Imaging of the Three-Dimensional Distribution of Small-Scale Heterogeneities in the Hidaka, Japan, Region Using Coda Wave Amplitudes

Although large-scale heterogeneities have been studied intensively by seismic tomography, small-scale hetenogeneities are not fully investigated yet. We estimated the 3-D spatial distribution of small-scale heterogeneities as scattering coefficients in the Hidaka, Japan, region, using coda envelopes. We analyzed 1280 seismograms in a frequency range of 1 - 32 Hz for 22 events recorded at 64 stations densely deployed by the Research Group of Hidaka Collision Zone and the Institute of Seismology and Volcanology, Hokkaido University. First of all, we estimated the site effect of each station using the coda normalization method with teleseismic data. All the rests of data processings were done after the correction of this site effect. Next, we determined the gross 3-D distribution of intrinsic attenuation by an inversion method with P/S spectral ratios. Finally, we assigned spatial and temporal variations of observed coda envelopes into the 3-D distribution of scattering coefficients. In this study, we used the following simplification: single as well as isotropic scattering in three-dimension half space with a constant S wave velocity, only S-to-S wave conversion. We obtained distributions of scattering coefficients in the Hidaka region in two frequency ranges: 2 and 16 Hz. Some resolution tests showed that distributions in deep parts are obtained reliably, although shallow images (< 50 km) may be not resolved so well due to relatively poor path coverage there. An area of large relative scattering coefficients at 2 Hz is located at depth of shallower than 50 km in the west of the Hidaka Mountains, correlating well with a high activity area of microearthquakes. Since this shallow feature is prominent only at lower than 2 Hz, we interpret it as heterogeneities of 2 km in length, responsible for both strong scattering of coda as well as shallow earthquakes activity. On the other hand, a zone of large relative scattering coefficients is clearly observed at the depth range around 100 km beneath the summit of the Hidaka Mountains in both 2 and 16 Hz. This zone may imply a layer of strong seismic-wave reflection, which is consistent with the upper interface of the subducting Pacific plate.