We investigated the spatial distribution of S-coda-wave energy in northeastern Honshu, Japan, where a volcanic front runs from north to south. Seismograms of a local event with a magnitude of 6.4 recorded by the Hi-net were analyzed in this study. The mean square amplitudes of coda waves at 82 seismic stations were calculated for bands with frequencies of 2–4, 4–8, 8–16, and 16–32 Hz. From these measurements, a clear spatial variation of coda-wave energy was observed across the volcanic front from east (forearc) to west (backarc). The coda-wave energy is uniformly distributed in the forearc, whereas an exponential decrease with horizontal offset from the volcanic front is found in the backarc. The decay rate increases with increasing frequency.
We propose a diffusion–absorption model to give a physical insight into the observed regional variation in coda-wave energy. This model explains the observed characteristics successfully and predicts that the exponential variation of coda-wave energy in the backarc is a function of the diffusivity and the absorption coefficient. The magnitude of the intrinsic attenuation factor in the backarc is estimated at about Qi−1 = 0.02 at a frequency of 10 Hz by assuming a value of 0.01 km−1 for the scattering coefficient. This intrinsic attenuation factor is about twice as large as those reported from previous studies on the forearc.