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1.
震中距在50°左右的深源远震,直达波S波震相波列可以与其他震相波列分离. 在层状介质中,本文通过传播矩阵方法得到由S波从台下底层输入获得地面理论地震图的新方法,并由此拟合了河北省两个地震台短周期S波记录,获得台站下方横波速度结构. 结果表明,红山台下13km和24km左右有S波低速层存在,地壳厚度为33.km;涉县台下18km和30km左右有S波低速层存在. 红山台下S波差异显著的2个低速层夹持的高速层、较薄地壳及上地幔顶部低S波速度结构,与166年邢台大地震有关. 考虑到S波速度对部分熔融体的敏感性,认为地壳S波速度结构可作为揭示强震深部背景的依据之一. 相似文献
2.
利用2004～2005年Hi-CLIMB计划架设在藏南日喀则附近由28台宽频带数字地震仪组成的二维台阵的地震记录,应用双差层析定位方法(TomoDD)对454个区域地震进行了精确重定位,共确定了340个地震的精确位置.发现区域震源深度呈明显的双层分布,其中有21 个地震的深度在50～80 km之间,与该地区的莫霍面深度相近.通过不同深度震源的理论地震图与实际地震图对比的方法,发现震源位置位于莫霍面上下地震图的震相存在明显差异,从而给出了存在地幔地震的直接观测证据.这一发现,对长期争论的地幔地震是否存在及大陆岩石圈流变结构的“三明治”模型有重要参考意义. 相似文献
3.
Measurements are taken of the mantle magnitudeM
m
, developed and introduced in previous papers, in the case of the 1960 Chilean and 1964 Alaskan earthquakes, by far the largest events ever recorded instrumentally. We show that theM
m
algorithm recovers the seismic moment of these gigantic earthquakes with an accuracy (typically 0.2 to 0.3 units of magnitude, or a factor of 1.5 to 2 on the seismic moment) comparable to that achieved on modern, digital, datasets. In particular, this study proves that the mantle magnitudeM
m
does not saturate for large events, as do standard magnitude scales, but rather keeps growing with seismic moment, even for the very largest earthquakes. We further prove that the algorithm can be applied in unfavorable experimental conditions, such as instruments with poor response at mantle periods, seismograms clipped due to limited recording dynamics, or even on microbarograph records of air coupled Rayleigh waves.In addition, we show that it is feasible to use acoustic-gravity air waves generated by those very largest earthquakes, to obtain an estimate of the seismic moment of the event along the general philosophy of the magnitude concept: a single-station measurement ignoring the details of the earthquake's focal mechanism and exact depth. 相似文献
4.
由深源远震体波记录反演华北北部地壳上地幔速度结构 总被引：5，自引：2，他引：5
重点研究的地区为河北省北部包括京津地区，以及山西、内蒙的部分地区，在此区选取了２９个台站；在向东延伸的背景区，选取了６个台站。利用这些台站的深源远震体波记录资料，通过选取介质结构模型，计算理论地震图，与观测图进行拟合，以确定地震台下介质结构的可接受模型。在拟合时，对每个地震台站都选择若干次深源远震的体波记录作为观测图。对每个地震台站一般都算出６０个不同模型的理论图。实际上做出的结果比地壳深，为大约８０ｋｍ（即大致相当于岩石层或称岩石圈）厚度内的地壳上地幔介质结构，它们大都由１０层左右的介质组成。通过波形拟合共给出了上述３５个台站下的壳幔介质分层结构。并由此给出了Ｍｏｈｏ面的轮廓。 相似文献
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7.
地震各向异性与地幔对流导致的变形存在因果关系,因此地幔对流模拟可被用来预测地震各向异性,并推测剪切波各向异性地幔源的深度.本文建立了基于地震速度结构的地幔对流模型来预测云南地区剪切波分裂的快波方向,它同时受地表板块运动和地幔内部的温度扰动所驱动.通过与观测结果进行对比分析,推测在云南地区西北部和东部区域,剪切波各向异性源主要存在于岩石圈中.在西南部和四川盆地及其西缘,地幔流动可能是剪切波各向异性的主要贡献者,各向异性层分别位于210~330 km和170~330 km深度,导致西南部剪切波各向异性的地幔可能处于大幅度的剪切变形状态,而四川盆地及其西缘主要处于中等强度的剪切变形状态. 相似文献
8.
