Envelope Widths of Volcano-Seismic Events and Seismic Scattering Characteristics Beneath Volcanoes

Volcanoes produce highly scattered seismic wavefields, which are characterized by the scattering mean free path (l₀) and the quality factor of medium attenuation for S waves (Q_i). Previous studies have estimated scattering mean free paths at various volcanoes to be around 100-1,000 m, but their depth distributions remain to be determined. We estimated one-dimensional l₀ and Q_i structures using the envelope widths of volcano-seismic events at Taal (Philippines) and at Nevado del Ruiz (Colombia) in a high-frequency band (5-10 Hz). We performed Monte Carlo envelope simulations in various one-dimensional models, and compared the simulated and observed envelope widths. Our results indicate that a highly heterogeneous (l₀ = 1,000 m) and attenuative (Q_i = 100) surface layer exists to a depth of 1 km, below which l₀ and Q_i are comparable to those in the normal crust. The surface layer may consist of unconsolidated materials with hydrothermal fluids and/or altered clay minerals, which cause strong scattering and attenuation effects beneath these volcanoes. Our results suggest that strong heterogeneities observed at volcanoes exist only in a thin surface layer.