【作者】 易桂喜；

【导师】 朱介寿；

【作者基本信息】 成都理工大学 ， 地球探测与信息技术， 2009， 博士

【摘要】 根据Rayleigh面波的频散特征,本文利用双台窄带通滤波-互相关方法与基于图像分析的相速度频散曲线提取技术,提取Rayleigh面波相速度频散,进而反演与讨论了中国大陆及边邻地区(70°~135°E,18°~55°N)地壳上地幔相速度结构与方位各向异性特征。取得的主要成果如下:1.充分收集了中国大陆及周边地区固定与流动台网记录的远震波形资料。根据记录波形质量,选定204个数字化地震台站所记录的1990年至2006年之间409个地震事件的长周期垂直向面波波形资料,获得了4941条双台路径上的Rayleigh面波相速度频散,并最终得到了2610条独立路径上20~120s周期基阶Rayleigh面波相速度频散资料。2.首次获得了中国大陆及边邻地区不同周期高分辨率Rayleigh面波相速度分布。检测板测试结果显示中国大陆西部及边邻地区横向分辨率大约5°,中东部地区3°,局部地区可达2°。研究结果显示中国大陆Rayleigh面波相速度分布横向差异显著,分区特征明显,可划分为具有不同相速度结构特征的东、西两部分。南北地震带在所有周期的相速度图上始终呈现出相对较低的速度特征,并成为划分中国大陆东、西部具有不同岩石圈相速度特征的天然分界。3.首次获得了中国大陆具有较高分辨率的不同周期Rayleigh面波相速度方位各向异性空间分布图像,该结果有助于理解板块之间运动的动力学模式以及变形过程中物质的运移方式。各向异性结果显示:青藏地块内各向异性强度存在明显的空间差异,中部强度大于两侧,最大强度达4%,出现在40~80s周期;喜马拉雅东构造结各向异性最大强度为3%,出现在40~60s周期;松潘-甘孜地块内各向异性强度也表现出一定的空间差异,靠近其北边界的东昆仑断裂带东端各向异性强度最大(3%),出现在35~40s周期。综合青藏地块及其东缘快波方向变化与各向异性强度随周期的变化,可以看出,该区域变形强烈,变形部位以下地壳与上地幔顶部最为突出。青藏地块内短周期与中长周期的快波方向基本保持一致,表明其地壳与上地幔顶部存在耦合关系;同时,快波方向自西向东围绕喜马拉雅东构造结顺时针旋转,可能指示在板块碰撞挤压过程中物质的流变方向。长周期各向异性图像显示顺时针旋转现象基本消失,且青藏地块内的快波优势方向与印度板块的运动方向一致,由于板块运动的主要驱动力来自于地幔对流,该区域长周期快波优势方向可能指示地幔流动的方向。位于青藏地块西北的塔里木盆地东部与西部各向异性强度大于盆地中部地区,最大强度达3%;短周期各向异性图像显示,盆地西部、中部与东部的中地壳快波优势方向分别为NE-SW、E-W、NW-SE向,存在自西向东的顺时针旋转,快波方向的方位变化与青藏地块的变化一致,应具有同源性。但当周期T≥30s时,盆地西部快波方向发生了明显变化,说明印度板块碰撞对塔里木盆地西部下地壳与上地幔的影响明显减弱。青藏地块东侧的四川盆地中地壳与下地壳上地幔快波方向明显不同,说明该盆地中地壳与下地壳变形状态具有明显差异,存在解耦关系,而30s以上周期快波方向保持一致则表明其下地壳与上地幔为强耦合;四川盆地较强的各向异性可能是其所经历的新近一次大规模构造运动遗留在岩石圈中的具有记忆性的“化石”各向异性。通过综合分析不同周期的相速度与方位各向异性图像发现,中国大陆104°E以东地区地壳上地幔构造变形总体弱于西部地区。但位于鄂尔多斯盆地西南缘、祁连褶皱带东段至秦岭-大别造山带西段的交汇地区例外,55~85s周期各向异性最大强度可达2%,显示该区域经历了强变形。中国大陆东部地壳上地幔存在的弱各向异性是中国大陆东部受印度-澳大利亚板块与欧亚板块碰撞以及太平洋板块与菲律宾海板块向欧亚板块下方俯冲共同作用的结果,进而也可以看出中国大陆东部受板块碰撞的影响明显弱于西部。随着周期的增大,中国大陆不仅上地幔相速度结构的横向非均匀性明显减小,除局部地区外,各向异性的强度也明显减小。但自65s周期开始,东部的日本海、朝鲜半岛以及南部海南岛以西的印支地块各向异性强度较中短周期有明显增大,处于岩石圈拉张减薄区的东南沿海地区上地幔各向异性增大可能与软流圈地幔对流有关,反映橄榄石晶格取向的快波方向可能指示地幔流动的方向。大约以103°E为界,龙门山断裂带可分为南西段和北东段。南西段处于低速区,北东段处于高速区;方位各向异性强度差异也较显著,北东段方位各向异性强度明显大于南西段,且30s以上周期的快波方向以NE-SW向为主。2008年5月12日汶川8.0级地震震中正好位于龙门山断裂带中部的南西段与北东段的分界过渡区,而该地震沿断裂带的单侧破裂模式除与北东段本身具有高应力积累直接相关外,与北东段地下介质物性也应存在密切关系,高速坚硬岩体的发育有利于应变能的积累与集中释放,同时该段方位各向异性结果所显示的NE-SW向快波方向的介质环境也可能是地震波能量传播的优势方向。单一周期的各向异性结果所显示的快波方向与研究区域内大型构造带走向之间没有一致性,应说明区域内大型构造带可能孕育于不同的构造运动时期,因而复杂的构造演化历史也就导致了构造活动存在显著的差异性与分段性。4.所获得的双台路径分布有利于获取可靠的相速度与方位各向异性。由于双台相速度频散资料主要集中在中国大陆内部,反演时受研究区域以外结构的影响较小;且很多频散资料路径距离较短,可以更好地约束小区域的相速度结构,从而可以更可靠地给出研究区内的相速度与方位各向异性分布,同时,对进一步反演该区可靠的横波速度结构具有重要意义。更多还原

【Abstract】 Based on the Rayleigh surface-wave dispersion characteristics, this paper will provide the Rayleigh-wave phase velocity dispersion curves by utilizing a two-station cross-correlation of narrow band-pass filter and an image analysis technique. The dispersion data were then used to invert phase velocity and azimuthal anisotropy distribution in the crust and upper-mantle beneath the continental China and its adjacent regions (70°-135°E, 18°-55°N). Several main results obtained are as follows:1. This paper will present a large amount of teleseismic surface waveform data, which had been collected from both permanent and portable digital seismic networks distributed in continental China and its adjacent areas. Further according to the waveform data quality, the vertical-component records of 409 earthquakes during 1990-2006 from 204 stations were processed to obtain inter-station phase velocity dispersions of fundamental-mode Rayleigh waves at periods between 20 and 120s along 4961 paths (including multiple paths) or 2610 independent paths.2. This paper will first provide relatively high resolution Rayleigh-wave phase velocity distributions at periods 20-120s in the continental China and its adjacent regions. Checkerboard tests show that the lateral resolution is about 5°in the western China and adjacent regions, about 3°in the central-eastern China, and even up to 2°for some local areas.The significant lateral variation of phase velocity distribution indicates various phase velocity structures in the studied area. The North-South