Inversion of Q value structure beneath the Tibet Plateau

A total of 11 earthquakes with 15 Rayleigh wave paths, recorded at 11 broadband digital PASSCAL seismometers installed in the Tibet Plateau by the Sino-U.S. joint research group, were used to determine the phase velocity and attenuation coefficient of surface waves in periods of 10–130 s. The average shear wave velocity and quality factor {ie271-1} structures in the crust and upper mantle were obtained in this region. The result shows the average {ie271-2} is low and there exists a high attenuation ({ie271-3}=93–141) layer in the crust. The depth range of the low {ie271-4} value layer (16–42 km) is consistent with the range of low velocity layer (21–51 km) in the crust. Below 63 km in the lower crust, {ie271-5} decreases with depth from 114 to 34 at depth of 180 km. The low shear wave velocity and low value of {ie271-6} at the same depth range in the crust indicate that the rocks in the range is probably melted or partially melted. According to the shear wave velocity structure, the average thickness of the crust is about 71 km and a clear velocity discontiniuty appears at the depth of 51 km. The low-velocity zone (4. 26 km/s) at depth of 96–180 km may be corresponding to the asthenosphere. Contribution No. 96A0047, Institute of Geophysics, SSB, China. This study was supported by the National Natural Science Foundation of China.     

利用远震接收函数方法研究南海西沙群岛下方地壳结构

利用西沙琛航岛流动地震台站和永兴岛固定地震台站的资料,提取了远震P波接收函数,结合正演和反演方法模拟了台站下方的地壳结构.模拟结果显示:西沙群岛地壳顶部存在2 km厚的新生代低速沉积层,横波速度只有2.0~2.2 km/s;上地壳为一速度梯度带,横波速度由2 km处的3.4 km/s逐渐增加到12 km深度时的3.8 ...     

中国中部的地壳及上地幔构造

本文利用中国数字地震台网的资料,对昆仑山—柴达木盆地—秦岭—大别山纬向构造带的地壳及上地幔构造进行了研究。沿这一构造带沉积层平均厚度为8km,剪切波速度β=2.62km/s,壳内平均速度较低,为3.5km/s左右,在下地壳还存在低速层。地壳平均厚度为48km,自东而西相差不很大。上地慢低速层在70—90km深处开始出现。这种构造特征有别于地台和高原,具有块体间的过渡性质,表明这是中国地壳块体中的一个独立构造单元。     

Shear Wave Velocity Structure of the Sinai Peninsula from Rayleigh Wave Analysis

The lithospheric structure of the Sinai Peninsula is shown by means of nine shear velocity profiles for depths ranging from zero to 50 km, determined from the Rayleigh wave analysis. The traces of 30 earthquakes, which occurred from 1992 to 1999 in and around the study area, have been used to obtain Rayleigh wave dispersion. These earthquakes were registered by a broadband station located in Egypt (KEG station). The dispersion curves were obtained for periods between 3 and 40 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were inverted according to generalized inversion theory, to obtain shear wave velocity models for each source-station path, which is the main goal of this study. The shear velocity structure obtained for the Sinai Peninsula is shown through the shear velocity distributions with depth. These results agree well with the geology and other geophysical results, previously obtained from seismic and gravity data. The obtained velocity models suggest the existence of lateral and vertical heterogeneity. The shear velocity increases generally with depth for all paths analyzed in the study area. Nevertheless, in some paths a small low velocity channel in the upper or lower crust occurs. Along these profiles, it is found that the crustal structure of the Sinai Peninsula consists of three principal layers: upper crust with a sedimentary layer and lower crust. The upper crust has a sedimentary cover of 2 km thick with an average S-velocity of 2.53 km/s. This upper crust has a variable thickness ranging from 12 to 18 km, with S-wave velocity ranging from 3.24 to 3.69 km/s. The Moho discontinuity is located at a depth of 30 km, which is reflected by a sharp increase in the S-velocity values that jump from 3.70–4.12 to 4.33–4.61 km/s.     

苏鲁地区剪切波速度结构研究及与地震关系探讨

利用S波纯波形拟合法以及T函数法反演了苏鲁地区壳幔剪切波速度结构,并利用长周期P波T函数反演得到了连云港和莱阳台下方800km深度的速度结构。结果显示:(1)苏鲁地区大部分台站地壳表层及上地壳浅部速度偏高,分别对应高压、超高压物质和古老基底出露地区;(2)沿郯庐断裂带分布的台站均显示明显低速层,并具有北浅南深的特点;(3)连云港和莱阳台超深度反演结果显示两台均在150km深度下出现高速层,反映扬子板块的俯冲深度为100km以下,俯冲板片厚度在100km以上;板片拆离下沉深度甚至达到300km或者更深;(4)地震深度分布与低速层关系密切,沿郯庐断裂和烟台—五莲断裂的中小地震震源深度都比较深,有的甚至达到地壳的底部,反映这两条断裂目前切割深度都比较大,而且地幔物质相对比较活跃。     

Crustal Structure of the Korean Peninsula Using Surface Wave Dispersion and Numerical Modeling

Surface wave dispersion is studied to obtain the 1-D average velocity structure of the crust in the Korean Peninsula by inverting group- and phase-velocities jointly. Group velocities of short-period Rayleigh and Love waves are obtained from cross-correlations of seismic noise. Multiple-filter analysis is used to extract the group velocities at periods between 0.5 and 20 s. Phase velocities of Rayleigh waves in 10- and 50-s periods are obtained by applying the two-station method to teleseismic data. Dispersion curves of all group and phase velocities are jointly inverted for the 1-D average model of the Korean Peninsula. The resultant model from surface wave analysis can be used as an initial model for numerical modeling of observations of North Korean events for a velocity model appropriated to the Korean Peninsula. The iterative process is focused especially on the surface sedimentary layer in the numerical modeling. The final model, modified by numerical modeling from the initial model, indicates that the crust shear wave velocity increases with depth from 2.16 km/s for a 2-km-thick surface sedimentary layer to 3.79 km/s at a Moho depth of 33 km, and the upper mantle has a velocity of 4.70 km/s.     

山西临汾震区地壳上地幔构造的研究

利用郑州－临汾－靖边深地震测深剖面临汾，阳城炮点所获得的太行山至靖边段的观测资料，在以往解释的基础上重新进行了对比解释。研究结果表明：临汾与其东西两侧壳幔结构与构造的差异是极其明显的，其主要特征如下：（１）对仅在临汾以西出现的强震相Ｐ２进行了解释，并在ＰＭ波之前识别出一组来自下地壳的反射波Ｐ５；（２）根据临汾以西Ｐｇ与Ｐ２波的特征，我们确认在临汾盆地下方及其西侧，中地壳的上部８－１２ｋｍ深度内存在     

四川盆地西部及邻区地壳上地幔精细结构研究

为获得四川盆地及其邻区地壳和上地幔精细结构,并据此对该区域整体构造运动初步进行动力学解释,本研究采用接收函数、线性反演与H-kappa扫描相结合的方法,获得以下结论:研究区域地壳厚度东西向差异巨大,其厚度差高达29 km。四川盆地地表具有较厚沉积层,属于刚性块体。龙门山南段与四川盆地交接地区存在某些区域的波速比较高,推测这些地区可能存在地幔物质上涌导致地壳中铁镁质成分增加。松潘-甘孜地块的地壳厚度存在58~65 km的横向变化,中下地壳存在梯形低速层,可能由于青藏高原向东俯冲过程中遭遇四川盆地刚性块体的阻挡而引起该地块的挤压变形,该区域具有较高的波速比,综合分析推测其可能存在区域性的部分熔融。      

长白山火山区壳幔S波速度结构研究

利用面波层析成像和远震接收函数方法对长白山地区的地壳上地幔速度结构进行了研究。结果表明:长白山火山区附近存在岩石圈减薄、上地幔软流圈增厚以及上地幔S波速度降低等与上地幔高温物质有关的现象,它表明长白山的岩浆系统一直延伸到上地幔软流圈范围。天池火山区地壳内部存在明显的S波低速层,在离天池火山口较近的WQD台附近,低速层顶部埋深约8km,厚度近20km,S波最小速度约2.2km/s。在距离天池火山北部50km的EDO台地壳中没有明显的低速层。火山区S波速度结构总体表现出距离天池越近,地壳的V_P/V_S越大,低速层的厚度和幅度增加的特征,表明天池火山口附近地壳内部存在高温物质或岩浆囊。CBS台站不同方位的接收函数及反演结果表明,地表低速层厚度以及莫霍面深度存在随方位的变化。地表低速层在南部方向明显较厚,莫霍面深度在南部天池火山口方向存在小幅度抬升。CBS台站附近特殊的近地表速度结构可能是该台站记录的火山地震波形主频较低的主要因素。天池火山口附近莫霍面的小幅度抬升意味着存在与火山作用有关的壳幔物质交换通道     

利用地震面波频散重建川滇地区壳幔S波速度

利用适配滤波频时分析技术分析覆盖川滇地区的长周期面波记录,计算了周期10~100 s内的面波群速度频散,对研究区进行划分尺度大小1.5°×1.5°分格后,采用射线追踪方法求取各分段射线的长度和时间,得到各个格子的纯路径频散.继而采用阻尼最小二乘法求解,反演得到该研究区壳幔S波速度分布.研究结果表明,川滇地区表现出地壳增厚和缩短,在地壳和上地幔顶部,川滇菱形块体内部与其外部相比,虽然存在局部速度负异常,总体上呈相对高速,其周边的走滑断裂带呈现深至上地幔顶部的负速度异常,这有助于地壳块体沿断裂的侧向挤出;此外,云南西部和四川西部壳内和上地幔高导层的存在被认为是与部分熔融的物质或与滑脱构造相关联;从纬向剖面和经向剖面可以得到四川盆地莫霍面平均深度大约为45 km,云南地区莫霍面深度南北方向不一致,云南地区最北端深度达到49 km,南端莫霍面深度大约为36 km,这说明不同构造块体在构造运动过程中受到影响的程度不同.     

中国西部及邻区岩石圈S波速度结构面波层析成像

本文利用瑞利波群速度频散资料和层析成像方法,研究了中国西部及邻近区域(20°N—55°N,65°E—110°E)的岩石圈S波速度结构.结果表明这一地区存在三个以低速地壳/上地幔为特征的构造活动区域:西蒙古高原—贝加尔地区,青藏高原,印支地区.西蒙古高原岩石圈厚度约为80 km,上地幔低速层向下延伸至300 km深度,说明存在源自地幔深部的热流活动.缅甸弧后的上地幔低速层下至200 km深度,显然与印度板块向东俯冲引起俯冲板片上方的热/化学活动有关.青藏高原地壳厚达70 km,边缘地区厚度也在50 km以上并且具有很大的水平变化梯度,与高原平顶陡边的地形特征一致.中下地壳的平均S波速度明显低于正常大陆地壳,在中地壳20~40 km深度范围广泛存在速度逆转的低速层,这一低速层的展布范围与高原的范围相符.这些特征说明青藏高原中下地壳的变形是在印度板块的北向挤压下发生塑性增厚和侧向流动.地幔的速度结构呈现与地壳显著不同的特点.在高原主体和川滇西部地区上地幔顶部存在较大范围的低速,低速区范围随深度迅速减小;100 km以下滇西低速消失,150 km以下基本完全消失.青藏高原上地幔速度结构沿东西方向表现出显著的分段变化.在大约84°E以西的喀喇昆仑—帕米尔—兴都库什地区,印度板块的北向和亚洲板块的南向俯冲造成上地幔显著高速;84°E—94°E之间上地幔顶部速度较低,在大约150~220 km深度范围存在高速板片,有可能是俯冲的印度岩石圈,其前缘到达昆仑—巴颜喀拉之下;在喜马拉雅东构造结以北区域,存在显著的上地幔高速区,可能阻碍上地幔物质的东向运动.川滇西部岩石圈底界深度与扬子克拉通相似,约为180 km,但上地幔顶部速度较低.这些现象表明青藏高原岩石圈地幔的变形/运动方式可能与地壳有本质的区别.     

