Anomalous Spatial Distribution Of Coda-Wave Energy Observed In Northeastern Honshu, Japan And Its Interpretation

Seismic coda waves of local earthquakes are considered to be scattered waves due to short wavelength heterogeneities in the lithosphere and to be distributed uniformly in space at large lapse times. This idea has been applied to the determination of earthquake magnitude of local events and to the measurement of seismic attenuation and site amplification. However, is the spatial uniformity of coda-wave energy observed even in tectonically very heterogeneous regions? We investigate the spatial distribution of S-coda-wave energy in northeastern Honshu, Japan, where a volcanic front is running from north to south. Seismograms recorded by the Hi-net from a large local event are used in this study. At large lapse times, root-mean-square amplitudes of coda waves at each station are calculated for frequency bands of 2-4, 4-8, 8-16 and 16-32 Hz. From this analysis, a clear spatial variation of coda-wave energy across the volcanic front from east (forearc-side) to west (backarc-side) is observed. The coda-wave energy is significantly smaller in the backarc-side than in the forearc-side. The energy is uniformly distributed in the forearc-side, whereas a monotonic exponential decrease with horizontal distance from the volcanic front is found in the backarc-side. The decay rate shows clear frequency dependence and increases with frequency, suggesting strong attenuation of high-frequency waves in the backarc-side. In order to explain this anomalous distribution of coda-wave energy, we propose a diffusion-absorption model that consists of two adjoined half-spaces with different absorption strength. We suppose strong absorption for one half-space to model significant seismic attenuation in the backarc-side. In the long-lapse-time limit, the spatial energy density in the strong absorption medium decays exponentially with the offset from the boundary. It is found that the magnitude of the decay rate is expressed by a simple function of the diffusion coefficient and the absorption coefficient. This indicates that, in northeastern Honshu, Japan, the volcanic front is a boundary between different seismic attenuation structures (forearc-side and backarc-side) and that the assumption of homogeneous spatial distribution of coda-wave energy is not valid in this case.