共查询到20条相似文献，搜索用时 31 毫秒
1.
Anatoli L. Levshin Johannes Schweitzer Christian Weidle Nikolai M. Shapiro Michael H. Ritzwoller 《Geophysical Journal International》2007,170(1):441-459
The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1°× 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions. 相似文献
2.
Rengin Gök Hanan Mahdi Haydar Al-Shukri Arthur J. Rodgers 《Geophysical Journal International》2008,172(3):1179-1187
We report the crustal structure for two locations in Iraq estimated by joint inversion of P -wave receiver functions (RFs) and surface (Rayleigh) wave group velocity dispersion. RFs were computed from teleseismic recordings at two temporary broad-band seismic stations located in Mosul (MSL) in the Zagros Fold Belt and Baghdad (BHD) in the Mesopotamian Foredeep. Group velocity dispersion curves at the sites were derived from continental-scale tomography. The inversion results show that the crustal thicknesses are 39 km at MSL and 43 km at BHD. We observe a strong Ps Moho at BHD consistent with a sharp Moho discontinuity. However, at MSL we observe a weak Ps Moho suggesting a transitional Moho where crustal thickening is likely to be occurring in the deep crust. Both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station MSL and 7 km at BHD and agreeing well with the previous reports. Ignoring the sediments, the crystalline crustal velocities and thicknesses are remarkably similar at both stations. The similarity of crustal structure suggests that the crust of the northeastern proto-Arabian Platform was uniform before subsidence and deposition of the sediments in the Cenozoic. If crystalline crustal structure is uniform across the northern Arabian Platform then crustal thickness variations in the Zagros Fold Belt and Thrust Zone should reveal the history of deformation and crustal shortening in the Arabian–Eurasian collision zone and not reflect pre-existing crustal thickness variations in the Arabian Plate. 相似文献
3.
Crustal and uppermost mantle structure in southern Africa revealed from ambient noise and teleseismic tomography 总被引：2，自引：0，他引：2
Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s–1 at 0–10 km depths to 3.86 km s–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic. 相似文献
4.
J. Julià C. J. Ammon R. B. Herrmann A. M. Correig 《Geophysical Journal International》2000,143(1):99-112
We implement a method to invert jointly teleseismic P wave receiver functions and surface wave group and phase velocities for a mutually consistent estimate of earth structure. Receiver functions are primarily sensitive to shear wave velocity contrasts and vertical traveltimes, and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages. Their combination may bridge resolution gaps associated with each individual data set. We formulate a linearized shear velocity inversion that is solved using a damped leastsquares scheme that incorporates a priori smoothness constraints for velocities in adjacent layers. The data sets are equalized for the number of data points and physical units in the inversion process. The combination of information produces a relatively simple model with a minimal number of sharp velocity contrasts. We illustrate the approach using noisefree and realistic noise simulations and conclude with an inversion of observations from the Saudi Arabian Shield. Inversion results for station SODA, located in the Arabian Shield, include a crust with a sharp gradient near the surface (shear velocity changing from 1.8 to 3.5 km s1 in 3 km) underlain by a 5kmthick layer with a shear velocity of 3.5 km s1 and a 27kmthick layer with a shear velocity of 3.8 km s1 , and an upper mantle with an average shear velocity of 4.7 km s1 . The crustmantle transition has a significant gradient, with velocity values varying from 3.8 to 4.7 km s1 between 35 and 40 km depth. Our results are compatible with independent inversions for crustal structure using refraction data. 相似文献
5.
We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50 s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18 km in the Norwegian Sea area and 9 km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9 km s−1 for most paths. An average lithospheric thickness of 60 km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30 km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region. 相似文献
6.
7.
Ambient noise Rayleigh wave tomography of New Zealand 总被引：16，自引：0，他引：16
Fan-Chi Lin Michael H. Ritzwoller John Townend Stephen Bannister Martha K. Savage 《Geophysical Journal International》2007,170(2):649-666
We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries. 相似文献
8.
本文依据南极智利贝纳尔多奥伊金斯将军站的来自南桑德韦奇群岛的Ｍｓ＝６．４级地震面波资料，采用最新的适配滤波频时分析技术和网格频散反演方法，计算了南极半岛地区的勒夫波群速度频散，并获得了该区岩石圈结构模型。结果表明，西南极南极半岛地区地壳为成层结构，可分为三层，其分层厚度分别为５ｋｍ、８ｋｍ和１０ｋｍ。地壳厚度为２３ｋｍ，上地幔顶部速度为５．３２ｋｍ／ｓ。在深度５３ｋｍ以下，地幔介质中速度逐步降低为５．１１ｋｍ／ｓ和４．９ｋｍ／ｓ，最低值达４．８ｋｍ／ｓ，可以推断南极地幔亦为成层结构。 相似文献
9.
