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During a phreatic explosion at Mayon Volcano, Philippines, on 7 May 2013, a very long period seismic event with a peak frequency of 0.4 Hz was observed. 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. We estimated the deconvolved forms of the source time functions (DSTFs), which are source time functions corrected for the effects of the band-pass filter. The DSTF of the crack can be approximated by an impulse-type function composed of inflation followed by deflation, whereas the DSTF of the single force can be approximated by 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, whereas 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 could not find clear precursors in seismic, thermography, geothermal, geodetic, and meteorological data. 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.

Taal Volcano, Philippines, is one of the world's most dangerous volcanoes given its history of explosive eruptions and its close proximity to populated areas. A real-time broadband seismic network was recently deployed and has detected volcano-tectonic events beneath Taal. Our source location analysis of these volcano-tectonic events, using onset arrival times and high-frequency seismic amplitudes, points to the existence of a region of strong attenuation near the ground surface beneath the east flank of Volcano Island in Taal Lake. This region is beneath the active fumarolic area and above sources of pressure contributing inflation and deflation, and it coincides with a region of high electrical conductivity. The high-attenuation region matches that inferred from an active-seismic survey conducted at Taal in 1993. These features strongly suggest that the high-attenuation region represents an actively degassing magma body near the surface that has existed for more than 20 years.

article i nfo The amplitude source location (ASL) method, which uses high-frequency amplitudes under the assumption of isotropic S-wave radiation, has been shown to be useful for locating the sources of various types of volcano-seismic signals. We tested the ASL method by using synthetic seismograms and examined the source amplitudes determined by this method for various types of volcano-seismic signals observed at different volcanoes. Our synthetic tests indicated that, although ASL results are not strongly influenced by velocity structure and noise, they do depend on site amplification factors at individual stations. We first applied the ASL method to volcano-tectonic (VT) earthquakes at Taal volcano, Philippines. Our ASL results for the largest VT earthquake showed that a frequency range of 7-12 Hz and a Q value of 50 were appropriate for the source location determination. Using these values, we systematically estimated source locations and amplitudes of VT earthquakes at Taal. We next applied the ASL method to long-period events at Cotopaxi volcano and to explosions at Tungurahua volcano in Ecuador. We proposed a practical approach to minimize the effects of site amplifications among different volcano seismic networks, and compared the source amplitudes of these various volcano-seismic events with their seismic magnitudes. We found a proportional relation between seismic magnitude and the logarithm of the source amplitude. The ASL method can be used to de- termine source locations of small events for which onset measurements are difficult, and thus can estimate the sizes of events over a wider range of sizes compared with conventional hypocenter determination ap- proaches. Previously, there has been no parameter widely used to quantify the sources of volcano-seismic signals. This study showed that the source amplitude determined by the ASL method may be a useful quan- titative measure of volcano-seismic event size.