In this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc-oblique, right-lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear-wave velocity models using data recorded by an ocean-bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary-prism origin of the latter. The resolved azimuthal anisotropy exhibits arc-parallel and arc-perpendicular fast shear-wave polarization directions in the upper to mid-crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc-parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right-lateral motion of the Ryukyu Arc. The underlying arc-perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc-perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear-dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.

中文翻译：

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环境噪声层析成像揭示琉球北部以剪切为主的大陆裂谷


在这项研究中，我们使用环境噪声层析成像揭示了琉球弧和冲绳海槽北部地壳的变形结构。与南琉球相比，北段呈现出宽而浅的盆地，地壳没有局部变薄，伸展速度缓慢，琉球弧高度弧斜，右侧后退。我们使用海底地震仪阵列和附近岛屿站记录的数据，提出了各向同性和方位各向异性剪切波速度模型。各向同性模型表明，从后弧到前弧的速度呈单调下降，这与前弧的增生棱柱原点一致。分辨的方位各向异性在上部到中部地壳和下部地壳到最上部地幔的大部分电弧和后电弧中分别表现出电弧平行和电弧垂直的快速剪切波极化方向。我们将弧平行各向异性解释为由琉球弧的右侧运动施加的垂直剪切对齐的各向异性织物。下面的弧垂直各向异性可能归因于地幔楔中角流驱动的水平剪切。我们在前弧上地壳中发现了弧垂直各向异性，这可能反映了奄美高原碰撞引起的裂纹排列。斜弧后退以及弧和背弧中分辨的变形结构共同证明了琉球北部大陆裂谷系统的剪切主导、跨张性质。从跨张裂谷的角度来看，琉球北部的一些特征可能可以更好地解释。