【作者】 孙安辉；

【导师】 陈颙；

【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， 2012， 博士

【摘要】 盆岭体系是板块构造演化中的两个主要末端,在岩石圈结构构造及其演化中扮演中重要的角色。本文针对对我国东西部两个不同区域的盆岭体系,以北天山东部与准噶尔盆地结合区域和华北首都圈区域为例,结合各自不同的数据基础特点,优化地壳一维速度模型以及应用全三维有限频方法进行全波层析成像,这将有助于我们深入理解盆岭体系中相邻板块间的相互作用以及构造与地震活动性之间的关系。三维地震层析成像是目前世界上研究地壳速度结构广泛使用的直接又有效的手段。精确的反演数据基础和先进的反演理论及方法技术都是提高反演结果可靠性的有力保障。数据基础中的初始参考速度结构和震源参数对层析成像的结果有着重要的影响。不合适的初始速度结构除了将影响震源参数确定的准确性,还将给层析成像带来虚假的结果。因此我们结合初始速度结构和震源参数耦合的特性,在目前尚缺乏充分研究的北天山东部与准噶尔盆地结合区域,采用新疆乌鲁木齐遥测台网和10个宽频带流动地震台站记录的173个良好约束的近震的1370个P波和1396个S波走时数据,应用“最小一维速度模型”方法,联合反演初始速度结构和震源参数,并选用多个参考模型库和地震事件库以避免出现反演得到“局部极小值”的问题,最终得到优化的区域地壳一维速度模型。结果表明,我们建立的北天山东部地壳一维P波和S波速度模型以及台站修正值,较好地反映了区域的大致地质构造特征。比较前人的地壳一维速度结构研究结果,我们的结果分别在地壳浅层0～10km、深度约30km的中地壳、下地壳底部45km深度的区域波速表现出较强烈的低速异常性质；并在深度48km以下存在地震波速度逐渐增加的壳幔过渡带,厚度约为12km,与横跨天山造山带的的转换波探测和地震成像剖面以及深地震测深剖面测量结果较为一致。基于优化后的地壳一维速度模型对研究区1988～2005年间2370个地震进行了重新定位,最后获得了781个地震事件分布,相对于基于已有的一维速度结构采用完全相同的定位参数的定位结果,我们的定位结果在水平和深度的平均误差有了较大幅度的改善。结果表明：地震事件大部分位于玛纳斯-吐谷鲁背斜和博格达山前推覆构造带以南的区域,并且可能预示着隐伏断层的分布和块体边界的存在。地震的平均深度为16.8km,大部分的地震位于深度5-25km。地质构造的恰当反映和精确定位结果的改善都表明了我们建立的该区域的地壳参考一维速度模型的可靠性,为将来积累充足的数据以开展地壳三维层析成像等相关地震科学研究打下基础,为推进该地区研究盆岭构造的演化和油气资源的探寻提供有效参考依据。另一方面,三维地震层析成像中传统的射线理论中存在着以下局限：低速体造成的射线路径徽、波前愈合问题、成像分辨率存在极限(?)(第一菲涅尔带宽度)、极高频近似造成波形信息的缺失等等。新近发展的全波有限频反演理论针对地震波频带本身所具有的有限频宽的真实物理特性,更多地考虑了地球内部介质非均匀性,提取地震波带来的更多结构信息,提高了对地球内部更精细构造的分辨能力。但受数据量不充分的局限,本文尚不能进一步在北天山地区开展地壳三维结构的有限频反演研究。相对地,首都圈地区有已有相当的数据基础,前人开展的速度结构研究可以为本文提供良好反演的所需数据。本研究在集群计算服务系统的支持下,在有限频全波反演理论框架中,基于三维速度模型计算了首都圈50个宽频地震台的应变格林张量(strain Green’s tensor,简称SGT)；然后通过三维交错网格有限差分法(3-Dstaggered-grid finite-difference method)计算得到了震源到台站的理论地震图；继而测量得到了与实际地震波形的到时差数据；根据SGT计算走时扰动的三维Frechet核,避开了时间与频率域的FFT变换造成的误差,并通过震源-台站问的互易原理(reciprocity),改善数值模拟计算的效率；由于该方法对数据质量的要求较高,首都圈近十年的地震波形数据经过处理筛选,尚不能提供方法所需的足够测量数据。最后择优选取分析了首都圈区域内1°×1°平面精度的波形层析成像初步结果,并在首都圈地区的东北区域在深度15km以上获得了较好的分辨率。经过LSQR迭代计算,走时测量数据的总体均方跟残差从反演前的0.756秒下降到反演后的0.710秒。结果显示研究区域内的中上地壳中,P波速度结构存在明显的横向不均匀性,上地壳的速度图像很好地反映了地表地质、地形和岩性的特征,凹陷或山间盆地呈现低速,而隆起的山区或基岩出露区显示为高速。块体的边界区域及渤海地区,特别是张家口-渤海断陷带主要表现为低速异常,与前人的研究结果具有一定的一致性。在太平洋板块至东向西俯冲产生的挤压的大环境中,受华北区域的最大主压应力的联合作用下,浅层5km处的渤海盆地的低速带在坚硬的燕山隆起影响,分别向北往燕山隆起的东部和向西往燕山隆起的南部蔓延。燕山隆起南部向华北盆地过渡的浅层区域并未如前人的结果一般显示高速分布,而是向北直到北京密云区域(北纬40.5°)附近的范围才渐渐显示明显的高速异常,而从断层的分布图显示,在燕山隆起南部向华北盆地过渡区的低速带区域分布着大量的断层带。虽然由于数据的限制导致成像分辨率的不足,但三维有限频全波有限频反演理论的优越性依然在分辨率改善和速度结构的细节上得到反映。这是国内首次应用全三维有限频全波层析成像方法在反演地壳的精细速度结构,即基于三维初始速度模型构建有限频三维敏感核,进行全波层析成像反演研究。本文在首都圈地区开展的试验性研究对该理论方法的应用和发展提供了一个良好的开端。更多还原

【Abstract】 The basin and range province is two primary tectonic unit in the of plate tectonics, and play important role in the lithospheric structure and its evolution. In this paper, two different basin and range system at northwest and north China, one is collision area between east part of the north Tianshan Mountains and Junggar Basin (TSM-JGB), the other is the North China metropolitan area, where to improve1-D crustal velocity model and conducte full-wave tomograply on full3-D finite-frequency theory, respectively, will help us further understand the interaction between adjacent plates in the basin and range system, and the relationship between tectonic structure and seismic activity.The widely used three-dimensional seismic tomography is a direct and effective means to study the crustal velocity structure in the world. The precise inversion data base, advanced inversion