[S-VC32] Active Volcanism

[SVC32-09] Seismic tremor associated with SO₂ gas emissions after the 2020 eruption at Taal volcano, Philippines

*Hiroyuki Kumagai¹, Ma. Antonia Bornas², Christian Joseph Clarito², Paolo Reniva², Raul Ryan Rebadulla², Eric Lino Arconado², Rudy Lacson², Yuki Hattori¹ (1.Graduate School of Environmental Studies, Nagoya University, 2.Philippine Institute of Volcanology and Seismology)

Intensive SO₂ gas emissions have continued after the eruption on January 13, 2020 at Taal volcano (Philippines) for more than four years, during which seismic tremor has been also observed. To investigate the relationship between tremor and SO₂ gas emissions, we performed systematic waveform analysis of tremor signals. We used continuous seismic waveform data recorded by the Taal seismic network from January 17, 2020, when the first post-eruption tremor was observed, until the end of 2023. Our tremor analysis results were compared with daily SO₂ fluxes measured by a correlation spectrometer at the Taal volcano observatory. Using a method based on displacement spectral slopes, we detected more than 3,300 tremor episodes. We selected the maximum amplitude tremor in each day, and estimated its source location by the amplitude source location method. The estimated sources were located beneath the northern to eastern flanks of Taal Volcano Island at various depths down to about 10 km. The tremor sources showed a shallowing trend from July, 2021 and reached almost the surface at the end of August, 2021. Then, very few tremor was observed between 6 September and 2 December, 2021, in which the maximum SO₂ flux more than 2 × 10⁴ t/d was detected. After the quiescence, tremor occurred at various depths with variable SO₂ emissions less than 10⁴ t/d. Our cross-correlation (CC) analysis between tremor source amplitudes (A_s) and SO₂ fluxes (W_SO2) indicated relatively large CC coefficients around 0.4 with lag times within 20 days. The shallowing tremor sources suggest that magma ascended during July−August, 2021. In this period, tremor was generated by gas releases from the magma to a closed conduit, which formed a gas pocket above the magma. Gases in the packet were gradually released to the summit crater with permeable flows, which caused the lags between A_s and W_SO2. The magma reached the surface, and most intensively degassed SO₂ without seismic tremor. This might have occurred because gas release channels in the magma were open to the atmosphere. As overpressure in the magma was gradually decreased by degassing, the magma dead was lowered. Then, tremor occurred again with the mechanism mentioned above. Using Darcy’s law, a lag time of 10 days can be explained by a permeable flow for a conduit length of 2 km with permeability of 10⁻¹² m², which may be reasonable for porous volcanic rocks. Our study indicates that seismic tremor at Taal is directly related to magma degassing and provides useful information to magma ascent and degassing processes.