Study on S wave velocity structure beneath part stations in Shanxi Province

Introduction Shanxi uplift area is bounded by Ordos block to the west, Taihang Mountain front fault zone to the east. On the north side, it is also adjoined with Yinshan fold belt and Hetao-Baotou fault depression basin accompanied with its southern active ruptures, as well as on the south side, connected to Weihe fault depression basin accompanied with large ruptures on the northern margin of Qinling fold belt. The interior of the Shanxi uplift area, with complicated tectonic structures, …     

南海西沙石岛地震台下的地壳结构研究

为加强海上天然地震观测和岩石圈结构研究，我们在南海西沙群岛建立了石岛流动地震台，进行了为期一年多的观测试验.观测结果表明，虽然岛礁区的地震数据受热带气旋影响期间背景噪声较大，但仍能记录到MW 6级以上的地震.本文对具有清晰P波波形的远震记录进行了接收函数处理和计算模拟，得到台站下简单的地壳结构模型，其莫霍面深度为28 km，上地壳顶部有一层2 km厚的低速层，横波速度只有23 km/s，向下逐渐过渡到横波速度为38 km/s的下地壳.与已有的研究结果比较，发现石岛台与琼中台的地壳结构模型是西沙海槽剖面的自然延伸，西沙地块的地壳结构属减薄型陆壳，可与华南地块的正常陆壳对比.     

青藏高原东部和周边地区地壳速度结构的背景噪声层析成像

利用连续地震背景噪声记录和互相关技术获得瑞利面波格林函数,进而反演获得了青藏高原东部和周边地区的地壳三维速度结构.地震数据源于北京大学宽频带流动观测地震台阵,国家数字测震台网数据备分中心提供的部分固定台站的连续记录及INDEPTH IV宽频带流动观测地震台阵.首先对观测数据进行处理和分析取得所有可能台站对的面波经验格林函数和瑞利波相速度频散曲线,反演得到了观测台阵下方周期从6~60s的瑞利波相速度异常分布图像.并且进一步反演获得研究区域三维剪切波速度结构和莫霍面深度分布.短周期(6~14s)相速度异常分布与地表地质构造特征吻合较好,在青藏高原和四川盆地之间存在一个明显的南北向转换带.而本文最重要的结果是周期大于25s的相速度异常分布图像显示,以昆仑断裂带为界,柴达木盆地和祁连山脉地区呈现与青藏高原截然不同的中地壳速度结构,反而与青藏高原东缘地区和川滇菱形块体速度结构相似.反演获得的剪切波速度在27.5~45km深度的切片也明显地揭示:青藏高原的松潘—甘孜地块和羌塘地块呈现均一的低速层;然而,柴达木盆地和祁连山脉地区则呈现较强的横向不均匀性,尤其是柴达木盆地的高速异常和四川盆地的高速异常相对应.这些结果为前人提出的青藏高原东北向台阶式增长模式提供了重要的地震学观测证据.与全球一维平均速度模型(AK135)相比较发现,本文测量和反演获得的研究区域内平均相速度和剪切波速度都比AK135模型慢很多,尤其是青藏高原的中地壳(25~40km)剪切波速度显著低于全球平均速度模型.进一步的层析成像反演证实松潘—甘孜和羌塘地块中地壳(27.5~45km)呈现大范围均一的低速层,为青藏高原可能存在大规模中下地壳"层流"提供地震学观测证据.在祁连山脉的27.5~45km深度观测到的明显低速异常体可能对应于该造山带下地幔岩浆活动导致的底侵作用,表明引起该地区地壳增厚的主要机制可能是来自地幔岩浆的底侵作用.     

Crustal structure of Northwest Zagros (Kermanshah) and Central Iran (Yazd and Isfahan) using teleseismic Ps converted phases

Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. In this study we used the P receiver function technique to investigate the velocity structure of the crust beneath the Northwest Zagros and Central Iran and map out the lateral variation of the Moho boundary within this area. Our dataset includes teleseismic data (M _b ≥ 5.5, epicentral distance from 30° to 95°) recorded at 12 three-component short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at the Moho boundary. The Moho depths were firstly estimated from the delay time of the Moho converted phase relative to the direct P wave beneath each network. Then, we used the P receiver function inversion to find the properties of the Moho discontinuity such as depth and velocity contrast. Our results obtained from PRF are in good agreement with those obtained from the P receiver function modeling. We found an average Moho depth of about 42 km beneath the Northwest Zagros increasing toward the Sanandaj-Sirjan Metamorphic Zone and reaches 51 km, where two crusts (Zagros and Central Iran) are assumed to be superposed. The Moho depth decreases toward the Urmieh-Dokhtar Cenozoic volcanic belt and reaches 43 km beneath this area. We found a relatively flat Moho beneath the Central Iran where, the average crustal thickness is about 42 km. Our P receiver function modeling revealed a shear wave velocity of 3.6 km/s in the crust of Northwest Zagros and Central Iran increasing to 4.5 km/s beneath the Moho boundary. The average shear wave velocity in the crust of UDMA as SSZ is 3.6 km/s, which reaches to 4.0 km/s while in SSZ increases to 4.3 km/s beneath the Moho.     

