首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
Summary. We have analysed the east-west tilt components, O1, K1, N2, M2 and S2 from a continuously recording tiltmeter located in Uwekahuna Vault on Kilauea Volcano, Hawaii, for the period 1971—79. Detailed analysis of the M2 component gives values of 30.9 ± 2.0 (95 per cent) nrad and 116.0 ± 2.0° for the amplitude and phase, respectively, compared to values of 48.5 nrad and 139.4° for the equilibrium tide. the total theoretical tide, found by summing the equilibrium and load tides, amounts to 37.2 nrad at a phase of 121.7°. the 20 per cent discrepancy with that observed may be due to an inaccurate cotical chart, cavity effects in the vault, strain—tilt coupling or an inappropriate solid earth model. In the vicinity of Hawaii (≤ 3°) two independent cotidal charts give almost identical results for the near field ocean load. At greater distances, we use the Schwiderski (1978) cotidal chart. We estimate that local cavity and strain—tilt coupling effects are less than 12 per cent owing to the agreement between geodetically determined and instrumental tilt but we can not rule out regional effects. Assuming these are small and the cotical charts correct, we find that the M2 results are brought into satisfactory agreement if, instead of using an average oceanic earth model in the (< 75 km) vicinity of Hawaii, we use an earth model with nearly one-half the oceanic rigidity. Such a low upper mantle and crustal rigidity is consistent with Kilauea's position above the thermal upwelling associated with the Hawaiian hotspot.  相似文献   

2.
b
The results are presented from tidal gravity measurements at five sites in Europe using LaCoste and Romberg ET gravimeters. Improvements that we have made to the accuracies of these gravimeters are discussed. It is shown that the 'standard' calibration of the International Center for Earth Tides, used for worldwide tidal gravity profiles, is 1.2 per cent too high. The M2 and O1 observations are compared with model calculations of the Earth's body tide and ocean tide loading and it is shown that there is a very significant improvement in the agreement between observations and models compared to that obtained with previous tidal gravity measurements. For O1, where the ocean tide loading and attraction in central Europe is only 0.4 per cent of the body tide, our measurements verify that the Dehant-Wahr anelastic body tide model gravimetric factor is accurate to 0.2 per cent. It is also shown that the effects of lateral heterogeneities in Earth structure on tidal gravity are too small to explain the large anomalies in previously published tidal gravity amplitudes. The observations clearly show the importance of conserving tidal mass in the Schwiderski ocean tide model. For sites in central Europe, the M2 and O1 observations and the models are in agreement at the 0.1 μgal (10−9 m s−2) level and tidal corrections to this accuracy can now be made to absolute gravity measurements.  相似文献   

3.
Numerical models of ductile rebound of crustal roots beneath mountain belts   总被引：3，自引：0，他引：3  
Crustal roots formed beneath mountain belts are gravitationally unstable structures, which rebound when the lateral forces that created them cease or decrease significantly relative to gravity. Crustal roots do not rebound as a rigid body, but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust. 2-D numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound. Numerical models of root rebound show three main features which may be of general application: first, with a low-viscosity lower crust, the rheology of the mantle lithosphere governs the rate of root rebound; second, the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere; and third, redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images.  相似文献   

4.
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   

5.
Summary Isotropic earth models are unable to provide uniform fits to the gross Earth normal mode data set or, in many cases, to regional Love-and Rayleigh-wave data. Anisotropic inversion provides a good fit to the data and indicates that the upper 200km of the mantle is anisotropic. The nature and magnitude of the required anisotropy, moreover, is similar to that found in body wave studies and in studies of ultramafic samples from the upper mantle. Pronounced upper mantle low-velocity zones are characteristic of models resulting from isotropic inversion of global or regional data sets. Anisotropic models have more nearly constant velocities in the upper mantle.
Normal mode partial (Frediét) derivatives are calculated for a transversely isotropic earth model with a radial axis of symmetry. For this type of anisotropy there are five elastic constant. The two shear-type moduli can be determined from the toroidal modes. Spheroidal and Rayleigh modes are sensitive to all five elastic constants but are mainly controlled by the two compressional-type moduli, one of the shear-type moduli and the remaining, mixed-mode, modulus. The lack of sensitivity of Rayleigh waves to compressional wave velocities is a characteristic only of the isotropic case. The partial derivatives of the horizontal and vertical components of the compressional velocity are nearly equal and opposite in the region of the mantle where the shear velocity sensitivity is the greatest. The net compressional wave partial derivative, at depth, is therefore very small for isotropic perturbations. Compressional wave anisotropy, however, has a significant effect on Rayleigh-wave dispersion. Once it has been established that transverse anisotropy is important it is necessary to invert for all five elastic constants. If the azimuthal effect has not been averaged out a more general anisotropy may have to be allowed for.  相似文献   

6.
A series of three‐dimensional models has been constructed for the structure of the crust and upper mantle over a large region spanning the NE Atlantic passive margin. These incorporate isostatic and flexural principles, together with gravity modelling and integration with seismic interpretations. An initial isostatic model was based on known bathymetric/topographic variations, an estimate of the thickness and density of the sedimentary cover, and upper mantle densities based on thermal modelling. The thickness of the crystalline crust in this model was adjusted to equalise the load at a compensation depth lying below the zone of lateral mantle density variations. Flexural backstripping was used to derive alternative models which tested the effect of varying the strength of the lithosphere during sediment loading. The models were analysed by comparing calculated and observed gravity fields and by calibrating the predicted geometries against independent (primarily seismic) evidence. Further models were generated in which the thickness of the sedimentary layer and the crystalline crust were modified in order to improve the fit to observed gravity anomalies. The potential effects of igneous underplating and variable upper mantle depletion were explored by a series of sensitivity trials. The results provide a new regional lithospheric framework for the margin and a means of setting more detailed, local investigations in their regional context. The flexural modelling suggests lateral variations in the strength of the lithosphere, with much of the margin being relatively weak but areas such as the Porcupine Basin and parts of the Rockall Basin having greater strength. Observed differences between the model Moho and seismic Moho along the continental margin can be interpreted in terms of underplating. A Moho discrepancy to the northwest of Scotland is ascribed to uplift caused by a region of upper mantle with anomalously low density, which may be associated with depletion or with a temperature anomaly.  相似文献   

7.
Summary . We report 35 measurements of Earth strain tides at 16 near-surface sites in Great Britain. This is the first widespread survey undertaken specifically to examine the problem of the inhomogeneity of elastic strain fields near the Earth's surface. Some sites were instrumented intensively in order to examine variations of tidal admittance over distances of several hundred metres, while measurements from single instruments at other sites were compared with theoretically predicted strain tides. After allowing for cavity and topographic effects, our data show variations of up to 50 per cent in tidal admittance. We interpret such large anomalies as being due to variations in the regional elastic parameters of wavelength about a 100 m or less. The data indicate that strain measurements from single instrument sites must be interpreted with caution.  相似文献   

8.
Summary. The mantle between 700 and 1200 km is modelled using mixed oxides (MgO + FeO + SiO2 (stishovite)) having olivine, pyroxene, and peridotite (67 per cent ol-33 per cent px) stoichiometries and third-order finite strain theory. It is possible to satisfy the densities, compressional and shear velocities, and bulk and shear moduli of seismic models B1 and PEM and to obtain adiabatic temperature profiles consistent with literature profiles with olivine and peridotite stoichiometries, but not with pyroxenes. Estimates of K and μ for stishovite satisfying Bl and PEM profiles indicate that the stishovite data of Mizutani, Hamano & Akimoto and Liebermann, Ringwood & Major are inconsistent with the assumed mixed-oxide assemblages, even with a large range of pressure and temperature derivatives for K and μ. Assuming perovskite structure materials below 700 km shows that pyroxenes transformed to the perovskite structure are too dense by 1–2.5 per cent. Taking a wide range of P and T derivatives for the K and μ of MgSiO3 (perovskite) indicates that the seismic models can be satisfied with olivines and peridotites transformed to perovskites and oxides. The MgSiO3 (perovskite) K and μ estimates bracket the systematics estimates of Liebermann, Jones & Ring-wood. We find only marginal evidence for relaxation of the shear modulus. The mean atomic weight calculated for the mixed-oxide models agrees with the results of Watt, Shankland & Mao for the same region and with previous estimates for the upper mantle. Thus, an increase in iron content below 700 km appears unlikely. This result and the corresponding temperatures (lower than the Clark & Ringwood conduction geotherm) are consistent with convection below 700 km.  相似文献   

