Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases
Seismic anisotropy and mantle deformation in NW Iran inferred from splitting measurements of SK(K)S and direct S phases
复制标题
DOI：
10.1093/gji/ggab181
复制
发表时间：
2021-05
影响因子：
2.8
通讯作者：
Shiva Arvin;Farhad Sobouti;K. Priestley;A. Ghods;K. Motaghi;F. Tilmann;T. Eken
中科院分区：
文献类型：
--
作者：
Shiva Arvin;Farhad Sobouti;K. Priestley;A. Ghods;K. Motaghi;F. Tilmann;T. Eken
本文对伊朗西北部阿拉伯-欧亚碰撞带北侧的68个临时宽带台站的远震SK(K)S和直接S波记录进行了横波分裂分析，研究了地幔变形。我们使用参考站技术来克服直接S波信号的源侧各向异性带来的潜在污染。这种方法使我们能够扩展我们的分裂测量数据库超出通常的SK(K)S阶段。SK(K) s波在整个区域的平均分裂延迟时间为1.14±0.42 s，直接s波为1.36±0.26 s。在大部分研究区，两个剪切相的快速极化方向一致，表现为均匀的NE-SW方向，SK(K)S波和S波的结果平均分别为36°和37°。该方向与伊朗西北部（N39°E）的绝对板块运动矢量方向非常吻合。快速方向既与地表地质趋势无关，也与大地应变场无关。我们认为观测到的各向异性主要是由岩石圈相对于深部地幔的运动导致软流层剪切而形成的LPO结构控制的。只有在伊朗中部与北西伊朗和阿尔博尔斯的构造边界附近的狭窄区域，北西-东南向的SK(K)S快速方向倾向于与主要地质构造对齐。直接S波快速方向在同一区域的变化较为平缓，且大部分继续沿NE-SW方向排列。这些差异与构造边界带岩石圈结构的变化有关。伊朗中部的西部边缘具有强烈的变形结构，主要的活动走滑带证明了这一点。考虑到这种结构的深度范围扩展在地幔中一个相对狭窄的区域，它可以局部影响SK(K)S相。另一方面，直接S波具有更大的足迹，因此在更大的区域内平均，并且相对于SK(K)S相，由于其更大的入射角，因此受软流层结构的影响更强，这导致台站平均各向异性参数的影响范围更大。
We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broad-band stations to investigate the mantle deformation on the northern side of the Arabia–Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE–SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39°E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW–SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE–SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.