华北东部上地幔破裂带

利用天然地震的走时和S波震相的层析成像方法可以提供上地幔S波速度扰动精细图像。我们对华北东部(32°～44°N,114°～126°E)进行了高分辨率S波地震层析成像研究,揭示了苏鲁及环渤海地区上地幔速度结构的差异,为研究该区域深部构造演化提供新的地球物理证据和制约。本次成像取得的最醒目的成果是揭示了由波速剧烈变化指示的两条上地幔破碎带,它们分别出现在渤海湾裂谷区与苏鲁超高压变质带下方。苏鲁超高压变质带下方上地幔破碎带包含多个有序排列的高速团块,推测与扬子走滑并向前俯冲、以及前锋超高压变质块体拆沉及折返作用有关。地幔地震S波速的成像结果支持华北东部渤海湾地区发育地幔热羽柱体系的观点,并对地幔热羽柱的鉴识提供了新的标识。     

塔里木盆地地震波速扰动及泊松比成像

大型克拉通盆地地壳上地幔组构有什么特征？其内部结构有何变化？这是目前备受关注的科学问题之一。在新世纪体波地震层析成像的研究进入区域地壳上地幔探测的视野,这个问题可通过体波地震层析成像来研究。文中结合塔里木盆地内的宽频地震记录以及新疆地震台网、中国数字地震台网资料对塔里木地区进行了同台同源的P波和S波地震层析成像, 重点研究区域分辨率横向达到0.5°×0.5°,纵向达到10 km间隔。通过体波到时地震层析成像反演,得到了研究区域精细地壳P波和S波速度结构。根据P波和S波速度扰动和泊松比三维图像,分析了塔里木盆地内部岩石圈波速和泊松比内部结构的信息,通过上中地壳的地震层析成像,地壳波速与泊松比结构反映了满加尔的基底、流体活动性和地温结构利于油气的成熟和保存。一般情况下莫霍面上下方波速及泊松比异常分布模式有明显区别,但塔里木盆地中央顺托果勒呈现穿透莫霍面的异常模式,解释为二叠纪岩石圈火山作用留下的“指纹”。这些信息对油气远景区定位有一定意义。     

太行山构造带及其以东地区上地幔地震层析成像

基于首都圈地区及河北邯郸台网共115个台站记录的地方震、近震和远震P波和S波走时,利用地震层析成像技术得到了太行山构造带及其以东地区下方300km深度范围内的P、S波速度结构。结果发现沿太行山构造带速度结构在上地幔中存在明显的横向不均匀性,其南、中、北段显示了各自不同的构造特征。太行山以东盆地区岩石圈厚度较薄,在约80km深度进入地幔软流层,但在160km深度下,P波和S波速度结构呈现较大差异,其中P波在华北东部地区逐渐以高速为主,而S波速度剖面上虽然低速体被切割,但仍然保持了大部分地区的相对低速。深部结构揭示,太行山中段受华北地区岩石圈减薄过程作用最为强烈,其速度结构与盆地区更为相似。而南段构造作用与浅部断裂关系明显,深部可能更多地保留了构造造山带岩石圈厚度大,高速介质多的特征。太行山北段处于多构造交界地区,速度结构比较复杂,部分S波低速区可能与深部地幔物质上涌作用有关。     

岩石脆性临界破坏的波速特征分析

根据岩石加载破坏过程中应力-应变和波速-应变曲线所反映的的波速变化特征,研究了岩石声波传播速度与其加载变形过程的相关性。研究表明:岩石破裂前波速及特征参数随着变形破坏阶段变化会有显著改变,特别是应力水平到达70%⁸0%之后,当加载破坏过程中岩石的裂隙大量增加后波速参数出现突变。这是由于在膨胀点附近岩石中微裂隙迅速丛集式增长,改变了岩石内部的微观结构,导致岩石波速降低。伴随着波速降低,波速走时急剧增大,离散度增大;S波和P波的振幅减小,但其振幅比急剧增大;S波、P的波Q值也会出现突变。通过对岩石加载过程中的波速变化与应力-应变曲线之间的关联性分析,利用波速信息变化可以实现对岩石脆性破裂前兆的识别。     

华南地区典型岩石的地震波性质研究

地震资料的解释具有多解性,为根据地震资料认识华南的地壳结构和物质组成,对江西一福建地区的典型岩石样品进行了岩石波速的实验测量和理论计算.使用美国MTS公司815岩石物理参数测试系统测量了5块典型岩石的物性参数.结果表明,岩石的P波和S波速度随着压力的增加而增加,随着温度的增加而降低;在相同温压下,大理岩化灰岩的波速＞花...     

三相非饱和土参数对波的传播的影响研究

基于已建立的非饱和土中的波动方程,导出4种体波的波速以及衰减的解析表达式。数值分析了4种体波的波速、衰减与孔隙率以及Lame常数 、 之间的关系。结果表明,随孔隙率的增大,P1波和S波的波速有所提高,P2波和P3波的波速随孔隙率的增大而有所减小;3种P波的衰减有所增大,其中以P3波变化最为显著。P1波、P3波和S波的波速基本上不随Lame常数 变化,P2波的波速随Lame常数 的增大而稍有增大。P2及S波的衰减基本不随Lame常数 变化,P1、P3波的衰减随Lame常数 的增大而有所减小。3种P波的波速基本上不随Lame常数 变化,S波的波速随Lame常数 的增大而有所增大。P2及S波的衰减基本不随Lame常数 变化,P1、P3波的衰减随Lame常数 的增大而有所减小,其中P3波的变化最为显著。     

地幔转换带条件下岩石矿物波速测量方法:超声波与多面砧技术的结合

地幔矿物的波速测量研究是认识地球深部物质组成和性质的重要方法.国际上在大压机中利用超声波技术对地幔矿物材料开展了广泛的波速测量研究,实验温压范围达到地幔转换带条件,而国内大压机超声波波速测量局限于6 GPa压力以内.在中国地质大学(武汉)地球深部研究实验室1 000 t Walker型多面砧大压机上,利用超声波技术,建立了一套高压波速测量系统,对地幔转换带矿物Mg₂SiO₄瓦兹利石多晶样品在18 GPa压力范围内的弹性波速进行了测量,测量结果与前人超声波波速测量结果相比总体吻合程度良好.利用多面砧大压机和超声波技术,在国内首次实现了地幔转换带高压条件下的波速测量,缩短了我国高压波速测量水平与国外先进水平的差距,同时可以为中国及周边地区地球物理观测资料的解析提供矿物物理方面的实验约束,为国内岩石矿物和固体材料的弹性研究提供实验技术支持.      

场地波速测量资料处理的小波变换方法

在场地波速测量中,由于噪声等因素的影响,很难准确识别P波、S波的初至时刻,致使波速结果存在很大的误差。本文给出了一种基于小波变换的波速测量的新方法。该方法利用波动信号的小波变换与弹性波群速度的关系准确识别弹性波初至时刻。弹性波小波变换的峰值时刻代表着以群速度传播的弹性波的初至时刻,使P波、S波的初至时刻的确定具有明确的物理意义,波速的结果准确、可靠、稳定。此外,利用波动信号的小波多尺度分析,还可以确定地层中传播的弹性波的频散特性。最后,该方法在场地波速测量的实测信号的应用表明,该方法可准确确定P波和S波速度。     

基于背景噪声面波的吉林省东南部S波速度结构成像

为研究吉林省东南部地区的S波速度结构,笔者使用吉林、黑龙江和辽宁测震台网共计39个固定台站的重采样连续波形数据,利用台站对的背景噪声互相关计算瑞利面波的经验格林函数,提取群速度和相速度频散曲线,用面波成像方法获得研究区深度为0～30 km的S波速度结构图像。成像结果表明:吉林省东南部地区地壳深度内的S波速度结构具有较强的横向不均匀性特征。研究区上地壳的速度异常与沉积盆地、断裂带和造山带等地质构造有较好的对应,中下地壳的速度异常表现了火山活动和岩浆作用的特征。     

华南地壳及上地幔三维速度结构成像

利用国家地震科学数据共享中心的地震目录及临时台网资料,挑选出11 113个区域地震的77 093条P波走时和93 541条S波走时,采用1°×1°的经纬度网格划分,反演获得了深至60km的华南南部地区的地壳及上地幔三维P波和S波的速度结构。研究结果表明,纵波速度结构与横波速度结构从整体来看具有较好的一致性,说明该研究获得的深部速度结果具有较高的可信性,但是在50km的深度纵、横波速度结构的一致性较差,可能是由于该深度的纵横波走时数据存在着较大的差异所导致的。本研究显示了研究区域内的速度结构存在着明显的横向不均匀性,东南沿海地区的地壳中出现了大规模的低速异常,可能与该区地幔物质的上涌有关;而在珠江三角洲、雷州半岛、北部湾及海南岛等地区莫霍面下方出现的低速异常,则与该区的热运动有关。经分析认为,华南南部地壳及上地幔的速度不均匀性和华南板块与扬子地块的相互作用有关,因此开展进一步研究能为探索和分析华南再造以及中国南海北部的构造演化提供重要信息。     

Structure of the crust and uppermost mantle of the Yanshan Belt and adjacent regions at the northeastern boundary of the North China Craton from Rayleigh Wave Dispersion Analysis