9.
Summary. Reduced Pn travel times from the Archaean Pilbara Craton of north-west Australia show a strong correlation with azimuth, which could be used as evidence of anisotropy. However, the azimuthal correlation could also be explained by a southerly dip of between 1 and 2° on the crust–mantle boundary, although the models from several reversed seismic profiles across the craton suggest a smaller dip.
A time-term analysis of the Pn date yielded several models. The preferred solution, in which the dip on the crust–mantle boundary is similar to that in the models from the reversed profiles, has approximately 2 per cent anisotropy in the uppermost mantle, with the direction of maximum velocity 30° east of north. One possible cause of the anisotropy is that olivine crystals were aligned by syntectonic recrystallization and/or power law creep in the tensional environment caused at the base of the lithosphere by flexure during loading of the lithosphere by the strata of the Hamersley Basin which overlies the Pilbara Craton.
A seismic discontinuity occurs about 15 km below the crust–mantle boundary under the craton. A qualitative analysis of all available seismic data suggests that the velocity below the boundary is probably also anisotropic, with the direction of maximum velocity between north and 40° west of north. The direction of minimum velocity below the sub-Moho boundary correlates loosely with the direction of basement lineaments in the Proterozoic Capricorn Orogenic Belt to the south of the craton, suggesting that the anisotropy under the boundary may be younger than that immediately under the crust/mantle boundary. This is consistent with the notion that the Archaean lithosphere was thinner than the present lithosphere.  相似文献   

10.
对研究区（23.9°N～25.1°N,97.8°E～99.0°E）内地壳和上地幔S波速度结构与强震的关系和强震活动的深部背景进行了探讨。活动断裂的运动是孕育和发生强震的重要诱因,而有利于高应变积聚的深部地质构造则是产生强震的关键条件。研究区内,以瑞丽—龙陵断裂为界,西北侧地壳和上地幔存在大范围低速区,不利于高应变的积聚,不容易孕育和发生强震;东南侧上地幔无明显低速层,地壳内有较小范围低速层,介质偏于刚性,易于积累高应变,孕育和发生强震的可能性较大。  相似文献   

11.
Summary. Numerical convection models are presented in which plates are simulated by imposing piecewise constant horizontal velocities on the upper boundary. A 4 × 1 box of constant viscosity fluid and two-dimensional (2-D) flow is assumed. Four heating modes are compared: the four combinations of internal or bottom heating and prescribed bottom temperature or heat flux. The case with internal heating and an isothermal base is relevant to lower mantle or whole mantle convection, and it yields a lower thermal boundary layer which is laterally variable and can be locally reversed, corresponding to heat flowing back into the core locally. When scaled to the whole mantle, the surface deflections and gravity and geoid perturbations calculated from the models are comparable to those observed at the Earth's surface. For models with migrating ridges and trenches, the flow structure lags well behind the changing surface 'plate'configurations. This may help to explain the poor correlation between the main geoid features and plate boundaries. Trench migration substantially affects the dip of the cool descending fluid because of induced horizontal shear in the vicinity of the trench. Such shear is small for whole mantle convection, but is large for upper mantle convection, and would probably result in the Tonga Benioff zone dipping to the SE, opposite to the observed dip, for the case of upper mantle convection.  相似文献   

