Seismic anisotropy within the Nankai area，Japan，using DONET seafloor observation network
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摘要: 基于DONET海底观测网的直达S波地震记录，采用波形旋转互相关方法和最小特征值最小化方法求得了日本南海海域俯冲带横波分裂快轴方向和分裂时间，获得了该俯冲带地震波的各向异性结果。结果显示：该俯冲带地震波的各向异性快轴方向基本平行于南海海槽走向，分裂时间为0.1—0.96 s。这表明：日本南海海域俯冲带各向异性来源于太平洋俯冲板块上覆地幔楔和菲律宾海俯冲板块；地幔楔各向异性产生于二维地幔楔拐角流所导致的各向异性矿物晶体的定向排列；菲律宾海俯冲板块的各向异性则产生于板块扩张时期形成的“化石各向异性”和俯冲过程中板块挠曲产生的断层；日本南海海域俯冲带大范围变化的分裂时间反映了该地区各向异性介质的强度和（或）厚度的不均匀性。Abstract: Based on the direct S-wave seismic records of the DONET seafloor observation network, this paper obtains the orientation of shear wave splitting fast axis and splitting time of Naikai area of Japan by using the cross-correlation method and the smallest eigenvalue minimization method. The results show the fast axis of the anisotropy in the subduction zone is sub-parallel to the strike of the Nankai trough, and the splitting time ranges from 0.1 s to 0.96 s. This indicates that the anisotropy of the Nankai subduction zone originates from the mantle wedge which overlies the subducted Pacific Plate and the subducted Philippine Sea slab. The anisotropy of the mantle wedge is caused by the existence of two-dimensional corner flow that causes the anisotropic mineral crystals to be oriented along the direction perpendicular to the trench. The anisotropy of the subducted Philippine Sea slab is caused by the “fossil anisotropy” and the faults related to the plate bending during the subduction. The widely varying splitting times reflect the inhomogeneous strength and/or thickness of the anisotropic structure beneath the area.
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Keywords:
- shear wave splitting /
- anisotropy /
- Izu-Bonin /
- seafloor observation network /
- subduction
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图 1 本文研究区域和前人的研究结果
（a） 研究区域的前人横波分裂结果 （引自Fouch，Fischer，1996；Salah et al，2008；Wirth，Long，2010）以及菲律宾海俯冲板块（引自Baba et al，2002；Hirose et al，2008）和太平洋俯冲板块（引自Hayes et al，2018）等深度图。短线方向代表快轴方向，短线长度代表相对分裂时间，横波分裂解绘制在震源与台站的中点；（b） 用于横波分裂的地震事件和台站
Figure 1. The studied region and previous results
（a） Previous shear wave splitting results of the studied region （after Fouch，Fischer，1996；Salah et al，2008；Wirth，Long，2010） and the iso-depth of the subducted plates （Baba et al，2002；Hirose et al，2008；Hayes et al，2018）. The orientations of the short lines represent fast directions，and the lengths of the short lines correspond to splitting times，the shear wave splitting results are plotted in the middle point of the event and station； （b） Seismic events and stations used for shear wave splitting
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图 2 用两种横波分裂方法对地震波形进行分析处理的例子
选取的地震发生于2017年1月11日，MJMA4.5，震源深度为351.95 km。（a） 波形旋转互相关方法校正前、后的快慢波波形图（上）和质点振动图（下）；（b） 最小特征值最小化方法校正前、后的快慢波波形图（上）和质点振动图（下）；（c） 利用波形旋转互相关方法进行横波分裂得到的相关系数等值线图；（d） 利用最小特征值最小化方法进行横波分裂得到的切向能量等值线图；（e） 对不同滤波频带进行横波分裂分析得到的快轴方向（上）和分裂时间（下）
Figure 2. Examples of shear wave splitting using two different methods
The selected earthquake occurred on January 11，2017，MJMA4.5，and the focal depth is 351.95 km. （a） The waveform （upper） and particle motion diagram （lower） of the fast and slow waves before and after the correction by the cross-correlation method；（b） Waveform （upper） and particle motion diagram （lower） of fast and slow wave before and after correction by the tangential energy minimization method；（c） The contours of the correlation coefficients obtained by the cross-correlation method；（d） The contours of the tangential energy obtained by the tangential energy minimization method；（e） The orientation of the fast axis （upper） and the splitting time （lower） obtained by the shear wave splitting analysis applying different filter bands