L. Knopoff R. G. Mitchel E. G. Kausel † F. Schwab ‡ 《Geophysical Journal International》1979,56(1):211-218
Summary. The Lg phase has been shown previously to be a collection of higher-mode surface waves guided by the continental crust (Knopoff, Schwab & Kausel). A simple scaling between continental and oceanic crustal thicknesses suggests that a search for an oceanic Lg phase should be made in the period range from 1 to 2s. In a search for SH polarized Lg arrivals over oceanic paths, we found that in addition to the fundamental mode, seismo-grams at relatively short ranges in the Pacific showed the presence of only the first higher mode with group velocities on the steep portion of the dispersion curve rather than at the group velocity minimum as expected. Numerical model analysis indicates that, contrary to the continental case, there is no strong confluence of stationary phases of higher-mode crustal waves in the appropriate period range to produce Lg wave packets; this is due to small but significant differences in scaled crustal structures. Further, lateral variations in the thickness of oceanic sediments are sufficient to scatter most of the crustal surface-wave energy within a relatively short distance. Even were this thickness uniform, attenuation in the sediments would be strong enough to absorb the Lg stationary phases in a short distance. 相似文献
10.
On crustal corrections in surface wave tomography 总被引：1，自引：0，他引：1
Mantle models from surface waves rely on good crustal corrections. We investigated how far ray theoretical and finite frequency approximations can predict crustal corrections for fundamental mode surface waves. Using a spectral element method, we calculated synthetic seismograms in transversely isotropic PREM and in the 3-D crustal model Crust2.0 on top of PREM, and measured the corresponding time-shifts as a function of period. We then applied phase corrections to the PREM seismograms using ray theory and finite frequency theory with exact local phase velocity perturbations from Crust2.0 and looked at the residual time-shifts. After crustal corrections, residuals fall within the uncertainty of measured phase velocities for periods longer than 60 and 80 s for Rayleigh and Love waves, respectively. Rayleigh and Love waves are affected in a highly non-linear way by the crustal type. Oceanic crust affects Love waves stronger, while Rayleigh waves change most in continental crust. As a consequence, we find that the imperfect crustal corrections could have a large impact on our inferences of radial anisotropy. If we want to map anisotropy correctly, we should invert simultaneously for mantle and crust. The latter can only be achieved by using perturbation theory from a good 3-D starting model, or implementing full non-linearity from a 1-D starting model. 相似文献
11.
Summary. Within the framework of site survey studies of the Deep Drilling Program of the Fedral Republic of Germany (KTB), coincident deep-seismic reflection and refraction experiments in the Black Forest, southwest Germany, were carried out. The simultaneous interpretation of the reflection and the refraction data reveals in particular both a strong velocity reduction in the upper crust and a laterally varying laminated structure of the lower curst. Additional refraction lines result in a three-dimensional crustal model which shows two distinct crustal types of different seismic properties. These crustal types seem to correlate with the major geologic units of Southwest Germany. Variations of Poisson's ratio derived from clearly recorded shear wave data show a similar trend. 相似文献
12.
Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland 总被引：1，自引：0，他引：1
Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s−1 in the upper 2 km, with a slow increase to around 3.8 km s−1 . P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments. 相似文献
13.
Summary. Rayleigh and Love wave group velocities were determined for 21 paths across the Barents shelf. Those group velocities exhibit regional variations of 1.0 km-1 or more at short periods, depending upon the location of the path within the shelf. Only two different crustal shear-velocity models beneath sedimentary layers are required, however, to explain all of the group velocity data. One model pertains to most of the shelf from a longitude near the eastern coast of Svalbard to Novaya Zemlya. The other pertains to a 200 or 300 km wide region at the western edge of the shelf. Shear velocities in the upper crust of the western region are significantly higher and the crust is much thinner than they are for the rest of the shelf. That region is known to have moved to its present prosition from a point several hundred kilometres to the north during the Caledonian orogeny.
Surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the Barents shelf. Some of these basins, in the southern portion of the shelf, may be 10km or more in thickness. 相似文献
Surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the Barents shelf. Some of these basins, in the southern portion of the shelf, may be 10km or more in thickness. 相似文献
14.
