We present a high-resolution P-wave azimuthally anisotropic velocity model for the upper and middle crust beneath southern California by a novel adjoint-state traveltime tomography technique. Our model reveals significant anisotropy variations between tectonic blocks that clearly reflect both past and current plate boundary deformation. In the shallow crust, seismic anisotropy is mostly controlled by the preferred alignment of microcracks related to the present N-S compressive stress; while at deeper depths (>∼6 km), seismic anisotropy mainly records paleofabrics formed during the long-lived Farallon subduction and later extension that have not been fully reset by the present transform motion. Interestingly, our model demonstrates distinct fast axes beneath the western Transverse Ranges from its neighboring blocks, probably reflecting the large-scale vertical axis clockwise rotation of the block. In addition, we identify layered structures with distinct anisotropy features beneath the Salton Trough, which could be a result of the current transtension.

中文翻译：

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地震方位角各向异性揭示了南加州过去和正在进行的地壳变形的复杂模式

我们提出了一个高分辨率的P波方位角各向异性速度模型，用于南加州下方的上地壳和中地壳，采用新的伴随状态走时层析成像技术。我们的模型揭示了构造块之间的显着各向异性变化，清楚地反映了过去和当前的板块边界变形。在浅层地壳中，地震各向异性主要受与当前 NS 压应力相关的微裂纹的优选排列控制；而在更深的深度（>~6 km），地震各向异性主要记录了在长期的 Farallon 俯冲和后来的伸展过程中形成的古织物，这些古织物尚未被当前的变换运动完全重置。有趣的是，我们的模型展示了西部横向山脉下方与其相邻区块不同的快轴，可能反映了块的大尺度垂直轴顺时针旋转。此外，我们确定了索尔顿槽下方具有明显各向异性特征的分层结构，这可能是当前过渡的结果。