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图 3 日本南海海域横波分裂结果
（a） 横波分裂结果平面统计图。快轴方向为短线的方向，短线的相对长度代表相应的分裂时间，短线绘制在震源与台站间的中点。不同颜色的玫瑰图代表类型Ⅰ −Ⅲ的快轴方向统计，相应长度代表解的个数；（b） 分裂时间与震源深度的关系
Figure 3. Shear wave splitting results of Nankai area，Japan
（a） Map view of the shear wave splitting results. The orientation of the short line indicates the fast direction，and the length of the short line represents the splitting time，they are plotted in the middle point of the event and station. The rose diagrams with different colors give the distribution of fast directions for types Ⅰ −Ⅲ results，corresponding length represents the number of solutions；（b） The relationship between splitting times and focal depths
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图 4 射线路径示意图
板块和射线路径的相对位置根据已有层析成像结果（Nakajima，Hasegawa，2007；Hirose et al，2008；Asamori，Zhao，2015）绘制
Figure 4. Schematic diagram of the ray paths
The relative positions of the plates and the seismic ray paths are drawn according to the previous tomography results （Nakajima，Hasegawa，2007；Hirose et al，2008；Asamori，Zhao，2015）
表 1 日本南海海域横波分裂结果
Table 1 Shear wave splitting results of Nankai area，Japan
射线路径
类型发震时刻 北纬/° 东经/° 震源深度/km 台站名称 台站位置 快轴方向 分裂时间/s 北纬/° 东经/° Ⅰ 2014−12−06 35.51 135.72 355.12 KMA01 33.80 136.56 20° 0.87 Ⅰ 2014−12−06 35.51 135.72 355.12 KMA03 33.65 136.60 68° 0.13 Ⅰ 2014−12−06 35.51 135.72 355.12 KMB05 33.48 136.93 58° 0.8 Ⅰ 2014−12−06 35.51 135.72 355.12 KMB06 33.36 136.92 51° 0.89 Ⅰ 2014−12−06 35.51 135.72 355.12 KMD13 33.22 136.69 55° 0.77 Ⅰ 2015−10−26 35.46 135.83 342.67 KMA01 33.80 136.56 18° 0.81 Ⅰ 2015−10−26 35.46 135.83 342.67 KMB06 33.36 136.92 61° 0.96 Ⅰ 2015−10−26 35.46 135.83 342.67 KMD13 33.22 136.69 67° 0.88 Ⅰ 2015−10−26 35.46 135.83 342.67 KMD16 33.30 136.60 9° 0.36 Ⅰ 2015−10−31 35.82 135.38 362.65 KMB08 33.47 136.80 35° 0.37 Ⅱ 2015−10−07 33.88 136.31 384.69 KMA02 33.75 136.65 61° 0.35 Ⅱ 2015−10−07 33.88 136.31 384.69 KMB08 33.47 136.80 9° 0.33 Ⅱ 2015−10−07 33.88 136.31 384.69 KMC11 33.00 136.78 14° 0.35 Ⅱ 2015−10−07 33.88 136.31 384.69 KMD13 33.22 136.69 16° 0.83 Ⅱ 2016−11−27 33.61 135.85 404.01 KMA02 33.75 136.65 62° 0.47 Ⅱ 2016−11−27 33.61 135.85 404.01 KMB05 33.48 136.93 47° 0.45 Ⅱ 2016−11−27 33.61 135.85 404.01 KMD13 33.22 136.69 30° 0.21 Ⅱ 2017−01−03 34.03 136.42 376.64 KMA02 33.75 136.65 31° 0.27 Ⅱ 2017−01−03 34.03 136.42 376.64 KMC11 33.00 136.78 10° 0.34 Ⅱ 2017−01−03 34.03 136.42 376.64 KMD16 33.30 136.60 9° 0.1 Ⅲ 2014−10−02 34.35 137.12 344.75 KMD16 33.30 136.60 39° 0.51 Ⅲ 2015−03−05 34.63 137.01 333.94 KMA02 33.75 136.65 −44° 0.29 Ⅲ 2015−03−05 34.63 137.01 333.94 KMB08 33.47 136.80 53° 0.65 Ⅲ 2015−09−21 34.32 137.20 340.06 KMA02 33.75 136.65 9° 0.45 Ⅲ 2015−09−21 34.32 137.20 340.06 KMB08 33.47 136.80 21° 0.32 Ⅲ 2015−09−21 34.32 137.20 340.06 KMC12 33.13 136.82 −86° 0.67 Ⅲ 2015−12−30 33.88 137.32 338.23 KMB05 33.48 136.93 66° 0.49 Ⅲ 2015−12−30 33.88 137.32 338.23 KMB07 33.36 136.81 58° 0.44 Ⅲ 2015−12−30 33.88 137.32 338.23 KMB08 33.47 136.80 77° 0.33 Ⅲ 2015−12−30 33.88 137.32 338.23 KME18 33.39 136.38 −81° 0.11 Ⅲ 2016−07−17 33.97 137.35 333.51 KMD13 33.22 136.69 35° 0.31 Ⅲ 2016−07−17 33.97 137.35 333.51 KMB08 33.47 136.80 17° 0.28 Ⅲ 2017−01−11 33.76 137.18 351.96 KMB08 33.47 136.80 −36° 0.35 Ⅲ 2017−07−18 34.42 137.56 306.79 KMD13 33.22 136.69 −47° 0.4 
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