スペクトルインバージョンと地震動予測式に基づく2018年大阪府北部の地震の地震動特性

抄録

We examine the source, path and site effects to strong ground motions during the 2018 northern Osaka earthquake (M_J6.1, M_w 5.5-5.7) by spectral inversion and ground motion prediction equations (GMPEs) using strong motion records. It is found that the Q for paths is modeled using frequency f as 38f ^1.34 by spectral inversion, which almost agrees with Q estimated by a previous study. The observed attenuation gradients for 5%-damped acceleration response spectra S_A and peak ground velocity PGV are consistent to GMPEs for crustal earthquakes in western Japan. These results show that path effects are average as crustal earthquakes. The site amplification factors from the seismic bedrock for PGV and Fourier spectra at periods of 1 to 2 s are large in the northeast and southwest directions from the source. This result shows that one of the causes of large instrumental seismic intensity in these directions is site amplification factors. The S_A at periods of 0.1 to 4 s after the correction by site amplification factors in the GMPEs are 1.3 to 1.6 times larger than the GMPEs for M_w 5.5 and 1.0 to 1.3 times larger than GMPEs for M_w 5.7. On the other hand, the S_A at periods of 4 to 5 s is the average level. The flat level of acceleration source spectra called as short-period spectral level A is estimated to 5.1−5.2×10¹⁸ Nm at periods of 0.2 to 2 s by spectral inversion. The estimated A is larger than A for previous crustal earthquakes with the same seismic moment M₀ and the empirical M₀-A relations. From both the spectral inversion and the comparison with GMPEs revealed that the generation of strong ground motions from the source at periods of 0.1 to 4 s are larger than the average of crustal earthquakes with the same M₀ in Japan. It was pointed out by previous studies that the northern Osaka earthquake was composed of a strike-slip fault and a dip-slip fault and that the rupture mainly propagated to the southwest direction on the strike-slip fault. Therefore, we examine the effects of near fault rupture directivity and the radiation pattern to strong ground motions by correcting the path and site effects using spectral inversion results. It is found that the spectra become larger at periods longer than about 0.5 s at stations in the southwest direction from the source. Especially the effects of transverse components are larger than radial components and transverse components have clear velocity pulses. On the other hand, the spectra at stations in the northeast direction become smaller. The effects of near fault rupture directivity and the radiation pattern for S_A are almost reproduced by the previous empirical model on the average by using both the strike-slip and dip-slip faults. However, the effects at periods of 0.5 to 1 s are noticeably stronger than those predicted by the model and are interpreted to be the other cause of large instrumental seismic intensity from 6 lower to 5 lower in the southwest direction.