青藏高原及邻近区域的S波三维速度结构

本文收集了WWSSN台网和我国台网中13个地震台站的长周期地震记录,用140条10-90s瑞利波频散曲线和作者提出的Tarantola-Backus面波频散层析成象方法,作了青藏高原及邻区的速度反演,得出该地区岩石层速度结构的三维图象.结果表明,1.在10-110km深度范围内,速度结构出现与大地构造特征相关的分区性,显示出四个构造单元:青藏块体、柴达木-巴颜喀拉-三江块体、塔里木块体和印度块体.2.高原内部,深度为10-70km内速度较低,莫霍界面呈不对称盆形分布,藏北那曲附近地壳厚度超过70km,高原边缘壳厚为45-50km,90-110km为高速异常,表明高原内部存在上地幔盖层.3.高原北部的班公湖断裂和东部的三江断裂系是该区重要的分界线,是岩石层结构存在明显差异的重要接触部位,可能是冈瓦纳古陆与欧亚古陆的缝合带.4.柴达木-巴颜喀拉-三江块体内部速度分布不均匀,地壳厚度由北向南从45km加深到60km;在深度90-110km存在一低速层.5.塔里木地块内速度随深度均匀增加,从地壳到上地幔110km内没有发现低速层.地壳厚度约50km.     

GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea

The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward electric field, and are not associated with the increase of the O/N₂ ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically coupled upward to generate ionospheric perturbations and oscillations.     

电阻率图像中的地下水和断层

目的：将电阻率层析成像应用于探测潜伏断层的研究中，本文发现了断层和地下水的一些基本电阻率分布特征，这对于工程物探意义重大，一般情况下，断层两侧具有不同的电阻率特征，但是，根据电阻率层析图像中的电阻率分布，通常难以区分断层和地下水层，这是因为两者不仅都具有低电阻率值，而且还具有非常相似的电阻率异常特征。资料和方法：运用电阻率层析图像的数据，电阻率层析图像中的断层会呈现如下特征：1）由于孔隙度的加大和地下水的存在，使得断层表现出高角度的低阻线性结构。它们既可以出现在浅部盖层中，也可以存在于深部基岩中，特别是在深部区域，它们尤为明显；2）它们还呈现出高角度的线性梯度带，在该梯度带两边的电阻率结构出现整体性的差异，通常情况下，正断层的上盘表现出低阻或／和班驳状的高阻和低阻扰动区，而下盘则为完整的高阻区，这与逆冲断层正好相反；3）与断层有关的电阻率异常区常常具有良好的大尺度水平连续性，并且可以追瞎异常区附近的精细电性结构。而地下水的电阻率特征为：1）如果没有裂隙，地表水所引起的低阻区非常浅，即使存在丰富的水源以及高孔隙度的砾岩和中粗砂。一般情况下，其底端深度不超过强风化区；2）地下水的电阻率值非常低，特别在高矿化度的地区。地下水，包括岩溶水和砂岩水，的电阻率总显示出局部水平延伸或／和面团状特征；3）地下水层的深度朝某个固定方向逐渐增加，并且其电阻率图像会随季节而变；4）一般情况下，在水下渗的地区，会出现降水漏斗，其上部为高阻，而下部为低阻，从而便形成了“Y”或“V”字型的典型结构。结果：利用上述的基本特征一般可以区分断层和地下水。结论：仅依靠电阻率层析图像，可能极难准确地区分断层和裂隙水，这是因为裂隙水不但可能具有高角度的低阻线性结构，而且在一定尺度上具有很好的水平连续性，还有，由于电阻率层析成像较差的垂直分辨率，难以精确确定断层的上端点位置，所以结合其它的物探手段如钻探和浅层地震勘探是非常必要的。     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

大别造山带上部地壳结构的有限差分层析成像

将有限差分层析反演方法用于大别造山带人工地震测深剖面上部地壳初至波的走时层析成像，得到这一剖面上部地壳横向不均匀结构图像表明，在南大别地区的超高压变质带发现榴辉岩相岩石，其下方3km深度以内的基底仍具有大陆地壳构造中正常的结晶基底速度（6．00km／s左右）；北大别地区在整个上地壳保持正常的速度，而在超高压带下方3km深度以下为6．20-630km／s的相对高速异常区，这一现象可能与超高压变质岩的含量增大有关．同时还表明，南大别和北大别之间至少在上地壳已有明显差异，它们之间的水吼一五河断裂可能是大别造山带内部的主要构造分界线．     

冲绳海槽地区地壳结构与岩石层性质研究

用冲绳海槽地区地质、地球物理、地球化学数据与资料，对横跨大陆边缘—大洋板块的中国东部—冲绳海槽—琉球岛弧地区进行综合研究. 根据地震体波、面波成像结果，空间与布格重力异常及磁力异常的综合解释等结果，利用调和系数法对该研究区的地壳厚度进行了计算，并对地壳属性进行分析. 同时，选择中国东部东北地区的宽甸、华北地区的汉诺坝、华南的闵溪和台湾海峡的澎湖列岛与冲绳海槽和日本岛弧的新生代玄武岩开展元素—同位素对比研究. 结果认为冲绳海槽岩石层明显减薄，上地幔低速层已达到莫霍面，在地幔中已形成交代软流层. 元素—同位素研究表明：它不同于中国东部，与琉球岛弧地区的特点也不相同. 综合分析认为：冲绳海槽地壳属于过渡型地壳，与大洋地壳的许多特点颇为相似.     

地基GPS气象学研究的主要问题及最新进展

目前大多数映射函数在低高度角时精度都比较差，发展适应低高度角的映射函数，提高低高度角观测值的利用率，研究大气的水平梯度是目前地基GPS气象学的主要研究任务，发展随时空变化的动态映射函数也是国际上的一个主要研究问题。由于PWV不能提供水汽分布的三维信息，求定GPS信号斜路径方向上的水汽SWV，以及利用SWV层析大气的垂直结构是目前国际上 GPS气象学的研究前沿。文中总结了目前国际上SWV的求定及层析水汽三维结构的主要方法，及存在的问题。把GPS探测的水汽同化到数值天气预报中，以提高数值预报的能力是GPS气象学研究的目的，作者介绍了目前水汽在数值预报模型中的同化情况。另外还回顾了利用GPS观测结果进行全球气候变化和大气折射环境研究的进展。     

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.     

Cenozoic rifting and volcanism in eastern China: a mantle dynamic link to the Indo–Asian collision?

The Indo–Asian continental collision is known to have had a great impact on crustal deformation in south-central Asia, but its effects on the sublithospheric mantle remain uncertain. Studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the Asian continent, similar to the observed lateral extrusion of Asian crustal blocks. Here we present supporting evidence from P-wave travel time seismic tomography and numerical modeling. The tomography shows continuous low-velocity asthenospheric mantle structures extending from the Tibetan plateau to eastern China, consistent with the notion of a collision-driven lateral mantle extrusion. Numerical simulations suggest that, at the presence of a low-viscosity asthenosphere, continued mass injection under the Indo–Asian collision zone over the past 50 My could have driven significant lateral extrusion of the asthenospheric mantle, leading to diffuse asthenospheric upwelling, rifting, and widespread Cenozoic volcanism in eastern China.     

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).