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Studies on the origin of Earth's background free oscillations

地球背景自由振荡起源的研究

基本信息

批准号：
11640415
负责人：
SUDA Naoki
金额：
$ 1.92万
依托单位：
Hiroshima University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

项目摘要

We have analyzed records from superconducting gravimeters at four stations : Canberra (Australia), Esashi (Japan), Metsahovi (Finland) and Syowa Station (Antarctica). We were able to detect the Earth's background free oscillations from the records at Canberra and Metsahovi in addition to Syowa Station. The background free oscillations were detected most consistently and distinctly at Canberra since the noise level was the lowest among the SG stations used in this study. Although the noise level at Canberra was higher than that at one of the quietest GEOSCOPE stations CAN (Canberra), it was comparable to those at two of the quietest IDA stations ESK (Scotland) and SUR (South Africa). At Esashi it was too high to detect the oscillations. All of the anomalous oscillation features observed at Syowa Station were absent at the other SG stations, and we conclude that they arise from some local phenomena.We have developed a normal mode theory of atmospheric excitation of the Earth's spheroidal oscillations to derive the expression that can be directly compared to the observed spectrum of background free oscillations. The calculated spectrum exhibits distinct peaks of fundamental modes and complex troughs consisting of overtone modes, which well simulate the peaks and troughs of the observed spectrum. The observed and calculated spectra are in marked contrast to the synthetic free oscillation spectrum for seismically quiet days not only in terms of the amplitude level but in terms of the spectral behavior. These results clearly rule out the possibility of excitation source of earthquake origin and strongly support the idea of atmospheric excitation of background free oscillations.

我们分析了澳大利亚堪培拉、日本江藏、芬兰梅察霍维和南极昭和站的超导重力仪记录。我们能够从堪培拉和Metsahovi以及Syowa站的记录中检测到地球的背景自由振荡。在堪培拉检测到的背景自由振荡最为一致和明显，因为噪声水平是本研究中使用的SG站中最低的。虽然堪培拉的噪音水平高于最安静的GEOSCOPE站之一CAN（堪培拉）的噪音水平，但与最安静的两个IDA站埃斯克（苏格兰）和SUR（南非）的噪音水平相当。在江藏，它太高了，无法检测到振荡。在昭和站观测到的所有异常振荡特征在其他SG站都不存在，我们认为它们是由某些局部现象引起的。我们发展了大气激发地球椭球振荡的简正模理论，导出了可以直接与观测到的背景自由振荡谱相比较的表达式。计算得到的光谱具有明显的基模峰和由泛音模组成的复杂的谷，很好地模拟了观测光谱的峰和谷。观测和计算的谱与静震日的合成自由振荡谱不仅在振幅水平上，而且在谱行为上都有明显的对比。这些结果清楚地排除了震源激发的可能性，并有力地支持了大气激发背景自由振荡的观点。