Variational characteristics of shear-wave splitting on the 2001 Shidian earthquakes in Yunnan, China

Introduction In the crust there exist widely vertical nearly-parallel fluid-saturated microcracks, which are nearly parallel. Shear-wave splitting can be observed when shear-wave travels through this kind of microcrack structure. Shear-wave splitting could be adopted to monitor the earthquake activity since its pattern is very sensitive to variation of crustal stress (GAO, et al, 1995, 1996), however, it needs lots of reliable 3-component waveform data. Previous studies mainly used some temp…     

Study on shear wave splitting in the after- shock region of the Yao′an earthquake in 2000

Introduction Rock microcracks in the crust play very important roles in all kinds of geophysical phe-nomenon variation (Aki et al, 1970; Crampin et al, 1980). The medium anisotropy of the upper crust has been studied extensively, mostly based on analysis on shear wave splitting from local earthquakes. Commonly, explanation for the upper crust anisotropy is the assumed ″extensive di-latancy anisotropy″ (Crampin, 1978) or its modified version ″anisotropic poroelasticity″ (Crampin and Zatse…     

Anisotropy of the apparent resistivity variation rate near the epicentral region for strong earthquakes

IntroductionThe geo-electrical anisotropy before earthquake is of special significance for the prediction of earthquake location and for the research on crustal stress status near the epicentral region. The anisotropy has been studied from various aspects (CHEN, et al, 1983; QIAN, et al, 1985, 1996; LU, et al, 1990a; MAO, et al, 1995; RUAN, et al, 1999). But the anisotropic phenomena presented by geo-electrical data are not practically satisfactory and not clear as compared with the res…     

Research on anisotropic parameters by syn-thetic seismogram

Introduction The anisotropy of the Earth crust medium is a common phenomenon(Crampin,1984).More and more observation materials and study results have proved that on most of the Earth′s upper crust is spread fluid-filled cracks and microcracks,which are aligned according to the contempo-rary stress-field.Such distributions of aligned cracks show effectively anisotropy to seismic wave and the phenomenon is called extensive-dilatancy anisotropy(EDA)(Crampin et al,1984).At the same time,it is …     

Azimuthal anisotropy of Rayleigh waves beneath the Tibetan Plateau and adjacent areas

The crustal and upper mantle azimuthal anisotropy of the Tibetan Plateau and adjacent areas was studied by Rayleigh wave tomography. We collected sufficient broadband digital seismograms trav-ersing the Tibetan Plateau and adjacent areas from available stations, including especially some data from the temporary stations newly deployed in Yunnan, eastern Tibet, and western Sichuan. They made an adequate path coverage in most regions to achieve a reasonable resolution for the inversion. The model resolution tests show that the anisotropic features of scope greater than 400 km and strength greater than 2% are reliable. The azimuthal anisotropy pattern inside the Tibetan Plateau was similar to the characteristic of tectonic partition. The crustal anisotropy strength is greater than 2% in most re-gions of East Tibet, and the anisotropy shows clockwise rotation surrounding the eastern Himalayan syntaxis. Vertically, the anisotropy direction indicates a coherent pattern within the upper crust, lower crust, and lithosphere mantle of the Tibetan Plateau, which also is consistent with GPS velocity field and SKS fast polarization directions. The result supports that the crust-mantle deformation beneath the Tibetan Plateau is vertically coherent. The anisotropy strength of crust and lithospheric upper mantle in Yunnan outside the Tibetan Plateau is lower than 2%, so SKS splitting from core-mantle boundary to station should largely be attributed to the anisotropy of asthenosphere.     

