Site Amplification in Tangshan Area, China: A Comparison of Site-Effects Estimated by Different Methods

Abstract

中国唐山地域の震源, 地震波の伝播経路及びサイト等の特性を評価するために, 同地域に強震観測ネットが構築された.同観測ネットは, 8地点での地表観測と1ヶ所でのボアホール観測(-32, -16, 0m)から成る.これまでに, マグニチュード(M)が2.2-5.9の地震が60個観測された.まず, 10個の地震のS波水平成分スペクトルのインバージョン解析から, 震源スペクトル, S波のquality factor, Qs値及び各観測点の基準観測点に対する相対的な増幅特性を分離・評価した。震源スペクトルは, 周波数(f)が12Hz以下であればω-2震源モデルで近似できる.Qs値は, 周波数が0.5-32Hzの時Qs=67f^1.1で近似できる.次に, 各観測点の増幅特性を他の方法(S波スペクトル比法, S波レシーバ関数法及びコーダ波スペクトル比法)で評価し, その結果を先にインバージョン法で得られた結果と比較・検討した.何れの方法も同一の共振周波数を抽出できる.S波スペクトル比法は増幅度についてもインバージョン法で得られた結果とfactor2-3以内で一致するが, 他の方法(S波レシーバ関数法及びコーダ波スペクトル比法)では大きく異なる.

A seismic observation array for strong motions were deployed to estimate seismic source, propagation path and local site effects in Tangshan, China. About sixty events ranging from M 2.2 to 5.9 were recorded in the past several years. We first separated seismic source, propagation path and local site effects from a linear inversion of S-wave spectra using the data set of 10 events recorded at 8 stations. We next compared site responses from the S-wave inversion and those from other techniques, such as traditional direct spectral ratios of S waves, receiver-function of S waves and spectral ratios of coda waves. From the separation, we found that S-wave quality factor, I.e. Qs-value, is approximately satisfied with the relation of Qs = 67f^1.1 in the range of frequency from 0.5 to 32 Hz and that the source spectra follow the ω-2 model of seismic source for low frequencies less than about 12 Hz. From the comparison of site responses estimated by the different methods for each soil site, we found that all the methods can extract the same predominant peaks from the responses and that the amplifications from direct S-wave spectral ratios are well correlated with those from the S-wave inversion within a factor of 2 to 3, while the correlation between the amplifications from S-wave receiver-function and coda-wave spectral ratios and those from the S-wave inversion is not good, especially for high frequencies more than 8 Hz.