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1.
Love-Rayleigh wave incompatibility and possible deep upper mantle anisotropy in the Iberian peninsula 总被引：1，自引：0，他引：1
Love and Rayleigh wave phase velocities are analyzed with the goal of retrieving information about the anisotropic structure of the Iberian lithosphere. The cross-correlation method is used to measure the interstation phase velocities between diverse stations of the ILIHA network at periods between 20 and 120 s. Despite the 2-D structure of the network, the Love wave data are too few to enable an analysis of phase velocity azimuthal variations. Azimuthal averages of Love and Rayleigh wave phase velocities are calculated and inverted both in terms of isotropic and anisotropic structures. Realistic isotropic models explain the Rayleigh wave and short-period Love wave phase velocities. Therefore no significant anisotropy needs to be introduced in the crust and down to 100 km depth in the upper mantle to explain our data. A discrepancy is observed only at long periods, where the data are less reliable. Love wave data at periods between 80 and 120 s remain 0.15 km/s faster than predicted by isotropic models explaining the long-period Rayleigh wave data. Possibilities of biases in the measurements due to interferences with higher modes are examined but seem unlikely. A transversely isotropic model with 8% of S-wave velocity anisotropy in the upper mantle at depths larger than 100 km can explain the whole set of data. In terms of a classical model of mantle anisotropy, this corresponds to 100% of the crystals perfectly oriented in the horizontal plane in a pyrolitic mantle. This is a rather extreme model, which predicts at time delay between 0 and 2 seconds for split SKS. 相似文献
2.
应用现代适配滤波频时分析技术对SRO/GDSN长周期面波记录进行处理,获得穿过太平洋盆地的117条波路径的勒夫波频散数据。我们用改进的分格频散反演方法,从混合路径频散数据中提取出对应于构成整个太平洋盆地的10°×10°分格的纯路径频散数据。所获得的18s到215s范围内的28个周期的太平洋勒夫波群速度的横向分布,揭示了速度随海底年龄的增大而逐渐增加的总趋势,以及其它变化细节。例如,对于同一等年龄线区而言,北太平洋及南太平洋的群速度要比中太平洋的低0.1-0.3km/s。在分格模型反演的基础上得出的深至300km的三维剪切波速度结构提供了太平洋盆地地壳及上地幔横向不均匀性的详尽的描述。 相似文献
3.
R. Pillet D. Rouland G. Roult D. A. Wiens 《Physics of the Earth and Planetary Interiors》1999,110(3-4):211-234
Direct earthquake-to-station Rayleigh and Love wave data observed on high gain broadband records are analyzed in order to improve the lateral resolution of the uppermost mantle in the southwest Pacific region. We used data of nine permanent Geoscope and Iris stations located in the southern hemisphere and nine other stations as part of two temporary networks, the first one installed in New Caledonia and Vanuatu (hereafter named Cavascope network) by ORSTOM and the EOST from Louis Pasteur University in Strasbourg (France) and the second one installed in the Fiji, Tonga and Niue islands (hereafter named Spase network) by Washington University in St. Louis (USA). In order to collect more significant details on the surficial structures, we included the analysis of short period waves down to 8 s. A multiple frequency filtering technique has been used to recover phase velocities of Rayleigh and Love waves for selected earthquakes with magnitude greater than 5.5 and with known centroid moment tensor (CMT). About 1100 well-distributed seismograms have been processed in the period range 8–100 s and corrections for topography and water depth have been applied to the observed phase velocities. The geographical distribution of phase velocity anomalies have then been computed using the tomographic method developed by Montagner [Montagner, J.P., 1986a. Regional three-dimensional structures using long-period surface waves. Ann. Geophys. 4 (B3), 283–294]. Due to a poor knowledge of dense, well-distributed, crustal thickness values and corresponding velocity models, we did not perform or speculate on the construction of an S-wave 3D velocity model; therefore, we limited this study to the interpretation of the phase velocity distribution. The location of phase velocity anomalies are well determined and the deviations are discussed within the framework of the geological context and compared with other tomographic models. At long periods, from 40 s to 100 s, our results agree well with most of previous studies: the tomographic imaging shows a large contrast between low and high phase velocities along the Solomon, New Hebrides and Fiji–Tonga trenches. The lowest phase velocity anomalies are distributed beneath northern and southern Fiji basins and the Lau basin (corresponding to the volume situated just above the dipping slabs), whereas the highest values are displayed beneath the Pacific plate and the eastern part of Indian plate downgoing under the North Fiji basin. At shorter periods, our results show that the phase velocity distributions are well correlated with the large structural crustal domains. The use of local temporary broadband stations in the central part of the studied area gives us the opportunity to observe surface waves showing well-dispersed trains, allowing extended velocity measurements down to 8 s although aliasing due to multipaths become important. The continental regions (Eastern Australia, New Guinea, Fiji islands and New Zealand) show low velocities which are likely due to thick continental crust, whereas the Tasmanian, D'Entrecasteaux, and the Northern and Southern Fiji basins are characterized by higher velocities suggesting thinner oceanic crust. Additional analysis including the anisotropic case and S-wave velocity inversion with depth is in progress. 相似文献
4.
Long period Rayleigh wave and Love wave dispersion data, particularly for oceanic areas, have not been simultaneously satisfied by an isotropic structure. In this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy, anelastic dispersion, sphericity, and gravity. We assume that the surface wave data represents an azimuthal average of actual velocities. Thus, we can treat the mantle as transversely isotropic. The resulting models for average Earth, average ocean, and oceanic regions divided according to the age of the ocean floor, are quite different from previous results which ignore the above effects. The models show a low-velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies. The correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive. For most of the Earth SH > SV in the vicinity of the low-velocity zone. Neat the East Pacific Rise, however, SV > SH at depth, consistent with ascending flow. Anisotropy is as important as temperature in causing radial and lateral variations in velocity. The models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age. This layer defines the LID, or seismic lithosphere. In the Pacific, the LID thickens with age to a maximum thickness of ～50 km. This thickness is comparable to the thickness of the elastic lithosphere. The LID thickness is thinner than derived using isotropic or pseudo-isotropic procedures. A new model for average Earth is obtained which includes a thin LID. This model extends the fit of a PREM, type model to shorter period surface waves. 相似文献
5.
Long-period Rayleigh wave group velocity dispersion curves are presented for paths across the east Pacific Ocean. The records are from the IPG site at Pamatai (Tahiti) and the IDA station at Nana. For the first time, direct-path observations of group velocities up to 300 s have been obtained. This study shows that for young oceanic regions group velocities are low even at long periods. The observations are interpreted in terms of an S-wave velocity model by a generalized inversion scheme. In the models for young ages, the low-velocity zone under the asthenosphere lid is well developed, with a strong velocity gradient at the bottom of this zone, followed at larger depths by a plateau representing a lower-velocity zone, and a marked gradient at 400 km. 相似文献
6.
根据德国卫星激光测距（ＳＬＲ）数据分析中心ＧＦＺ对１９８０年１月－１９９１年７月获取的ＳＬＲ观测数据处理后得到的４１个ＳＬＲ站的站速度，解算了北美、欧亚、太平洋、南美和澳大利亚板块之间的相对运动欧拉矢量，得到了第１个ＳＬＲ实测的板块运动模型ＳＰＭＭ１．与地学板块模型ＲＭ２和ＮＵＶＥＬ－１的比较指出，ＳＰＭＭ１大体上与地学模型一致，与ＮＵＶＥＬ－１更为接近；ＳＰＭＭ１的欧亚与北美板块相对运动欧拉极与ＮＵＶＥＬ－１的相应极很接近，但旋转速率明显偏小．还分析了各板块上ＳＬＲ站的局部形变特征．欧亚板块东部和西部存在１０－２０ｍｍ／ａ的相对运动；板块边界附近ＳＬＲ站的残差站速度基本上反映了该边界的构造形变特征． 相似文献
7.
Zygmunt Kowalik William Knight Tom Logan Paul Whitmore 《Pure and Applied Geophysics》2007,164(2-3):379-393
A numerical model for the global tsunamis computation constructed by Kowalik et al. (2005), is applied to the tsunami of 26 December, 2004 in the World Ocean from 80°S to 69°N with spatial resolution of one
minute. Because the computational domain includes close to 200 million grid points, a parallel version of the code was developed
and run on a Cray X1 supercomputer. An energy flux function is used to investigate energy transfer from the tsunami source
to the Atlantic and Pacific Oceans. Although the first energy input into the Pacific Ocean was the primary (direct) wave,
reflections from the Sri Lankan and eastern shores of Maldives were a larger source. The tsunami traveled from Indonesia,
around New Zealand, and into the Pacific Ocean by various routes. The direct path through the deep ocean to North America
carried miniscule energy, while the stronger signal traveled a considerably longer distance via South Pacific ridges as these
bathymetric features amplified the energy flux vectors. Travel times for these amplified energy fluxes are much longer than
the arrival of the first wave. These large fluxes are organized in the wave-like form when propagating between Australia and
Antarctica. The sources for the larger fluxes are multiple reflections from the Seychelles, Maldives and a slower direct signal
from the Bay of Bengal. The energy flux into the Atlantic Ocean shows a different pattern since the energy is pumped into
this domain through the directional properties of the source function. The energy flow into the Pacific Ocean is approximately
75% of the total flow to the Atlantic Ocean. In many locations along the Pacific and Atlantic coasts, the first arriving signal,
or forerunner, has lower amplitude than the main signal which often is much delayed. Understanding this temporal distribution
is important for an application to tsunami warning and prediction. 相似文献
8.
用高分辨率地震体波速度成像以及相关的地球物理资料，计算地幔垂直流动形式及流动速度，得到全球地幔流垂直运动模式.从全球尺度来看，地幔流基本可划分为以下几个区域：欧亚大陆—澳大利亚、北美洲—南美洲为两个大规模下降流区域，西印度洋—非洲及大西洋、中南太平洋及东太平洋为两个大规模地幔上升流区域.地幔上升流起源于核幔边界，主要表现在地幔中部和上地幔下部.地幔垂直流动速度约每年1～4cm.地幔流动对地表板块运动、海洋中脊和中隆、俯冲带和碰撞带的分布起着控制作用.地幔上升流与地表现代热点有密切关系.从东亚尺度看，地幔流大体分为三个区域：东亚边缘裂谷系和西太平洋边缘海为上升流、西伯利亚地幔深度表现为物质下降流、青藏高原—缅甸—印度尼西亚特提斯俯冲带地幔下降流，这三个区域地幔流动与地表的西太平洋构造域、亚洲构造域和特提斯构造域相吻合.勾勒出南海地区构造特征：从上到下的大体结构是上部呈“工"字型、中间为圆柱型、底部呈盾形的地幔上升流. 相似文献
9.
Abstract Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum-Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific-type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward-dipping subduction zone(s). The 650-500 Ma arc-related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490-470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present-day western Pacific. At 480-470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460-430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc-continent collision(s). From 435-400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430-400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro-Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390-370 Ma collision of Gondwana-derived Aquitaine-Cantabrian blocks with Eastern Avalonia-Baltica and removed the part of the Iapetus south of Baltica. Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present-day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid-Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a < 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably < 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present-day Mediterranean. 相似文献
10.
清华大学地球系统科学研究中心在一个标准耦合模式（SC）的基础上建立了交互集合耦合模式系统（IE）,该系统可以实现多个不同大气模式或者同一大气模式采取不同初值组成的多个分量集合之后与海、陆、冰模式进行耦合.本文利用同一大气模式七个不同初值分量与其它模式分量开展在线集合耦合试验,利用积分稳定之后100年的试验结果,分析了IE在减小海-气界面大气噪音的情况下,对北太平洋海表面温度（SST）变率和ENSO的模拟,并与SC模拟结果进行了对比.分析表明,IE减小了北太平洋中高纬度SST方差的85%以上,表明该区域SST变率主要受大气的影响,且主要是通过改变海表湍流热通量实现的.黑潮延伸体区和北太平洋中部副热带涡旋区域平均SST 8年左右的低频周期主要受来自大气内部动力过程的驱动.在集合耦合模拟中,无论是副热带涡旋区SST与ENSO的联系,还是ENSO与北太平洋中高纬度SST的联系都能模拟出来,而标准模式未能模拟出这些现象,意味着大气噪音过强将掩盖ENSO与太平洋热带外SST的联系.IE对与ENSO关联的“太平洋-北美”（PNA）遥相关型的合理模拟,并通过湍流热通量对海表温度的影响,是其能够更好模拟ENSO与北太平洋中高纬度SST关系的重要原因.本文通过分析验证了所建立的交互集合耦合模式系统的合理性,揭示了该系统在海-气相互作用研究领域方面具有一定应用前景. 相似文献
11.
Abstract North Pacific fossil sirenians comprise representatives of three subfamilies of the Dugongidae: Halitheriinae ( Metaxytherium arctodites , Middle Miocene, North America), Hy-drodamalinae ( Dusisiren spp., Early-Late Miocene, and Hydrodamalis spp., Late Miocene-Pleistocene, North America and Japan), and Dugonginae ( Dioplotherium allisoni , Early-Middle Miocene, North America). Indeterminate dugongid remains are also known from the Late Oligocene of Japan, and the discovery of additional taxa in the western Pacific, especially in Paleogene rocks, can be anticipated. The known North Pacific Neogene taxa apparently dispersed into the Pacific from the Caribbean. Metaxytherium gave rise in the Pacific to Dusisiren ; a series of chronospecies of the latter genus eventually culminated in Hydrodamalis , which was exterminated by humans circa AD 1768. Dioplotherium left no known descendants in the Pacific. The Recent Dugong probably entered the Pacific from the Indian Ocean. The presence in the North Pacific Miocene of at least three sympatric dugongid lineages, together with desmostylians, is evidence for a diversity of marine plants that was reduced by subsequent climatic cooling. 相似文献
12.
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. 相似文献
13.
S. N. Bhattacharya 《Pure and Applied Geophysics》1976,114(6):1021-1029
Love wave dispersion in various semi-infinite media consisting of inhomogeneous layers is discussed. The phase and group velocities are computed when shear wave velocity and density in each inhomogeneous layer are varying exponentially with depth. At the beginning one or two inhomogeneous layers over a homogeneous semi-infinite medium are considered. The dispersion results for these structures are compared with those for their approximations with homogeneous layers. Comparisons show that differences of phase and group velocities for the original models from those for their approximated models (i) increase with the increase of wave number and (ii) are larger for group velocity than for phase velocity. The difference is approximately proportional to the rate of change of parameters in the layers. Finally, dispersion curves are obtained for model IP3MC, which consists of many inhomogeneous and homogeneous layers over a homogeneous semi-infinite medium. The results are compared with the observed group velocity data across the Indian Peninsula. 相似文献
14.
V. P. Trubitsyn 《Izvestiya Physics of the Solid Earth》2006,42(2):93-113
The present Pacific Ocean differs significantly in its structure and evolution from the expanding Atlantic Ocean. The Pacific is asymmetric. Its mid-ocean ridge is located not along its median line but is closer to South America and adjoins North America. The Pacific is surrounded by a ring of subduction zones but has marginal seas only at its Eurasian margins. After the breakup of Pangea, the Atlantic began to open and the Pacific began to close. This paper examines the evolution of the Pacific Ocean and, in particular, the formation mechanisms of its present structures. Numerical modeling of the long-term drift of a large continent is performed, with the initial position of the continent corresponding to the state after the breakup of the supercontinent. At first the continent, driven by the nearest descending mantle flow, begins to approach a subduction zone. Since the mantle flows beneath a large continent have different directions, its velocity is a few times lower than that of the mantle flows near the subduction zone. As a result, a zone of extension arises at the active continental margin and a fragment is broken off from the continent; this fragment rapidly moves away and stops above the descending mantle flow as in a trap. A marginal sea forms at the active continental margin. The continent continues its slow movement toward the subduction zone. The oceanic lithosphere, which earlier sank vertically, begins to descend obliquely. This evolutionary stage corresponds to the present position of Eurasia. The modeling shows how the interaction of the continent with the mantle causes the subduction zone to roll back toward the ocean. Subsequently, the continent nevertheless catches up with the subduction zone, and they move together for a while. The marginal sea then closes and high compressive stresses arise at the active continental margin. This state corresponds to the present position of South America. During the subsequent drift, the continent together with the subduction zone reaches the mid-ocean ridge and partially overrides it. This state corresponds to North America, which was the first to break off from Pangea and passed through the stages of both Eurasia and South America. The large and slowly moving Eurasia, which formed only at the time of Pangea, is still in the first evolutionary stage of the Pacific Ocean closure. 相似文献
15.
在利用远震面波资料研究小区域内的地壳上地幔速度结构时,传统的面波层析成象所依据的大圆路径和平面波入射假设不能成立．本文利用德国区域地震台网记录的Love波资料,根据Friederich和Wielandt提出的面波波场和速度结构联合反演方法,对德国南部地区的Love波相速度横向不均匀分布作了初步研究．反演得到各个周期面波相速度分布的主要特征是稳定的,不随资料选取而改变．30s左右Love波相速度主要受地壳厚度控制,反演得到的相速度分布与该区域已知的莫霍面深度分布有很好的对应关系．39-66s周期的Love波相速度图像随周期逐渐改变,过渡到一个主要受岩石圈下部上地幔构造活动控制的格局．在75-113s周期范围内的相速度分布图像表明,莱茵地堑等构造活动区为明显的低速区．在研究区域东北部的构造稳定地区,从20-113s周期始终显示为高速区． 相似文献
16.
本文使用适配滤波频时分析技术首次对中国数字地震台网的中长周期面波记录进行处理,获得穿过东南地区的82条勒夫波频散数据.使用随机反演理论,获得了东南沿海地区4°×4°网格的纯路径频散数据.这些频散的周期为1.95-68.27s,弥补了长周期面波所不能分辨的浅层结构.在网格反演的基础上,使用Harkrider的面波及演程序得出了中国东南地区的地壳和上地幔结构,浅部可分辨到1km,深部可达80km.在分辨率保证的前提下得出东南地区深至80km的三维剪切波速度结构. 相似文献
17.
B. H. Vaid C. Gnanaseelan P. S. Polito P. S. Salvekar 《Pure and Applied Geophysics》2007,164(8-9):1765-1785
The sea-surface height anomalies derived from Simple Ocean Data Assimilation (SODA) during 1958–2001, Topex/Poseidon satellite
during 1993–2001 and the SODA heat content anomalies (125 m depth) during 1958–2001 are filtered into annual and biennial
Rossby wave components using a two-dimensional Finite Impulse Response filter. The filtered Rossby wave components (both annual
and biennial) in the southern Pacific and Indian Oceans have considerable strength and variability. The propagation of annual
and biennial Rossby waves in the Indonesian through-flow region [12.5°S–7.5°S] of the Indian Ocean is in phase with the southern
Pacific Ocean waves. So it is speculated that the Pacific Ocean influences the Indian Ocean, especially through the region
17.5°S to 7.5°S and thus the southern Pacific Rossby waves could be an unexplored contributor to the Indian Ocean Rossby waves.
We also carried out Fast Fourier Transform (FFT) and wavelet analysis on the tide gauge sea-level data along the Australian
coast to support our claim. Filtered annual and biennial components of SODA heat content anomalies (125 m depth) also support
these findings. 相似文献
18.
Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas 总被引：8，自引：0，他引：8
Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet. 相似文献
19.
Alexander B. Rabinovich Richard E. Thomson Fred E. Stephenson 《Surveys in Geophysics》2006,27(6):647-677
The M
w=9.3 megathrust earthquake of December 26, 2004 off the coast of Sumatra in the Indian Ocean generated a catastrophic tsunami that caused widespread damage in coastal areas and left more than 226,000 people dead or missing. The Sumatra tsunami was accurately recorded by a large number of tide gauges throughout the world's oceans. This paper examines the amplitudes, frequencies and wave train structure of tsunami waves recorded by tide gauges located more than 20,000 km from the source area along the Pacific and Atlantic coasts of North America. 相似文献
20.
Gerhard Müller Abdel Hafiz Mula Søren Gregersen 《Physics of the Earth and Planetary Interiors》1977,14(1):30-40
The amplitudes of the core reflection PcP are sensitive to the wave velocities and densities in the neighborhood of the core-mantle boundary (CMB). We study the amplitude ratio of the long-period phases PcP and P from two South American deep-focus earthquakes with favorable fault-plane solution, depth and magnitude, as recorded by WWNSS and CSN stations in North America.Comparison is made with long-period PcP/P amplitude ratios, derived from theoretical seismograms for a variety of CMB models. Models from previous studies, which were mainly derived from short-period PcP observations and which are characterized by discrete layers above the CMB, are almost all inconsistent with the long-period data. The data also discriminate against low nonzero S velocities below the CMB. Simple first-order-discontinuity models of the CMB, for instance according to the Jeffreys-Bullen earth model or according to recent models based mainly on free oscillations, explain the data reasonably well.Model improvements are attempted by varying the P-velocity gradient above the CMB. The best amplitude fit is obtained for a rather strong decrease in P velocity with depth in this zone which, however, gives no acceptable traveltime fit for PcP. The scatter in body-wave amplitudes is considerable even for long-period waves and may prevent the correct assessment of that part of the amplitude variation of a phase with distance that is due to the variation of velocities and densities with depth alone. 相似文献