We have used Rayleigh wave dispersion analysis and inversion to produce a high resolution S-wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC). Using waveform data from 45 broadband NCISP stations, Rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the S-wave velocity structure from 15 km down to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are low compared with the global average. The Moho was constrained in the depth range of 30–40 km, indicating a typical crustal thickness along the profile. However, a thin lithosphere of no more than 100 km was imaged under a large part of the profile, decreasing to only ~ 60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60 km depth. The overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during Mesozoic–Cenozoic time. Distinctly high velocities, up to ~ 4.6 km/s, were found immediately to the south of the IMA beneath the northern Yanshan Belt (YSB), extending down to > 100-km depth. The anomalous velocities are interpreted as the cratonic lithospheric lid of the region, which may have not been affected by the Mesozoic–Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our S-wave velocity structural image and other geophysical observations, we propose a possible lithosphere–asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic–Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA zone. The relatively thick lithospheric lid in the northern YSB may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Local modification of the lithosphere beneath the central and western North China Craton: 3-D constraints from Rayleigh wave tomography

We have imaged the lithospheric structure beneath the central and western North China Craton (NCC) with Rayleigh wave tomography. The Rayleigh waveforms of 100 teleseismic events recorded by 208 broadband stations are used to yield high-resolution phase velocity maps at 13 periods from 20 s to 143 s. A 3-D S-wave velocity model is constructed based on the phase velocity maps. Our S-wave velocity model is broadly consistent with the results of previous tomography studies, but shows more detailed variations within the lithosphere. The Trans-North China Orogen (TNCO) is generally characterized by low-velocity anomalies but exhibits great heterogeneities. Two major low-velocity zones (LVZs) are observed in the north and south, respectively. The northern LVZ laterally coincides with sites of Cenozoic magmatism and extends to depths greater than 200 km. We propose that a small-scale mantle upwelling is present, confined to the north of the TNCO. A high-velocity patch in the uppermost mantle is also observed between the two LVZs adjacent to the narrow transtensional zone of the Cenozoic Shanxi–Shaanxi Rift (SSR). We interpret this as the remnant of a cratonic mantle root. The Ordos Block in the western NCC is associated with high-velocity anomalies, similarly reflecting the existence of cratonic mantle root, but a discernible low-velocity layer is observed at depths of 100–150 km in this location. We interpret that this mid-lithospheric structure was probably formed by metasomatic processes during the early formation of the NCC. Based on the observations from our S-wave velocity model, we conclude that the current highly heterogeneous lithospheric structure beneath the TNCO is the result of multiphase reworking of pre-existing mechanically weak zones since the amalgamation of the craton. The latest Cenozoic lithospheric reworking is dominated by the far-field effects of both Pacific plate subduction and the India–Eurasia collision.     

Lower-crust S-wave velocity beneath western Yunnan Province from waveform inversion of dense seismic observations

We use teleseismic body waveforms to explore S-wave layered velocity structures beneath 30 portable digital seismic stations deployed around western Yunnan Province. Results show that the Moho depth in this region is ∼40 km and decreases in general from north to south, consistent with previous geophysical studies. Associated with this lateral variation of the Moho depth, the lower crust above the Moho discontinuity has a 15–25 km thick zone with an S-wave velocity lower than that of the upper crust. This lower velocity zone might be interpreted as a lower crust weak channel, which may mechanically partially decouple the upper-crust deformation from the underlying mantle. Thus, the inverted S-wave velocity structure could provide new evidence for the lateral flow of lower crust in the build-up of the south-eastern Tibetan plateau.     

1976年唐山大地震的孕震环境:密集地震台阵观测得到的结果

1976-07-28唐山地区发生了震惊中外的7.8级大地震。为什么在华北古老克拉通内部的唐山地区能够发生如此的大地震一直是一个令人费解的问题。是否会在唐山地区再次出现同样的破坏性地震值得认真研究。利用流动地震观测台阵数据和接收函数反演方法,我们研究了唐山地区60 km深度范围内的三维地壳上地幔速度结构。结果表明:(1)由活动断裂切割的唐山断块与周围介质存在明显差异,围限唐山断块的断裂均为超壳的活动断裂;(2)唐山大震区中上地壳具有明显的非均匀壳内低速体;(3)该地区壳幔界面表现为明显的断块式隆升,与两侧相比,唐山菱形地块下方的上地幔顶部异常隆起的高度达到10 km左右,下伏的上地幔具有异常的非均匀结构;(4)唐山大震区可能有幔源物质较大规模的侵入,形成了中、上地壳内的低速体。由于较已往的研究结果有更高的空间分辨率,我们得到了一些以往尚未发现的有关唐山地区深部结构的异常特征;(5)首都圈地区内破坏性地震发生的地点绝非偶然,它们均与其相应的深部构造背景密切有关,这为强震发生地点的预测提供了可能。根据本文结果,我们认为,1976年唐山大地震的主因源于上地幔的垂向运动变形及壳幔之间物质及能量的交换,区域水平向应力场为次要作用。这有助于解释为什么能够在我国华北古老克拉通地区发生7级以上强震,在唐山地区再次发生7级以上大地震的可能性值得给予进一步的研究和关注。     

