Coupling characteristics of stress and strain at different layers of different sub-regions in Yunnan and its adjacent areas

In this paper,we collect 6 361 waveform data to calculate the shear wave splitting parameters from a regionalseismic network of 22 digital stations in Yunnan and its adjacent area from July 1999 to June 2005.By using thecross-correlation method,64 splitting events of 16 stations are processed.We also collect the splitting results ofeight earthquake sequences to present the characteristics of shear wave splitting in Yunnan and its adjacent areas.The orientations of maximum principal compressive stress of three sub-regions in this area are derived from theCMT focal mechanism solutions of 43 moderate-strong earthquakes provided by Harvard University by the P axisazimuth-averaging method.The principal strain rate at each observatory is deduced from the observations ofCrustal Movement Observation Network of China during the period from 1999 to 2004.In addition,the data of Pnaniso-tropy and SKS splitting of Yunnan and its adjacent areas are also collected.We have discovered from thisstudy that the continental lithosphere,as a main seismogenic environment for strong earthquake,can be dividedinto blocks laterally;the mechanical behavior of lithosphere varies with depth and can be divided into differentlayers in the vertical orientation;the information of crustal deformation obtained from GPS might be affected bythe type of blocks,since there are different types of active blocks in Yunnan and its adjacent areas;the shear wavesplitting in this region might be affected mainly by the upper crust or even the surface tectonics.     

Crustal and upper mantle structure and the deep seismogenic environment in the source regions of the Lushan earthquake and the Wenchuan earthquake

Following the M w 7.9 Wenchuan earthquake, the M w 6.6 Lushan earthquake is another devastating earthquake that struck the Longmenshan Fault Zone (LFZ) and caused severe damages. In this study, we collected continuous broadband ambient noise seismic data and earthquake event data from Chinese provincial digital seismic network, and then utilized ambient noise tomography method and receiver function method to obtain high resolution shear wave velocity structure, crustal thickness, and Poisson ratio in the earthquake source region and its surroundings. Based on the tomography images and the receiver function results, we further analyzed the deep seismogenic environment of the LFZ and its neighborhood. We reveal three main findings: (1) There is big contrast of the shear wave velocities across the LFZ. (2) Both the Lushan earthquake and the Wenchuan earthquake occurred in the regions where crustal shear wave velocity and crustal thickness change dramatically. The rupture faults and the aftershock zones are also concentrated in the areas where the lateral gradients of crustal seismic wave speed and crustal thickness change significantly, and the focal depths of the earthquakes are concentrated in the transitional depths where shear wave velocities change dramatically from laterally uniform to laterally non-uniform. (3) The Wenchuan earthquake and its aftershocks occurred in low Poisson ratio region, while the Lushan earthquake sequences are located in high Poisson ratio zone. We proposed that the effect of the dramatic lateral variation of shear wave velocity, and the gravity potential energy differences caused by the big contrast in the topography and the crustal thickness across the LFZ may constitute the seismogenic environment for the strong earthquakes in the LFZ, and the Poisson ratio difference between the rocks in the south and north segments of the Longmenshan Fault zone may explain the 5 years delay of the occurrence of the Lushan earthquake than the Wenchuan earthquake.     

Study on shear wave splitting in the aftershock region of the Yao＇an earthquake in 2000

After Ms=6.5 Yao＇an earthquake on January 15, 2000, a large amount of aftershock waveforms were recorded by the Near Source Digital Seismic Network （NSSN） installed by Earthquake Administration of Yunnan Province in the aftershock region. It provides profuse data to systematically analyze the features of Yao＇an earthquake. The crustal anisotropy is realized by shear wave splitting propagating in the upper crust. Based on the accurate aftershock relocations, the shear wave splitting parameters are determined with the cross-correlation method, and the results of different stations and regions are discussed in this paper. These conclusions are obtained as follows： firstly, the average fast directions of aftershock region are controlled by the regional stress field and parallel to the maximum horizontal compressive stress direction; secondly, the average fast directions of disparate stations and regions are different and vary with the structural settings and regional stress fields; finally, delay time value is affected by all sorts of factors, which is affinitive with the shear wave propagating medium, especially.     

Study on Correlation and Quantitative Error Estimation Method Among the Splitting Shear Wave Identification Methods

Based on the propagation characteristics of shear wave in the anisotropic layers,thecorrelation among several splitting shear-wave identification methods hasbeen studied.Thispaper puts forward the method estimating splitting shear-wave phases and its reliability byusing of the assumption that variance of noise and useful signal data obey normaldistribution.To check the validity of new method,the identification results and errorestimation corresponding to 95% confidence level by analyzing simulation signals have beengiven.     