Shear wave anisotropy in the crust around the San Andreas fault near Parkfield: spatial and temporal analysis 总被引：1，自引：0，他引：1
We systematically analysed shear wave splitting (SWS) for seismic data observed at a temporary array and two permanent networks around the San Andreas Fault (SAF) Observatory at Depth. The purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the SAF in this segment and its temporal behaviour in relation to the occurrence of the 2004 Parkfield M 6.0 earthquake. The dense coverage of the networks, the accurate locations of earthquakes and the high-resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the SAF zone. The results show that the primary fast polarization directions (PDs) in the region including the SAF zone and the northeast side of the fault are NW–SE, nearly parallel or subparallel to the SAF strike. Some measurements on the southwest side of the fault are oriented to the NNE–SSW direction, approximately parallel to the direction of local maximum horizontal compressive stress. There are also a few areas in which the observed fast PDs do not fit into this general pattern. The strong spatial variations in both the measured fast PDs and time delays reveal the extreme complexity of shear wave anisotropy in the area. The top 2–3 km of the crust appears to contribute the most to the observed time delays; however substantial anisotropy could extend to as deep as 7–8 km in the region. The average time delay in the region is about 0.06 s. We also analysed temporal patterns of SWS parameters in a nearly 4-yr period around the 2004 Parkfield main shock based on similar events. The results show that there are no appreciable precursory, coseismic, or post-seismic temporal changes of SWS in a region near the rupture of an M 6.0 earthquake, about 15 km away from its epicentre. 相似文献
15.
16.
R. de Franco G. Biella G. Boniolo A. Corsi M. Demartin M. Maistrello A. Morrone 《Geophysical Journal International》1997,128(3):723-736
A seismic-array study of the continental crust and upper mantle in the Ivrea-Yerbano and Strona-Ceneri zones (northwestern Italy) is presented. A short-period network is used to define crustal P - and S -wave velocity models from earthquakes. The analysis of the seismic-refraction profile LOND of the CROP-ECORS project provided independent information and control on the array-data interpretation.
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s−1 , 6.5±0.3 km s−1 and 7.8±0.3 km s−1 respectively. Pronounced low-velocity zones in the upper and lower crust are not observed. A clear change in the velocity structure between the upper and lower crust is documented, constraining the petrological interpretation of the Ivrea-type reflective lower continental crust derived from small-scale petrophysical data. Moreover, we found a V P / V S ratio of 1.69±0.04 for the upper crust and 1.82±0.08 for the lower crust and upper mantle. This is consistent with the structural and petrophysical differences between a compositionally uniform and seismically transparent upper crust and a layered and reflective lower crust. The thickness of the lower crust ranges from about 8 km in front of the Ivrea body (ARVO, Arvonio station) in the northern part of the array to a maximum of about 15 km in the southern part of the array. The lower crust reaches a minimum depth of 5 km below the PROV (Provola) station. 相似文献
Apparent-velocity measurements from both local and regional earthquakes, and time-term analysis are used to estimate the velocity in the lower crust and in the upper mantle. The geometry of the upper-lower crust and Moho boundaries is determined from the station delay times.
We have obtained a three-layer crustal seismic model. The P -wave velocity in the upper crust, lower crust and upper mantle is 6.1±0.2 km s
17.
《Geophysical Journal International》1969,18(5):461-476
Summary Recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the Yellow-knife seismic array. In the immediate vicinity of the array the crust is found to be very uniform. A superficial layer with an intercept time of 0–172 ± 0–012s and unknown velocity is underlain by a crust with a P wave velocity of 6.04 ± 0–01 km s-1 near the top: assuming this velocity constant throughout the second layer, the total thickness of the crust is about 34 ± 2 km. The Mohorovicic discontinuity is horizontal under the array within the resolution of this experiment and the apparent Pn velocity is 8.15 km s-1 . At a distance of a few tens of kilometres the crustal uniformity breaks down. The distances are such that, for most teleseismic signals, the effect of these in homogeneities should be negligible. 相似文献
18.
According to the theory of isostasy, the Earth has a tendency to deform its surface in order to reach an equilibrium state. The land-uplift phenomenon in the area of the Fennoscandian Shield is thought to be a process of this kind. The geoid, as an equipotential surface of the Earth's gravity field, contains information on how much the Earth's surface departs from the equilibrium state. In order to study the isostatic process through geoidal undulations, the structural effects of the crust on the geoid have to be investigated.
The structure of the crust of the Fennoscandian Shield has been extensively explored by means of deep seismic sounding (DSS). The data obtained from DSS are used to construct a 3-D seismic-velocity structure model of the area's crust. The velocity model is converted to a 3-D density model using the empirical relationship that holds between seismic velocities and crustal mass densities. Structural effects are then estimated from the 3-D density model.