Seismic evidence for deep low-velocity anomalies in the transition zone beneath West Antarctica 总被引：1，自引：0，他引：1
Anne Sieminski Eric Debayle Jean-Jacques Lévêque 《Earth and Planetary Science Letters》2003,216(4):645-661
We present a three-dimensional (3D) SV-wave velocity model of the upper mantle beneath the Antarctic plate constrained by fundamental and higher mode Rayleigh waves recorded at regional distances. The good agreement between our results and previous surface wave studies in the uppermost 200 km of the mantle confirms that despite strong differences in data processing, modern surface wave tomographic techniques allow to produce consistent velocity models, even at regional scale. At greater depths the higher mode information present in our data set allows us to improve the resolution compared to previous regional surface wave studies in Antarctica that were all restricted to the analysis of the fundamental mode. This paper is therefore mostly devoted to the discussion of the deeper part of the model. Our seismic model displays broad domains of anomalously low seismic velocities in the asthenosphere. Moreover, we show that some of these broad, low-velocity regions can be more deeply rooted. The most remarkable new features of our model are vertical low-velocity structures extending from the asthenosphere down to the transition zone beneath the volcanic region of Marie Byrd Land, West Antarctica and a portion of the Pacific-Antarctic Ridge close to the Balleny Islands hotspot. A deep low-velocity anomaly may also exist beneath the Ross Sea hotspot. These vertical structures cannot be explained by vertical smearing of shallow seismic anomalies and synthetic tests show that they are compatible with a structure narrower than 200 km which would have been horizontally smoothed by the tomographic inversion. These deep low-velocity anomalies may favor the existence of several distinct mantle plumes, instead of a large single one, as the origin of volcanism in and around West Antarctica. These hypothetical deep plumes could feed large regions of low seismic velocities in the asthenosphere. 相似文献
9.
A radial velocity anomaly in the lower mantle may cause a triplication in the travel-time curve for short-period P waves, but the first two arrivals may not be separable visually on seismograms over a distance range of about 4–10°. However, the changes of slowness and azimuth as a function of time can be used to infer the presence of interfering signals. Some of the interference effects that can be generated synthetically are often observed on seismograms of earthquakes recorded at the Yellowknife array at distances close to 50°, 80° and 90°. The data from Yellowknife provide evidence for the presence of regions of high velocity gradients at depths of about 1250, 2400 and 2730 km that also show rapid lateral variations. Numerous P arrivals from South American earthquakes that traverse the lowest 500 km of the mantle beneath the Caribbean region have been used to illustrate the main features of the interference method. 相似文献
10.
利用牡丹江(MDJ)和海拉尔(HIA)台1988-2000年的宽频带三分量数字地震记录,通过地幔间断面产生的P-SV转换波,研究台站下方地幔间断面的分布以及日本海俯冲带对660km间断面的影响.为了提高所得结果的可靠性,采用了线性-偏振滤波方法和修正的N次根倾斜叠加方法来提取P-SV转换波.前一方法通过实验选取适当的光滑平均和提出立体角的约束;对后一方法进行了震源深度校正和间断面倾斜的修正.结果表明,在MDJ和HIA台下方,除在660,410,220和520km深度附近均存有间断面外,在140,350,570,740和1080km深度附近也存在间断面,显示了地幔中径向分层的复杂性;另外,MDJ台下方660-840km之间的间断面结果比HIA台下方的复杂,似有分层结构,可能反映了俯冲带的影响. 相似文献
11.
A detailed dispersion analysis of Rayleigh waves generated by local earthquakes and occasionally by blasts that occurred in
southern Spain, was undertaken to obtain the shear-wave velocity structure of the region at shallow depth. Our database includes
seismograms generated by 35 seismic events that were recorded by 15 single-component short-period stations from 1990 to 1995.
All these events have focal depths less than 10 km and body-wave magnitudes between 3.0 and 4.0, and they were all recorded
at distances between 40 and 300 km from the epicentre. We analysed a total of 90 source-station Rayleigh-wave paths. The collected
data were processed by standard digital filtering techniques to obtain Rayleigh-wave group-velocity dispersion measurements.
The path-averaged group velocities vary from 1.12 to 2.25 km/s within the 1.0-6.0 s period interval. Then, using a stochastic
inversion approach we obtained 1-D shear-wave velocity–depth models across the study area, which were resolved to a depth
of circa 5 km. The inverted shear-wave velocities range approximately between 1.0 and 3.8 km/s with a standard deviation range
of 0.05–0.16 km/s, and show significant variations from region to region. These results were combined to produce 3-D images
via volumetric modelling and data visualization. We present images that show different shear velocity patterns for the Betic
Cordillera. Looking at the velocity distribution at various depths and at vertical sections, we discuss of the study area
in terms of subsurface structure and S-wave velocity distribution (low velocity channels, basement depth, etc.) at very shallow
depths (0–5 km). Our results characterize the region sufficiently and lead to a correlation of shear-wave velocity with the
different geological units features. 相似文献
12.
Digital seismograms from 25 earthquakes located in the southeastern part of Europe, recorded by three-component very broadband seismometers at the stations Vitosha (Bulgaria) and Muntele Rosu (Romania), were processed to obtain the dispersion properties of Rayleigh and Love surface waves. Rayleigh and Love group-velocity dispersion curves were obtained by frequency–time analysis (FTAN). The path-averaged shear-wave velocity models were computed from the obtained dispersion curves. The inversion of the dispersion curves was performed using an approach based on the Backus–Gilbert inversion method. Finally, 70 path-averaged velocity models (35 R-models computed from Rayleigh dispersion curves and 35 L-models computed from Love dispersion curves) were obtained for southeastern Europe. For most of the paths, the comparison between each pair of models (R-model and L-models for the same path) shows that for almost all layers the shear-wave velocities in the L-models are higher than in the R-models. The upper sedimentary layers are the only exception. The analysis of both models shows that the depth of the Moho boundary in the L-models is shallower than its depth in the R-models. The existence of an anisotropic layer associated with the Moho boundary at depths of 30–45 km may explain this phenomenon. The anisotropy coefficient was calculated as the relative velocity difference between both R- and L-models at the same depths. The value of this coefficient varies between 0% and 20%. Generally, the anisotropy of the medium caused by the polarization anisotropy is up to 10–12%, so the maximum observed discrepancies between both types of models are also due to the lateral heterogeneity of the shear-wave velocity structure of the crust and the upper mantle in the region. 相似文献
13.
O'Leary González Leonardo Alvarez Mariangela Guidarelli Giuliano F. Panza 《Pure and Applied Geophysics》2007,164(10):1985-2007
An overview of the crust and upper mantle structure of Central America and the Caribbean region is presented as a result of
the processing of more than 200 seismograms recorded by digital broadband stations from SSSN and GSN seismic networks. Group
velocity dispersion curves are obtained in the period range from 10s to 40s by FTAN analysis of the fundamental mode of the
Rayleigh waves; the error of these measurements varies from 0.06 and 0.09 km/s. From the dispersion curve, seven tomographic
maps at different periods and with average spatial resolution of 500 km are obtained. Using the logical combinatorial classification
techniques, eight main groups of dispersion curves are determined from the tomographic maps and eleven main regions, each
one characterized by one kind of dispersion curves, are identified. The average dispersion curves obtained for each region
are extended to 150s by adding data from a larger-scale tomographic study (Vdovin et al., 1999) and inverted using a nonlinear procedure. A set of models of the S-wave velocity vs. depth in the crust and upper
mantle is found as a result of the inversion process. In six regions we identify a typically oceanic crust and upper mantle
structure, while in the other two the models are consistent with the presence of a continental structure. Two regions, located
over the major geological zones of the accretionary crust of the Caribbean region, are characterized by a peculiar crust and
upper mantle structure, indicating the presence of lithospheric roots reaching, at least, about 200 km of depth. 相似文献
14.
Robert P. Massé 《Pure and Applied Geophysics》1987,125(2-3):205-239
From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere. 相似文献
15.
Abstract Phase transformations in model mantle compositions and those in subducting slabs have been reviewed to a depth of 800 km on the basis of recent high-pressure experimental data. Seismic velocity and density profiles in these compositions have also been calculated using these and other mineral physics data. The nature of the seismic velocity and density profiles calculated for a pyrolite composition was found to generally agree with those determined by seismic observations (e.g. PREM). The locations of the seismic discontinuities at 400 and 670 km correspond almost exactly to the depths where the transformations of the olivine component to denser phases take place. Moreover, the steep gradients in the seismic velocity/density profiles observed between these depths are qualitatively consistent with those expected from the successive transformations in the complementary pyroxene-garnet component in the pyrolite composition. Further, the calculated seismic velocity and density values agree well with those observed in the upper mantle and mantle transition region within the uncertainties attached to these calculations and observations. Pyrolite or peridotite compositions are thus most likely to represent the composition of the mantle above 670 km depth, although some degrees of chemical heterogeneity may exist in the transition region. The observed sharp discontinuous increases of seismic velocities and density at this depth may be attributed either to the phase transformation to a perovskite-bearing assemblage in pyrolite or to chemical composition changes. Density profiles in subducted slabs have been calculated along adequate geotherms assuming that the slabs are composed of the former oceanic crust underlain by a thicker harzburgitic layer. It is shown that the former oceanic crust is substantially less dense than the surrounding pyrolite mantle at depths below 670 km, while it is denser than pyrolite in the upper mantle and the transition region. The subducted former oceanic crust may be trapped in this region, forming a geochemically enriched layer at the upper mantle-lower mantle boundary. Thick and cool slabs may penetrate into the lower mantle, but the chemically derived buoyancy may result in strong deformation and formation of megalith structures around the 670 km seismic discontinuity. These structures are consistent with those detected by recent seismic tomography studies for subduction zones. 相似文献
16.
目前人们利用4种基本的地震波现象研究地震各向异性,如横波双折射、面波散射、与传播方向有关的走时异常和PS转换波震相.本文利用面波散射产生的Quasi-Love（QL）波研究青藏高原上地幔顶部的各向异性结构特征.首先利用中国地震台网昌都（CAD）台记录的地震波形资料识别出产生QL波的路径,并利用合成地震记录和垂直偏振极性分析证实所观测到的为QL波,而不是高阶振型的Rayleigh波或其他体波震相;然后由Rayleigh波、Love波和QL波的群速度估算了各向异性结构横向变化的转换点;不同周期时,转换点的位置不同,这种频率依赖性还需要进一步的模拟研究.Love波向Rayleigh波耦合（产生QL波）的转换点位置揭示了青藏高原面波方位各向异性变化特征,并以南北向构造带的东西分段性、上地幔流引起的地球内力诱导岩石形变解释了青藏高原各向异性的东西向差异性. 相似文献
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Teleseismic receiver function using stacking and smoothing of multi seismic-records at a single station 
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下载免费PDF全文 Receiver functions (RFs) obtained using teleseismic wave records at a seismic station and synthetic seismograms indicate that RF with a single teleseismic wave record is related to the selection of record section and to the calculating parameters of the RF. The scatter noise contained in the seismogram also affects the quality of RF. A new method for calculating receiver function, stacking and smoothing multi-seismic records in a single station, is presented in this paper. The RF results using some records and some synthetic seismograms with different noises indicate prominent mantle discontinuity and thus prove that the method is effective and satisfied. 相似文献
19.
J.H. Leven 《Physics of the Earth and Planetary Interiors》1985,38(1):9-27
During the period 1974 to 1977, a long range seismic refraction project was conducted in Central Australia, along a profile extending south from Darwin. Earthquakes from the Banda Sea region were used as seismic sources for this experiment. An analysis by Hales and co-workers of the resulting data based on travel times, and using geometric raytracing techniques, has resulted in the construction of an upper mantle velocity model. Using synthetic seismograms to model amplitudes, it is shown that additional constraints can be placed on the derived velocity profile. The low velocity zone beneath the “200 km” discontinuity is found to have a more abrupt onset than was previously suggested. A smaller discontinuity at 325 km depth is now implied. The analysis suggests that the “400 km” discontinuity is a first order velocity increase, whereas all other observed upper mantle discontinuities are more satisfactorily modelled as second order type structures. 相似文献
20.
Eleven PASSCAL broadband digital seismic stations were employed in the Tibetan Plateau by the Sino-US team from September,
1991 to June, 1992. Seven of them were distributed along the Qinghai-Tibet highway, others in Maqin and Yushu of Qinghai Province,
Linzhi and Xigatze of Tibet. The data included 31 local earthquakes recorded by these stations from July, 1991 to January,
1992. Considering the characters of digital data, we identified the seismic phases carefully in several methods using SAC
softwares (Seismic Analysis Code) in SUN workstation. We compared the seismic phases after converting the seismograms of the
single stations to the seismic profiles; analyzed the first arrivals of P waves in the incident planes by rotating 3 component
seismic records; identified the seismic phases from the particle motion pictures. The Pn apparent velocities were calculated
in the distance range of 230–1200 km from Linzhi earthquakes, western Changtang earthquakes, Xitieshan and Gonghe earthquakes
and the earthquakes in India. The results show that the Pn velocities change slightly in the Tibetan Plateau (8.0–8.1 km/s).
These values near the velocities at the uppermost mantle of the normal continents. The Moho undulation in the Tibetan Plateau
was also studied by using Pn data by the time-term method. The primary results indicate that the Moho beneath the Tibetan
Plateau is flat, and its depths are 67–70 km.
The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, Supp., 593–600, 1992. 相似文献