Imaging of three‐dimensional small‐scale heterogeneities in the Hidaka, Japan region: coda spectral analysis

SUMMARY While large‐scale heterogeneities have been studied intensively by seismic tomography with traveltime data, small‐scale heterogeneities are not fully investigated yet. We obtained the 3‐D spatial distribution of small‐scale heterogeneities as relative scattering coefficients in the Hidaka, Japan, region using the spatial variation of high‐frequency coda power spectra. We analysed 530 seismograms recorded at 62 stations for nine local earthquakes in the frequency range of 1–32 Hz. After correcting source, station and propagation effects, we assigned the spatial and temporal variations of observed power spectra of coda waves into the 3‐D distribution of scattering coefficient, based on single and isotropic scattering models. Small‐scale heterogeneities are distributed not only in a non‐uniform manner but also differently among scale lengths estimated from their frequency dependence. An area of large relative scattering coefficient in 2 and 16 Hz is located at a depth shallower than 45 km in the southwest of the Hidaka Mountains, correlating well with a highly active area of microearthquakes. At a depth of 60–120 km, two dipping zones of large scattering coefficient are imaged beneath the summit of the Hidaka Mountains. These zones may imply two layers of strong seismic‐wave reflection, consistent with the upper and lower planes of the double seismic zone in the subducting Pacific Plate, as determined by the hypocentral distribution in this region.