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

Abstract

AbstractVolcanoes 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 (Qi). 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 Qi 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 (Qi = 100) surface layer exists to a depth of 1 km, below which l0 and Qi 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.