A Compositional Component to the Samoa Ultralow‐Velocity Zone Revealed Through 2‐ and 3‐D Waveform Modeling of SKS and SKKS Differential Travel‐Times and Amplitudes
A Compositional Component to the Samoa Ultralow‐Velocity Zone Revealed Through 2‐ and 3‐D Waveform Modeling of SKS and SKKS Differential Travel‐Times and Amplitudes
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DOI：
10.1029/2021jb021897
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发表时间：
2021-06
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影响因子：
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通讯作者：
J. Krier;M. Thorne;K. Leng;T. Nissen‐Meyer
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作者：
J. Krier;M. Thorne;K. Leng;T. Nissen‐Meyer
我们分析了来自萨摩亚超低速度带（ULVZ）的13个高质量事件的4,754次宽带地震记录，包括SKS、SKKS和SPdKS波场。我们测量了SKKS和SKS到达之间的差分旅行时间和振幅，它们对SPdKS地震相的出现高度敏感，而SPdKS地震相又对ulvz产生的最低地幔速度扰动高度敏感。我们使用二维轴对称波形建模方法对这些数据进行建模，并能够用单个ULVZ解释这些数据。为了预测行程时间和振幅扰动，我们发现在大圆弧方向上需要10°或更大的ULVZ长度。较大的ULVZ长度限制了可接受的ULVZ弹性参数。在这里，我们发现δVS和δVP分别从20%到22%和15%到17%的减少给了我们最好的拟合，厚度为26 km。最初的3 - D建模工作不能恢复差分测量中的极端值，这表明3 - D效应很重要，必须在未来加以考虑。然而，三维模型总体上与二维模型恢复的速度降低一致。这些速度降低与ULVZ的组成部分是兼容的。此外，对一个正在移动的组成ULVZ的地球动力学预测预测了一个与我们在本研究中证实的萨摩亚ULVZ形状相似的长线性形状。
We analyzed 4,754 broadband seismic recordings of the SKS, SKKS, and SPdKS wavefield from 13 high quality events sampling the Samoa ultralow‐velocity zone (ULVZ). We measured differential travel‐times and amplitudes between the SKKS and SKS arrivals, which are highly sensitive to the emergence of the SPdKS seismic phase, which is in turn highly sensitive to lowermost mantle velocity perturbations such as generated by ULVZs. We modeled these data using a 2‐D axi‐symmetric waveform modeling approach and are able to explain these data with a single ULVZ. In order to predict both travel‐time and amplitude perturbations we found that a large ULVZ length in the great circle arc direction on the order of 10° or larger is required. The large ULVZ length limits acceptable ULVZ elastic parameters. Here we find that δVS and δVP reductions from 20% to 22% and 15% to 17% respectively gives us the best fit, with a thickness of 26 km. Initial 3‐D modeling efforts do not recover the extremes in the differential measurements, demonstrating that 3‐D effects are important and must be considered in the future. However, the 3‐D modeling is generally consistent with the velocity reductions recovered with the 2‐D modeling. These velocity reductions are compatible with a compositional component to the ULVZ. Furthermore, geodynamic predictions for a compositional ULVZ that is moving predict a long linear shape similar to the shape of the Samoa ULVZ we confirm in this study.