3-D Intrinsic and Scattering Seismic Attenuation Structures Beneath Kyushu, Japan

The attenuation of seismic wave energy is caused by intrinsic absorption and scattering. The former involves the conversion of seismic wave energy to heat energy via internal friction due to the anelasticity of the medium, whereas the latter involves the scattering of seismic wave energy due to random elastic heterogeneities in the medium. Quantifying both intrinsic (Q(i)(-1)) and scattering (Q(s)(-1)) attenuation is therefore important for understanding the physical properties of the Earth's interior and predicting seismic wave propagation. Here we develop a new procedure to separately map three-dimensional (3-D) Q(i)(-1) and Q(s)(-1) structures. The path-averaged Q(i)(-1) and Q(s)(-1) values are obtained via an envelope-fitting approach that employs a multiple-scattering model. The path-averaged Q(i)(-1) and Q(s)(-1) structures are then mapped into 3-D space using a tomographic inversion technique based on sensitivity kernels, which are calculated from a Monte Carlo simulation of the radiative transfer equations. We apply this method to map the crustal structure beneath Kyushu, Japan, and obtain 3-D Q(i)(-1) and Q(s)(-1) structures for the 1-2, 2-4, and 4-8 Hz frequency bands. The spatial attenuation patterns are similar for each of the analyzed frequency bands, with the spatial variability in Q(s)(-1) being more pronounced than that in Q(i)(-1). The high-Q(i)(-1) and Q(s)(-1) regions correspond to the locations of active volcanoes. Conversely, the northern Kyushu area possesses low-Q(i)(-1) and Q(s)(-1) zones that correspond to low heat flow and Cretaceous granite, respectively. The overall Q(i)(-1) and Q(s)(-1) distributions correlate with thermal and geological structures, respectively.