The structural effects computed from the crustal model show that the mass deficiency of the crust in Fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid. It proves again that the crust has been isostatically compensated by the upper mantle. In other words, an anomalously high-density upper mantle must exist beneath Fennoscandia. 相似文献
The structure of the crust of the Fennoscandian Shield has been extensively explored by means of deep seismic sounding (DSS). The data obtained from DSS are used to construct a 3-D seismic-velocity structure model of the area's crust. The velocity model is converted to a 3-D density model using the empirical relationship that holds between seismic velocities and crustal mass densities. Structural effects are then estimated from the 3-D density model.
The structural effects computed from the crustal model show that the mass deficiency of the crust in Fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid. It proves again that the crust has been isostatically compensated by the upper mantle. In other words, an anomalously high-density upper mantle must exist beneath Fennoscandia. 相似文献
19.
The dispersive properties of surface waves are used to infer earth structure in the Eastern Mediterranean region. Using group velocity maps for Rayleigh and Love waves from 7 to 100 s, we invert for the best 1-D crust and upper-mantle structure at a regular series of points. Assembling the results produces a 3-D lithospheric model, along with corresponding maps of sediment and crustal thickness. A comparison of our results to other studies finds the uncertainties of the Moho estimates to be about 5 km. We find thick sediments beneath most of the Eastern Mediterranean basin, in the Hellenic subduction zone and the Cyprus arc. The Ionian Sea is more characteristic of oceanic crust than the rest of the Eastern Mediterranean region as demonstrated, in particular, by the crustal thickness. We also find significant crustal thinning in the Aegean Sea portion of the backarc, particularly towards the south. Notably slower S -wave velocities are found in the upper mantle, especially in the northern Red Sea and Dead Sea Rift, central Turkey, and along the subduction zone. The low velocities in the upper mantle that span from North Africa to Crete, in the Libyan Sea, might be an indication of serpentinized mantle from the subducting African lithosphere. We also find evidence of a strong reverse correlation between sediment and crustal thickness which, while previously demonstrated for extensional regions, also seems applicable for this convergence zone. 相似文献
20.
J. P. Narayan 《Geophysical Journal International》1999,139(3):879-887
This paper presents the development of a 2.5-D simulation technique for acoustic wave propagation in media with variable density and velocity. A comparative study of the 2-D and 2.5-D responses of a model reveals the spatially and temporally damped nature of the 2.5-D acoustic wave equations. The simulated results for constant and variable density models show that the density variation affects only the reflectivity of the layer. The computational cost for variable density models is 2.17 and 2.26 times that for constant density models for the 2.5-D and 2-D cases, respectively. Furthermore, the 2.5-D computational cost in the time domain is only about 10–15 per cent more than that for two dimensions, so this modest increase in computational cost can avoid the exorbitant 3-D computational cost.
Snapshots for a crosshole geometry were computed at various times in order to study the effect of heterogeneity on the amplitude and shape of the wave front. Extensive analysis of an oil-bearing reservoir with and without the inclusion of a gas zone was performed using a point source as well as multiple sources. In addition, the effects of the thickness of a low-velocity layer (oil-bearing) and of the location of the source have been studied. It is concluded from the numerical response that the waveguide action of the low-velocity layer depends on its thickness in terms of the dominant wavelength. Trapping of waves was not observed when the source was outside the low-velocity layer. Furthermore, the presence of heterogeneity in the low-velocity layer contributes considerably to the leakage of energy in the adjacent layers due to scattering/diffraction. It was found that, in the 2.5-D numerical simulation, the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D case. 相似文献
Snapshots for a crosshole geometry were computed at various times in order to study the effect of heterogeneity on the amplitude and shape of the wave front. Extensive analysis of an oil-bearing reservoir with and without the inclusion of a gas zone was performed using a point source as well as multiple sources. In addition, the effects of the thickness of a low-velocity layer (oil-bearing) and of the location of the source have been studied. It is concluded from the numerical response that the waveguide action of the low-velocity layer depends on its thickness in terms of the dominant wavelength. Trapping of waves was not observed when the source was outside the low-velocity layer. Furthermore, the presence of heterogeneity in the low-velocity layer contributes considerably to the leakage of energy in the adjacent layers due to scattering/diffraction. It was found that, in the 2.5-D numerical simulation, the stability condition and the requirement of the number of grid points per wavelength to avoid grid dispersion are the same as for the 2-D case. 相似文献