Crustal S-velocity structure and radial anisotropy beneath the southern part of central and western North China Craton and the adjacent Qilian Orogenic Belt from ambient noise tomography

The crustal S-velocity structure and radial anisotropy along a dense linear portable seismic array with 64 broadband seismic stations were investigated from ambient noise tomography with about one-year-long ambient noise recordings. The array transverses the southern part of the central North China Craton(CNCC) and western NCC(WNCC) from east to west and reaches the adjacent Qilian Orogenic Belt(QOB). The phase velocity structures of Rayleigh waves at 5–35 s and Love waves at 5–30 s were measured. The crustal S-velocity structures(Vsv and Vsh) were constructed from the dispersion data(Rayleigh and Love waves,respectively) from point-wise linear inversion with prior information of the Moho depth and average crustal Vp/Vs ratio. The radial anisotropy along the profile was calculated based on the discrepancies between Vsv and Vsh as 2×(Vsh.Vsv)/(Vsh+Vsv). The results show distinct structural variations in the three major tectonic units. The crustal architecture in the southern CNCC is complicated and featured with wide-distributed low-velocity zones(LVZs), which may be a reflection of crustal modification resulting from Mesozoic-Cenozoic tectonics and magmatic activities. The pronounced positive radial anisotropy in the lower-lowermost crust beneath the Shanxi-Shaanxi Rift and the neighboring areas could be attributed to the underplating of mantle mafic-ultramafic materials during the Mesozoic-Cenozoic tectonic activation. In southern Ordos, the overall weak lateral velocity variations, relative high velocity and large-scale positive radial anisotropy in mid-lower crust probably suggest that the current crustal structure has preserved its Precambrian tectonic characteristics. The low-velocity westward-dipping sedimentary strata in the Ordos Block could be attributed to the Phanerozoic whole-basin tilting and the uneven erosion since late Cretaceous. Integrated with previous studies, the systematic comparison of crustal architecture was made between the southern and northern part of CNCC-WNCC. The similarities and differences may have a relation with the tectonic events and deformation histories experienced before and after the Paleoproterozoic amalgamation of the NCC. The nearly flat mid-crustal LVZ beneath the southern QOB weakens gradually as it extends to the east, which is a feature probably associated with crustal vertical superpositionand ductile shear deformation under the intensive compressional regime due to the northeastward growth and expansion of the Tibetan Plateau.     

Rheology of the lower crust beneath the northern part of North China: Inferences from lower crustal xenoliths from Hannuoba basalts, Hebei Province, China

Lower crustal xenoliths brought up rapidly by basaltic magma onto the earth surface may provide di-rect information on the lower crust. The main purpose of this research is to gain an insight into the rheology of the lower crust through the detailed study of lower crustal xenoliths collected from the Hannuoba basalt, North China. The lower crustal xenoliths in this area consist mainly of two pyroxene granulite, garnet granulite, and light-colored granulite, with a few exception of felsic granulite. The equilibration temperature and pressure of these xenoliths are estimated by using geothermometers and geobarometers suitable for lower crustal xenoliths. The obtained results show that the equilibration temperature of these xenoliths is within the range of 785―900℃, and the equilibrium pressure is within the range of 0.8―1.2 GPa, corresponding to a depth range of 28―42 km. These results have been used to modify the previously constructed lower crust-upper mantle geotherm for the studied area. The dif-ferential stress during the deformation process of the lower crustal xenoliths is estimated by using recrystallized grain-size paleo-piezometer to be in the range of 14―20 MPa. Comparing the available steady state flow laws for lower crustal rocks, it is confirmed that the flow law proposed by Wilks et al. in 1990 is applicable to the lower crustal xenoliths studied in this paper. The strain rate of the lower crust estimated by using this flow law is within the range of 10-13―10-11 s-1, higher than the strain rate of the upper mantle estimated previously for the studied area (10-17―10-13 s-1); the equivalent viscosity is estimated to be within the range of 1017―1019Pa·s, lower than that of the upper mantle (1019―1021 Pa·s). The constructed rheological profiles of the lower crust indicate that the differential stress shows no significant linear relation with depth, while the strain rate increases with depth and equivalent vis-cosity decrease with depth. The results support the viewpoint of weak lower continental crust.     

