The P and S wave velocity structures of the crust and upper mantle beneath Tibetan Plateau

Using the P-and S-wave arrivals from the 150 earthquakes distributed in Tibetan Plateau and its neighboring areas, recorded by Tibetan seismic network, Sichuan seismic network, WWSSN and the mobile network situated in Tibetan Plateau, we have obtained the average P-and S-wave velocity models of the crust and upper mantle for this region:

The crust and upper mantle structure beneath southeastern China

We analyzed teleseismic waveforms recorded by 44 stations in the Fujian and Taiwan provinces of China and obtained 5344 high quality receiver functions. The crustal thickness (H) and average crustal V_P/V_S ratio (k) beneath every station were estimated using the H–k stacking method. Crustal thicknesses near the Fujian Province range from 28.3 to 32.8 km with an average of 31.1 km, and the corresponding V_P/V_S ratios vary from 1.70 to 1.84 with a mean of 1.76. From inland to offshore of the Fujian Province, the crustal thicknesses decrease and Poisson's ratios increase. These may indicate decreasing SiO₂ and increasing calc-alkaline contents in the crust. The discontinuity structures such as the Moho, subducting slab, the 410- and 660-km discontinuities (hereafter we call them the 410 and the 660) are also studied using common converted point (CCP) stacking of receiver functions. Along two NW–SE lines of central and northern Taiwan, the CCP stacking results show a western dipping structure at depths above 50 km, suggesting that the Philippine Sea plate is probably subducting beneath the Eurasian continent plate near the central and northern Taiwan. The CCP stacking results show sharp and flat 410- and 660-km discontinuities, and the transition zone thickness (TZT) is the same as that of ambient mantle beneath Fujian and Taiwan Strait, but thickens in the east of Taiwan. These results suggest that (1) the subducting Eurasian continent plate is confined to the depths above 410 km beneath Fujian and Taiwan Strait; and (2) the South China Sea slab may reach the transition zone beneath the east of Taiwan.     

Travel-time residuals and three-dimensional velocity structure of Italy

Jeffreys-Bullen P and PKP travel-time residuals observed at more than 50 seismic stations distributed along Italy and surrounding areas in the time interval 1962–1979, indicate the complex velocity pattern of this region. Strong lateral velocity inhomogeneities and low velocity zones are required to explain the observed pattern of residuals. In particular, late arrivals of about 1 sec are observed in the Apenninic mountain range, requiring both greater crustal thickness and low velocity layers, coherent with seismic refraction data and surface wave dispersion measurements. The seismic stations located in the Western and Eastern Alps indicate the presence of high velocities. In the Western Alps the strong azimuthal variation of residuals and the high values of early arrivals have a close relationship to the Ivrea body, an intrusive crustal complex characterized by a velocity as high as 7–7.2 km/sec.A travel-time inversion performed with theAki et al. (1977) block model, confirms the peculiar characteristics and the sharp variations in the lithosphere of the whole Italian region, with values of velocity perturbations between many adjacent blocks, ranging in size from 50 to 100 km, and independent from the earth parametrization chosen, reaching values up to 10% in the lithospheric part and 5% in the asthenosphere. 3-D inversion requires also high velocity along the Tyrrhenian coastal margin, equivalent to an uprise of major crustal and lithospheric discontinuities along this part of the Italian peninsula. Moreover low velocity material must be present in the northern part of the Adriatic foreland, in the lithosphere-asthenosphere system, closely related to the stress and seismicity pattern, and the lateral bending of the lithosphere in the same region.     

