Studies on the origin of Earth's background free oscillations

我们分析了四个台站的超导重力仪记录：堪培拉(澳大利亚)、Esashi(日本)、Metsahovi(芬兰)和Syowa站(南极洲)。除了西奥瓦站外，我们还能从堪培拉和梅特萨霍维的记录中探测到地球的背景自由振荡。堪培拉的背景自由振荡是最稳定和明显的，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)，但与IDA最安静的两个站Esk(苏格兰)和Sur(南非)的噪音水平相当。在Esashi，它太高了，检测不到振荡。在Syowa站观测到的所有异常振荡特征在其他SG站都是不存在的，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球面振荡的简正模理论，得到了可以直接与观测到的背景自由振荡谱相比较的表达式。计算的频谱显示出明显的基模峰值和由泛音模式组成的复谷，很好地模拟了观测频谱的峰和谷。观测和计算的频谱不仅在幅度水平上，而且在频谱行为方面，都与地震平静日的合成自由振荡频谱形成了明显的对比。这些结果清楚地排除了震源激发的可能性，有力地支持了大气激发背景自由振荡的观点。

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.