Crustal anisotropy of Taihang Mountain Range using azimuthal variation of receiver functions

We discussed the possibility of studying crust anisotropy by analyzing azimuthal variation of the receiver functions and presented a technique for computing the transmission response of a flat-layered medium with arbitrarily ori-ented hexagonally symmetric anisotropy using the reflectivity algorithm. Using this method we investigated the crust anisotropy of Taihang Mountain Range (TMR). Our result shows that there is significant anisotropy with a slow symmetry axis in the upper crust and a fast symmetry axis in the lower crust. The anisotropy magnitude of about 8%~15% is found in the upper crust and a smaller magnitude of about 3%~5% in the lower crust. Orienta-tion of the symmetry axes and the depth of anisotropy appearance as deduced from the seismic records of four individual seismic stations are different from each other. The crust anisotropy beneath the four stations may be associated with the local crustal fabrics in a small area.     

Variation of shearwave splitting in earthquake clusters with very similar waveforms

IntroductionAfter the confirming of anisotropy theory and the shear wave splitting, Crampin suggested that the accumulation and release of stress before and after a large earthquake could be observed by investigating the behavior of shear-wave (Crampin, et al, 1984; Crampin, 1987). Since that time, seismologists tried to find evidence to prove the hypothesis. Peacock, et al (1988) and Crampin, et al (1990) declared that they find a variation of time delay before and after the North Palm Spr…     

Preliminary study on characteristics of earthquake precursor of earth resistivity anisotropy degree in East China

Introduction The degree of earth-resistivity anisotropy was described (MAO, et al, 1995, 1998) as follows: 31EWSN1011-==niinSrr n=6 (1) where rNS and rEW are monthly mean values of earth resistivity in the direction of NS and EW, respectively, S is the half-year value. Equation (1) shows that if rNS=rEW, then S =0 and the electrical property of medium is isotropic; if rNSrEW, then S0 and the electrical property of medium is anisotropic. When S increases, the anisotropy of electri…     

云南地区地壳介质各向异性-快剪切波偏振特性

通过对云南遥测地震台网2000年1月1日——2003年12月31日4年资料的分析， 使用剪切波分裂SAM综合分析方法，获得了云南地区10个数字地震台站的快剪切波偏振结果. 结果表明， 云南地区大部分台站的快剪切波偏振优势方向主要为近N——S或NNW方向； 位于活动断裂上的台站的快剪切波偏振优势方向与活动断裂的走向一致；与GPS主压应变方向一致，与区域主压应力方向基本一致；少数台站的快剪切波偏振较为复杂，或与活动断裂的走向及GPS主压应变方向不一致. 这样的台站总是位于几个断裂的交会处，反映了复杂的断裂背景和复杂的应力分布特征. 快剪切波偏振优势方向代表了原地最大主压应力方向，受到区域应力场和断裂分布等多种因素的控制.      

山西地区地壳剪切波分裂特征

通过对山西数字地震台网2000年6月—2012年12月的波形记录资料的分析, 使用剪切波分裂系统分析方法, 即SAM综合分析方法, 获得了山西地区18个数字地震台站的快剪切波偏振结果. 结果表明: 位于活动断裂上的台站的快剪切波偏振优势方向与活动断裂的走向基本一致; 个别距离断裂较远的台站的快剪切波偏振优势方向与震源机制解及GPS主压应变方向完全一致; 少数位于几条断裂交汇处的台站的快剪切波偏振优势方向则较为复杂, 与活动断裂的走向和GPS主压应变方向均不一致, 反映了该地区断裂背景和应力分布特征的复杂性.      

