Modeling Seismic Attributes of Pn Waves using the Spectral-Element Method

To investigate the nature of Pn propagation, we have implemented the spectral-element method (SEM) for vertically and laterally varying media with and without attenuation. As a practical measure, essential features of the Pn waves are distilled into seismic attributes including arrival times, amplitudes and pulse frequencies. To validate the SEM simulations, we first compare the SEM results with reflectivity calculations of Braile and Smith (Geophys. J.R. Astr. Soc. 40, 145–176, 1975) and then to the asymptotic results of ?erveny and Ravindra (Theory of Seismic Headwaves, University of Toronto Press, pp. 235–250, 1971). Models with random, laterally varying Moho structures are then simulated, where the amplitude and pulse frequency characteristics are found to be stable to small Moho interface perturbations. SEM calculations for models with different upper-mantle velocity gradients are next performed where it is found that interference effects can strongly influence the Pn amplitudes and pulses frequencies. For larger-scale, laterally varying structures, SEM models similar to that found along the Hi-CLIMB array in Tibet are then performed. It is observed that large-scale structures, along with small-scale structures, upper-mantle velocity gradients and attenuation, can all significantly affect the Pn attributes. Ambiguities between upper-mantle velocity gradients and attenuation are also found when using Pn amplitudes and pulse frequency attributes. These ambiguities may be resolved, to some degree, by using the curvature of the travel times at longer regional distance, however, this would also be complicated by lateral variability.     

青藏高原东缘上地幔顶部Pn波速度结构及各向异性研究

本研究使用中国地震局地壳应力研究所2010—2011年期间在云南地区布设流动地震台站以及青藏高原周边地区固定地震台站记录到的波形资料,提取了大量高质量Pn波到时资料.联合中国地震台网观测报告,我们获得了一个新的青藏高原东缘上地幔顶部Pn波速度和各向异性结构模型.结果显示,研究区内上地幔顶部存在明显横向不均匀性.古老盆地和稳定地台区如四川盆地、柴达木盆地、拉萨地块和阿拉善块体呈现为明显高波速异常,而祁连山至西秦岭褶皱带和川滇菱形块体北部等为相对弱高波速异常.在龙日坝断裂带以东的松潘—甘孜地块往南沿安宁河—则木河断裂至川滇菱形块体南部显示为一条近南北向明显低波速异常.三江褶皱系、缅甸弧俯冲带以及四川盆地东南等地区为明显低波速异常.地壳强震多发生在高波速异常边缘或高低波速异常过渡带上,表明地壳强震的孕育可能还与地幔构造作用存在一定相关性.青藏高原东构造结的各向异性快波方向呈顺时针旋转分布,与印度—欧亚碰撞密切相关.龙门山断裂带东西两侧的各向异性快波方向发生明显变化,由其西侧松潘—甘孜地块下方的NE向转变为四川盆地下方的近EW向,说明青藏高原物质流动遇四川盆地后分为NE和SW向两支.在川滇地区26°N以南地区上地幔顶部各向异性呈现近NS向与地表GPS观测相一致,但与SKS分裂结果存在较大差异,可能表明地壳与上地幔顶部形变表现为耦合现象,而上地幔顶部至岩石圈内部则存在解耦现象.     

Velocity and Density Heterogeneities of the Tien-Shan Lithosphere

—The Tien-Shan orogene is a region in which the earth’s crust undergoes considerable thickening and tangential compression. Under these conditions the lithosphere heterogeneities (composi tion, rheological) create the prerequisites for the development of various phenomena of tectonic layering (lateral shearing, different deformation of layers). To study the distribution of velocity, density and other elastic parameters, the results from a seismic tomography study on P-wave as well as S-wave velocities were used. Using empirical as well as theoretical formulas on the relationship between velocity, density and silica content in rocks, their distribution in the Tien-Shan’s lithosphere has been calculated. In addition, other elastic parameters, such as Young’s modulus, shear modulus, Poisson’s ratio and coefficient of general compressions have been determined. Zoning of different types of crust was carried out for the region investigated. The characteristics of the "crust-mantle" transition have been investi gated. Large blocks with different types of the earth’s crust were distinguished. Layers with inverse values of velocity, density and shear and Young modulus are revealed in the Tien-Shan lithosphere. All of the above described features open new ways to solve geodynamics problems.     