中国大陆东南缘地震接收函数与地壳和上地幔结构

从2008—2011年,分别在中国大陆东南缘沿海和内陆两条NE向剖面上进行了宽频地震观测,利用记录到的远震波形资料提取得到1446个远震P波接收函数,用H-κ叠加扫描和CCP偏移叠加方法研究了中国大陆东南缘地壳及上地幔过渡带的结构及其变化特征.结合固定台网25个台站的H-κ结果,获得中国大陆东南缘(福建地区)地壳厚度从内陆到沿海逐渐减薄的图像:地壳从闽西北山区的33 km减薄到厦门沿海一带的29 km以下,平均地壳厚度为31.3 km,具有陆地向洋壳过渡的特征;地壳泊松比从内陆到沿海显示出分带特征,闽中西部内陆地区小于0.26,沿海地带高于0.26,且在断裂带的交汇区域表现为相对异常高值.地壳上地幔顶部(0~200 km)的CCP偏移叠加成像结果显示闽江断裂等NW向断裂深切Moho界面,在断裂两侧Moho面急剧抬升或下沉,产状改变,这些特征向内陆地区逐渐变得不明显.闽江等NW向断裂对研究区地壳厚度、地震等有明显控制作用.上地幔尺度(300~700 km)的CCP偏移叠加成像,未见410 km和660 km速度间断面突变和起伏异常,其绝对深度略大于IASP91模型的,上地幔转换带厚度正常(250±5 km),表明中国大陆东南缘上地幔转换带未受欧亚与菲律宾板块碰撞的明显影响,推断中国大陆东南缘及台湾海峡下方不存在俯冲板块,或俯冲前缘未扰动到410 km的深度.     

The crust and upper mantle discontinuity structure beneath Alaska inferred from receiver functions

In this study, three receiver function stacking methods are used to study the detailed crust and upper mantle structure beneath south-central Alaska. We used teleseismic waveform data recorded by 36 stations in the Broadband Experiment Across the Alaska Range (BEAAR) and 4 permanent stations in Alaska. H − κ stacking method using P-to-S converted wave and its multiply reflected waves between the Earth's surface and the Moho discontinuity is adopted to estimate the crustal thickness (H) and average crustal V_P/V_S ratio (κ) in this region. The receiver function results for 24 stations show that the crustal thickness under Alaska ranges from 26.0 to 42.6 km with an average value of 33.8 km, and the V_P/V_S ratio varies from 1.66 to 1.94 with an average value of 1.81 which corresponds to an average Poisson's ratio of 0.277 with a range from 0.216 to 0.320. High Poisson's ratios under some stations are possibly caused by partial melting in the crust and the uppermost mantle. Common converted point (CCP) stacking results of receiver functions along three lines show clear Moho and slab images under this subduction zone. The depths of the slab from our CCP stacking images are consistent with those estimated from the Wadati–Benioff Zone (WBZ). In the area between two stations DH2 (147.8°W, 63.3°N) and DH3 (147.1°W, 63.0°N), a Moho depth offset of about 10 km is found by both the H − κ and CCP stacking techniques. Common depth point (CDP) stacking of receiver functions shows not only the 410-, 520- and 660-km discontinuities, but also significant variations (−30 to 15 km) in the transition zone thickness under the southwest and southeast parts of the study region. The transition zone becomes thinner by 20–30 km, indicating that the temperature there is 150–200 K higher than that of the normal mantle.     

华北地区地壳上地幔S波三维速度结构

利用华北地区大型流动地震台阵的记录资料,采用近震和远震联合成像方法,得到了水平分辨率0.5°×0.5°、深至600 km的S波速度结构.研究结果表明,上地壳S波速度结构与地表地质构造基本一致,燕山—太行山山脉均呈现高速异常,延庆—怀来盆地、大同盆地表现为低速异常,华北盆地内部的拗陷和隆起分别呈现低速和高速.唐山地区中地壳、山西裂陷盆地中下地壳存在明显的低速异常,可能分别与流体和热物质作用有关,有利于形成孕育强震的地质构造环境.90 km的速度结构图像依然与地表的构造特征有较大的相关性,可能说明深部结构对地表构造有一定的控制作用.燕山隆起区岩石圈的厚度可达120～150 km左右,华北盆地的岩石圈厚度可能在80 km左右,太行山地区的岩石圈厚度介于两者之间.山西裂陷盆地上地幔低速层较厚,反映了该区不稳定的构造环境造成了地幔热物质的上涌.华北盆地下方220～320km出现的高速异常体,可能揭示了华北盆地上地幔仍然存在拆沉后残留的难熔、高密度的古老岩石圈地幔.研究区东部地幔转换带呈低速异常,推测可能与太平洋板块俯冲至该区下方地幔转换带前缘120°E左右的俯冲板块相变脱水有关.     

