D″ anisotropy inverted from shear wave splitting intensity

The D″ layer, located at the bottom of the mantle, is an active thermochemical boundary layer. The upwelling of mantle plumes, as well as possible plate subduction in the D″ layer, could lead to large-scale material transformation and mineral deformation, which could result in significant seismic anisotropy. However, owing to limited observations and immense computational cost, the anisotropic structures and geodynamic mechanisms in the D″ layer remain poorly understood. In this study, we proposed a new inversion method for the seismic anisotropy in the D″ layer quantitatively with shear wave splitting intensities. We first proved the linearity of the splitting intensities under the ray-theory assumption. The synthetic tests showed that, with horizontal axes of symmetry and ray incidences lower than 30º in the D″ layer (typical SKS phase), the anisotropy is well resolved. We applied the method to the measured dataset in Africa and Western Europe, and obtained strong D″ anisotropy in the margins of the large low shear-wave velocity provinces and subducting slabs. The new method makes it possible to obtain D″ anisotropy, which provides essential constraints on the geodynamical processes at the base of the mantle.     

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 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…     

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.     

Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

ＶａｒｉａｔｉｏｎｓｉｎｓｈｅａｒｗａｖｅｓｐｌｉｔｔｉｎｇｄｕｒｉｎｇａｆｔｅｒｓｈｏｃｋｓｏｆｔｈｅＬｕｑｕａｎｅａｒｔｈｑｕａｋｅｉｎＹｕｎｎａｎＰｒｏｖｉｎｃｅＢＡＩ－ＪＩＬＩ（李白基）（ＳｅｉｓｍｏｌｏｇｉｃａｌＢｕｒｅａｕｏｆＹｕｎｎａｎ...     

Temporal changes in shear wave splitting after the M=5. 2 Shimian Earthquake

ＴｅｍｐｏｒａｌｃｈａｎｇｅｓｉｎｓｈｅａｒｗａｖｅｓｐｌｉｔｔｉｎｇａｆｔｅｒｔｈｅＭ＝５．２ＳｈｉｍｉａｎＥａｒｔｈｑｕａｋｅ　ｏｆ　Ｊｕｎｅ　９，１９８９Ｔｉａｎ－ＣｈａｎｇＣＨＥＮ；Ｚｈｉ－ＪｕｎＬＩ；Ｒｕｎ－ＳｈｅｎｇＺＨＡＮＧ；ＧｕｏＧｕ...     

Seismic anisotropy of the crust in Yunnan, China： Polarizations of fast shear-waves

Introduction Anisotropy of the crust is a common phenomenon(Crampin,1984).Shear-wave splitting can be used to study the earthquake anisotropic characteristic in crust,to analyze crustal stress field condition,and to describe the static and the dynamic state of the related anisotropic parameters(GAO et al,1999).Shear-wave splitting is quite sensitive to anisotropy.The domestic scholars applied shear-wave splitting to studying the crustal anisotropy(YAO et al,1992;GAO and FENG,1990).The st…     

Analysis of the rising and sinking movement of the crust in Qinghai－Xizang Plateau，China

ＡｎａｌｙｓｉｓｏｆｔｈｅｒｉｓｉｎｇａｎｄｓｉｎｋｉｎｇｍｏｖｅｍｅｎｔｏｆｔｈｅｃｒｕｓｔｉｎＱｉｎｇｈａｉ－ＸｉｚａｎｇＰｌａｔｅａｕ，ＣｈｉｎａＺｉ－ＬｉｎｇＬＵ（吕梓龄），ＺｈｅｎｇＺＨＵＡＮＧ（庄真），Ｚｈｕ－ＷｕＦＵ（傅竹武），Ｊｉａ－...     