用背景噪声和地震面波反演东北地区岩石圈速度结构

本文利用东北地区的黑龙江、 吉林、 辽宁和内蒙古等四省区区域数字地震台网122个宽频带地震台站记录的波形数据, 分别通过背景噪声互相关及地震面波提取8~25 s和25~70 s的瑞利面波频散曲线, 进而反演得到东北地区从浅到深直至约100 km的岩石圈速度结构。 结果表明, 周期为8 s至15 s的短周期群速度分布与地表构造特征有较好的对应关系, 盆岭边界的大型断裂对上地壳速度结构的控制作用明显, 松辽盆地呈现较厚的低速沉积盆地特征; 周期为20 s至30 s的群速度与短周期时相比出现明显变化, 反映了以大兴安岭—太行山重力梯度带为界, 西部地区莫霍面深度大于东部地区; 周期为50 s至70 s的长周期群速度图表现为随着周期的增加, 东部低速区域西移而西部显示稳定高速, 可能反映了研究区受太平洋板块俯冲影响, 大兴安岭以东地区软流圈热物质上涌的特征。     

Receiver-Function Velocity Structure of the Lithosphere beneath the Khibiny and Lovozero Massifs,Northeastern Baltic Shield

Izvestiya, Physics of the Solid Earth - Abstract—The velocity structure of the lithosphere is studied beneath the Khibiny and Lovozero massifs in the northeastern part of the Baltic shield....     

青藏高原Pn波研究进展及问题

简要总结了青藏高原地区Pn波速度结构、各向异性研究进展；介绍了Pn波速度结构、各向异性等在岩石圈结构、构造背景反映等方面的应用研究进展。分析了目前青藏高原Pn波研究中存在的一些问题。     

2000年云南姚安地震余震区的速度和衰减结构层析成像分析

文中根据2000年云南姚安6.5级地震后,流动数字化地震监测台网记录到的地震序列走时和波形资料,采用震源位置和速度结构联合反演的方法,反演了震源区的V_P、V_S和Q_S的三维结构。 层析成像结果显示,姚安6.5级地震发生在低V_P、高Q_S扰动异常的边缘,大部分余震均发生在高、低V_P、Q_S过渡区域内。 V_P的低值异常表明姚安震区存在一条NWW向断裂带,且异常深度达到10 km左右。 该文通过研究地震序列的分布特征并结合震源区速度结构和衰减结构的特点,进一步探讨了姚安地震的发震构造。     

青藏高原东缘深部速度结构远震层析成像

本文利用FMTOMO层析成像软件以及远震P波走时数据,获得青藏高原东缘地区300km深度范围内的P波速度结构,得到以下结果: ①在60~140km深度范围内,102°E~104°E、29°N~31°N,四川盆地西南部存在一个相对低速区域;②在60~140km深度范围内,沿着龙门山断裂带,在龙门山断裂带的西北侧区域出现相对高速的现象;③在190~290km深度范围内,青藏高原东缘西南区域存在着高速现象,龙门山断裂带、安宁河断裂带不再是高低速区域的分界带。     

The Pn wave velocities and the relief of Moho in the Tibetan Plateau

Eleven PASSCAL broadband digital seismic stations were employed in the Tibetan Plateau by the Sino-US team from September, 1991 to June, 1992. Seven of them were distributed along the Qinghai-Tibet highway, others in Maqin and Yushu of Qinghai Province, Linzhi and Xigatze of Tibet. The data included 31 local earthquakes recorded by these stations from July, 1991 to January, 1992. Considering the characters of digital data, we identified the seismic phases carefully in several methods using SAC softwares (Seismic Analysis Code) in SUN workstation. We compared the seismic phases after converting the seismograms of the single stations to the seismic profiles; analyzed the first arrivals of P waves in the incident planes by rotating 3 component seismic records; identified the seismic phases from the particle motion pictures. The Pn apparent velocities were calculated in the distance range of 230–1200 km from Linzhi earthquakes, western Changtang earthquakes, Xitieshan and Gonghe earthquakes and the earthquakes in India. The results show that the Pn velocities change slightly in the Tibetan Plateau (8.0–8.1 km/s). These values near the velocities at the uppermost mantle of the normal continents. The Moho undulation in the Tibetan Plateau was also studied by using Pn data by the time-term method. The primary results indicate that the Moho beneath the Tibetan Plateau is flat, and its depths are 67–70 km. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, Supp., 593–600, 1992.     

