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Determining the location and the amount of volume change of the pressure source beneath a volcano during the eruption preparation stage is an important issue in monitoring the magma accumulation. To do so, we have implemented a GPS campaign survey network around the Mayon volcano and monitored ground deformation since 2005. Rapid grounddeflating deformation was detected accompanied by the 2009 eruption. The Mogi model pressure source was estimated to be 8.5 km deep beneath the summit and the amount of volume change –13 × 106 m3. In magma accumulation preceding the 2009 eruption, ground deformation showed a weak inflationary trend, but it was difficult to evaluate the source parameters definitively. After the 2009 eruption, no deformation has been detected by the Continuous GPS observation network since 2012. Trend of many baselines of continuous and campaign network turned to extension since 2014. Magma may have started accumulating beneath the Mayon volcano.

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.