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 ( l 0 ) 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 l 0 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 ( l 0  = 1,000 m) and attenuative ( Q i  = 100) surface layer exists to a depth of 1 km, below which l 0 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.