唐海-商都地震台阵剖面下方的地壳上地幔S波速度结构研究

2006年底,我们沿“张渤地震带”布设了一条从唐海—北京—商都的宽频带地震台阵剖面.本文利用台阵记录的远震波形资料,通过接收函数和面波联合反演对剖面下方100 km深度范围内地壳上地幔S波速度结构进行了研究.结果表明剖面东段莫霍面深度约30～34 km,西段深度约38～42 km,平原与山区的过渡地带地壳厚度变化较快.地壳内部10～20 km深度范围内存在多个低速体.在唐山7.8级地震震区附近Moho面出现小幅度隆起,中地壳存在明显的S波低速体.张家口以西,剖面下方10～20 km范围内存在两个S波低速体,张北6.2级地震发生在这两个低速体之间狭小的高速区. 在观测剖面附近,历史上发生的4个大震都与壳内低速体的分布有关. 张家口以东,上地幔普遍存在低速层,顶部埋深在60~80 km之间,并表现出明显的东部浅西部深的特点.     

3-D velocity structure beneath the crust and upper mantle of Aegean Sea region

The region of the Aegean Sea and the surrounding areas in the Eastern Mediterranean lies on the boundary zone between the Eurasian and the African plates. It is a zone of widespread extensive deformation and, therefore, reveals a high level of seismicity.Three-dimensional velocity structure, beneath the crust and upper mantle of the region between 33.0°N–43.0°N and 18.0°E–30.6°E, is determined.The data used are arrival times ofP-waves from 166 earthquakes, recorded at 62 seismological stations. In total, 3973 residual data are inverted.The resultant structure reveals a remarkable contrast of velocity. In the top crustal layer, low velocities are dominant in Western Turkey and on the Greek mainland, while a high velocity zone is dominant in the Ionian Sea and in the southern Aegean Sea.In the upper mantle, high velocity zones dominate along the Hellenic arc, corresponding to the subducting African plate and in the northern part of the region, corresponding to the subducting African plate and in the northern part of the region, corresponding to the margin of Eurasian plate.A low velocity zone is dominant in the Aegean Sea region, where large-scale extension and volcanic activity are predominant, associated with the subduction of the African plate.     

华北地区地壳上地幔三维P波速度结构

利用华北地震科学台阵和首都圈地震台网记录的4511次近震和625次远震的P 波到时数据,采用纬度和经度方向分别为0.5deg;times;0.5deg;的网格划分,反演得到了华北北部地区（111deg;E——120deg;E,37deg;N——42deg;N）深至400km 的地壳上地幔三维P 波速度结构.层析成像结果表明,研究区的速度存在明显的横向不均匀性,随着深度增加横向不均匀性总体呈现减弱趋势.燕山隆起带在60——120km 深度内存在明显的高速异常,这与较大的岩石圈厚度有关;山西裂陷盆地、华北平原下方60km 深度存在明显低速异常,与软流圈的出现有关.燕山隆起带岩石圈厚度在120km 以上,明显比太行山隆起的岩石圈厚度大,与稳定大陆地区的岩石圈厚度一致.太行山山前断裂已切穿莫霍面,贯入岩石圈.研究区上地幔顶部大范围的低速异常反映了软流圈上隆的特点.在华北平原及燕山隆起下方200——300km 存在高速异常可能与太古代大陆板块岩石圈的残留体有关.     

