To discern spatial and explore possible existence of temporal variations of upper crustal anisotropy in an ∼15 km section of the San Jacinto Fault Zone (SJFZ) that is composed of the Buck Ridge and Clark faults in southern California, we conduct a systematic shear wave splitting investigation using local S‐wave data recorded by three broadband seismic stations located near the surface expression of the SJFZ. An automatic data selection and splitting measurement procedure is first applied, and the resulting splitting measurements are then manually screened to ensure reliability of the results. Strong spatial variations in crustal anisotropy are revealed by 1,694 pairs of splitting parameters (fast polarization orientation and splitting delay time), as reflected by the dependence of the resulting splitting parameters on the location and geometry of the raypaths. For raypaths traveling through the fault zones, the fast orientations are dominantly WNW‐ESE which is parallel to the faults and may be attributed to fluid‐filled fractures in the fault zones. For non‐fault‐zone crossing raypaths, the fast orientations are dominantly N–S which are consistent with the orientation of the regional maximum compressive stress. A three‐dimensional model of upper crustal anisotropy is constructed based on the observations. An increase in the raypath length normalized splitting times is observed after the 03/11/2013 M4.7 earthquake, which is probably attributable to changes in the spatial distribution of earthquakes before and after the M4.7 earthquake rather than reflecting temporal changes of upper crustal anisotropy.

中文翻译：

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加利福尼亚南部圣哈辛托断裂带上地壳各向异性的空间变化：剪切波分裂分析的约束

为了辨别空间并探索南加利福尼亚州巴克里奇和克拉克断层组成的圣哈辛托断裂带（SJFZ）约15公里段上地壳各向异性的时空变化，我们进行了系统的剪切波分裂使用由SJFZ地表表情附近的三个宽带地震台站记录的本地S波数据进行调查。首先应用自动数据选择和拆分测量程序，然后手动筛选结果拆分测量以确保结果的可靠性。1,694对分裂参数（快速极化取向和分裂延迟时间）揭示了地壳各向异性的强烈空间变化，如所得到的分裂参数对射线路径的位置和几何形状的依赖性所反映的那样。对于穿过断层带的射线路径，快速取向主要是WNW-ESE，它与断层平行，并且可能归因于断层带中充满流体的裂缝。对于无断裂带交叉的射线路径，快速方向主要是N–S，与区域最大压应力的方向一致。基于这些观测资料，建立了上地壳各向异性的三维模型。在03/11/2013 M4.7地震后，观察到的射线路径长度归一化分裂时间增加了，这可能归因于M4地震前后的空间分布变化。