2014 年 67 巻 2 号 p. 57-79
In this study, spectral inversion of high-density strong motion data is done to evaluate source effects of the intraslab and interplate earthquakes off Miyagi Prefecture in northeastern Japan. Then, seismic moments, corner frequencies, and high-frequency levels of acceleration spectra are calculated from the obtained source effects. From the comparison between the intraslab and interplate earthquakes, the high-frequency levels of the former are systematically higher than those of the latter. On the other hand, from the viewpoint of source depth, a clear trend is found that deeper earthquakes have higher high-frequency spectral levels. Additionally, we find no significant difference between the spectral levels of intraslab and interplate earthquakes that have approximately the same source depths. Based on these results, we conclude (1) the high-frequency level does not depend on the difference of tectonic environments, such as intraslab or interplate earthquakes, but on the source depth, (2) deeper earthquakes have higher high-frequency spectral levels, and (3) the trend that intraslab earthquakes have higher high-frequency level than interplate earthquakes is apparent due to the fact that the former have systematically deeper source depths than the latter. Difference of ∼4 times is seen between the high-frequency levels of deeper (∼80 km depth) and shallower (∼30 km depth) earthquakes. Finally, we pick up two factors, other than source-originated ones, that may effect the evaluation of the high-frequency level: effect of waveform variation with source depth and effect of depth-dependent attenuation structure. These effects are evaluated quantitatively, and we conclude that they cannot bring such significant biases as can change the above-mentioned depth-dependent trend of the high-frequency level. Thus, we have successfully enhanced the reliability of our interpretation that deeper earthquakes have higher high-frequency spectral levels.