广州地区三维地下速度结构模型建立及优化研究

利用广州地区详尽的深、浅部地震勘探资料,大深度钻孔和测速等资料,运用自主开发的建模程序和多功能商业软件的二次开发功能高速处理海量数据进行建模,采用开发的二元三次B—Spline函数建模方法解决大城市圈建模数据空间分布不均匀的问题并进行模型优化,建立了广州地区符合地质学和地形学理论三维不均匀地下速度结构模型,为基于活断层探测和地震动危险性评价基础上的强地震动预测（模拟）奠定了基础。     

鄂尔多斯块体及其东南缘剪切波速度结构与波速比研究

利用深源远震的纯S波波形拟合方法研究了鄂尔多斯块体及其东、南缘的剪切波速度结构。结果显示，鄂尔多斯块体内部榆林台站下方速度结构相当稳定，没有很明显的高低速层变化，随着深度的增加，剪切波速度稳步升高；在该台下方25km深处有一个厚度达12km的层位(ys=3．90km／s)，该层的波速度相对其东部同等深度处的波速度偏低(以≥4．00km／s)。鄂尔多斯地块东部边缘的地壳厚度都比榆林厚，而且呈现高、低速层相间结构，低速层明显增多，与块体内部比较一致的特征为下地壳是一个厚达10km的速度较低的下地壳(yS=3．8km／s)，但埋深在35km左右。另外反演得到了各个台站的P波速度结构，并对相应的波速比(Vp/Vs)进行了研究，发现位于鄂尔多斯块体内部的榆林台波速比平均值仅为1．68，上地壳波速比处在非常低的状态(1．60左右)，中下地壳一直到上地幔的波速比稳定在1．73左右，反映介质均匀稳定，处于一种刚性状态；而鄂尔多斯块体东、南边缘的台站，都有几个波速比较高(1．80左右)的层位，其中东部边缘苛岚和离石地区的波速比平均值系统超过一般的弹性体波速比平均值1．732，反映了边缘地区地壳活动相对比较活跃，介质层中可能有其他物质的影响。     

Seismic Velocity and Q Structure of the Middle Eastern Crust and Upper Mantle from Surface-wave Dispersion and Attenuation

—Observed velocities and attenuation of fundamental-mode Rayleigh waves in the period range 7–82 sec were inverted for shear-wave velocity and shear-wave Q structure in the Middle East using a two-station method. Additional information on Q structure variation within each region was obtained by studying amplitude spectra of fundamental-mode and higher-mode Rayleigh waves. We obtained models for the Turkish and Iranian Plateaus (Region 1), areas surrounding and including the Black and Caspian Seas (Region 2), and the Arabian Peninsula (Region 3). The effect of continent-ocean boundaries and mixed paths in Region 2 may lead to unrealistic features in the models obtained there. At lower crustal and upper-mantle depths, shear velocities are similar in all three regions. Shear velocities vary significantly in the uppermost 10 km of the crust, being 3.21, 2.85, and 3.39 km/s for Regions 1, 2, and 3, respectively. Q models obtained from an inversion of interstation attenuation data show that crustal shear-wave Q is highest in Region 3 and lowest in Region 1. Q’s for the upper 10 km of the crust are 63, 71, and 201 for Regions 1, 2, and 3, respectively. Crustal Q’s at 30 km depth for the three regions are about 51, 71, and 134. The lower crustal Q values contrast sharply with results from stable continental regions where shear-wave Q may reach one thousand or more. These low values may indicate that fluids reside in faults, cracks, and permeable rock at lower crustal, as well as upper crustal depths due to convergence and intense deformation at all depths in the Middle Eastern crust.     

Age-dependent Large-scale Fabric of the Mantle Lithosphere as Derived from Surface-wave Velocity Anisotropy

—Systematic variations of the seismic radial anisotropy ξ to depths of 200–250 km in North America and Eurasia and their surroundings are related to the age of continental provinces, and typical depth dependences of ξ _R are determined. The relative radial anisotropy ξ _R in the mantle lithosphere of Phanerozoic orogenic belts is characterized by ν _SH > ν _SV , with its maximum depth of about 70 km, on the average, while beneath old shields and platforms, it exhibits a maximum deviation from ACY400 model (Montagner and Anderson, 1989) at depths of about 100 km with ν _SV ≥ν _SH signature. An interpretation of the observed seismic anisotropy by the preferred orientation of olivine crystals results in a model of the mantle lithosphere characterized by anisotropic structures plunging steeply beneath old shields and platforms, compared to less inclined anisotropies beneath Phanerozoic regions. This observation supports the idea derived from petrological and geochemical observations that a mode of continental lithosphere generation may have changed throughout earth's history.     

