用远震接收函数研究滇西地区的深部结构

【作者】 贺传松；

【导师】 王椿镛；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理学， 2003， 博士

【摘要】 滇西地区以其独特的构造环境，成为大陆动力学研究的热点地区之一；而火山活动的成因、监测与预报则是国际减灾计划的重要组成部分，宽频带地震资料的接收函数反演能够为滇西地区和火山地区的深部结构研究提供高分辨的信息。 本文在前人工作的基础上，收集了滇西地区和五大连池以及腾冲火山区远震波形数据，开展了接收函数非线性反演研究，揭示出滇西地区以及五大连池和腾冲火山区深部结构方面的一些重要地学特征，并探讨了相关的地球动力学问题。 本文详细阐述了接收函数方法及非线性反演的研究进展以及相邻算法的基本理论。对不同噪声背景下的合成接收函数采用相邻算法进行反演，结果表明相邻算法具有较强的抗干扰能力，即使在噪音背景较大的情况下，算法仍能分辨出一些主要的速度结构，并能反映S波速度结构的整体轮廓。相邻算法具有全局搜索，提取健全模型的特征。在此基础上，进一步将这一非线性反演方法应用于滇西及五大连池与腾冲火山区的深部结构研究，得出了一些非常有意义的结果。 五大连池火山区接收函数的反演结果表明：五大连池火山区下部存在广泛分布的低速结构，厚度大约在15—22km的深度范围内，S波速度在2.5～3.0km／s范围之间。低速体沿北北东向构造线性排列，与该火山区的构造格局基本吻合。个别埋藏深度较浅的低速结构可能是上次火山喷发的岩浆在原火山口固结，阻塞了岩浆上涌的原始通道，下部岩浆寻找新的上涌通道而形成更浅部的岩浆囊。地震活动与低速体的存在部位具有对应关系。 腾冲火山区接收函数的反演结果表明：腾冲火山区的形成与活动明显受到区域构造环境的影响，特别是NE向大盈江断裂的影响。在此构造南侧10～20km的深度范围内S波为低速层，速度在2.5km／s左右。总体上看在大盈江断裂以南，台站下方的S波低速结构的深度范围基本一致，在该断裂的北部，台站下方的S波低速结构并不明显，这说明低速结构受到构造的阻控作用。 由于五大连池和腾冲火山区所处的构造背景有较大的差别，深部结构即有共同的特征，又有一定的区别。主要表现在：(1)腾冲火山区S波低速体埋藏比较浅，一般在10～20km的深度范围内；五大连池火山区则相对较深，一般在15～25km的深度范围内。(2)腾冲火山区的莫霍界面与五大连池火山区的莫霍界面的埋藏深度有一定的差异，壳幔界限不十分清楚。(3)腾冲火山区较浅的低速结构在4～12km之间；五大连池火山区较浅的低速体在5～25km之间。(4)腾冲火山区的横向速度结构变化比五大连池火山区更为显著。(5)两个火山区的深部结构均与地震活动性有一定的关系。(6)两个火山区一定范围内存在的低速结构表明五大连池和腾冲火山区具有活动火山的深部结构特征，存在再次活动的基本条件。 滇西地区接收函数反演的结果支持了滇西地区S波速度整体偏低、地壳厚度由北向南渐次变薄的研究成果。提供了攀西构造带上地幔隆起、地壳减薄的地震学证据。揭示出横跨怒江、红河和小江断裂地壳厚度发生显著变化这一重要的地学现象。总体上看，怒江西侧的地壳厚度明显小于东侧，这表明怒江断裂可能是一条重要的缝合线。红河断裂和小江断裂两侧地壳厚度呈现急剧减薄的特征，在断裂处或附近地壳最厚，这表明红河断裂和小江断裂两侧的块体呈断块碰撞接触关系。研究还揭示出一个有待进一步证实的观测事实，洱源台可能属于二该L褶皱系，因为洱源台和云龙台地壳中的S波速度结构非常相似，如果这一认识成一立的话，那末，浪西地区所表现出空间上由西向东规律分布的造山带一缝合线、造山带一碰撞带，这种现象可能反映了滇西地区大陆动力学的时空演化过程，值得进一步的深入研究。更多还原

【Abstract】 The West Yunnan area is a hotspot for geodynamical study, due to its peculiar geotectonic, both of genesis, monitoring and prediction of volcanic activity form the major objectivity in The International Disaster Reduction Program, broadband seismic data will inverse high-resolution information for the underneath structure in the West Yunnan and volcanic area.Based on the previous study, teleseismic waveform data, observed in the West Yunnan and volcanic area, is gathered to study their S wave velocity structures by the receiver function. Underneath structure features are shown in those areas, and their geodynamical implication is also discussed.The receiver function approach and nonlinear inversion proceedings are reviewed in this paper; the basic theory of the neighborhood algorithm is also analyzed. The receiver functions with different-level noise backgrounds are inverted using the neighborhood algorithm. The results show that the neighborhood algorithm is nearly noise free; even under large noise background, the neighborhood algorithm can also distinguish main velocity structures and reflect the whole character of S wave velocity. The neighborhood algorithm possesses global search ability and can recover robust models. The neighborhood algorithm adapts to the receiver function inversion, a kind of non-linear problem. The neighborhood algorithm is then used to infer underneath structures of Western Yunnan area and Wudalianchi and Tengchong volcanic areas, and some important results are obtained.The main conclusions from the receiver function inversion about the Wudalianchi volcanic area are summarized as: Low velocity structure is widely distributed in Wudalianchi volcanic area, with thickness between 15 and 22km and S velocity between 2.5km and 3.0km/s. The lower velocity structure spreads in linearity and consistent to the tectonic pattern. Possibly, some lava was blocked during the latest eruption due to the original vent being choked; lower lava was forced to find new channels and formed shallow lava. Seismic activity was correlated with lower velocity structure. It is shown by the receiver function inversion in Tengchong volcanic area: tectonic environments influence the volcanic activity, especially, influenced by the Dayingjiang fault. Low velocity zone is between 10 and 20km to the south of this fault, and velocity is about 2.5km/s or so. As a whole the low velocity zone are within same depth at Dayingjiang fault south. The low velocity zone isn’t evident to the north of the Dayingjiang fault. It shows that low velocity structure is obstructed by the fault.Due to the different tectonic backgrounds, the underneath structure share some common characters as well as certain differences, mainly shown by: (1) The S wave velocity structure is imbedded at shallow position within 10~20km in Tengchong volcanic area, in contrast to underneath position within 15~25km in Wudalianchi volcanic area. (2) Moho in Tengchong volcanic area is deeper than that of Wudalianchi volcanic area. (3) Shallow lower velocity structure is within 4-12km in Tengchong volcanic area, contrast to 5~25km in Wudalianchi volcanic area. (4) The lateral variation in Tengchong volcanic area is larger than that in Wudalianchi volcanic area. (5) The lower velocity structure in two volcanic areas quite correlates with seismicity. (6) The low velocity structure located at certain depth implies that the reoccurrence possibility of Tengchong and Wudalianchi volcanic activity.This receiver function study in Western Yunnan area supports the previous results that the S wave velocity is lower globally and the crustal thickness becomes thin from north to south. It shows seismic evidences for the upper mantle upwelling and crust thinner in Panxi tectonic zone. It is shown that the crustal thickness significantly varies across the Nujiang River fault, Honghe fault and Xiaojiang fault. As a whole it is thinner in the west of Nujiang River than that in the east of Nujiang River. It is shown that Nujiang River is an important suture. Crustal t更多还原