Study on the Anisotropy of the Underground Medium in Hohhot

In this paper, the data of earthquake events of magnitude M_S6.0 and above produced in Hohhot Seismic Station from 2008 to 2015 and the data of M_L ≥ 1.0 seismic events from 2015 to 2016 in Horinger Seismic Station and the surrounding mobile stations in southern Hohhot are selected. Using Splitlab and SAM software, the spin-correlation method, the least-energy method and the cross-correlation coefficient method are used to analyze the teleseismic and near-seismic phases (SKS, S). The results of this study are in good agreement with the results previously obtained by other researchers. The study of teleseismic SKS splitting reflects the characteristics of the anisotropy of the upper mantle beneath Hohhot, that is, the anisotropy of the upper mantle shows NW, which reflects "fossil" Anisotropy, mainly in the continental structure of stable units and preserves the history of mantle deformation information. The crustal anisotropy reflected by the near-earthquake S-wave splitting study is similar to that of the active fault zone, trending NE as a whole and is consistent with the tectonic stress field background of the northeastern margin of Ordos block.     

Estimation of residual shear strength ratios of liquefied soil deposits from shear wave velocity

For sites susceptible to liquefaction induced lateral spreading during a probable earthquake, geotechnical engineers often need to know the undrained residual shear strength of the liquefied soil deposit to estimate lateral spreading displacements, and the forces acting on the piles from the liquefied soils in order to perform post liquefaction stability analyses. The most commonly used methods to estimate the undrained residual shear strength (Sur) of liquefied sand deposits are based on the correlations determined from liquefaction induced flow failures with SPT and CPT data. In this study, 44 lateral spread case histories are analyzed and a new relationship based on only lateral spread case histories is recommended, which estimates the residual shear strength ratio of the liquefiable soil layer from normalized shear wave velocity. The new proposed method is also utilized to estimate the residual lateral displacement of an example bridge problem in an area susceptible to lateral spreading in order to provide insight into how the proposed relationship can be used in geotechnical engineering practice.     

An evaluation of force-based design vs. direct displacement-based design of jointed precast post-tensioned wall systems

The unique features of jointed post-tensioned wall systems, which include minimum structural damage and re-centering capability when subjected to earthquake lateral loads, are the result of using unbonded post-tensioning to attach the walls to the foundation, along with employing energy dissipating shear connectors between the walls. Using acceptance criteria defined in terms of inter-story drift, residual drift, and floor acceleration, this study presents a multiplelevel performance-based seismic evaluation of two five-story unbonded post-tensioned jointed precast wall systems. The design and analysis of these two wall systems, established as the direct displacement-based and force-based solutions for a prototype building used in the PREcast Seismic Structural Systems （PRESSS） program, were performed at 60% scale so that the analysis model could be validated using the PRESSS test data. Both buildings satisfied the performance criteria at four levels of earthquake motions although the design base shear of the direct displacement-based jointed wall system was 50% of that demanded by the force-based design method. The study also investigated the feasibility of controlling the maximum transient inter-story drift in a jointed wall system by increasing the number of energy dissipating shear connectors between the walls but without significantly affecting its re-centering capability.     

Mantle anisotropy across the southwestern boundary of the Ordos block,North China

Located at the northeastern margin of the Tibetan plateau,the Ordos block is a stable tectonic unit in North China.With its active boundary fault zones,the Ordos block played an important role in the eastward extrusion mechanism of the Tibetan plateau.Peking University deployed a linear array of 15 portable broadband seismometers across the western Weihe graben during September 2005 to August 2006 and later a 2-D seismic array(Southwest Ordos Array) of 14 portable broadband seismometers during 2007-2008 at its southwestern boundary.Analyses of shear wave splitting of SKS and SKKS phases at these stations show that the fast directions trend ～110° with an average delay time of 0.9 s in the southwestern margin of the Ordos block.The agreement between the lithosphere deformation indicated by GPS data and Quaternary fault slip-rate observations and the mantle flow represented by shear wave splitting implies that accordant deformation patterns from lithosphere to asthenosphere in relation to the eastward extrusion of the Tibetan plateau could extend at least to 200 km depth.Spatial distribution of splitting polarization directions indicates that the mantle flow driven by the Tibetan plateau is blocked by the Ordos block and locally restricted in a narrow channel along the Qinling-Dabie fault zones between the Ordos block and Sichuan basin.     