Velocity and Density Cross Sections of the Upper Part of the Lithosphere within the North Urals Segment

Izvestiya, Physics of the Solid Earth - Abstract—The seismic data acquired by the Bazhenov Geophysical Expedition on the regional DSS profiles are revised to study the structure of the...     

Velocity structure of uppermost mantle beneath North China from Pn tomography and its implications

20301 Pn arrival time data are collected from the seismological bulletins of both national and regional seismic networks. Pn travel time residuals are tomographically inverted for the Pn velocity structure of uppermost mantle beneath North China. The result indicates that the average Pn velocity in North China is 7.92 km/s, and the velocity varies laterally from ?0.21 to +0.29 km/s around the average. The approximately NNE trending high and low velocity regions arrange alternatively west-eastward. From west to east we can see high velocity in the middle Ordos region, the Shanxi graben low, the Jizhong depression high, the west Shandong uplift and Bohai Sea low, and the high velocity region to the east of the Tanlu fault. In the southern boundary zone of the North China block, except for the high velocity in the Qingling Mountains region, the velocity is generally lower than the average. Obvious velocity anisotropy is seen in the Datong Cenozoic volcanic region, with the fast velocity direction in NNE-SSW. Notable velocity anisotropy is also seen around the Bay of Bohai Sea, and the fast velocity directions seem to show a rotation pattern, possibly indicating a flow-like deformation in the uppermost mantle there. The Pn velocity variations show a reversed correlation with the Earth's heat flow. The low Pn velocity regions generally show high heat flow, e.g., the Shanxi graben and Bohai Sea region. While the high Pn velocity regions usually manifest low heat flow, e.g., the region of Jizhong depression. This indicates that the Pn velocity variation in the study region is mainly aroused by the regional temperature difference in the uppermost mantle. Strong earthquakes in the crust tend to occur in the region with the abnormal low Pn velocity, or in the transition zone between high and low Pn velocity regions. The earthquakes in the low velocity region are shallower, while that in the transition zone are deeper.     

基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义

青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明：①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km,导致地幔过渡带增厚。     

Velocity structure of uppermost mantle beneath North China from Pn tomography and its implications

20301 Pn arrival time data are collected from the seismological bulletins of both national and regional seismic networks. Pn travel time residuals are tomographically inverted for the Pn velocity structure of uppermost mantle beneath North China. The result indicates that the average Pn velocity in North China is 7.92 km/s, and the velocity varies laterally from ?0.21 to +0.29 km/s around the average. The approximately NNE trending high and low velocity regions arrange alternatively west-eastward. From west to east we can see high velocity in the middle Ordos region, the Shanxi graben low, the Jizhong depression high, the west Shandong uplift and Bohai Sea low, and the high velocity region to the east of the Tanlu fault. In the southern boundary zone of the North China block, except for the high velocity in the Qingling Mountains region, the velocity is generally lower than the average. Obvious velocity anisotropy is seen in the Datong Cenozoic volcanic region, with the fast velocity direction in NNE-SSW. Notable velocity anisotropy is also seen around the Bay of Bohai Sea, and the fast velocity directions seem to show a rotation pattern, possibly indicating a flow-like deformation in the uppermost mantle there. The Pn velocity variations show a reversed correlation with the Earth”s heat flow. The low Pn velocity regions generally show high heat flow, e.g., the Shanxi graben and Bohai Sea region. While the high Pn velocity regions usually manifest low heat flow, e.g., the region of Jizhong depression. This indicates that the Pn velocity variation in the study region is mainly aroused by the regional temperature difference in the uppermost mantle. Strong earthquakes in the crust tend to occur in the region with the abnormal low Pn velocity, or in the transition zone between high and low Pn velocity regions. The earthquakes in the low velocity region are shallower, while that in the transition zone are deeper.     

由微振动记录用剥层法研究浅层S波速度结构

根据江苏华电句容发电一期2×1000 MW发电机组场地情况,布设了三角形嵌套和同心圆台阵进行微振动观测,采用空间自相关法计算频散曲线,并用剥层法反演得到该工程场地浅部的S波速度结构。反演结果与钻孔探测结果吻合较好,表明用微振动台阵记录反演浅部地壳S波速度结构在工程上是可行的。该方法具有不需要人工源,对避开噪声要求不苛刻、成本低且精度可满足工程要求等优点,故有良好的应用前景。