Spatial Variations of Upper Crustal Anisotropy Along the San Jacinto Fault Zone in Southern California: Constraints From Shear Wave Splitting Analysis
Spatial Variations of Upper Crustal Anisotropy Along the San Jacinto Fault Zone in Southern California: Constraints From Shear Wave Splitting Analysis
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DOI：
10.1029/2020jb020876
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发表时间：
2020-08
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通讯作者：
Enyuan Jiang;Kelly H. Liu;Yuan Gao;Xiaofei Fu;Stephen S. Gao
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作者：
Enyuan Jiang;Kelly H. Liu;Yuan Gao;Xiaofei Fu;Stephen S. Gao
为了弄清加州南部圣哈辛托断裂带∼段上地壳各向异性可能存在的空间和时间变化，利用位于圣哈辛托断裂带地表表达式附近的三个宽带地震台记录的当地S波资料，进行了系统的横波分裂研究。首先应用自动数据选择和分割测量程序，然后手动筛选所得到的分割测量，以确保结果的可靠性。1694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，这反映在所产生的分裂参数与射线路径的位置和几何形状的关系上。对于穿过断裂带的射线路径，快速方向主要是WNW-ESE向，它平行于断层，可能归因于断裂带中充满流体的裂缝。对于非断裂带跨越射线路径，快速取向以N-S为主，与区域最大压应力方向一致。在此基础上建立了上地壳各向异性的三维模型。3/11/2013级4.7级地震后，射线路径长度归一化分裂时间增加，这可能是由于4.7级地震前后地震空间分布的变化，而不是反映了地壳上部各向异性的时间变化。
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.