The Dʺ layer, defined as 200–400 km in the lowermost mantle, is a thermal and chemical boundary layer between the solid silicate mantle and the liquid outer core. Deciphering the detailed structures of the Dʺ region is essential for unlocking the thermal and chemical states in the deep Earth. Here, we precisely measure the slowness and back-azimuth of the direct P-waves by beamforming based on the F-trace stack at the KZ Array in Kazakhstan, to investigate the detailed Dʺ structures beneath the East China Sea. The P-wave slowness for rays turning beneath the East China Sea exhibits a significant anomaly as a function of the P-wave turning depth. Strong correlations between slowness and back-azimuth anomalies for rays from different directions suggest a tilted Moho, with a tilting direction of ∼103° and a dip angle of ∼15°, beneath the KZ Array, further supported by radial receiver functions. After correcting for the slowness anomalies caused by the tilted Moho and heterogeneities outside the Dʺ layer, we construct a series of Vp Dʺ models to fit the remaining slowness anomalies for rays sampling the East China Sea. We obtain the best Dʺ model with a height of 360 km, a maximum δVp of +1.4%, a Dʺ discontinuity thickness of 120 km, and an 80-km low-velocity layer at the base of the mantle by minimizing residuals between the predicted and observed slowness anomalies. Combining the sharpness of the Dʺ discontinuity imaged here with mineralogical analysis suggests a Fe-enriched region in a cold subduction environment beneath the East China Sea.

中文翻译：

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东海海底的 Dʺ 构造被 P 波缓慢异常解析


Dʺ 层定义为最下层地幔中 200-400 公里，是固体硅酸盐地幔和液体外核之间的热和化学边界层。破译 Dʺ 区域的详细结构对于解开地球深处的热状态和化学状态至关重要。在这里，我们通过在哈萨克斯坦 KZ 阵列的 F 迹线堆栈上通过波束成形精确测量直接 P 波的缓慢性和反向方位角，以研究东海海底的详细 Dʺ 结构。东海海底光线转动的 P 波缓慢性与 P 波转向深度呈显著异常。来自不同方向的射线的缓慢性和反向方位角异常之间的强相关性表明，在 KZ 阵列下方有一个倾斜的 Moho，倾斜方向为 ∼103°，倾角为 ∼15°，进一步得到径向接收器函数的支持。在校正了倾斜的 Moho 和 Dʺ 层外的异质性引起的慢速异常后，我们构建了一系列 Vp Dʺ 模型来拟合对东海采样的射线的剩余慢速异常。我们通过最小化预测和观测的慢速异常之间的残差，获得了高度为 360 公里、最大 δVp 为 +1.4%、Dʺ 不连续厚度为 120 公里、地幔底部有 80 公里低速层的最佳 Dʺ 模型。将这里成像的 Dʺ 不连续性的清晰度与矿物学分析相结合，表明在东海下方的寒冷俯冲环境中存在富铁区域。