Some evidences for earthquake occurrence in the shallow crust in some areas of North China and Southwest China

IntroductionThe hypocentral location parameters (the longitude and latitude of epicenter and the focal depth) are important fundamental data for the study of seismology and the earth interior physics. To locate the seismic source precisely is the basis for the study of seismicity patterns, relation between earthquake and active structures, engineering seismology, etc (WANG, et al, 1995). Among these parameters, the precise measurement of focal depth is closely related to the correct understa…     

Crack-induced anisotropy in the crust from shear wave splitting observed in Tangshan region,North China

Using the cross correlation function analysis method, this paper discusses shear wave splitting and crack-induced anisotropy in the crust beneath Tangshan, North China, by the digital data from Tangshan strong ground motion temporary arrays. Sixteen of twenty-one stations in the arrays recorded earthquake events available for studying from 1982 to 1984. Having calculated 131 available records, we get slower shear wave time delayτ and faster shear wave polarization azimuthPaz in Tangshan region, and the cracks densityε is got further from them. The analysis shows that the stress field is very complicated in Tangshan region and has strongly regional feature. Because of the complicated distribution of faults, different shear wave splitting characteristics are shown in 16 stations, scatteredτ and differentPaz. And they also were observed that theτ andPaz values were diverse within the time scale of hours in more than one station. In Tangshan region the average results ofτ,Paz andε are 0.0071 s · km^?1, northwest-west near to east-west and 0.022 respectively. Meantime, the standard deviations were calculated in this paper.     

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.     

新疆地区S波分裂研究h

利用国家地震台网及中国地震局ldquo;十五rdquo;期间在新疆地区布设的宽频地震台站记录到的远震波形数据,采用最小能量法和旋转相关法分别对SKS、 SKKS震相进行了偏振分析,计算了台站下方介质各向异性的分裂参数:快波的偏振方向（phi;）和慢波延迟时间（delta;t）．研究结果表明,塔里木盆地北缘、天山造山带和阿尔泰造山带大多数台站的快波偏振方向与台站下方构造走向方向接近,其快慢波分裂延迟介于0.8——1.8 s之间. 这与印度 欧亚碰撞导致的岩石圈缩短有关．相比而言,塔里木盆地和准噶尔盆地内部的各向异性强度明显要弱,表明其自前寒武形成以来并没有经历强烈的变形作用．阿尔金断裂带附近台站下方各向异性快波方向与断裂带的走向具有很强的相关性,表明该断裂已经切穿整个岩石圈．      

Insight into craton evolution: Constraints from shear wave splitting in the North China Craton

The multi-episodic tectonic activities from the Precambrian to Cenozoic, including nucleus formation, cratonic amalgamation, and rejuvenation, make the North China Craton (NCC) an ideal natural laboratory for studying craton evolution. Spatial change in the upper deformation records is an important aspect for understanding cratonic formation and rejuvenation. In this study, we performed seismic shear wave splitting analysis using SKS phases from 50 portable stations. Two different methodologies, shear wave splitting measurement and amplitude analysis of transverse/radial components, produced mutually consistent splitting results. These results showed that the seismic anisotropy beneath the Ordos Block can be divided into three subgroups reflecting the tectonic control. Combining these results with those from previous splitting studies in the eastern NCC, we suggest that the Proterozoic amalgamation generated the seismic anisotropy in the boundary zone between the Ordos Block and the Trans-North China Orogen, while the anisotropy in the eastern Trans-North China Orogen and eastern NCC were possibly associated with the lithospheric rejuvenation during the Late Mesozoic to Cenozoic.     

