Multiple scattering and mode conversion revealed by an active seismic experiment at Asama volcano, Japan

Volcanoes are one of the most heterogeneous fields in the Earth's crust, and the understanding of such inhomogeneities may provide us important information on various volcanic processes. In the previous studies on the seismic wave propagation at active volcanoes, the diffusion model has been widely used to model the energy transportation and the contribution of P and S waves and their mode conversion have not been well recognized partly due to lack of dense observations capturing spatiotemporal pattern of energy propagation. In this study, we present observational evidence of mode conversions and multiple scattering at Asama volcano, Japan, revealed by an active seismic experiment with a dense seismic network. The observed spatial distribution of propagating energy emitted from the explosive P source shows a wavefront of direct P wave and a pattern exhibiting two slopes which is indicative of multiple scattering and conversion scattering of two modes having different scattering coefficients. These facts suggest that the radiative transfer theory is more preferable than the diffusion theory to explain the observed characteristics. We thus modeled the energy propagation using the radiative transfer theory assuming multiple isotropic scattering including conversion scatterings and quantitatively estimated scattering parameters. Estimated total scattering coefficients for P-to-S and S-to-S scattering are about three times larger than that of P-to-P scattering, and the mean free path of S wave is about 1 km for an 8-16 Hz band. These results suggest that the mode conversion and multiple scattering have an indispensable effect on the seismic energy propagation in heterogeneous volcanic environments.