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1.
Seismic Wave Velocities in Deep Sediments in Poland: Borehole and Refraction Data Compilation 总被引：1，自引：0，他引：1
Sedimentary cover has significant influence on seismic wave travel times and knowing its structure is of great importance for studying deeper structures of the Earth. Seismic tomography is one of the methods that require good knowledge of seismic velocities in sediments and unfortunately by itself cannot provide detailed information about distribution of seismic velocities in sedimentary cover. This paper presents results of P-wave velocity analysis in the old Paleozoic sediments in area of Polish Lowland, Folded Area, and all sediments in complicated area of the Carpathian Mountains in Poland. Due to location on conjunction of three major tectonic units — the Precambrian East European Craton, the Paleozoic Platform of Central and Western Europe, and the Alpine orogen represented by the Carpathian Mountains the maximum depth of these sediments reaches up to 25 000 m in the Carpathian Mountains. Seismic velocities based on 492 deep boreholes with vertical seismic profiling and a total of 741 vertical seismic profiles taken from 29 seismic refraction profiles are analyzed separately for 14 geologically different units. For each unit, velocity versus depth relations are approximated by second or third order polynomials. 相似文献
2.
Introduction The Yunnan region is located on the east margin of the collision zone between the Indian and Eurasian plates; it belongs to the south section of the N-S Seismic Belt of China and is the junc-ture of the Yangtze metaplatform, Songpan-Garz?fold system, Sanjiang fold system and South China fold system. This region has complex tectonic movements, crisscross faults and frequently occurring strong earthquakes, and hence it is one of the regions with the strongest earthquake ac-tivi… 相似文献
3.
S. E. Hazler A. F. Sheehan D. E. McNamara W. R. Walter 《Pure and Applied Geophysics》2001,158(8):1475-1493
—?Rayleigh wave group velocity dispersion measurements from 10?s to 160?s periods have been made for paths traversing Northern Africa. Data were accumulated from the IRIS DMC, GEOSCOPE, and MEDNET seismic networks covering the years 1991–1997. The group velocity measurements are made including the effects of debiasing for instantaneous period and a single-iteration, mode-isolation (phase match) filter. The curves are grouped by tectonic province and compared to tomographic model-based curves in an effort to test and validate the tomographic models. Within each tectonic category (rift, orogenic zone, or craton) group velocity curves from various provinces are similar. Between tectonic categories, however, there are marked differences. The rift related paths exhibit the lowest group velocities observed, and cratonic paths the fastest. One-dimensional shear velocity inversions are performed, and while highly nonunique, the ranges of models show significant differences in upper mantle velocities between the tectonic provinces.¶This work is part of a larger project to determine group velocity maps for North Africa and the Middle East. The work presented here provides important tools for the validation of tomographic group velocity models. This is accomplished by comparing group velocity curves calculated from the tomographic models with carefully selected high-quality group velocity measurements. The final group velocity models will be used in M s measurements, which will contribute to the m b :M s discriminant important to the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The improved shear wave velocity models provided by this study also contribute to the detection, location, and identification of seismic sources. 相似文献
4.
Frequency-time analysis was applied to records of Rayleigh surface waves due to teleseismic earthquakes (M ≥ 6.0) to obtain
dispersion curves of fundamental mode phase velocities for paths between 51 pairs of digital seismic stations in the Asian
IRIS networks; the range of periods was 10–200 s. For each of these pairs we derived 1D-shear-velocity sections that fit the
corresponding dispersion curves and image the integral earth structure down to depths of ∼650–700 km beneath each of the interstation
seismic paths, which traverse the area of study in different directions. These structures were used by Yanovskaya (2001) to
derive a 3D-shear-velocity model by 2D tomography, imaging the distribution of the larger horizontal inhomogeneities in the
Central Asia mantle for depths of 50 to 600 km, and to construct 2D-velocity sections for five lines passing through the major
tectonic features of Central Asia. 相似文献
5.
