Large low‐velocity provinces (LLVPs) are hypothesized to be purely thermal features or possess some chemical heterogeneity but which exactly remains ambiguous. Regional seismology studies typically use travel time residuals and multipathing identification in the waveforms to infer properties of LLVPs. These studies have not fully analyzed all available information such as measuring the direction and inclination of the arrivals. These measurements would provide more constraints of LLVP properties such as the boundary velocity gradient and help determine their nature. Here, we use array seismology to measure backazimuth (direction) and horizontal slowness (inclination) of arriving waves to identify structures causing multipathing and wavefield perturbation. Following this, we use full‐wavefield forward modeling to estimate the gradients required to produce the observed multipathing. We use SKS and SKKS data from 83 events sampling the African LLVP, which has been extensively studied providing a good comparison to our observations. We find evidence for structures at heights of up to 600 km above the core‐mantle boundary causing multipathing and wavefield perturbation. Forward modeling shows gradients of up to 0.7% δ V _s per 100 km (0.0005 km s⁻¹ km⁻¹) can produce multipathing with similar backazimuth and horizontal slowness to our observations. This is an order of magnitude lower than the previous strongest estimates of −3% δ V _s per 50 km (0.0044 km s⁻¹ km⁻¹). As this is lower than that predicted for both thermal and thermochemical structures, lateral velocity gradients capable of producing multipathing are not necessarily evidence for a thermochemical nature.

中文翻译：

---

从多径慢速空间观测结果推断非洲下地幔的横向速度梯度

大型低速省（LLVP）被假定为纯粹的热力特征或具有某些化学异质性，但实际上仍然模棱两可。区域地震学研究通常在波形中使用传播时间残差和多径识别来推断LLVP的特性。这些研究还没有完全分析所有可用信息，例如测量到达方向和倾斜度。这些测量将提供更多的LLVP属性约束，例如边界速度梯度，并有助于确定其性质。在这里，我们使用阵列地震学来测量到达波的后方位角（方向）和水平慢度（倾斜度），以识别引起多径和波场扰动的结构。按照此，我们使用全波场正演模型来估计产生观测到的多径所需的梯度。我们使用来自非洲LLVP采样的83个事件的SKS和SKKS数据，该数据已得到广泛研究，可与我们的观察结果进行很好的比较。我们发现证据表明，在高于地幔边界的高度高达600 km的结构会引起多径和波场扰动。正向建模显示梯度高达0.7％δ V _š每100公里（0.0005公里小号^-1 千米^-1）可以产生具有类似backazimuth和水平缓慢我们的观察多路径。这是一个数量级低于先前最强估计- 3％ δ V_小号每50公里（0.0044公里小号^-1 千米^-1）。由于这低于热化学结构和热化学结构的预测值，因此能够产生多径效应的横向速度梯度不一定是热化学性质的证据。