High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics
High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics
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DOI：
10.1093/gji/ggv381
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发表时间：
2015-12
影响因子：
2.8
通讯作者：
W. Zürn;A. Ferreira;R. Widmer-Schnidrig;K. Lentas;L. Rivera;E. Clévédé
中科院分区：
文献类型：
--
作者：
W. Zürn;A. Ferreira;R. Widmer-Schnidrig;K. Lentas;L. Rivera;E. Clévédé
我们展示了 2004 年苏门答腊岛西北海岸和 2011 年日本东北附近海岸发生灾难性地震后，从位于德国黑森林天文台的殷钢线应变计和 STS-1 宽带地震仪的记录中获得的地球最低频率简正模态的频谱。我们将这些频谱与使用模式求和技术计算的合成地震图获得的频谱进行了比较 一个迟弹性、椭圆形、旋转、球对称的地球模型。合成包括应变-应变耦合效应，通过使用从应变计记录的地球潮信号与合成潮汐记录之间的比较获得的耦合系数。我们表明，对于高达 1 mHz 的低频环形和球状模式，应变仪比宽带水平地震仪产生更好的信噪比。总体而言，与合成材料的比较令人满意，但不如垂直加速度。特别是，我们通过在单个记录中首次显示环形模式的科里奥利分裂、首次清晰地观察到单线态 2S0/1 以及检测到具有良好信噪比和应变幅度为 10^−11 的基本径向模式 0S0 来证明应变仪数据的高质量。我们还在 1960 年智利大地震后伊莎贝拉应变仪的记录中识别出了后一种模式，而最初的研究未能检测到该模式。
We present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strain–strain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2S0/1 and the detection of the fundamental radial mode 0S0 with good signal-to-noise ratio and with a strain amplitude of 10^−11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies.