Ultra-low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core-mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core-reflected PcP waveforms recorded by seismic stations across Antarctica using a double-array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1-D synthetics; however, others are more complex, which may reflect 2-D or 3-D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability.

中文翻译：

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使用 PcP 波形双阵列叠加成像的南半球下方的超低速区

超低速区 (ULVZ) 是一种异常结构，通常与地震速度降低有关，有时与密度增加有关，已在地球核幔边界 (CMB) 顶部的某些位置检测到。之前的研究已经报道了多种 ULVZ 特征，导致人们对它们的起源提出了许多疑问。南极洲及其周边地区下方的最低地幔并不位于当前地幔上涌或下涌活跃区域附近，这使其成为研究超低电压区来源的独特环境；然而，这部分宇宙微波背景的地震采样很少。在这里，我们使用双阵列叠加技术检查了南极洲各地地震台记录的岩心反射 PcP 波形，以进一步阐明南半球下方的 ULVZ 结构。我们的结果显示了广泛的、可变的 ULVZ，其中一些可以使用一维合成材料进行稳健建模；然而，其他的则更复杂，可能反映 2-D 或 3-D ULVZ 结构和/或具有内部速度变化的 ULVZ。我们的发现与 ULVZ 很大程度上可以通过 CMB 上俯冲洋壳的可变堆积来解释这一概念是一致的。俯冲地壳和其他含水俯冲物质的部分熔融也可能导致 ULVZ 的变化。