Observations of Mantle Seismic Anisotropy Using Array Techniques: Shear‐Wave Splitting of Beamformed SmKS Phases
Observations of Mantle Seismic Anisotropy Using Array Techniques: Shear‐Wave Splitting of Beamformed SmKS Phases
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DOI：
10.1029/2022jb025556
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发表时间：
2022-12
期刊：
影响因子：
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通讯作者：
J. Wolf;D. Frost;M. Long;E. Garnero;Adeolu O. Aderoju;N. Creasy;E. Bozdağ
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文献类型：
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作者：
J. Wolf;D. Frost;M. Long;E. Garnero;Adeolu O. Aderoju;N. Creasy;E. Bozdağ
剪切波分裂测量通常用于解决上地幔和下地幔中的地震各向异性。通常，这种技术应用于SmKS相位，这些相位在被记录之前已经从核幔边界的下侧反射了（m-1）次。剪切波分裂研究的实际限制包括合适相位的数量有限，以及由于信噪比（SNR）差或测量不确定性大而丢弃的大部分可用数据。阵列技术，如波束形成，通常用于观测地震学，以提高信噪比，但还没有被应用到以前提高剪切波分裂研究的SmKS信号强度和相干性。在这里，我们研究了基于慢度和后方位角vespagrams来确定最相干的入射波方向的波束形成方法如何提高横波分裂测量置信区间。通过对真实的和合成地震图的分析，我们表明：（a）从波束形成的地震图（波束）获得的分裂测量值反映了对波束有贡献的单站分裂参数的平均值;（B）波束具有（平均）信噪比比对波束有贡献的单站地震图大两倍以上;（c）增加的信噪比允许从波束可靠地测量剪切波分裂参数，平均单站信噪比为1.3。因此，波束形成可能有助于更可靠地测量由于上地幔各向异性引起的分裂。此外，我们表明，波束形成有可能大大提高最低地幔各向异性的检测，通过使用波束形成的USAray数据演示差分SKS-SKKS分裂分析。
Shear‐wave splitting measurements are commonly used to resolve seismic anisotropy in both the upper and lowermost mantle. Typically, such techniques are applied to SmKS phases that have reflected (m‐1) times off the underside of the core‐mantle boundary before being recorded. Practical constraints for shear‐wave splitting studies include the limited number of suitable phases as well as the large fraction of available data discarded because of poor signal‐to‐noise ratios (SNRs) or large measurement uncertainties. Array techniques such as beamforming are commonly used in observational seismology to enhance SNRs, but have not been applied before to improve SmKS signal strength and coherency for shear wave splitting studies. Here, we investigate how a beamforming methodology, based on slowness and backazimuth vespagrams to determine the most coherent incoming wave direction, can improve shear‐wave splitting measurement confidence intervals. Through the analysis of real and synthetic seismograms, we show that (a) the splitting measurements obtained from the beamformed seismograms (beams) reflect an average of the single‐station splitting parameters that contribute to the beam; (b) the beams have (on average) more than twice as large SNRs than the single‐station seismograms that contribute to the beam; (c) the increased SNRs allow the reliable measurement of shear wave splitting parameters from beams down to average single‐station SNRs of 1.3. Beamforming may thus be helpful to more reliably measure splitting due to upper mantle anisotropy. Moreover, we show that beamforming holds potential to greatly improve detection of lowermost mantle anisotropy by demonstrating differential SKS–SKKS splitting analysis using beamformed USArray data.