汶川地震区地壳上地幔三维S波速度结构初步研究

2008年5月12日汶川MW7.9地震发生在龙门山断裂带。龙门山断裂带及其邻域的地壳上地幔三维速度结构的研究对于理解汶川大地震的动力学背景具有重要的意义。2006年10月至2009年10月,在国家重大基础研究项目(973)的支持下,中国地震局地质研究所地震动力学国家重点实验室在川西地区(26°~32°N,100°~105°E)布设了由297台宽频带数字地震仪组成的流动观测台阵(简称川西台阵)。根据川西台阵记录的环境噪声和远震波形数据,利用噪声成像技术和接收函数方法,我们研究了川西地区(29°~32°N,100°~105°E)地壳上地幔100km深度范围内的三维S波速度结构。本文得到的结果为研究川西高原和四川盆地的地壳结构提供了新的高分辨率观测证据。我们的结果表明:1)观测台阵覆盖的川滇地块、松潘-甘孜地块和四川盆地的地壳上地幔S波速度结构具有显着差异,龙门山断裂和鲜水河断裂带,作为地块间的边界断裂带,对两侧地壳结构具有明显的控制作用。2)观测台阵覆盖区域的地壳厚度存在明显差异,川滇地块的地壳厚度为60~64km,松潘-甘孜地块的地壳厚度为52~56km,四川盆地前陆的地壳厚度为46~52km,沿龙门山断裂带松潘-甘孜地块和四川盆地形成镶嵌结构,汶川地震震中处南北两侧的壳幔边界存在约6km的断错。3)四川盆地前陆低速特征表明相应区域存在厚度8~10km的沉积盖层,松潘-甘孜地块和川滇地块的中下地壳具有大面积分布的S波低速区,松潘-甘孜地块地壳平均泊松比高达0.29~0.31,汶川地震余震绝大多数分布在低速区上方的高速介质区域内,而四川盆地的中下地壳呈现整体性的高速特征,以汶川地震的震中为界,龙门山断裂带北段和南段的S波速度结构显示了明显的速度分段特征,其北段的S波速度总体上高于南段。4)本文给出的研究区地壳三维S波速度结构表明,川西高原中下地壳较为软弱,而四川盆地中下地壳的强度应明显高于松潘-甘孜地块,意味着四川盆地坚硬中下地壳可以阻挡松潘-甘孜地块向东的逃逸;另一方面,川西高原和川滇地块的中下地壳虽然均存在大面积的S波低速区,但松潘-甘孜地块内的地壳速度结构相对来说较为复杂,并形成了高、低速相间的结构特征,表明在四川盆地的阻挡作用下,该地块形成了折皱变形的结构。5)与S波低速区相应,松潘-甘孜地块和川滇地块中下地壳应处于部分熔融的状态,这对该区域存在中下地壳通道流(Channelflow)的推断是一个支持;但是,松潘-甘孜地块内是否存在中下地壳通道流仍有待进一步的深入研究。6)接收函数方位各向异性的偏振分析表明,以汶川地震震中为界,龙门山断裂西南侧处于挤压状态,而其东北侧的主压应力方向与断层走向大体平行,推断先存应力场可能驱动了汶川地震逆冲破裂之后沿龙门山断裂向北东方向的走滑破裂。     

环渤海地区的地震层析成像与地壳上地幔结构

利用环渤海地区的天然地震P波到时资料,采用纬度和经度方向分别为0.5°×0.6°的网格划分,反演了该地区地壳上地幔的三维P波速度结构.初步结果表明,环渤海地区地壳上地幔的速度结构具有明显的横向不均匀性：京津唐地区地壳中上部的速度异常反映了浅表层的地质构造特征,造山带和隆起区对应于高速异常,坳陷区和沉积盆地对应于低速异常;地壳下部出现大规模的低速异常与华北地区广泛存在的高导层相对应,估计与壳内的滑脱层和局部熔融、岩浆活动有关;莫霍面附近的速度异常反映了地壳厚度的变化及壳幔边界附近热状态的差异;上地幔顶部大范围的低速异常可能是上地幔软流层热物质大规模上涌所致.     

