Large earthquakes can diminish and redistribute stress, which can change the stress field in the Earth’s crust. Seismic anisotropy, measured through shear-wave splitting (SWS), is often considered to be an indicator of stress in the crust because the closure of cracks due to differential stress leads to waves polarized parallel to the cracks traveling faster than in the orthogonal direction. We examine spatial and temporal variations in SWS measurements and the V_p/V_s ratio associated with the 2013 Cook Strait (Seddon, Grassmere) and 2016 Kaikōura earthquakes in New Zealand. These earthquake sequences provide a unique dataset, where clusters of closely-spaced earthquakes occurred. We use an automatic, objective splitting analysis algorithm and automatic local S-phase pickers to expedite the processing and to minimize observer bias. We present SWS and V_p/V_s measurements for over 40,000 crustal earthquakes across 36 stations spanning close to |$5\frac{1}{2}$| years between 2013 and 2018. We obtain a total of 102,260 (out of 398,169) high-quality measurements. We observe significant spatial variations in the fast polarization orientation, φ. The orientation of gravitational stresses are consistent with most of the observed anisotropy. However, multiple mechanisms (such as structural, tectonic stresses and gravitational stresses) may control some of the observed crustal anisotropy in the study area. Systematic analysis of SWS parameters and V_p/V_s ratios revealed that apparent temporal variations are caused by variation in earthquake path through spatially varying media.

中文翻译：

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新西兰库克海峡与凯库拉地震序列相关的地震各向异性的时空分析

大地震可以减少并重新分配应力，从而可以改变地壳中的应力场。通过剪切波分裂（SWS）测量的地震各向异性通常被认为是地壳应力的指标，因为由于应力差导致的裂缝闭合会导致平行于裂缝极化的波传播得快于正交方向。我们研究了SWS测量值和V _p / V _s的时空变化与2013年库克海峡（Seddon，Grassmere）和2016年Kaikōura地震在新西兰相关的比率。这些地震序列提供了一个独特的数据集，在这里发生了间隔很近的地震簇。我们使用自动，客观的分裂分析算法和自动局部S相选择器来加快处理速度，并最大程度地减少观察者的偏见。我们介绍了在36个台站上近40,000美元的地壳地震的SWS和V _p / V _s测量结果，其范围接近| $ 5 \ frac {1} {2} $ |。2013年至2018年之间，我们总共获得了102,260项（在398,169项中）高质量测量。我们在快速极化方向φ中观察到明显的空间变化。重力应力的方向与大多数观察到的各向异性一致。但是，多种机制（例如结构应力，构造应力和重力应力）可能会控制研究区域内某些观测到的地壳各向异性。对SWS参数和V _p / V _s比的系统分析表明，明显的时间变化是由于通过空间变化介质的地震路径变化引起的。