2015 年 68 巻 4 号 p. 91-105
For the Japanese train system, two kinds of the earthquake early warning systems (EEWS) have been installed to protect high-speed running vehicles against earthquakes. One is the on-site EEWS, which issues a warning to the areas where earthquake ground motions larger than a certain threshold are observed. The other is the regional EEWS, which issues a warning to the areas where railway damages will be expected by the information of the magnitude and the epicentral distance. These source parameters are estimated using the initial P-wave data at the closest seismic station to the epicenter. However, a method to directly predict S-wave amplitudes on the ground surface from the P-wave amplitudes at the seismic bedrock could be able to issue earthquake early warnings more simply and surely than the present method. Therefore, we quantitatively investigated the influence of source, path and site effects on the empirical relationship between P-waves at the seismic bedrock and S-waves on the ground surface, using seismic records of KiK-net in the Kanto Basin.
At first, spectral ratios and peak amplitude ratios of S-waves to P-waves observed at the seismic bedrock (α1), and those of S-waves observed on the ground surface to S-waves observed at the seismic bedrock (α2), were evaluated theoretically and empirically. Here, α1 and α2 correspond to the influence of source effects (VP/VS around the source region and radiation pattern) and path effects (Attenuation of P-wave and S-wave) and the influence of site effects (Green’s function of S-wave in the sedimentary basin), respectively. Our evaluation shows that α2 are larger α1; therefore, site amplification factors more largely affect the prediction of S-waves on the ground surface using P-waves at the seismic bedrock than the source and path effects. On the other hand, logarithmic standard deviations of α1 (σα1) were larger than those of α2 (σα2); therefore, the variability of source and path effects by individual earthquakes more largely affected the variability of the prediction of S-waves on the ground surface using P-waves at the seismic bedrock.
Secondly, we computed α1 from the theoretical equation as a function of VP/VS around the source region, radiation coefficients, and the wave velocity and the quality factor beneath the Kanto Basin. As a result, α1 values were mainly affected by VP/VS around the source region and the path effects in the high-frequency range. On the other hand, σα1 values were mainly affected by radiation patterns in the low-frequency range and path effects in the high-frequency range.