Source distribution of Earth's background free oscillations
Source distribution of Earth's background free oscillations
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DOI：
10.1029/2006jb004720
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发表时间：
2007-06-21
影响因子：
3.9
通讯作者：
Fukao, Y.
中科院分区：
文献类型：
--
作者：
Nishida, K.;Fukao, Y.
最近，一些研究小组报告说，即使在地震平静的日子里，地球的背景自由振荡也存在。观测到的特征表明，它们在地球表面或就在地球表面上方持续受到激发，这很可能是大气和/或海洋扰动造成的。为了限制它们的激发机制，我们开发了一种新的方法，通过模拟成对站之间的交叉谱来估计它们的激发源的空间分布。该方法是计算空间均匀分布的随机源的合成互谱，并用它们将观测到的互谱反演为非均匀源分布。我们应用这种方法在1988-2000年期间在54个站的IRIS记录。结果表明，空间格局具有明显的时间变化。11月至次年4月，南海海域的空间分布为1度型，最大值出现在北太平洋。从5月到10月，强激发源位于沿着东太平洋和西太平洋边缘通过印度洋。在所有的时间段内，大陆上的激发源都比海洋地区弱。这种空间格局的时间变化与Rhie和Romanowicz（2004）报告的结果在定性上一致。然而，激发源并不局限于浅海，正如从海洋亚重力波激发的假设所预期的那样，而是必须分布在整个海面上。
Recently a few groups reported existence of Earth's background free oscillations even on seismically quiet days. Observed features suggest that they are persistently excited on or just above the Earth's surface owing most likely to atmospheric and/or oceanic disturbances. To constrain their excitation mechanisms we developed a new method for estimation of spatial distribution of their excitation sources by modeling cross spectra between pairs of stations. The method is to calculate synthetic cross spectra for spatially homogeneous distribution of random sources and invert with them the observed cross spectra to the heterogeneous source distribution. We applied this method to the IRIS records at 54 stations during 1988-2000. The result showed clear temporal variations of spatial patterns. From November to April the spatial distribution shows a degree 1 pattern with the maximum in the North Pacific Ocean. From May to October strong excitation sources are located along the eastern and western Pacific rims through the Indian Ocean. In all the time periods, excitation sources on continents are weaker than in oceanic areas. This temporal variation of the spatial pattern is qualitatively consistent with that reported by Rhie and Romanowicz (2004). However, the excitation sources are not localized in shallow seas, as might be expected from the hypothesis of excitation by ocean infragravity waves, but must be distributed on the whole sea surface.