Crustal seismic anisotropy in Yunnan, Southwestern China

Using seismic data recorded by Yunnan Telemetry Seismic Network from January 1, 2000, to May 31, 2005, the polarization directions of fast shear waves are obtained at 15 seismic stations by SAM technique, which is a systematic analysis method on shear-wave splitting. The results show that predominant directions of polarizations of fast shear waves at most stations are mainly nearly in the N–S or NNW directions in Yunnan. The predominant polarization directions of fast shear waves at stations located on the active faults are consistent with the strike of active faults, directions of regional principal compressive strains from GPS measurement, and directions of regional principal compressive stress. A few of the stations show that polarization patterns of fast shear waves are more complicated or inconsistent with the strike of active faults and the directions of principal GPS compressive strains; these stations are always located at the junction of several faults. We conclude that the predominant polarization direction of fast shear waves indicates that the direction of the in situ maximum principal compressive stress is controlled by multiple tectonic aspects, such as the regional stress field and faults.     

四川紫坪铺水库库区地震剪切波分裂研究

本研究利用四川紫坪铺水库数字地震台网2004年8月17日~2008年5月11日的地震观测波形资料,使用剪切波分裂SAM系统分析方法,获得了四川紫坪铺水库库区8个数字地震台站的快剪切波偏振结果.结果表明,紫坪铺库区台站的快剪切波偏振优势方向主要为NE或NW方向;台站的快剪切波偏振优势方向与区域主压应力方向或活动断裂走向一致;快剪切波偏振方向变化可能与汶川大地震前区域应力场的增加和龙门山断裂带微破裂增加有关,慢剪切波时间延迟的变化与四川紫坪铺水库水位升降变化相关.     

2013年芦山MS7.0地震序列S波分裂特征

本文测定了2013年4月20日芦山M_S7.0地震震源区及其附近台站的S波分裂参数,包括快波偏振方向和慢波延迟时间,最终得到了40个台站的S波分裂结果．结果显示:在地震主破裂区内观测到的快波优势取向为NE向,与余震分布的长轴方向一致;位于双石—大川断裂以西台站的快波偏振优势方向为NW向,与区域最大主压应力轴方向一致;位于荥经断裂附近台站的快波偏振优势方向为NW向,与该断裂走向一致．快波偏振优势方向随时间的变化结果显示:主震前位于地震破裂区附近的TQU和BAX台站的快波偏振优势方向均呈NE向;主震后TQU台站的快波偏振优势方向为近EW向,而BAX台站的快波偏振优势方向则不突出,反映出芦山地震主震前快波偏振方向受控于龙门山断裂带,而主震后受构造应力场的作用更加明显．此外,各台站的慢波延迟时间为1.25—5.40ms/km,在余震覆盖密集区域,台站的慢波延迟时间均大于3.0ms/km,反映出震源区的各向异性程度较强．芦山主震后,各台站的延迟时间随时间变化持续减小,反映出震源区地壳应力随余震活动逐渐减小．      

辽宁区域地震台网的地壳剪切波分裂研究

主要利用辽宁区域地震台网8个台站记录到的1999年6月至2004年12月的波形数据,采用剪切波分裂SAM分析方法,对辽宁地区的剪切波分裂特征进行了研究。发现大部分台站的快剪切波优势偏振方向为NEE(近EW)向,与原地主压应力方向一致,也与华北北部的区域构造应力场方向一致。然而,辽宁中部的SJ台站和东部的KD台站的快剪切波优势偏振方向分别为近NS向和NW向,与其他台站的结果有差异,可能是受到复杂的局部构造的控制和影响,这2个台站的结果还需要更多资料的证实。根据GPS、地震和地球物理等其它资料,对该地区断层的区域分布、主压应力以及剪切波分裂参数的空间分布特征进行了讨论     

汶川地震余震序列的地震各向异性

利用2008年5月12日汶川地震震源区及周边地震台站记录的余震序列资料,使用剪切波分裂系统分析法,分析了汶川地震发震构造龙门山断裂带及周边地区的地壳各向异性特征,推断了地壳最大主压应力方向及空间分布特征.研究结果表明：大致以安县为界,位于龙门山北东段的台站快剪切波的偏振方向为北东向,与断裂带走向一致;而位于龙门山西南段的台站快剪切波的偏振方向为北西向,与断裂带走向垂直;这个特征同样揭示出龙门山断裂带西南段逆冲、北东段带有明显走滑性质的特征.研究还显示,靠近龙门山与鲜水河、安宁河小江断裂交汇区附近的台站快剪切波的偏振方向表现比较离散,这可能是由震源区局部的复杂地质构造引起,与该地区复杂的主压应力方向特点一致.     