青藏高原及邻近地区地壳与上地幔剪切波三维速度结构

本文利用30个基准台所记录的238条长周期面波资料,经过适配滤波和分格频散反演,得到中国大陆及邻区147个分格10-105s的纯路径频散,进而反演出青藏高原及邻近地区深至170km的剪切波三维速度结构.研究表明,青藏高原中西部地区和东部地区的地壳平均厚度分别为70±7km和65±7km,地壳平均剪切波速度分别为3.55和3.62km/s,上地幔顶盖平均速度分别为4.63和4.61km/s; 岩石层厚度均为120±10km;东部地区下地壳内30-40km深度处普遍存在低速层;青藏高原及其东侧的上地幔低速层内有横贯东西且明显向上隆起的低速腔.滇西缅北地区的地壳厚45±5km,上地壳及下地壳内都有低速层;上地幔顶盖的速度为4.42km/s,比青藏高原本体及恒河平原都低.恒河平原地壳厚34±2km,速度平均为3.45km/s;上地幔顶盖厚86±10km,速度平均为4.63km/s,顶盖内55-83km深处有一个低速夹层.     

长白山火山区地壳S波速度结构的背景噪声成像

利用探测深俯冲的中国东北地震台阵NECsaids的60个流动台与固定地震台2010年7月至2014年12月的垂向连续波形数据,采用地震背景噪声成像方法获得了研究区6~40 s周期的瑞雷波相速度分布,并通过相速度频散反演得到了研究区下方0~50 km的三维S波速度结构.结果表明:研究区下方地壳S波速度结构存在明显的横向和纵向不均匀性,浅部速度结构与浅表地质构造单元有较好的对应,深部速度结构较好地反映了区域火山活动及深部热物质作用的结构特征;在长白山火山下方9~30 km深度范围内存在明显低速区并有向下延伸的趋势,推测可能为长白山火山地壳岩浆囊;在龙岗火山下方12~30 km深度范围内发现较弱的低速区,可能代表火山喷发后的残留物,而在镜泊湖火山下方没有明显的低速异常,说明镜泊湖火山地壳内可能不存在部分熔融的岩浆物质.     

Crustal S-wave velocity structure of the Yellowstone region using a seismic ambient noise method

The Yellowstone volcano is one of the largest active volcanoes in the world, and its potential hazards demand detailed seismological and geodetic studies. Previous studies with travel time tomography and receiver functions have revealed a low-velocity layer in the crust beneath the Yellowstone volcano, suggesting the presence of a magma chamber at depth. We use ambient seismic noise from regional seismic stations to retrieve short-period surface waves and then study the shallow shear velocity structure of the Yellowstone region by surface wave dispersion analysis. We first obtained a crustal model of the area outside of the Yellowstone volcano and then constructed an absolute shear wave velocity structure in combination with receiver function results for the crust beneath the Yellowstone volcano. The velocity model shows a low-velocity layer with shear velocity at around 1.3 km/s, suggesting that a large-scale magma chamber exists at shallow levels within the crust of the Yellowstone volcanic region.     

Crustal seismic structure and depth distribution of earthquakes in the Archean Kuusamo region,Fennoscandian Shield

Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average V_p/V_s ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.     

S-wave velocity structure beneath Changbaishan volcano inferred from receiver function

The S wave velocity structure in Changbaishan volcanic region was obtained from teleseismic receiver func-tion modeling. The results show that there exist distinct low velocity layers in crust in volcano area. Beneath WQD station near to the Tianchi caldera the low velocity layer at 8 km depth is 20 km thick with the lowest S-wave velocity about 2.2 km/s. At EDO station located 50 km north of Tianchi caldera,no obvious crustal low velocity layer is detected. In the volcanic re-gion,the thickness of crustal ...