A knowledge of seismic wave velocities in the sedimentary cover is of great importance for interpreting reflection and refraction
seismic data, deep seismic soundings and regional and global seismic tomography. This is particularly true for regions characterized
by significant thicknesses and a complex sedimentary cover structure. This paper presents the results of an analysis of seismic
P-wave velocities in the sedimentary cover of Poland, a complex area of juxtaposition of major tectonic units: the Precambrian
East European Craton, the Palaeozoic Platform of Central and Western Europe, and the Alpine orogen represented by the Carpathian
Mountains. Based on vertical seismic profiling data from 1188 boreholes, the dependence of velocity versus depth was determined for regional geological units and for successions from the Tertiary and Quaternary to the Cambrian.
The data have been approximated by polynomials, and velocity-depth formulas are given down to 6000 m depth. The velocities
in the sedimentary cover have been compared with those from other areas in Europe. 相似文献
6.
本文利用喜马拉雅二期台阵674个流动地震台一年的远震垂直向观测资料,获取了18491条独立路径上的Rayleigh波相速度频散曲线,并反演得到了周期10~80 s的Rayleigh波的相速度分布图.通过对比,本文与已有成像结果具有较一致的高低速分布特征,表明了本文结果的可靠性.结果显示,研究区的相速度分布存在明显的横向非均匀性.短周期(如10~15 s)的相速度分布与地表地形密切相关,中等周期(如20~40 s)的相速度分布受地壳厚度的影响较大.在长周期(如60~80 s),鄂尔多斯块体的高速比阿拉善块体更显著、完整,表明同属于华北板块的阿拉善地块,其上地幔结构并没有鄂尔多斯稳定.从短周期至长周期,与周缘地块相比,青藏高原始终表现出较明显的低速异常,可能暗示其具有较活跃的地壳上地幔结构.松潘一甘孜和北祁连块体的中上地壳均存在低速层.全球参考模型Crust1.0和Lithos1.0均不能很好地解释我们的观测频散,基于本文获得的相速度结果可在很大程度上对Crust1.0和Lithos1.0模型进行补充和完善. 相似文献
7.
—?We present results from a large-scale study of surface-wave group velocity dispersion across the Middle East, North Africa, southern Eurasia and the Mediterranean. Our database for the region is populated with seismic data from regional events recorded at permanent and portable broadband, three-component digital stations. We have measured the group velocity using a multiple narrow-band filter on deconvolved displacement data. Overall, we have examined more than 13,500 seismograms and made good quality dispersion measurements for 6817 Rayleigh- and 3806 Love-wave paths. We use a conjugate gradient method to perform a group-velocity tomography. Our current results include both Love- and Rayleigh-wave inversions across the region for periods from 10 to 60 seconds. Our findings indicate that short-period structure is sensitive to slow velocities associated with large sedimentary features such as the Mediterranean Sea and Persian Gulf. We find our long-period Rayleigh-wave inversion is sensitive to crustal thickness, such as fast velocities under the oceans and slow along the relatively thick Zagros Mts. and Turkish-Iranian Plateau. We also find slow upper mantle velocities along known rift systems. Accurate group velocity maps can be used to construct phase-matched filters along any given path. The filters can improve weak surface wave signals by compressing the dispersed signal. The signals can then be used to calculate regionally determined M S measurements, which we hope can be used to extend the threshold of m b :M S discriminants down to lower magnitude levels. Other applications include using the group velocities in the creation of a suitable background model for forming station calibration maps, and using the group velocities to model the velocity structure of the crust and upper mantle. 相似文献
8.
Group velocities of Rayleigh and Love waves along the paths across the Black Sea and partly Asia Minor and the Balkan Peninsula are used to estimate lateral variations of the crustal structure in the region. As a first step, lateral variations of group velocities for periods in the range 10–20 s are determined using a 2D tomography method. Since the paths are oriented predominantly in NE–SW or N–S direction, the resolution is estimated as a function of azimuth. The local dispersion curves are actually averaged over the extended areas stretched in the predominant direction of the paths. The size of the averaging area in the direction of the best resolution is approximately 200 km. As a second step, the local averaged dispersion curves are inverted to vertical sections of S-wave velocities. Since the dispersion curves in the 10–20 s period range are mostly affected by the upper crustal structure, the velocities are estimated to a depth of approximately 25 km. Velocity sections along 43° N latitude are determined separately from Rayleigh and Love wave data. It is shown that the crust under the sea contains a low-velocity sedimentary layer of 2–3 km thickness, localized in the eastern and western deeps, as found earlier from DSS data. Beneath the sedimentary layer, two layers are present with velocity values lying between those of granite and consolidated sediments. Velocities in these layers are slightly lower in the deeps, and the boundaries of the layers are lowered. S-wave velocities obtained from Love wave data are found to be larger than those from Rayleigh wave data, the difference being most pronounced in the basaltic layer. If this difference is attributed to anisotropy, the anisotropy coefficient = (SH - SV)/Smean is reasonable (2–3%) in the upper layers, and exceeds 9% in the basaltic layer. 相似文献
9.
