Extrusion tectonics driven by the collision of Indian-Eurasian continents and the relative motion among microcontinents (e.g. South China, Simao-Indochina blocks) has led to multi-phase and complex geological activity and structures in Northern Vietnam. While extensively studied, its detailed crustal architecture in association with multiple large-scale shear zones and plate boundaries remains unclear and is key to resolving long-lasting tectonic debates. This study constructs the first 3-D regional S-wave velocity (Vs) model with consistent P-wave velocity (Vp) and Vp/Vs ratio models by jointly using the crustal and head waves (e.g. Pg/Sg and Pn/Sn phases) from an augmented earthquake catalog combining the Vietnam National Earthquake Catalog (VNCAT) and the data from multiple adjacent seismic networks. The quality of phase picking and earthquake location incorporating seismic data from various sources are carefully examined and collected for the tomographic inversion. We use a stepwise joint inversion scheme to subsequently constrain the earthquake location, 1-D and 3-D velocity models. The output models suggest that the Song Ma suture represents a better candidate for the Indochina-South China boundary rather than the Red River Shear Zone. The latter seems to act as a crustal structure while the former and the Dien Bien Phu Fault are likely lithospheric ones. Several other geologic features such as the Tu Le volcanic basin, the Song Hong basin, and the Day Nui Con Voi metamorphic complex are also clearly depicted and discussed in the model. This new set of the Vp, Vs, and Vp/Vs models could further contribute to better assessment to seismic hazards in the northern Vietnam.

中文翻译：

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当地地震层析成像结合多个网络的地震数据揭示了越南北部下方的大地构造结构


印度-欧亚大陆碰撞和微大陆（如华南、思茅-中南陆块等）之间的相对运动驱动的挤压构造导致了越南北部多期、复杂的地质活动和构造。虽然经过广泛研究，但其与多个大规模剪切带和板块边界相关的详细地壳结构仍不清楚，这是解决长期构造争论的关键。本研究联合使用地壳波和头波（例如Pg/Sg和Pn/Sn相位）构建了第一个具有一致P波速度（Vp）和Vp/Vs比模型的3D区域S波速度（Vs）模型）来自增强地震目录，该目录结合了越南国家地震目录 (VNCAT) 和多个相邻地震台网的数据。结合不同来源的地震数据，仔细检查和收集相位拾取和地震定位的质量，以进行层析成像反演。我们使用逐步联合反演方案来随后约束地震位置、一维和三维速度模型。输出模型表明，松马缝合带比红河剪切带更适合作为印度支那-华南边界。后者似乎充当地壳结构，而前者和奠边府断层可能是岩石圈结构。模型中还清晰地描述和讨论了其他几个地质特征，例如 Tu Le 火山盆地、Song Hong 盆地和 Day Nui Con Voi 变质杂岩。这套新的 Vp、Vs 和 Vp/Vs 模型可以进一步有助于更好地评估越南北部的地震灾害。