玉树地区地壳介质的各向异性特征

本文利用2010年5月7日到10月18日玉树地震震源区及其周边22个地震台站记录的2010年4月14日玉树地震余震数据资料,使用剪切波分裂分析方法初步得到了每一个台站的剪切波快波偏振方向和慢波时间延迟,并分析了玉树及周边地区地壳介质各向异性特征.在该研究区域,甘孜-玉树断裂上部分台站快波偏振方向近于东西向,这一结果与该区域的水平主压应力方向一致.甘孜-玉树断裂带南段玉树周边的台站快剪切波偏振方向为南东向,与断裂带的走向一致,显示了此次地震断裂走滑性质的特征.位于杂多断裂和清水河断裂上的台站及其附近的台站,快波偏振方向与所处的断裂走向基本一致,多为南东东向.各个台站的慢波延迟时间结果分布在4.23~7.01 ms/km范围内,平均慢波延迟时间是5.68 ms/km.在甘孜-玉树断裂带和乌兰乌拉湖-玉树南断裂相交的位置慢波延时总体较高;而低值区位于打贝通-小苏莽断裂的北西端与杂多断裂之间的位置.沿玉树断裂带,慢波延迟时间的梯度值较大,本文这一结果揭示了慢波延迟时间的分布和破裂带的走向、余震的分布有很大关系.     

张家口-渤海地震带与山西地震带交汇区的地壳剪切波分裂

张家口—渤海地震带(简称张渤地震带)与山西地震带交汇区域(113°E～117°E,39°N～42°N)是华北地区北部三个构造单元华北沉降、燕山隆起和太行山隆起的交界处.本文采用张渤带西端的流动地震台网和华北地区北部的区域数字地震台网的地震波形数据,使用SAM方法对该区域地壳介质剪切波分裂参数进行研究.结果表明,北部的燕...     

中国地震科学台阵两期观测资料近场记录揭示的南北地震带地壳剪切波分裂特征

利用中国地震科学台阵第一期(2011-01-2014-06)及部分中国地震科学台阵第二期(2013-02-2015-12)的流动地震台阵记录到的小震波形资料,运用剪切波分裂系统分析(SAM)方法,分析南北地震带的地壳各向异性,对剪切波分裂参数所反映的区域应力环境及构造特征,以及区域内主压应力方向与断裂分布的关系展开讨论.研究结果表明,南北地震带快剪切波偏振方向自北向南由NE向逐渐转变为NNW向,与南北地震带区域主压应力的方向变化具有一致性.区域内分布的大量NE及WNW或NW向断裂构造同样对快波偏振方向有比较大的影响,位于走滑断裂附近的台站,其快波方向与断裂走向大致平行,部分位于走滑断裂附近的台站其快波方向几乎垂直于断裂走向,而与构造应力场方向一致性较好.个别台站表现出复杂快波优势方向特征,反映出研究区内构造环境的复杂性.慢波时间延迟结果显示,南北地震带南段的平均时间延迟高于北段,反映了受印度板块和欧亚板块的碰撞挤压作用,南段地壳介质各向异性程度更大,构造变形更加剧烈.对比南北地震带上地幔各向异性特征,推测在川滇菱形块体内部可能存在复杂的壳幔耦合现象,地壳剪切波分裂除了反映区域应力特征,还可以揭示出区域构造信息.