Experimental dispersion curves of Rayleigh and Love waves along the Uppsala-Prague profile have been determined using records of several Italian earthquakes. To interpret the dispersion data, results of previous geophysical investigations in this region were first analyzed. Seven blocks of the crust and upper mantle were distinguished along the profile on the basis of deep seismic sounding and other seismic data. Layered models were proposed for these blocks. Computation of Rayleigh and Love waves shows a large differentiation of theoretical dispersion curves for the northern (Precambrian) and southern (Palaeozoic) part of the profile. A laterally inhomogeneous model for theUppsala - Prague profile, composed of the seven blocks, satisfies the surface wave data for the profile. Moreover, a mean layered model for the whole profile has also been proposed. 相似文献
10.
Summary The records at Athens of 85 earthquakes with epicenters in several regions on the earth were used to determine group velocities along thirty five paths. The mean crustal thickness along each path has been estimated by comparing the observations withPress's standard curves. A linear relation has been found between the mean crustal thickness and mean elevation along each path. This relation is in agreement with Airy's isostatic hypothesis. Determination of Love wave dispersion along five paths and Rayleigh wave dispersion along two paths in southeastern Europe and northern Asia Minor gave values from 35 to 45 km for the crustal thickness in this region. 相似文献
11.
Lateral variation of Rayleigh wave dispersion character 总被引：1，自引：0，他引：1
Tetsuo Santô 《Pure and Applied Geophysics》1965,62(1):49-66
Summary Group velocities of Rayleigh waves were measured along various paths, nearly 200 in all, covering the continents of Eurasia and Africa and the Atlantic and Indian Oceans, using long period seismograms of the World-Wide Standardized Seismograph Network. Dispersion characters thus obtained were compared with standard disperision curves. Several maps showing the paths along which Rayleigh waves have similar dispersion character are presented. The results are compared with dispersion data obtained by other authors.On leave from the International Institute of Seismology and Earthquake Engineering, Tokyo, Japan 相似文献
12.
13.
M.V. Manakou D.G. Raptakis F.J. Chávez-García P.I. Apostolidis K.D. Pitilakis 《Soil Dynamics and Earthquake Engineering》2010
The 3D structure of the Mygdonian sedimentary basin (N. Greece) is investigated. The aim of this study is to propose a 3D model of this sedimentary structure that can later be used to model the seismic records currently being obtained by the permanent accelerograph network operating in the area. This model builds on previous efforts and incorporates new data. The geometry and dynamic properties of the soil layers were inverted using data from microtremor array measurements, seismic refraction profiles, boreholes, and geotechnical investigations. Phase-velocity dispersion curves of Rayleigh waves were determined at 27 sites in the basin using the spatial autocorrelation method (SPAC) introduced by Aki [1]. S-wave velocity profiles were inverted from these dispersion curves and the whole valley structure was interpolated using our new results and all previously available data. The proposed 3D model describes the geometry and shear-wave velocities of the Mygdonian and pre-Mygdonian sedimentary systems, and the top bedrock surface. Our results indicate that this 3D model correctly reflects the geometry and dynamic properties of the sedimentary layers. The case of Euroseistest, where the subsoil structure is the result of bringing together many disparate data, could be used as an example for similar alluvial basins throughout the world, where usually only scarce data is available. 相似文献
14.