theory and technology are necessary guarantee to improve the reliability of the inversion results. The data base as the initial reference velocity model and source parameters has an strong impact for seismic tomography. Inappropriate initial velocity structure could affect determining of the source parameters inaccurately, and also bring unreal results for tomography. Therefore, to improve1-D crustal velocity model for TSM-JGB region, considering coupling of the initial velocity structure and source parameters, we selected1370P-wave and1396S-wave travel time data of172high quality local earthquake recorded Urumqi Telemetry Network and10temprary broadband seismic stations, apply the’minimum1-D model’approach to carry out jointly inversion of the initial velocity structure and hypocenter. Particularly, we prepared several reference models and seismic events datebase to avoid’local minima’problem during inversion, and eventually achieved optimized the regional1-D crustal velocity model The results show that the one-dimensional P-wave and S-wave velocity model we build as well as the stations correction well represent the general geological characteristics for regional structure of the area. Compared with previous model, our result indicated relative low velocity at shallow depth range (0-10km), middle crust (about30km) and lower crust (about45km), respectively. Additional, we revealed several transition layers with thickness about12km below lower crust (about48km) which may imply complexity of uppermost mantle, which is consistent with deep seismic sounding measurements by the converted wave detection and seismic imaging profiles across the Tianshan orogenic belt. Then we relocated2370earthquakes occurred from1988to2005on this improve1-D crustal model. The most relocated output781epicenters are distinctly aligned south of the surface trace of the Manas-Tugulu anticline and foldbelts of north Bogda piedmont, and distributed at depth from5to25km with the average depth16.8km. The average horizontal and vertical error outputted by method were reduced greatly, which suggests that we improve the accuracy of hypocenter determination by this new model.Properly reflecting the geological structure and the improvement of precise location suggest that we have established the regional one-dimensional crustal velocity model is reliable, which lay the foundation for future3D crustal seismic tomography with sufficient data and other related seismology research, provide evidence for evolution of basin and range tectonics and exploration of oil and gas resources.Additionally,3-D seismic tomography in traditional ray theory has the following limitations:extremely high frequency approximation, the ray path migration caused by low velocity zone, wave front healing problem, imaging resolution cannot beyond λL (width of the first Fresnel zone), waveform information invalidity, etc. Recent developments in the full-wave finite-frequency theory reveal more information for real physical characteristics of the seismic wave and its propagation in the earth, which could extract more structure information from seismic waves, thus improve resolution of the earth interior medium. However, we could not further conduct3-D finite-frequency inversion in the TSM-JGB