天津滨海地区浅层剪切波速与基底构造关系研究

Shear wave velocity is one of the important dynamic characteristics of soil layers and applied widely in aseismic engineering. In this paper, 500 drill logging data are used to make a linear interpolation based on 0.01°×0.01°×1m grid. A shallow 3-D shear wave velocity structure of Tianjin coastal area is obtained. According to the data and geological background, we selected two typical velocity profiles to try to introduce and explain its relationship to basement structure. The results show that the shear wave velocity structure clearly presents the characteristic of stratification and lateral inhomogeneity. Furthermore, the difference of the shear wave structure between tectonic elements is clear and the velocity structure between the two sides of the local or border fault in the Quaternary is disturbed or affected significantly. It intuitively shows that the basement structure and fault activity of this region had good control of sedimentation development and strata formation in the Quaternary period which would have an important effect on engineering seismic and geological condition evaluation.     

Shear wave anisotropy in D＂ region beneath the western Pacific

Using seismic shear phases from 47 Tonga-Fiji and its adjacent region events recorded by the CENC and IRIS, and from 26 northeast Asia and north Pacific events recorded by IRIS, we studied the shear wave anisotropy in D" region beneath the western Pacific utilizing the ScS-S differential travel time method and obtained the splitting time values between the radial and transverse components of each ScS wave corresponding to each core-mantle boundary (CMB) reflection point. We found that most shear waves involved horizontally polarized shear wave components traveling faster than vertically polarized shear wave components through the D" region. The splitting time values of ScS wave range from ?0.91 s to 3.21 s with an average value of 1.1 s. The strength of anisotropy varies from ?0.45% to 1.56% with an average value of 0.52%. The observations and analyses show that in the D" region beneath the western Pacific the lateral flow is expected to be dominant and the vertical transverse isotropy may be the main anisotropic structure. This structure feature may be explained by the shape preferred orientation of the CMB chemical reaction products or partial melt and the lattice preferred orientation of the lower mantle materials caused by the lateral flow at lowermost mantle.     

荣昌台S波分裂研究

采用SAM方法对重庆市荣昌台2000～2006年（缺2001年资料）的数字地震资料进行S波分裂的计算分析,并讨论了其与该区构造的关系。计算结果表明,S波快波偏振方向主要集中分布在NW向40°～45°、75°～85°。认为S波快波优势分布方向为NW向,与荣昌地区所受区域应力场方向大致一致,与该区的主要基底断裂走向接近垂直,反映了该区应力场的大致轮廓,为地震预报提供信息。     

水库气枪震源产生的S波及其分裂

人工气枪震源在陆地水库可以有效激发S波,S波能量较强,与ML1.6天然地震相当。气枪可用于S波分裂研究,对布置在燕山隆起带的流动地震台的气枪信号进行了S波分裂参数分析,结果表明,快剪切波偏振优势方向为NWW和NNE向,偏振方向和断裂的性质密切相关。气枪是高度可重复性人工震源,利用气枪定点激发和定点接收有可能精确获取S波分裂参数随时间的变化规律,为地震预测探索实践提供可靠的物理途径     

剪切波分裂分析系统SAM(2007)——区域地震台网资料应用实例

通过对云南、辽宁、福建遥测地震台网的波形资料分析实例,介绍了"剪切波分裂系统分析方法"软件SAM(2007)的用法。通过对区域台网资料的处理分析,表明SAM(2007)是研究剪切波分裂的一个比较实用的软件,能够有效的处理快、慢剪切波识别的问题,研究地壳介质的地震各向异性问题,能普遍用于分析国内区域地震台网资料。     

云南及邻区应力应变场分区耦合特性初步研究

利用云南及邻区遥测台网记录到的6 361条近震数据，采用互相关系数法计算16个台站共64条S波分裂事件，同时收集了8个序列的S波分裂研究结果，初步给出了云南及邻区横波分裂的分布特征；应用哈佛大学给出的43个中强震震源机制解，利用其P轴方位角平均结果，给出了云南及邻区3个主要分区地壳应力场的最大主压应力方位；根据ldquo;中国地壳运动观测网络rdquo;基本网1999——2004年的观测资料，按观测点给出了云南及邻区的主应变率分布；此外，还搜集到云南及邻区的SKS分裂结果和Pn波各向异性资料.通过研究发现: ① 作为强震孕育主体环境的大陆岩石圈在横向上具有明显的分块特点 ② 大陆岩石圈的力学行为随深度而变化，具有明显的分层特性；③云南及邻区所含的各活动地块可能具有不同类型，GPS所反映的地壳变形信息可能受到块体类型控制；④云南及邻区的地壳S波分裂可能受其上地壳甚至地表构造影响较大.      