12.
Body tides on an elliptical, rotating, elastic and oceanless earth   总被引：17，自引：0，他引：17  
Summary. The Earth's deformation caused by the luni-solar tidal force is defined as the 'body tide'. We compute the effects of the Earth's rotation and elliptical stratification on the body tide for a number of modern elastic structural models. Rotation and ellipticity within the mantle are found to affect tidal observations by about 1 per cent. A consequence is an improved estimate for the fluid core resonance in the diurnal tidal band. Agreement between results for the different structural models is very good. As a result, the results computed here can be used to model the tidal effects of a globally averaged, oceanless, rotating, elliptical and elastic earth to an accuracy of at least one part in 300.  相似文献   

13.
Detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers, including the lower crust and possibly the uppermost mantle, is horizontally displaced with respect to that of the upper crust. We examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper- and lower-crustal magnetization patterns. Thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the Nusselt number and the depth of hydrothermal circulation in the crust, and the temperature range of serpentinization. The models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 C successfully explain the anomalous skewness and the 'hook shape' of observed sea-level magnetic anomalies created at slow and intermediate spreading rates.  相似文献   

14.
Focal mechanisms determined from moment tensor inversion and first motion polarities of the Himalayan Nepal Tibet Seismic Experiment (HIMNT) coupled with previously published solutions show the Himalayan continental collision zone near eastern Nepal is deforming by a variety of styles of deformation. These styles include strike-slip, thrust and normal faulting in the upper and lower crust, but mostly strike-slip faulting near or below the crust–mantle boundary (Moho). One normal faulting earthquake from this experiment accommodates east–west extension beneath the Main Himalayan Thrust of the Lesser Himalaya while three upper crustal normal events on the southern Tibetan Plateau are consistent with east–west extension of the Tibetan crust. Strike-slip earthquakes near the Himalayan Moho at depths >60 km also absorb this continental collision. Shallow plunging P -axes and shallow plunging EW trending T -axes, proxies for the predominant strain orientations, show active shearing at focal depths ∼60–90 km beneath the High Himalaya and southern Tibetan Plateau. Beneath the southern Tibetan Plateau the plunge of the P -axes shift from vertical in the upper crust to mostly horizontal near the crust–mantle boundary, indicating that body forces may play larger role at shallower depths than at deeper depths where plate boundary forces may dominate.  相似文献   

15.
Summary. P -wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the Adamawa Plateau and the Central African Shear Zone of central Cameroon, to determine the variation in crust and upper mantle velocity associated with these structures. A plot of the mean relative residuals for the stations shows a long wavelength (> 300 km) variation of amplitude 0.45 s. the slowest arrivals are located over and just to the north, of the faulted northern margin of the Adamawa Plateau. the residuals do not correlate with topography, surface geology or the previously determined crustal structure, in any simple way.
The Aki inversion technique has been used to invert the relative residuals into a 3-D model of velocity perturbations from a mean earth model. the results show the region is divided roughly into three blocks by two subvertical boundaries, striking ENE and traversing both the crust and upper mantle down to depths greater than 190km. the central block, which is 2 per cent slower than the adjacent blocks, roughly corresponds to the Central African Shear Zone. the Adamawa Plateau, as an individual uplifted area, is explained by the interaction of a regional anomalous upper mantle associated with the West African Rift System, and the Central African Shear Zone, which provided a conduit for heat flow to the surface.  相似文献   

16.
We explore the possible stress triggering relationship of the   M ≥ 6.4  earthquakes that occurred in Kerman Province, southern Iran since 1981. We calculated stress changes due to both coseismic sudden movement in the upper crust and the time-dependent viscous relaxation of the lower crust and/or upper mantle following the event. Four events of   M ≥ 6.4  occurred between 1981 and 2005, on and close to the Gowk fault, show a clear Coulomb stress load to failure relationship. The  2003 M = 6.5  Bam earthquake, however, which occurred approximately 95 km SW of the closest Gowk event, shows a very weak stress relation to preceding earthquakes. The coseismic static stress change at the hypocentre of the Bam earthquake is quite small (∼0.006 bars). The time-dependent post-seismic stress change could be 26 times larger or 7 times lower than that of coseismic static stress alone depending on the choice of viscoelastic crustal model and the effective coefficient of friction. Given the uncertainties in the viscoelastic earth models and the effective coefficient of friction, we cannot confidently conclude that the 2003 Bam event was brought closer to failure through coseismic or post-seismic stress loading. Interestingly, the southern Gowk segment with a similar strike to that of the Bam fault, experienced a stress load of up to 8.3 bars between 1981 and 2003, and is yet to have a damaging earthquake.  相似文献   

