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Atmospheric disturbance and oscillations of the Earth

大气扰动和地球振荡

基本信息

批准号：
15540406
负责人：
KOBAYASHI Naoki
金额：
$ 1.66万
依托单位：
Tokyo Institute of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2003
资助国家：
日本
起止时间：
2003 至 2005
项目状态：
已结题

项目摘要

Recently it has been established that the Earth's free oscillations are continuously excited by phenomena other than earthquakes. The excited modes are almost fundamental spheroidal modes with amplitudes slightly less than 1 ngal and their intensities clearly show annual variations (e.g.Nishida et al. 2000) with some biannual component. In particular the mode, 0S29 located at the branch crossing with the acoustic modes has excess amplitudes having an annual variation of 30%. As the cause of such oscillations, random disturbances in the atmosphere and/or the oceans has been considered. In the previous works, pressure disturbances are treated as random vertical traction forces on the bottom floors of the atmosphere or the oceans. Such disturbances has successfully explained the observed amplitudes of the free oscillations. However the treatments are insufficient conceptually and for detailed features of the observed spectra. In this study, we treat the pressure disturbances volumetricall … More y extending in the bottom atmosphere or the oceans and we use normal mode eigenfunctions of an earth model including the atmosphere in the calculation of the excitation problem.As the atmospheric disturbances, we used observed power spectral densities (PSDs) of wind speeds recorded by a super sonic wind meter at Kiyosumi station, the Boso peninsula. The PSDs show relatively flat spectra in the mHz band and a power low with steep slope in the higher frequency range. Clearly the process in the bottom atmosphere seem to change around 10 mHz and well known energy cascade of fully developed turbulence are observed above the frequency. Assuming this spectral feature is globally, we calculated free oscillation amplitudes from 0.4 mHz to 1Hz using eigenfunctions of a combined model of PREM and NRLMSISE-00 (an atmospheric model), which are calculated by a normal mode calculation method developed by the author (Kobayashi 2005). The method can fully treat anelasticity and leaking of energy at the top of the model atmosphere.In the mHz band, the results show that only the dynamic pressure of atmospheric disturbances can explain the observed amplitudes of 0S29 and the detailed features of the background spectra in the mHz band. This means that the volumetric effect of the atmospheric disturbances is necessary for the details. In 10 mHz band, there is an plateau of PSDs of the background ground motions (e.g.the New Low Noise Model (NLNM) of the Peterson 1993). Such a feature can be reproduced by the given atmospheric disturbances which change the property around 10 mHz. In the tremor band, we can produce a spectral peak which is due to natural responses of oceanic surface modes and the second spectral peak near the double frequency by including the first overtone modes. With the Newtonian attraction effect by air mass passing below mHz, the overall pattern of the calculated spectra is similar to the NLNM of Peterson. From those results, we conclude that atmospheric disturbances play an important role in the background seismic noises. Less

最近已经确定，地球的自由振荡是由地震以外的现象持续激发的。激发模式几乎是振幅略小于1 ngal的基本球状模式，其强度清楚地显示出年度变化（例如Nishida等人，2000年），其中包含一些两年分量。特别地，位于与声学模式交叉的分支处的模式0 S29具有年变化为30%的过量振幅。大气和/或海洋中的随机扰动被认为是这种振荡的原因。在以前的工作中，压力扰动被视为随机的垂直牵引力的大气或海洋的底部地板。这种扰动成功地解释了所观察到的自由振荡的振幅。然而，治疗是不够的概念和所观察到的光谱的详细功能。在这项研究中，我们处理的压力扰动体积 ...更多信息在激发问题的计算中，我们使用了包含大气的地球模式的简正模本征函数，作为大气扰动，我们使用了房总半岛清隅站的超音速风速仪所记录的风速观测功率谱密度（PSD）。PSD在mHz频带中示出相对平坦的频谱，并且在较高频率范围中示出具有陡峭斜率的功率低。显然，底部大气中的过程似乎在10 mHz左右变化，在该频率以上观察到充分发展的湍流的众所周知的能量级联。假设这种谱特征是全局的，我们使用PREM和NRLMSISE-00（一种大气模式）的组合模式的本征函数计算了0.4 mHz至1 Hz的自由振荡振幅，该模式是通过作者（小林2005）开发的简正模计算方法计算的。结果表明，在mHz波段，只有大气扰动的动压才能解释0 S29的观测振幅和mHz波段背景谱的细节特征。这意味着大气扰动的体积效应对于细节是必要的。在10 mHz频段，背景地震动的PSD存在一个平台（例如Peterson 1993年的新低噪声模型（NLNM））。这样的特征可以通过改变10 mHz附近的属性的给定的大气扰动来再现。在地震带中，我们可以产生一个谱峰，这是由于海洋表面模式的自然响应和第二个谱峰附近的倍频包括第一泛音模式。在低于mHz的空气质量通过的牛顿吸引效应下，计算的光谱的整体图案类似于Peterson的NLNM。从这些结果可以看出，大气扰动在背景地震噪声中起着重要的作用。少