抄録
We proposed the method for estimating inhomogeneous attenuation structures by dividing the propagation path regions from the source to sites. In the proposed method, it is not necessary to assume the divided region in advance. On the basis of the statistical test, the divided region is estimated. Once statistically significance level is satisfied in each block under the minimum block size are assumed, we can estimate inhomogeneous attenuation structure with the resolution according to the number of data. We applied the proposed method to the source region of the 2008 Iwate-Miyagi Nairiku Earthquake. The obtained results are summarized as follows.
1) Estimated inhomogeneous attenuation structure shows low-Q on the block including Mt. Kurikoma, which is an active volcano, and the west side of the volcanic region. The short-period level of the main shock source spectrum estimated from the proposed method corresponds to the average level estimated from the empirical formula by Dan et al. (2001).
2) Compared with the conventional method which assume homogeneous attenuation structure, site amplification factors were estimated to be large at the site where volcanos are in the vicinity such as the IWTH25, and the short-period level of source spectra was estimated to be small.
3) Interpretation of small source spectra and large site amplification factors is as follows. In the source region, several sites are located in the high attenuation zone. This means that by appropriately evaluating the influence of high attenuation zone, amplitude of seismic bedrock motions at the observation point near the source region are evaluated to be small, resulting in large site amplification factors in comparison with conventional method. At these sites, since the influence of the Q value is not dominant because the distance is short, source spectra are estimated to be small. Site amplification factors of east side of target region did not change regardless of methods. Q value of east side according to proposed method tends to be larger than the conventional method. This large Q also contributes to estimating small source spectra.
4) It is concluded that inhomogeneous modeling of the propagation path is important especially in the volcanic area, such as the source region of Iwate-Miyagi Nairiku Earthquake, when separating the observation records into the source, path, and site effects.
