Temporal Variations in Seismic Scattering Characteristics in a Shallow S-wave Attenuation Region at Taal Volcano, Philippines

Taal volcano in the Philippines is one of the most active volcanoes in the world. Previous studies suggest the existence of magma and hydrothermal fluids near the surface. We analyzed high-frequency seismic waveforms of volcano-tectonic (VT) earthquakes at this volcano to investigate its scattering characteristics represented by the mean free path (l₀) and the quality factor of S waves (Q_i). The earthquakes analyzed in this study occurred at similar locations NW of the volcano in May 2012-January 2013. The VT waveform on 15 May 2012 at VTCT located SE of the volcano showed a clear direct S wave with small coda amplitudes, while those at other stations have larger coda amplitudes characterized by multiple scattering. We found that the coda amplitudes at VTCT in the event on 25 June 2012 were clearly larger than those at VTCT in the event on 15 May 2012. These features suggest that l₀ and/or Q_i were not spatially homogeneous and changed temporally within one month. We used the Monte Carlo method to synthesize seismogram envelopes in 3D media consisting of two layers and a localized region beneath the eastern flank of the volcano, where strong S-wave attenuation was indicated by previous studies. We performed a grid search with respect to l₀ and Q_i in each of those layers and region to minimize the residuals between observed and calculated envelopes assuming Q_i = 100 for the two layers. Our results indicate a highly heterogeneous (l₀ = 0.5 km) surface layer underlain by a less heterogeneous (l₀ = 10 km) layer. Estimated values of l₀ in the localized region were 10 and 1 km for the events on 15 May 2012 and 25 June 2012, respectively. The large and rapid change of l₀ suggests fluid activity in the localized region. These spatio-temporal variations in l₀ may be explained by the existence of magma containing volatiles near the surface, in which the amount of bubbles generated by degassing temporally changed.