[S03-2] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise II

[S03-2-03] Scattering and attenuation structures beneath volcanoes inferred from envelope widths of volcano-seismic events

^○Hiroyuki Kumagai¹, Cristian Lopez², John Londono², Yuta Maeda¹, Rudy Lacson³ (1.Nagoya University, Nagoya, Japan, 2.Colombian Geological Survey, Manizales, Colombia, 3.Philippine Institute for Volcanology and Seismology, Quezon, Philippines)

It has been known that volcanoes exhibit highly heterogeneous structures with strong attenuation. Investigating scattering and attenuation structures is important to understand magma and hydrothermal systems beneath active volcanoes. In previous studies, the transport mean free paths (lm) and quality factors (Q) at volcanoes have been estimated by using seismic waveform data acquired by seismic experiments using artificial explosions, in which envelope waveforms (Wegler, JVGR, 2003) and spatio-temporal seismic energy distributions (Yamamoto and Sato, JGR, 2010) were analyzed. In this study, we propose a new approach using envelope widths of volcano-seismic events. The envelope width (p) is defined by the ratio of the cumulative amplitude (I) to the peak amplitude (A) of an envelope waveform of a volcano-seismic event in a 5-10 Hz band at each station. We used the analytical equation of Paasschens (Phys. Rev. E, 1997) for the radiative transfer theory in 3D isotropic scattering medium to derive the relationship of p with lm and Q. The estimated relationship indicated that p increases with decreasing lm and Q at a constant source-station distance, and p also increases with the source-station distance. We estimated p values for volcano-tectonic events at Taal volcano, Philippines, and at Nevado del Ruiz volcano, Colombia. Our estimated p values ranged up to 5 s and increased with increasing source-station distance. Our estimated relations between p and source distances at both the volcanoes were not explained by uniform lm and Q structures. We used the Monte Carlo simulation method of Yoshimoto et al. (JGR, 2000) to interpret the estimated relations. Our results indicated that the relations can be explained by depth dependent lm and Q structures: lm and Q are 500 m and 50, respectively, down to a depth of 1000 m, below which lm and Q are 10000 m and 100. The pulse width may be used as a new measure to investigate scattering and attenuation structures at volcanoes.