Ocean bottom seismograph orientation and crustal structure of the Woodlark Rift
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摘要: 伍德拉克裂谷位于巴布亚新几内亚东南部，是发育在澳大利亚板块和西南太平洋板块碰撞带中的年轻大陆裂谷，为研究汇聚构造背景下裂谷起始演化的地壳结构提供了理想场所。伍德拉克裂谷海域地区海水层的存在使得获取高质量地震数据成为难题，而数据主要通过海底地震仪（ocean bottom seismograph，缩写为OBS）获取。OBS的布放一般是自由下落式，其地震计的北向水平分量方位与地理北向通常不一致，这使得利用三分量波形数据获取的反演结果产生了较大误差甚至失效，例如接收函数方法。为确定伍德拉克裂谷地区OBS水平分量的方位偏转角度，本文同时引入纵波和瑞雷面波偏振分析方法进行方位校正，并利用校正后的三分量波形数据开展接收函数研究，进而约束该裂谷海域地区的地壳结构。结果分析表明，OBS方位校正后，其获得的可用接收函数波形数量显著增多，并且利用纵波偏振分析校正后的数据处理获得了更加合理的地壳结构。基于在该裂谷地区获得的地壳构造结果，基里比斯盆地和裂谷扩张轴所在的古迪纳夫盆地呈现对比鲜明的地壳结构特征：古迪纳夫盆地的地壳厚度朝着裂谷扩张轴处减薄，其平均值为（33.3±2.42） km；基里比斯盆地的地壳厚度更薄，平均值为（24.1±5.44） km。此外，研究区域内所有OBS处均观测到了较高的地壳纵横波速比值，这可能是巴布亚超镁铁质岩体富集和古俯冲残片脱水熔融共同作用的结果。Abstract: The Woodlark Rift in southeastern Papua New Guinea is a young continental rift and develops within the collision zone between the Australian and SW Pacific Plates, which offers an ideal location to explore the crustal structure beneath the incipient rift under a convergent setting. However, the sea water layer makes it difficult to collect high-quality seismic data, and the common step is to deploy the ocean bottom seismographs （OBSs） in a free-fall way. Therefore, mis-orientation of horizontal components of the OBS usually leads to failure of applying the inversion techniques such as the receiver function to the three-component waveforms. In this study, we employed both P-wave and Rayleigh-wave polarization analyses to determine all available OBS orientations, and then used the recorded teleseismic waveforms to conduct a receiver function study on the crustal structure beneath the Woodlark rift. The number of the receiver function traces has greatly increased after the orientation corrections and the crustal structures can be better constrained based on the results from P-wave polarization analysis. Contrasting crustal structures were revealed beneath the Kiribishi Basin and the Goodenough Basin where the rift axis is located. The crust beneath the Goodenough Basin is deciphered to thin towards the rift axis with an average of （33.3±2.42） km, while a much thinner crust is observed beneath the Kiribisi Basin with a mean value of （24.1±5.44） km. High vP/vS ratios were determined at all stations, which may be attributed to the Papuan ultramafic body and dehydration melting of fossil subducted slab segments.
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Keywords:
- Woodlark rift /
- crustal structure /
- OBS orientation /
- receiver function
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图 1 伍德拉克裂谷地区海底地震仪的分布
紫色和黑色的圆圈分别表示含有效和无效数据的海底地震仪；红色三角为新生代火山；黑色虚线代表裂谷扩张轴，红色实线为欧文—斯坦利断裂带，绿色实线为地震测线（Fitz，Mann，2013）。右上插图代表了纵波（蓝色圆圈）和瑞雷波偏振分析（红色圆圈）所用的地震事件分布图，其中绿色三角形表示研究区域的中心位置。底部右侧插图中的黄色方框显示了研究区域的具体位置，其中红线为板块边界（Bird，2003）
Figure 1. Topographic map of the Woodlark rift showing the locations of the ocean bottom seismographs （OBSs）
The purple and black dots represent OBSs with and without valid data，respectively. Red triangles denote Cenozoic volcanos. The black dashed line indicates the rift axis. The red line represents the Owen-Stanley fault zone. The green lines are seismic profiles from previous study （Fitz，Mann，2013）. The top-right inset presents the events used for the polarization analysis of Rayleigh wave （red circles） and P-wave （blue circles）. The green triangle marks the center of the study area. The bottom right inset displays the location of thestudy area highlighted by the yellow rectangle. The red lines denote the plate boundaries （Bird，2003）
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图 2 （a） OBS水平分量的方位偏转示意图，χ和θ分别为OBS方位偏转角度和相对于北向水平分量的地震后方位角；（b） 地震台站B的瑞雷波偏振分析结果，图中蓝色圆圈为对应单事件最优方位偏转角度，红色虚线为该地震台站最终方位偏转角度115.1°；（c） 地震台站B的纵波偏振分析结果，蓝色虚线表示最佳方位偏转角度108°