Variations of shear wave splitting in the 2008 Wenchuan earthquake region

Through the analysis of S-wave particle motion of local events in the shear wave window, the polariza-tion directions of the faster shear wave and the delay times between the faster and the slower shear waves were derived from seismic recordings at the stations near the fault zones. The shear wave split-ting results of seven stations in the area of Longmenshan fault zone reveal spatial variation of the po-larization directions of the fast shear wave. The directions at stations in the southeastern side of the Longmenshan fault zone (in the Sichuan Basin area) are in the NE direction, whereas the direction at station PWU (in the Plateau), which is in the northwestern side of the faults, is in the EW direction. Systematic changes of the time delays between two split shear waves were also observed. At station L5501 in the southern end of the aftershock zone, the delay times of the slower shear wave decrease systematically after the main shock. After the main shock, the delay times at station PWU were longer than those before the earthquake. Seismic shear wave splitting is caused mostly by stress-aligned microcracks in the rock below the stations. The results demonstrate changes of local stress field dur-ing the main-shock and the aftershocks. The stress in the southern part of Wenchuan seismogenic zone was released by the main-shock and the aftershocks. The crustal stresses were transferred to the northeastern part of the zone, resulting in stress increase at station PWU after the main-shock.     

远震剪切波分裂研究华南西南部上地幔各向异性特征

华南板块由扬子克拉通和华夏地块在新元古代碰撞拼合形成, 其西南部的缝合带位置存在强烈争议.本文基于一条南北向宽频带流动台阵天然地震数据, 利用SKS波分裂研究华南西南部上地幔各向异性特征的南北向变化.分裂测量结果显示, 26°N以南的右江盆地快波偏振方向为E-W或NEE-SWW方向, 分裂时差为0.5~2.5 s, 变化范围较大; 川东褶皱带存在较多空解, 快波偏振方向以近东西向为主, 分裂时差在0.5~1.5 s范围内变化; 在右江盆地北缘和川东褶皱带的过渡区域(约26°N附近)各向异性明显较弱.结合前人的上地幔速度成像结果, 我们认为川东褶皱带与右江盆地过渡带是扬子克拉通的南边界, 在其下方北侧的川东褶皱带巨厚的岩石圈向南转变成较薄的岩石圈, 陡变的岩石圈边界引起了软流圈上涌, 软流圈的垂向运动导致其水平方位各向异性较弱.

     

华北地区地壳上地幔速度各向异性研究

本文介绍面波反演得到的华北地区地壳上地幔速度各向异性分布图像,并与S波分裂的结果作初步的定性比较.不同周期瑞利波群速度的方位各向异性图像呈现显著的横向变化,与华北地区地壳上地幔的构造分块和垂直分层结构有比较密切的联系.在鄂尔多斯和阿拉善等稳定地块中,岩石圈地幔到160 km深度都保持比较一致的显著各向异性;而在发生过岩石圈减薄的华北克拉通东部,大约80~150 km深度范围内基本没有探测到方位各向异性,可能说明岩石圈减薄过程抹去了原有的各向异性印迹而且没有显著的水平构造运动造成新的方位各向异性.地球介质的各向异性具有明显的分层特征,面波的反演结果是如此,而S波分裂测量所表现的离散性,也可能是由各向异性的分层差异和倾斜的各向异性对称轴等因素引起.采用多层的各向异性模型,在多数情况下可以定性地解释面波反演和S波分裂结果之间的差异.进一步的工作要求增大面波的探测深度和改善分辨能力,获取更多的S波分裂测量资料,从而建立定量或半定量的三维各向异性模型.     

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.     

基于地幔动力学模拟推断云南地区剪切波各向异性源的深度

地震各向异性与地幔对流导致的变形存在因果关系,因此地幔对流模拟可被用来预测地震各向异性,并推测剪切波各向异性地幔源的深度.本文建立了基于地震速度结构的地幔对流模型来预测云南地区剪切波分裂的快波方向,它同时受地表板块运动和地幔内部的温度扰动所驱动.通过与观测结果进行对比分析,推测在云南地区西北部和东部区域,剪切波各向异性源主要存在于岩石圈中.在西南部和四川盆地及其西缘,地幔流动可能是剪切波各向异性的主要贡献者,各向异性层分别位于210~330 km和170~330 km深度,导致西南部剪切波各向异性的地幔可能处于大幅度的剪切变形状态,而四川盆地及其西缘主要处于中等强度的剪切变形状态.