1. Higher Mode Interference on Oceanic Love Waves Excited by Shallow Earthquakes as Inferred through Synthetic Waves
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The interference of the fundamental and higher modes of Love waves is an important subject to be solved for the inversion problem of the velocity of surface waves. After a sufficient analysis of Love waves with respect to the contamination of the fundamental and higher modes, the inversion of the group and phase velocities of the waves seems to be meaningful for a searching examination of surface wave polarization anisotropy. For making clear the problem about the contamination of different modes the excitation of the fundamental and first higher modes of oceanic Love waves is investigated numerically by generating synthetic waves based on a couple of representative fault models. The wave form and velocity of the two modes which travelled about 7000km, comparable to the length of the great circle path over the western Pacific, are investigated with special reference to the wave interference between the two modes. It is understood that the contamination of the first higher mode on the fundamental mode is severe for periods less than 100sec, showing that the intensity of the amplitude spectra of the first higher mode is close to that of the fundamental mode near a period of 40sec, and that the former is about one third of the latter at the period of 100sec. The group velocities of Love waves contaminated by the two modes showed values lower and higher than those of the fundamental mode for periods shorter and longer than70 sec, respectively. This group velocity dispersion curve has a trend approaching that of the first higher mode. The influence of the first, second, and third higher modes upon the fundamental mode is studied by calculating the theoretical amplitude spectral ratios of those higher modes to the fundamental mode.ラブ波基本モードと高次モードの干渉は,表面波速度の逆問題にとって,解かれるべき一つの重要な課題である.基本モードと高次モードの混合に関するラブ波の十分な解析の後に,表面波群速度と位相速度のインバージョンは表面波Polarization anisotropyの厳重な検討に対して意味をもつと思われる.異種モードの混合に関する問題を明らかにする為に,海洋性ラブ波の基本及び高次モードの励起が,二,三の代表的な断層モデルに基づいて合成波をつくる事により数値的に見積られた.西太平洋上の大円弧の長さに匹敵する約7000km伝播した二つのモードの波形と速度を,特に二種モードの干渉に関連して調べた.基本モードに対する一次高次モードの混合は周期100秒以内で激烈であり,40秒付近では,一次高次モードの振幅スペクトルの強度は基本モードのそれに近く,100秒では前者は後者のおよそ3分の1である事が理解出来た,二種モードが干渉した波の群速度は基本モードのそれよりも70秒以下では低く,70秒以上では高くなる事を示している.この群速度分散曲線は一次高次モードのそれに近づく傾向がみられる.二次,三次高次モードが基本モードに及ぼす影響の度合を高次モードの基本モードに対する振幅スペクトル比から推定した.高次モードが無視出来ない周期範囲はその振幅スペクトル比の基準を仮に30%とすると,一次高次モードが周期40秒から130秒まで,二次高次モードが70秒まで,三次高次モードが50秒までと推定される.基準レベルを50%とすると一次高次モードは周期40秒から80秒までは無視出来ず,二次,三次高次モードは周期40秒以上のどの周期でも無視出来る.
- 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Earthquake Research Instituteの論文
- 1983-07-28
東京大学地震研究所,Earthquake Research Institute, University of Tokyo,Earthquake Research Institute | 論文
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