Seismic Belt in China has relatively low phase velocity at all periods, and therefore becomes a natural boundary between eastern continental China and western portion with different lithospheric phase velocity features.3. It’s also the first time that relatively high resolution azimuthal anisotropy distributions of Rayleigh-wave phase velocity at various periods in the continental China and its adjacent regions have been obtained, and this knowledge is valuable for understanding the dynamics of plate-motion model and material migration pattern during deformation. The azimuthal anisotropy distributions in the study area also display spatial heterogeneity.The amplitude of azimuthal anisotropy beneath the central Qinghai-Tibet plateau is higher than that both in the western and eastern parts, and the maximum amplitude of azimuthal anisotropy is up to 4% at periods 40-80s; 3% appears near the eastern Himalayan syntaxis at periods 40-60s. It is also indicated that the anisotropy distribution beneath Songpan-Garze orogenic belt has significant spatial variation, and the maximum amplitude of anisotropy up to 3% is observed near the northern boundary at the east end of the east Kunlun fault zone at periods 35-40s. By considering the correlated variation between fast propagation direction, the anisotropy amplitude and different periods, it can be deduced that strong deformation has been occurred especially in the lower crust and the upper-mantle lid beneath Qinghai-Tibet block and its eastern margin. It is also recognized that fast propagation direction at short periods basically coincides with directions at mid-long periods beneath Qinghai-Tibet block, which indicates that the crust and upper-mantle lid are coupled there. Clockwise rotation is presented around the eastern Himalayan syntaxis from the west to the east within the block, which may indicate the direction of material flow (or escaping) in the block after the Eurasian and Indian collision. Azimuthal anisotropy maps at long periods show that clockwise rotation is disappeared and the dominant direction of fast propagation in Qinghai-Tibet block correlates well with the direction of the Indian plate movement. Since mantle convection mainly drives plate motion, the dominant direction maybe is indicative of the mantle flow direction beneath the Qinghai-Tibet block. Azimuthal anisotropy maps also indicate that the anisotropy amplitude of the eastern and western parts of the Tarim basin in front of the northwest of Qinghai-Tibet block is greater than that of the central part and the maximum amplitude is up to 3%. The maps at short periods show that the dominant directions of fast propagation in middle crust in the western, central and eastern parts are NE-SW, E-W and NW-SE, respectively, and present a clockwise rotation from west to east. This finding is in great accordance with Qinghai-Tibet block deformation and they may result from the same source property. Notable changes of fast propagation direction can be detected in western basin at periods above 30s; therefore, it becomes an indicator that the Eurasia-India collision effect on lower crust and upper mantle beneath the western Tarim basin is weakened clearly. It is recognized that fast propagation direction in the middle crust is clearly different from those both in the lower crust and upper mantle underlying the Sichuan basin adjacent to the eastern plateau margin. This finding suggests that decoupled deformation processes between middle crust and lower crust beneath the basin have been occurred. But further analysis of the maps at periods equal to or longer than 30s indicates that the lower crust and upper mantle are strongly coupled. The relatively large amplitude of azimuthal anisotropy in Sichuan basin may be the‘fossil’anisotropy frozen in the lithosphere, and this‘fossil’anisotropy could have been from more recent large scale