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，地震以外的现象不断激发地球的自由振荡。激发模式几乎是基本的球体模式，其放大器略小于1 ngal，其强度清楚地显示出每年的每年一年成分的年变化（例如Nishida等，2000）。特别是该模式，位于与声学模式的分支交叉处的0S29超过了每年30％变化的放大器。作为这种振荡的原因，已经考虑了大气和/或海洋中的随机距离。在先前的作品中，压力距离被视为大气或海洋底部楼层上的随机垂直牵引力。这样的距离成功地解释了自由振荡的观察到的放大器。但是，这些治疗方法在概念上不足，并且对于观察到的详细特征。 In this study, we treat the pressure disasters 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 excitement problem.As the atmospheric disasters, we used observed power spectral densities (PSDs) of wind speeds recorded by a super sonic wind meter at Kiyosumi station, the Boso peninsula. PSD在MHz频段中显示出相对较平坦的光谱，并且在较高频率范围内具有茎斜率的功率低。显然，底部大气中的过程似乎在10 MHz左右发生了变化，并且在频率上方观察到了完全发达的湍流的众所周知的能量级联。假设该光谱特征在全球范围内，我们使用PER和NRLMSISE-00（大气模型）组合模型的本征函数计算了自由振荡放大器从0.4 MHz到1Hz，该模型是由作者开发的正常模式计算方法计算的（Kobayashi 2005）。该方法可以完全处理模型大气顶部的能量的无弹性和泄漏。在MHz频带中，结果表明，只有大气灾害的动态压力才能解释观察到的0S29的放大器以及MHz频段中背景光谱的详细特征。这意味着大气灾害的体积效应对于细节是必需的。在10 MHz频段中，有一个背景地面运动的PSD高原（例如，Peterson 1993的新的低噪声模型（NLNM））。这样的特征可以由给定的大气灾害复制，这些灾难将特性造成10 MHz的变化。在震动带中，我们可以产生一个光谱峰，这是由于海洋表面模式的自然响应以及在双频率附近的第二个光谱峰的自然响应所致，包括第一张泛音模式。由于空气质量通过MHz以下的牛顿吸引力效应，计算出的光谱的总体模式与彼得森的NLNM相似。从这些结果来看，我们包括大气灾害在背景地震噪声中起着重要作用。较少的