTFs), which may be distorted from the source time functions without filters (deconvolved forms of the source time functions; DSTFs). To estimate the DSTFs, we assumed several simple trial functions as candidates for the DSTFs and applied a band-pass filter of 0.1-0.6 Hz to them. A comparison with the FSTFs suggested that the DSTFs of both the crack and single force are better approximated by an impulse-type function than by a step-type function. The estimated DSTF of the crack showed inflation followed by deflation, whereas that of the single force suggested a downward impulse. The inflation of the crack may be attributed to boiling of underground water and its deflation can be attributed to discharge of water vapor. The downward force may be understood as the counterforce of the explosion. Our results suggest that only a portion of the crack wall was destroyed by the explosion. We present a model of repeated explosions in which an explosion can occur once the fragmented portion of the crack is sealed by precipitation of clay minerals or hydrothermal secondary deposits. This model may explain the absence of clear precursory signals before the 2013 explosion.