中国东北地区上地幔P波方位各向异性研究

地震体波走时层析成像是以地震体波到时作为观测数据,反演地下介质的地震波传播速度分布的一种反演技术方法.基于弱各向异性介质的假定,通过在走时反演方程中引入各向异性参数,可以同时进行介质的速度扰动和各向异性属性的联合反演.对三维介质的各向异性分析,有利于对区域结构构造演化、深部物质变形、动力学模式等问题的进一步解释.本文基...     

青藏高原中部地壳和上地幔各向异性分析

对布设于青藏高原中部INDEPTH-III宽频带数字地震台阵的41个台站记录的远震体波资料所提取出的P波接收函数和SKS波形资料做偏振分析,并采用以误差为权的叠加分析方法求得每一个台站的Pms和SKS快波偏振方向和快慢波的时间延迟,获得了从拉萨块体中部,经喀喇昆仑—嘉黎断裂系和班公湖—怒江缝合带,到羌塘块体中部的地壳和岩石圈地幔的地震波各向异性图像.从各向异性分析结果可以看到:Pms快慢波的时间延迟为0.3~0.5 s,在拉萨块体,快波方向主要为NE-SW向,在羌塘块体,快波方向为近E-W向.SKS快慢波的时间延迟为1~2 s,主要分布在拉萨块体的北端和羌塘块体,并且向靠近班公湖—怒江缝合带和昆仑—嘉黎断裂带的方向时差增大,快波方向基本与Pms快波方向一致.在喀喇昆仑—嘉黎断裂带以南的拉萨块体中部没有测量到明显的SKS分裂,这可能与该区存在双层快轴方向近垂直的各向异性层有关.结合研究区已有的研究成果可以推测:拉萨块体地壳各向异性层的快轴方向与印度—欧亚板块汇聚方向一致,可能与地壳较强刚性有关,其在板块汇聚过程中不易发生流展变形;而羌塘块体地壳和岩石圈中各向异性层的快轴方向与青藏高原物质逃逸方向一致,表明这一块体流变性均较强,在板块汇聚挤压力的作用下发生了侧向流变变形.     

中国大陆及邻区海域地壳上地幔各向异性研究

利用分别由Love波和Rayleigh波得到的S波速度结构的差值（VSH-VSV）对中国大陆及邻区海域（70°E～145°E,15°N～55°N）地壳上地幔中的偏振各向异性进行研究.初步研究结果表明,各向异性在空间分布上存在不均匀性：（1）在小于150 km的深度范围内,VSH>VSV的各向异性体占主导地位,反映出在地球的浅部岩石圈内的水平应力作用及软流圈顶部物质的水平向流动对各向异性的形成起主导作用.在大陆地区,各向异性的强度随深度有显著变化.上地壳和上地幔盖层中的各向异性普遍较弱,而在流变性较强的下地壳和软流圈存在较大范围的各向异性.这一现象说明下地壳在岩石圈变形中可能有解耦作用.（2）在大于200 km深度的软流圈下部主要表现为VSHVSV的各向异性比较显著,而软流圈中VSHVSV各向异性.     

重震反演中国东北地壳上地幔三维密度结构

本文利用重力和地震P波到时数据反演得到了中国东北地区地壳上地幔三维密度结构.与单一的重力或地震反演相比,重震反演一方面有效地克服了重力反演结果垂向分辨率低的问题,另一方面也提高了地震反演结果的可靠性.结果显示:中国东北地区的地壳及上地幔剩余密度异常分布与构造单元具有明显的相关性,造山带对应低密度异常,盆地对应高密度异常;区域内火山下方有明显的低密度体存在,可能是由于太平洋板块俯冲进入上地幔并部分滞留,在滞留板块深部脱水和软流圈热物质共同作用下产生了上涌岩浆,喷发后形成了火山.     