Crustal and upper mantle S-wave velocity structure beneath the Bransfield Strait (West Antarctica) from regional surface wave tomography

Regional surface wave tomography in the sub-Antarctic Scotia Sea is helpful in revealing the nature of the crust and the S-wave seismic velocity profile beneath the Bransfield Strait. The joint use of our regional network, global seismographic network stations and local temporary arrays provide better lateral resolution than that obtained in our previous studies concerning the Scotia Sea region.Tomographic analysis of data obtained using 10 broad band seismic stations and more than 300 regional events, shows that the Bransfield Basin is characterised by a strong group velocity reduction of 8% with respect to the surrounding areas, in the period range from 15 s to 50 s.The crustal and upper mantle models of the eastern, central and western Bransfield Basin are obtained by joint inversion of Rayleigh and Love local dispersion curves from 15 s to 50 s. In addition our data set is expanded to a broader period interval (1–80 s), in central Bransfield Strait in order to better constrain the upper mantle and shallow crust.The main results can be summarized as follows: (a) the crust thins distinctly from W toward E; the variation is consistent with the type of volcanism, earthquake distribution and bathymetric observations, (b) low upper mantle velocities (soft lid) extend down to depths exceeding 70 km as a consequence of elevated temperatures, (c) the crust beneath the central Bransfield Basin displays continental characteristics with a gradually increasing S-wave velocity distribution versus depth analogous to the East African Rift structure of Kenya, (d) negative velocity gradients are present in the lower crust beneath the eastern Bransfield Basin; these could be interpreted as magmatic bodies originating from decompression melting of the mantle.     

Crustal structure in Tengchong Volcano-Geothermal Area, western Yunnan, China

Based upon the deep seismic sounding profiles carried out in the Tengchong Volcano-Geothermal Area (TVGA), western Yunnan Province of China, a 2-D crustal P velocity structure is obtained by use of finite-difference inversion and forward travel-time fitting method. The crustal model shows that a low-velocity anomaly zone exists in the upper crust, which is related to geothermal activity. Two faults, the Longling–Ruili Fault and Tengchong Fault, on the profile extend from surface to the lower crust and the Tengchong Fault likely penetrates the Moho. Moreover, based on teleseismic receiver functions on a temporary seismic network, S-wave velocity structures beneath the geothermal field show low S-wave velocity in the upper crust. From results of geophysical survey, the crust of TVGA is characterized by low P-wave and S-wave velocities, low resistivity, high heat-flow value and low Q. The upper mantle P-wave velocity is also low. This suggests presence of magma in the crust derived from the upper mantle. The low-velocity anomaly in upper crust may be related to the magma differentiation. The Tengchong volcanic area is located on the northeast edge of the Indian–Eurasian plate collision zone, away from the eastern boundary of the Indian plate by about 450 km. Based on the results of this paper and related studies, the Tengchong volcanoes can be classified as plate boundary volcanoes.     

Upper mantle structure of the South American continent and neighboring oceans from surface wave tomography

We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP).     

Tomographic evidence for the subducting oceanic crust and forearc mantle serpentinization under Kyushu, Japan

We determined high-resolution three-dimensional P- and S-wave velocity (Vp, Vs) structures beneath Kyushu in Southwest Japan using 177,500 P and 174,025 S wave arrival times from 8515 local earthquakes. A Poisson's ratio structure was derived from the obtained Vp and Vs values. Our results show that significant low-Vp, low-Vs and high Poisson's ratio zones are extensively distributed along the volcanic front in the uppermost mantle, which extend and dip toward the back-arc side in the mantle wedge. In the crust, low-Vp, low-Vs and high Poisson's ratio anomalies exist beneath the active volcanoes. The subducting Philippine Sea slab is clearly imaged as a high-Vp, high-Vs and low Poisson's ratio zone from the Nankai Trough to the back-arc. A thin low-velocity zone is detected above the subducting Philippine Sea slab in the mantle wedge, and earthquakes in the upper mantle are distributed along the transition zone between this thin low-velocity zone and the high-velocity Philippine Sea slab, which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration. The seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies. The serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge.     

Upper mantle structure of the Saharan Metacraton

The ∼500,000 km² Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean–Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian–Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175–250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies.