[S-VC28] Active Volcanism

[SVC28-08] Temporal variations in seismic scattering anomalies estimated by inversion of envelope widths of volcano-tectonic earthquakes at Taal volcano, Philippines.

*Miki Hamamoto ¹, Hiroyuki Kumagai¹, Rudy Lacson² (1.Graduate School of Environmental Studies, Nagoya University, 2.PHIVOLCS (Philippine Institute of Volcanology and Seismology))

Keywords:Volcano-tectonic earthquake, Taal volcano, envelope width, scattering, inversion

Volcano-tectonic (VT) earthquakes have frequently occurred at Taal volcano. We analyzed envelope widths (p) of VT earthquakes to investigate scattering and attenuation structures at this volcano. The envelope width is defined by the ratio of the peak amplitude to the cumulative amplitude in a high-frequency (5−10 Hz) envelope waveform. We performed inversion of p values to estimated spatial distributions of mean free path (l₀) and intrinsic attenuation (Q_i) at Taal. One-dimensional (1D) depth-dependent l₀ and Q_i structures were estimated by a previous study with a grid-search approach to fit observed envelope widths to those calculated with Monte Carlo envelope waveform simulations. Using these models as initial models and 224 VT earthquakes during 2011−2017 including those not used by the previous study, we performed inversion of p values with a damped least-squares method to estimate new 1D models, in which partial derivatives with respect to l₀ and Q_i in each layer were calculated with Monte Carlo simulations. Plots of the differences between observed p values and those calculated with the new 1D models (dp) in individual ray paths show that positive and negative values exist beneath a geothermal area in Main Crater and the eastern flank of Volcano Island, respectively. We thus assumed these two anomalies and estimated their l₀ and Q_i values from the new 1D models by inversion of dp values of all VT earthquakes. We also performed inversion of dp values of each earthquake to estimate temporal variations in l₀ and Q_i in the two anomalies. Our inversion results show smaller l₀ values or stronger scattering in the anomaly beneath the geothermal area in Main Crater without their temporal variations. In the anomaly beneath the eastern flank of Volcano Island, we found temporal variations in l₀ values, in which small l₀ values occurred when seismicity was high. This anomaly has been interpreted as an actively degassing magma body, and our estimated temporal variations may be caused by heterogeneous distributions of degassed bubbles and microlites in the magma as follows. An intense supply of volatile-rich magma into the magma reservoir produces stress changes in the surrounding medium, which induce more frequent VT earthquakes. The vesiculated magma with microlites ascends in the conduit, and releases degassed bubbles near the surface. This activates the magma convection in the conduit, and enhances heterogenous distributions of bubbles, microlites, and degassed magma, which result in small l₀ values in the anomaly beneath the eastern flank.