deformation.By analyzing phase velocity and azimuthal anisotropy maps at various periods, the present paper indicates that the tectonic deformation of the crust and upper-mantle east of approximately 104°E is weaker than that of the west. But local exceptions are also found near the intersection of the southwest Ordos, eastern segment of Qilian fold belt and the western segment of Qinling-Dabie orogenic belt. The maximum amplitude of anisotropy is up to 2% at periods 55-85s, and is representative of strong deformation in geological history. It is observed that weak azimuthal anisotropy exists in the crust and upper mantle beneath the eastern continental China. This might result from the synthetic effects from the collision between India-Australia and Eurasian plates, as well as the Pacific plate and Philippine Sea plate subduction under the Eurasia plate along the margins of the continental China. The overall effect on the eastern flank is obviously weaker than that on the western continental China.The lateral heterogeneity of phase velocity structure and amplitude of azimuthal anisotropy in the upper mantle beneath continental China decreases as period increases, except for some local areas. However, anisotropy amplitudes underlying the Sea of Japan, Korean peninsula and Indo-China block located in the west of Hainan Island increase apparently as the periods equal to or greater than 65s. Increasing of upper-mantle anisotropy amplitude along southeast China coast, where the lithosphere is experiencing extension and thinning processes, may be related to mantle convection in asthenosphere, and the fast propagation direction is an indicator of crystal lattice orientation of olivine representing the mantle flow direction.According to the phase velocity distribution and the azimuthal anisotropy amplitude, the Longmen Shan tectonic zone can be divided into southwestern and northeastern sections at about 103°E. The southwestern section has relatively lower phase velocity, while the northeastern area has higher phase velocity and stronger anisotropy with NE-SW Rayleigh-wave fast-propagation direction at periods above 30s. This conclusion suggests that the NE striking unilateral rupture propagation of the Wenchuan Ms8.0 earthquake on May 12, 2008, which occurred at the central segment of the Longmen Shan tectonic zone, may be related not only to the cumulated high stress of the northeastern section, but also to the underlying medium property along the segment where high phase velocity (suitable for energy accumulation and concentrated release) and NE-SW fast-propagation direction (suitable for seismic energy propagation) are expressed.By synthesizing all the data, it is concluded that the fast anisotropy direction at any period isn’t consistent with the strike of the large-scale geological structures in the studied area, and further this cognition maybe means that large-scale geological structures have been formed and evolved in different tectonic episodes, and therefore the activities along large-scale structures behave obvious regionalization and segmentation.4. Distribution of inter-station paths has great advantage of determining reliable phase-velocity and azimuthal anisotropy maps. Because most paths are located within the continental China, this ray path distribution therefore can consequently minimize the influence from the structures outside of the research area on the inversion results. Additionally, many measurements of phase velocity dispersions are along relatively short paths, and then they can further constrain the phase velocity structure within the local area, and finally this advantage makes the distribution of the phase velocity and azimuthal anisotropy more reliable in the studied area. Consequently, the result is of important significance for further inverting reliable shear wave velocity structure of the studied region.更多还原