鉛直アレー地震観測記録から推定された奈良盆地北部・平城宮跡地の地盤のS波の速度とQ値

Abstract

奈良盆地北部の平城宮跡地で実施されている鉛直アレー地震観測では, 基盤境界生成SP変換波と地表面からの反射S波が明瞭に記録される。地表/地中スペクトル比は, 概ね奇数倍の周波数列にシャープなピークを有する。また, 水平動/上下動スペクトル比には盆地の基盤構造に由来する山谷が顕著に現れる。これらのデータを基に, ‐深さ約600mの基盤岩までの地盤のS波速度とQ値のモデル化を行った。S波のQ値は, 周波数の1乗に, またs波速度の2‐3乗に依存していた。構造モデルから推定される地盤の固有周期は基本モードで約3秒, 増幅率は広い周波数範囲で約4倍であった。

To elucidate subsurface structure and ground vibration characteristics at Heijo, Nara basin, where 'Daigokuden', government office in the Nara era is planned to be restored to its original state, seismic observation has been carried out at GL Om, -20m, -42m and - lOOm in a borehole. The depth to bedrock is known to be about 600m from nearby deep boreholes. It is notable that, almost all the seismograms showremarkable SP phases generated at the soil-bedrock boundary and down-going S-waves reflected at groundsurface. We made spectral analyses of S wave parts including preceding SP phases; surface-to-subsurface spectral ratios (Ho/H-d) and horizontal-to-vertical spectral ratio (H/V). Ho/l-l-d shows sharp peaks at oddmultiple frequencies. Remarkable peaks and troughs of fundamental mode in H/V are observed, even though incident angle of SV phases at bedrock exceed the critical angle (arcsin (Vs/Vp); V in bedrock). We estimated S wave velocity and Q structure using Haskell method. As a result, it was found that, Qs depends on 1stpower of frequency and 2nd to 3rd power of Vs, fundamental natural-period of ground is 3s, andsoil-amplification is about 4 for 1-20Hz.