area with limitations of inadequate date. Comparing with the TSM-JGB region, there is relatively richer database and previous structural study in the North China metropolitan area.We calculated50Strain Green Tensors (SGTs) on3-D velocity model for broadband seismic station with the full-wave finite-frequency theory on cluster computing sever; then achieved calculated seismograms on previous3-D velocity structure through3-D staggered-grid finite-difference method; and measured the delay travel time by cross-correlation between the observed and calculated seismic waveforms; and then we computed the3-D Frechet sensitive kernel for delay travel time by SGTs with lateral velocity variation, which avoid the error caused by FFT transform in time-frequency domain, and improve the efficiency of the numerical simulation by the hypocenter-station reciprocity principle; Due to the high quality requirements for the seismic waveform data recorded for nearly a decade in the metropolitan area, however, after careful selection we could not afford sufficient measurements to gain higher resolution than before. As a result, finally, we conducted full3-D waveform tomography with1°×1°horizontal resolution, and produced good resolution for northeast study area above15km depth. After LSQR iteration, the total RMS travel time residual of measurement dropped to0.710sec from0.756sec after inversion.The results show that the P-wave velocity structure presents significant lateral heterogeneity in the mid-upper crust, the image in upper crust show good coherence with surface geology and lithology, like low-velocity anomalies beneath depression or mountain basin area, high-velocity anomalies beneath uplift mountains or exposed bedrock area. And distinct low-velocity anomalies exist beneath some board area of regional block and Bohai region, especial under Zhangjiakou-Bohai depression belt zone, which is similar to preview research. Under the combined effects of the extrusion stress produced by the Pacific Plate subduction from west to east and the maximum principal stress in the North China region, the low-velocity zone below Bohai Basin at5km depth spread to north by the east side of Yanshan Mountains, and to west by the south side of the Yanshan Mountains, respectively, when colliding with the hard uplift Yanshan Mountains tectonic block. And the shallow transition zone between Yanshan Mountains from and the North China basin indicate low velocity, not as previous studies with high-velocity distribution, where distribute a large number of fault zones,and gradually appear distinct high-speed anomaly from south to the north until near the range of the Beijing Miyun region (latitude40.5°).Although inadequate data limit image resolution, the advantages of3-D full-wave finite-frequency theory remains on the resolution improvements and the details of velocity structure. This is the first application in China of3-D finite-frequency tomography method for crustal velocity structure, which build the finite-frequency sensitive kernel based on the3-D initial velocity model to conduct full wave tomography. This pilot study for North China metropolitan area in this paper provide a meaningful practice, which is befit to utilize and develope of the promising theory.更多还原