南海南部陆缘的深部地壳结构对揭示西南次海盆渐进式扩张过程和古南海俯冲至关重要。2015年在南海西南部陆缘和南沙海槽新采集的2条海底地震仪（OBS）人工源广角地震数据，为研究南海南部陆缘的深部地壳结构提供了宝贵机遇。本申请拟利用这2条新采集的OBS数据，通过分析处理51台OBS记录的气枪信号，采用射线追踪正、反演模拟，获得该区域精细的纵、横波速度结构。结合南部陆缘东侧已有的速度结果，对比分析地壳厚度、莫霍面埋深和形态、地震波速度和泊松比的横向变化特征等，重点解决以下两个问题：（1）南海南部陆缘地壳结构的空间变化特征及其对西南次海盆渐进式扩张过程的响应；（2）南沙海槽的深部地壳结构特征及其构造属性，并根据南沙海槽两侧的地壳厚度、莫霍面变化和速度分布特征来探讨海槽下方是否存在古俯冲。本申请将为认识西南次海盆扩张过程和古南海俯冲提供深部地壳结构方面的依据，从而完整地认识南海的形成演化过程。

Deep crustal structure of the southern continental margin is important for us to understand the propagating rifting and spreading process of the southwest sub-basin and proto-subduction of the South China Sea (SCS). It is a chance for us to study the crustal structure of southern margin using the two new profiles of ocean bottom seismometers (OBS) data acquired in the southwest continental margin (SWCM) and Nansha Trough in 2015. Based on the OBS data, this application will image the 2D P-wave, S-wave and Vp/Vs ratios structures of SWCM and Nansha Trough. Combined with previous studies in the east part of the southern margin, we will compare the crustal thickness, the depth and form of Moho, the lateral variations of velocity and Vp/Vs ratios to solve the following questions: (1) Spatial variation characteristic of the crustal structure in the southern margin and the tectonic implication of propagating rifting and spreading process of the southwest sub-basin; (2) Deep crustal structure and tectonic attribute of the Nansha Trough, especially the crustal thickness and Moho variation and velocity distribution characteristics around the Nansha Trough, and then discuss the proto-subduction. This study will provide the evidence of crustal structure for us to understand the spreading process of southwest sub-basin and proto-subduction of the SCS, and thus to comprehend the evolution process of the SCS completely.