Figure 2. （a） A schematic map of coordinate system exhibiting the relationship between the geographical North direction，the North component and the back-azimuth （BAZ） of an event. $ \chi $ and θ indicate the sensor orientation and the BAZ of the event relative to the North component，respectively；（b） Example of the Rayleigh-wave polarization analysis from the station B. The blue circles represent the optimal sensor orientations determined from each event based on the maximum correlation coefficient between the vertical and the Hilbert transformed radial components. The red dash line depicts the resulting station orientation 115.1°；（c） The P-wave polarization analysis for determining orientation of station B. The blue dashed line depicts the optimal sensor orientation，which is 108°
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图 3 地震台站B纵波偏振分析方位偏转校正前（a）和后（b）的h-κ叠加结果
上图中黑色和红色线分别为每条接收函数和时间域所有接收函数简单叠加后的波形，蓝色线为校正前接收函数的简单叠加波形；下图左侧表示归一化后的h-κ叠加能量图，图中红点为叠加能量最强点，表示最终确定的地壳厚度h和纵横波速比值κ （vP/vS），NRFs为接收函数数量；右下图表示h-κ叠加能量图中不同纵横波速比值所对应的最大能量点连线
Figure 3. h-κ stacking results from representative station B before （a） and after （b） misorientation correction from the P-wave polarization analysis
The upper panel shows individual （black trace） receiver functions （RFs） and a simple time-domain stack （red trace） of all RFs，and the blue trace represents the simple stacked RF trace in time domain before misorientation correction. The lower left panel illustrates the h-κ plot in which the maximum stacking amplitude （red dot） determines the optimal pair of crustal thickness h and vP/vS ratio κ. NRFs is number of receiver functions. The lower right panel displays the maximum stacking amplitude for each candidate vP/vS ratio in the h-κ plot
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图 4 瑞雷波和纵波偏振分析得到的OBS方位结果对比图（a）以及本文与Abers等（2016）的地壳厚度结果对比（b）
Figure 4. Comparisons of OBS orientations from the Rayleigh-wave and P-wave polarization analyses （a） and crustal thickness from this study and Abers et al （2016） （b），respectively
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图 5 方位校正后地壳的平面（a，b）与剖面（c）结果图
（a） 地壳厚度平面分布图；（b） 纵横波速比值κ平面分布图；（c） 地壳厚度（蓝色三角形）和地壳纵横波速比值（红色圆圈）沿图（a）中AA′测线的剖面结果图，其中上图为地形起伏，中图红色波形为时深转换后的叠加接收函数
Figure 5. Planar （a，b） and vertical （c） display of the resulting crustal measurements after misorientation corrections
（a，b） Crustal thickness and vP/vS ratio in planar view，respectively；（c） Vertical display of crustal thickness （blue triangles） and vertical display of crustal vP/vS ratio （red circles） along the AA′ profile in fig.（a）. The upper panel shows the topography，red traces in the middle panel are the stacked receiver function traces after time-depth conversion
表 1 纵波和瑞雷波偏振分析得到的每台OBS方位偏转角χ及其标准差STD
Table 1 The resulting OBS orientations χ from analysis of the Rayleigh-wave and P-wave polarization analyses and their standard error STD for each station
台站 瑞雷波偏振分析 纵波偏振分析 χ*/° χ/° STD/° χ/° STD/° B 115.1 0.55 108 3.03 116.8 D 3.0 1.48 356 2.68 0.3 E 176.0 1.65 168 3.17 183.1 F 293.1 1.78 310 5.26 301.0 G 44.2 1.92 29 7.68 47.2 H 323.0 1.88 319 1.12 324.5 J 131.8 2.53 144 12.97 143.9 注：χ*为Eilon等（2014）基于瑞雷波偏振分析所得。 
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表 2 本文和前人获得的地壳厚度h和纵横波速比值κ
Table 2 Crustal thickness h and vP/vS ratios κ from this and previous studies
台站 南纬/° 东经/° h /km h*/km κ B 9.749 150.350 30.4±0.21 27.8±0.53 1.94±0.028 D 9.943 150.707 36.7±0.16 44.9±0.73 1.89±0.005 E 10.080 150.621 34.2±2.69 − 1.82±0.059 F 9.950 150.200 31.7±0.96 36.2±0.61 1.90±0.072 G 9.333 149.667 25.8±0.73 31.5±0.84 2.04±0.039 H 9.000 149.668 16.8±0.71 36.8±0.74 1.94±0.085 J 8.870 149.934 29.8±0.60 39.2±0.38 1.99±0.043 注：h*为Abers等（2016）的结果。
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