Abstract Although previous tomographic studies found a large low S-velocity province (LLSVP) in the lowermost mantle beneath the Pacific, due to a lack of resolution it remains unclear whether the LLSVP consists of clusters of small-scale low-velocity anomalies or large-scale anomalies. We recently deployed a seismic-array in Thailand which provides a dataset with wide azimuthal coverage at the western Pacific LLSVP. We analyze the new dataset using waveform inversion, and find high-velocity anomalies extending vertically to a height of ~400 km above the core-mantle boundary (CMB) beneath the Philippine Sea and small-scale low-velocity patches with a diameter of ~300 km at the CMB beneath New Guinea. The locations of the high-velocity anomalies are consistent with the past Izanagi-plate subduction boundary, and the low-velocity anomalies can be interpreted as a small-scale plume cluster. Hence we conclude that vertical flow (upwelling plumes and downwelling of slabs) is dominant in the lowermost mantle beneath the western Pacific region.

中文翻译：

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太平洋 LLSVP 西缘 D" 区域的高分辨率 3-D S 速度结构：小尺度羽流和古板岩的证据

摘要 尽管先前的层析成像研究在太平洋最下地幔中发现了一个大的低 S 速度区（LLSVP），但由于分辨率的缺乏，目前尚不清楚 LLSVP 是由小尺度低速异常群还是大-尺度异常。我们最近在泰国部署了一个地震阵列，它在西太平洋 LLSVP 提供了一个具有广泛方位角覆盖的数据集。我们使用波形反演分析了新数据集，并发现了垂直延伸至菲律宾海下方核心-地幔边界 (CMB) 上方约 400 公里高度的高速异常和直径约在新几内亚下方的 CMB 处 300 公里。高速异常的位置与过去伊邪那岐板块俯冲边界一致，低速异常可以解释为小规模的羽流星团。因此我们得出结论，垂直流（上升的羽流和下流的板片）在西太平洋地区最下地幔中占主导地位。