2015 年 67 巻 4 号 p. 105-124
We investigate three-dimensional attenuation structure of the P wave in the crust and upper mantle beneath the Chugoku, Shikoku and Kyushu districts of southwest Japan. The attenuation structure is estimated by inversion analysis of the P-wave attenuation factors obtained for a large number of ray paths covering the study area. To determine the attenuation factor from observed P-wave spectrum, the P-wave source spectrum is needed because the observed spectrum is convolved with the source spectrum. After estimating the S-wave corner frequencies from spectral ratio of the S coda waves of two events observed at a single station, we determine the P-wave corner frequency by assuming that it is 1.73 times higher than the S-wave corner frequency. The P-wave source spectrum is calculated by incorporating the estimated P-wave corner frequency into the ω-2 source model. The P-wave attenuation factor is estimated from amplitude decay rate of the P-wave spectrum corrected for the source spectrum. The P-wave t* data are inverted for QP structure by non-negative least squares method. Several features are found in the QP structure. In the upper crust down to 10 km depth, a high-attenuation zone extends in the Beppu-Shimabara rift valley. A prominent high-attenuation zone exists in the crust and uppermost mantle beneath Miyazaki Prefecture. The high-attenuation zone is interpreted as being an accretionary prism which was formed by adjunction of the Kyushu-Palau Ridge to the Kyusyu district due to the PHS slab subduction. Around 40 km depth in the uppermost mantle, a high-attenuation zone exists beneath the volcanic front and extends toward the fore-arc side of the Kyushu district. The high-velocity PHS slab shows low attenuation. A negative correlation is established between the Q-1 and velocity of P-wave.