5:15 PM - 6:30 PM
[SVC28-P23] Estimation of seismic scattering coefficients at active volcanoes
Keywords:Shallow volcanic structure, Seismic scattering, Active seismic experiments
Volcanoes are considered one of the most heterogeneous field in the Earth’s crust, and the understanding of such small-scale inhomogeneities in volcanoes may provide us important information on the various volcanic processes including fluid transport related to past and present volcanic activities. In heterogeneous media, like shallow volcanic structure, the seismic wave propagation is strongly affected by the scattering, and the conversion scattering between P and S waves and their multiple scattering play an important role in wave propagation and energy transportation. Such mode conversion and multiple scattering are also important to consider the applicability of seismic interferometry and Green’s function extraction which are often related to the requirement of equipartitioning. In our previous work, we estimated the total scattering coefficients including those of conversion scattering using the spatial distribution of propagating energy from an active seismic source at Asama volcano, Japan (Yamamoto and Sato, 2010). In this presentation, we report the results of systematic estimation of the scattering coefficients at several volcanoes and discuss the relation between the estimated scattering coefficients and the structure of each volcano.
In this study, we estimated the scattering coefficients at Aso, Bandai, Iwate, and other volcanoes using the data of active seismic experiments which were conducted as a part of the national project for the prediction of volcanic eruptions in 1990s and 2000s. The most remarkable characteristic of observed seismograms is the existence of long and dominant coda waves after the onset of P wave, and the spatial distribution of propagating energy at fixed time shows a clear pattern exhibiting two slopes which are indicative of multiple scattering P and S modes having different scattering coefficients. From the comparison between the observed energy distributions and those calculated by the radiative transfer model considering the mode conversions between P and S energies and the multiple isotropic scattering of these two modes in the full space, we found that the total scattering coefficients of P-S scattering at 4-8 Hz and 8-16 Hz bands are approximately 1.5 km at all volcanoes we examined. These estimated values of the total scattering coefficients were close to those estimated in our previous study at Asama and are also consistent with those estimated by an array observation at Sakurajima. In this study, we also verified the validity and reliability of the use of the radiative transfer model in the full space by analyzing the energy propagation in the half space and the layered models which are computed by numerical means (i.e., finite-difference and Monte Carlo methods). We found that the maximum error of the estimated total scattering coefficients is about 40%, and thus the wave scattering at shallow volcanic structure is still one or two order stronger than that in the ordinary crust.
These results suggest that the conversion scattering between P and S waves and significant multiple scattering of each mode have an indispensable effect in the modeling and analysis of seismic wave propagation in heterogeneous volcanic environments. Since we have been assuming the scattering caused by statistical random fluctuations, it is also necessary to consider the scattering by discrete cavities as pointed out by Sato (2019, GJI; 2020, JpGU) for a better understanding of wave scattering in the shallow volcanic structure in the future.
In this study, we estimated the scattering coefficients at Aso, Bandai, Iwate, and other volcanoes using the data of active seismic experiments which were conducted as a part of the national project for the prediction of volcanic eruptions in 1990s and 2000s. The most remarkable characteristic of observed seismograms is the existence of long and dominant coda waves after the onset of P wave, and the spatial distribution of propagating energy at fixed time shows a clear pattern exhibiting two slopes which are indicative of multiple scattering P and S modes having different scattering coefficients. From the comparison between the observed energy distributions and those calculated by the radiative transfer model considering the mode conversions between P and S energies and the multiple isotropic scattering of these two modes in the full space, we found that the total scattering coefficients of P-S scattering at 4-8 Hz and 8-16 Hz bands are approximately 1.5 km at all volcanoes we examined. These estimated values of the total scattering coefficients were close to those estimated in our previous study at Asama and are also consistent with those estimated by an array observation at Sakurajima. In this study, we also verified the validity and reliability of the use of the radiative transfer model in the full space by analyzing the energy propagation in the half space and the layered models which are computed by numerical means (i.e., finite-difference and Monte Carlo methods). We found that the maximum error of the estimated total scattering coefficients is about 40%, and thus the wave scattering at shallow volcanic structure is still one or two order stronger than that in the ordinary crust.
These results suggest that the conversion scattering between P and S waves and significant multiple scattering of each mode have an indispensable effect in the modeling and analysis of seismic wave propagation in heterogeneous volcanic environments. Since we have been assuming the scattering caused by statistical random fluctuations, it is also necessary to consider the scattering by discrete cavities as pointed out by Sato (2019, GJI; 2020, JpGU) for a better understanding of wave scattering in the shallow volcanic structure in the future.