中国边缘海及邻区地壳和上地幔速度结构特征

本文利用中国地震台网和世界地震台网1 222个地震台记录到的27 909个研究区内地震的479 707条P波到时数据和4 995个研究区外远震事件的90 530条P波到时数据,采用多波多震相地震层析成像法重建了中国边缘海及其邻近地区（10°~45°N,105°~145°E,深至1 061 km）的三维速度图像,揭示了中国边缘海及其邻近地区地壳和上地幔三维速度结构特征.     

中国东北地区地壳、上地幔速度结构及其对矿产能源形成的控制作用

中国东北地区处于古亚洲洋和滨太平洋构造域叠合部位, 地质构造极其复杂. 利用东北及华北地区部分台网所接收的近震及远震走时资料获得东北地区地壳与上地幔三维P波速度结构, 成像分辨率在80 km左右. 成像结果表明东北地区地壳与上地幔具有较强的横向不均匀性. P波速度异常走向大体呈北东向, 与该区地表构造走向一致. 5 km深度的速度异常与地表起伏形态有很好的对应关系: 阴山、燕山与大兴安岭造山带表现为高速异常; 渤海湾盆地, 松辽盆地表现为低速异常, 渤海湾低速异常一直延伸至 >200 km深度. 东北地区呈现两个明显的低速异常条带, 分别是沿着长白山一线与松辽盆地地区, 其中长白山低速异常延伸至400 km深度附近. 地幔转换带 (410~660 km)表现为显著高速异常体, 这与西向俯冲的高速太平洋板块在地幔转换带中的滞留相对应. 较高分辨率的三维速度成像结果表明东北地区构造演化 (如岩浆活动、岩石圈演化、深震与火山活动等) 与东北亚地区大地幔楔内软流圈上涌与西太平洋板块在地幔转换带中的滞留密切相关. 东北地区的岩浆活动在矿产与油气资源的形成过程中起着提供物源和深层控制作用.     

海南岛及邻区地壳三维P波速度结构

利用海南岛及邻区1999——2005年9个地震数字化台站记录到的3 500余条区域初至波到时资料,确定了海南岛及邻区地壳三维P波速度结构. 结果表明,该区域浅部上地壳的波速异常与地表地质构造有一定的相关性,即王五——文教断裂带以北地区波速较低,而以南地区为相对高的波速异常区. 这与琼北地区为凹陷、并伴有多期火山活动和较高的地热背景,而琼中南地区为较稳定的隆起等构造特征相一致. 在其中下地壳,琼东地区相对琼西地区波速较低,可能暗示了地幔热物质上涌等动力学过程. 波速异常分布模式还暗示出北西向断裂带(如铺前——清澜断裂带)较浅,而近东西向的王五——文教断裂带较深,有可能延伸至莫霍面或更深.      

Structure of the crust and upper mantle beneath Australia from Rayleigh- and Love-wave observations

Fundamental and first higher modes of the Rayleigh- and Love-wave group velocities along seven paths in Australia were jointly inverted by a controlled Monte Carlo procedure to obtain regional shear-wave velocity structures of the crust and upper mantle. Our data support the results of Gonez and Cleary which show an S-wave low velocity zone centred near 110 km depth in eastern Australia. However, the thickness-velocity contrast of the low velocity zone is significantly smaller. The crustal models for eastern Australia are characterized by upper crusts which are both thicker and have lower velocities than those in western Australia and have a less sharp crust-upper mantle boundary. The S-wave velocities for the upper mantle appear to be similar (～ 4.55 km s^?1) throughout the continent, with no obvious dependence on the age of cratonization or crustal thickness.