T. B. Yanovskaya V. M. Kozhevnikov O. A. Solovei K. R. Akchurin 《Izvestiya Physics of the Solid Earth》2008,44(8):622-630
Dispersion curves of phase velocities of Rayleigh waves are determined by the method of frequency-time analysis in a range of periods of 10–200 s from data of 43 interstation traces in Central Asia. Because the joint use of phase and group velocities significantly decreases the uncertainty in the determination of S wave velocity structures, the same traces were used for calculating group velocities from tomographic reconstructions obtained in [Yanovskaya and Kozhevnikov, 2003, 2006] and determining average velocity structures along these traces. The velocity structures were calculated by the Monte Carlo and linear inversion methods, which gave consistent results. Using velocity values obtained at fixed depths by the 2-D tomography method, lateral variations in velocities at these depths were estimated, which allowed us to construct smoothed vertical velocity structures at some points in the region. The resulting structures were used as initial approximations for constructing local velocity structures solely from previously obtained local dispersion curves of group velocities in the area (32°–56°N, 80°–120°E). Based on these structures, we mapped the lateral distribution of velocity variations at upper mantle depths of 75–400 km and along three vertical profiles. The inferred velocity variations are in good agreement with data on the tectonics of the region. 相似文献
15.
—More than 60 events recorded by four recently deployed seismic broadband stations around Scotia Sea, Antarctica, have been collected and processed to obtain a general overview of the crust and upper mantle seismic velocities.¶Group velocity of the fundamental mode of Rayleigh waves in the period between 10 s to 30–40 s is used to obtain the S-wave velocity versus depth along ten different paths crossing the Scotia Sea region. Data recorded by two IRIS (Incorporated Research Institutions for Seismology) stations (PMSA, EFI) and the two stations of the OGS-IAA (Osservatorio Geofisico Sperimentale—Instituto Antarctico Argentino) network (ESPZ, USHU) are used.¶The Frequency-Time Analysis (FTAN) technique is applied to the data set to measure the dispersion properties. A nonlinear inversion procedure, "Hedgehog," is performed to retrieve the S-wave velocity models consistent with the dispersion data.¶The average Moho depth variation on a section North to South is consistent with the topography, geological observations and Scotia Sea tectonic models.¶North Scotia Ridge and South Scotia Ridge models are characterised by similar S-wave velocities ranging between 2.0 km/s at the surface to 3.2 km/s to depths of 8 km/s. In the lower crust the S-wave velocity increases slowly to reach a value of 3.8 km/s. The average Moho depth is estimated between 17 km to 20 km and 16 km to 19 km, respectively, for the North Scotia Ridge and South Scotia Ridge, while the Scotia Sea, bounded by the two ridges, has a faster and thinner crust, with an average Moho depth between 9 km and 12 km.¶On other paths crossing from east to west the southern part of the Scotia plate and the Antarctic plate south of South Scotia Ridge, we observe an average Moho depth between 14 km and 18 km and a very fast upper crust, compared to that of the ridge. The S-wave velocity ranges between 3.0 and 3.6 km/s in the thin (9–13 km) and fast crust of the Drake Passage channel. In contrast the models for the tip of the Antarctic Peninsula consist of two layers with a large velocity gradient (2.3–3.0 km/s) in the upper crust (6-km thick) and a small velocity gradient (3.0–4.0) in the lower crust (14-km thick). 相似文献
16.
W. Friederich 《Pure and Applied Geophysics》1998,151(2-4):649-667
—A general relation linking the elasticity tensor of an anisotropic medium with that of the constituting single crystals and the function describing the orientation distribution of the crystals is derived. By expanding the orientation distribution function (ODF) into tensor spherical harmonics and using canonical components of the elasticity tensors, it is shown that the elastic tensor of the medium is completely determined by a finite number of expansion coefficients, namely those with harmonic degree l≤ 4. The number of expansion coefficients actually needed to determine the elastic constants of the medium depends on the symmetry of the single crystals. For hexagonal symmetry of the single crystals it is shown that only 8 real numbers are required to fix the 13 elastic constants which are for example needed to determine the azimuthal dependence of surface wave velocities. Thus, inversions of observations of seismic anisotropy are feasible which do not make any a priori assumptions on the orientation of the crystals. As a byproduct of the derivation, a formula is given which allows the easy calculation of the elastic constants of a medium composed of hexagonal crystals obeying an arbitrary ODF. An application of the theoretical results to the inversion of surface wave dispersion curves for an anisotropic 1D-mantle model is presented. For the S-wave velocities the results are similar to those of previous inversions but the new approach also yields P-wave velocities consistent with the assumption of oriented olivine. Moreover it provides a hint of the orientation distribution of the crystals. 相似文献
17.
