Modification of crustal seismic anisotropy by geological structures (“structural geometric anisotropy”)
Modification of crustal seismic anisotropy by geological structures (“structural geometric anisotropy”)
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DOI：
10.1130/ges01655.1
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发表时间：
2019-02
期刊：
影响因子：
2.5
通讯作者：
D. Okaya;S. Vel;W. Song;S. E. Johnson
中科院分区：
文献类型：
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作者：
D. Okaya;S. Vel;W. Song;S. E. Johnson
宏观地质结构可以将局部岩石材料各向异性几何地映射到更大的体积中，该更大的体积可以在地震波响应的尺度上具有不同的净各向异性特性。块体结构的各向异性强度、对称性类型和对称轴的方向通常与当地岩石不同;具有显示慢轴横向各向同性的材料组构的典型地壳岩石可以转换为例如较弱的快轴正交或较低对称性的块体结构。我们定义这种修改为“结构几何各向异性”（SGA）。这种结构产生的地震各向异性信号受地震波长度尺度的影响：较短的波长响应于结构的每个较大部分（路径积分），而较长的波长仅响应于所有部分的整体平均值（有效介质）。我们提出了一个张量公式，在某些条件下可以将各向异性填充结构分解为与填充岩石类型分离的宏观结构几何形状。当一个单一的代表性岩石材料可以取代当地岩石与织物，方位算子，描述结构的几何形状可以单独体积平均，以产生一个独特的“结构几何算子”，然后可以用来定义等效结构的有效介质。我们说明这些原则，使用常见的几何结构，并显示作为一个例子，逐步修改地震各向异性产生的圆柱形褶皱。由于地壳构造的广泛分布，其对地震各向异性的影响应纳入地震各向异性的解释中。地壳慢轴横观各向同性的假设并不总是成立的。
A macroscopic geological structure can geometrically map a local rock material anisotropy into a larger volume that may have different net anisotropic properties on a scale to which seismic waves respond. The bulk structure’s anisotropy intensity, symmetry type and orientation of symmetry axes will generally be different from the local rock; a typical crustal rock with material fabric showing slow-axis transverse isotropy can be converted, for example, into a bulk structure that is weaker fast-axis orthorhombic or lower symmetry. We define this modification as “structural geometric anisotropy” (SGA). The seismic anisotropy signals produced by this structure are influenced by the length scale of seismic waves: shorter wavelengths respond to each larger part of the structure (path integration) whereas longer wavelengths respond to just the bulk average of all parts (effective medium). We present a tensor formulation that under certain conditions can decompose an anisotropy-filled structure into its macroscale structural geometry separated from infilling rock types. When a single representative rock material can be substituted for local rocks with fabric, the orientation operators that describe the structure’s geometry can be separately volume averaged to produce a unique “structural geometry operator” that can then be used to define the equivalent structure’s effective medium. We illustrate these principles using common geometrical structures and show as an example the progressive modification of seismic anisotropy produced by cylindrical folding. Due to the widespread distribution of crustal tectonic structures, their effects on seismic anisotropy should be incorporated into interpretations of seismic anisotropy. The assumption of slow-axis transverse isotropy in crustal volumes is not always valid.