孕震过程中波速比及S波分裂时空变化的数值模拟

本文利用有限元方法模拟计算了强震发生前孕震区的波速比及Ｓ波分裂的时空变化，结果表明，孕震过程中，随着应力的局部集中，岩体发生扩容，引起裂隙的增生及定向排列，从而使孕震区产生各向异性，引起波速比的变化及Ｓ波分裂现象，在不同阶段，穿过同一路径的地震射线其波速比和Ｓ波分裂特征不同；在同一时刻，沿不同路径传播的地震射线其波速比及Ｓ波分裂也不同。总体看来，平行于孕震断层传播的地震波速比变化比垂直于其波速比及     

2006年3月31日吉林乾安M5．0余震序列S波分裂研究

2006年03月31日20时23分,松原市乾安县与前郭县交界处的查干花乡附近（44°36′N,124°03′E）发生5．0级地震,是吉林省近40年来最大一次地震。利用流动台记录到的地震波形资料对乾安地震余震序列进行s波分裂研究。结果表明,S波快波偏振方向的优势取向为NNE,与乾安地震主震的最大主压应力方向偏差较大,与该区区域应力场方向不一致,表明这次地震与区域的构造变化和应力场调整有关。s波分裂时间延时大小受多种因素影响,与S波传播时经过的介质性质关系密切,慢波延迟时间在0．02～0．38S之间变化。     

肃南5.7级地震过程剪切波分裂特征

剪切波分裂应用于地震预报中的３个主要困难是：（１）如何自动定量识别剪切波快慢波到时；（２）用以认识其基本特征的震例太少；（３）因各次地震的复杂性，对所给剪切波分裂结果难以给出恰当的解释，本文研究了１９８８年１１月２２日甘肃肃南５．７级地震过程剪切波分裂的特征。所用资料为１９８８年８月到１９９０年８月中法合作张掖数字台网的记录。通过对张掖～肃南地区地壳结构模型的研究，确认了所用各次地震的真实出射角基     

唐山地区慢剪切波的时间延迟:强地面运动台阵资料分析

利用唐山地区强地面运动台阵资料 ,根据剪切波分裂的 CDP分析方法 ,对唐山地区剪切波分裂时间延迟参数进行了讨论。通过选取具有相同射线方向的波形记录的剪切波分裂分析 ,得到较大地震 (ML 5.2 )之前的时间延迟值高于震后的时间延迟值。特别是 ,文章提出的研究剪切波分裂时间延迟的时间变化的思路 ,是实际分析中需要加以注意的。文章认为 ,利用剪切波分裂进行较大地震的监控和预测 ,具有重要的意义     

安庆4.8级地震的余震S波分裂研究

利用安徽数字地震台网安庆台记录的地震波形资料,运行Matlap S波分裂程序,对2011年1月19日安徽安庆4.8级地震的余震序列开展S波分裂研究。结果表明,快S波偏振方向有两个优势取向,分别为N73°E和N94°E,初步判断为区域应力场导致的EDA裂隙系各向异性结果。其中快S波N73°E偏振方向与区域应力场的主压应力方向N74°E基本相同,而快S波N94°E偏振方向稍有偏差,可能是由于4.8级主震产生的应力扰动引起的。慢S波延迟时间在0.82～10.92范围内变化,平均为2.84,主震后,延迟时间明显下降,可能反映了震后应力的释放过程,而几次较大余震前则处于上升高值状态,可能反映了较大余震前的应力积累过程。     

基于横波分裂的青藏高原多圈层各向异性研究进展

有关青藏高原横波分裂的各向异性研究已经开展了近30年,在理论方法和实际应用方面取得了重要进展,并获取了大量的横波分裂测量结果,为认识青藏高原壳幔各向异性变形特征和动力学机制提供了重要的依据.本文首先介绍了地震各向异性的来源与应用,随后回顾了横波分裂分析方法的发展,简述了各种横波分裂方法的原理,最后通过总结近30年来青藏高原上地壳、整个地壳和上地幔横波分裂各向异性研究成果,系统分析了青藏高原壳幔各向异性变形特征.基于各横波分裂结果的对比分析来看,XKS波分裂测量结果最为稳定,近震直达S波分裂测量结果次之,而Pms波分裂测量结果相对离散,往往相同区域内不同的研究结果差异较大,主要原因可能是相比XKS波和近震直达S波,Pms波的信噪比较低,次要原因可能是各研究在方法和处理分析等方面的差异.