利用中国大陆中东部地区以国家台网为主的100个分布均匀的宽频带地震台记录到的21个月的连续波形数据, 经过单台数据处理和互相关叠加计算后, 由时频分析法提取了研究区各台站对间瑞雷波的格林函数. 为了检验经验格林函数的可靠性和稳定性, 对沿部分路径的经验格林函数和频散曲线进行了质量评估. 检测结果表明, 自21个月叠加的台站对间背景噪声中提取的经验格林函数与实际的地震面波一致, 提取的格林函数可靠. 此外, 统计了使用从3—21个月不同长度数据叠加后, 经验格林函数信噪比大于10的频散曲线数目. 结果表明, 至少要使用12个月的数据才能提取到信噪比足够大, 数目足够多, 可用于反演面波速度结构的经验格林函数; 12个月的叠加时长, 可以保证30 s以下周期的频散曲线在时间上稳定. 相似文献
18.
Andriy Gnyp 《Acta Geophysica》2009,57(2):330-345
Based on the results of application of correlation analysis to records of the 2005 Mukacheve group of recurrent events and
their subsequent relocation relative to the reference event of 7 July 2005, a conclusion has been drawn that all the events
had most likely occurred on the same rup-ture plane. Station terms have been estimated for seismic stations of the Transcarpathians,
accounting for variation of seismic velocities beneath their locations as compared to the travel time tables used in the study.
In methodical aspect, potentials and usefulness of correlation analysis of seismic records for a more detailed study of seismic
processes, tectonics and geodynamics of the Carpathian region have been demonstrated. 相似文献
19.
This paper aims at investigating possible regional attenuation patterns in the case of Vrancea(Romania) intermediate-depth earthquakes.Almost 500 pairs of horizontal components recorded during 13 intermediate-depth Vrancea earthquakes are employed in order to evaluate the regional attenuation patterns.The recordings are grouped according to the azimuth with regard to the Vrancea seismic source and subsequently,Q models are computed for each azimuthal zone assuming similar geometrical spreading.Moreover,the local soil amplification which was disregarded in a previous analysis performed for Vrancea intermediate-depth earthquakes is now clearly evaluated.The results show minor differences between the four regions situated in front of the Carpathian Mountains and considerable differences in attenuation of seismic waves between the forearc and backarc regions(with regard to the Carpathian Mountains).Consequently,an average Q model of the type Q(f) = 115×f~(1.25) is obtained for the four forearc regions,while a separate Q model of the type Q(f) = 70×f~(0.90) is computed for the backarc region.These results highlight the need to evaluate the seismic hazard of Romania by using ground motion models which take into account the different attenuation between the forearc/backarc regions. 相似文献
20.
The lithosphere of the South American continent has been studied little, especially in northern Brazil (the Amazonian region). A 3D lithospheric S-velocity model of South America was obtained by first carrying out Rayleigh and Love wave group-velocity tomography, and then inverting the regionalized dispersion curves. Fundamental mode group velocities were measured using a Multiple Filtering Technique. More than 12,000 paths were examined and about 6000 Rayleigh- and 3500 Love-wave dispersion curves with good quality were retrieved. Checkerboard tests showed that our dataset permits the resolution of features 400-800 km across laterally in the central part of the continent from crustal to upper mantle depths. Our results confirm previous tomographic results and correlate well with the major geological provinces of South America. The 3D S-velocity model confirms both regional features of SE Brazil from P-wave travel-time tomography and continental-scale features of central and western South America from waveform inversion, e.g., lowest velocities in the Andean upper mantle; three parts of the Nazca plate with flat subduction; strong low-velocity anomalies in the upper-mantle depth beneath the Chaco basin. Furthermore, our 3D model revealed new features in the South American continent: (1) high velocities in the lower crust were consistently found in regions with high Bouguer or free-air anomalies; (2) the NE-SW trending TransBrasiliano shear zone was delineated by a NE-SW low-velocity belt at lithospheric depths; (3) the eastern Amazonian craton appears to have thicker lithosphere than the western craton; (4) in areas of Archean nuclei located in the northeastern Guaporé shield and southeastern São Francisco craton, high velocity anomalies were found down to 150 km. 相似文献