17.
Upper mantle shear structure of North America   总被引：5，自引：0，他引：5  
Summary. The waveforms and travel times of S and SS phases in the range 10°–60° have been used to derive upper mantle shear velocity structures for two distinct tectonic provinces in North America. Data from earthquakes on the East Pacific Rise recorded at stations in western North America were used to derive a tectonic upper mantle model. Events on the north-west coast of North America and earthquakes off the coast of Greenland provided the data to investigate the upper mantle under the Canadian shield. All branches from the triplications due to velocity jumps near 400 and 660 km were observed in both areas. Using synthetic seismograms to model these observations placed tight constraints on heterogeneity in the upper mantle and on the details of its structure. SS–S travel-time differences of 30 s along with consistent differences in waveforms between the two data sets require substantial heterogeneity to at least 350 km depth. Velocities in the upper 170 km of the shield are about 10 per cent higher than in the tectonic area. At 250 km depth the shield velocities are still greater by about 4.5 per cent and they gradually merge near 400 km. Below 400 km no evidence for heterogeneity was found. The two models both have first-order discontinuities of 4.5 per cent at 405 km and 7.5 per cent at 695 km. Both models also have lids with lower velocities beneath. In the western model the lid is very thin and of relatively low velocity. In the shield the lid is 170 km thick with very high elocity (4.78 km s-1); below it the velocity decreases to about 4.65 km s-1. Aside from these features the models are relatively smooth, the major difference between them being a larger gradient in the tectonic region from 200 to 400 km.  相似文献   

18.
Planetary topography can either be modelled as a load supported by the lithosphere, or as a dynamic effect due to lithospheric flexure caused by mantle convection. In both cases the response of the lithosphere to external forces can be calculated with the theory of thin elastic plates or shells. On one-plate planets the spherical geometry of the lithospheric shell plays an important role in the flexure mechanism. So far the equations governing the deformations and stresses of a spherical shell have only been derived under the assumption of a shell of constant thickness. However, local studies of gravity and topography data suggest large variations in the thickness of the lithosphere. In this paper, we obtain the scalar flexure equations governing the deformations of a thin spherical shell with variable thickness or variable Young's modulus. The resulting equations can be solved in succession, except for a system of two simultaneous equations, the solutions of which are the transverse deflection and an associated stress function. In order to include bottom loading generated by mantle convection, we extend the method of stress functions to include loads with a toroidal tangential component. We further show that toroidal tangential displacement always occurs if the shell thickness varies, even in the absence of toroidal loads. We finally prove that the degree-one harmonic components of the transverse deflection and of the toroidal tangential displacement are independent of the elastic properties of the shell and are associated with translational and rotational freedom. While being constrained by the static assumption, degree-one loads can deform the shell and generate stresses. The flexure equations for a shell of variable thickness are useful not only for the prediction of the gravity signal in local admittance studies, but also for the construction of stress maps in tectonic analysis.  相似文献   

19.
Seismic anisotropy — the state of the art: II   总被引：1，自引：0，他引：1  
Summary. The theory, causes, observations, and possible applications of seismic anisotropy in the Earth have developed considerably since the previous state of the art paper was published in 1977. The behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle. The top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress-field. The upper part of the crust is frequently anisotropic, probably due to cracks differentially aligned by the non-lithostatic stresses. The possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities.  相似文献   

20.
We have used tidal gravity measurements from six stations in central Europe to investigate the resonance in the diurnal tidal band, caused by inertial coupling between the mantle and outer core of the Earth. By the use of stacking it was possible to determine the eigenfrequency and quality factor of this eigenmode, commonly called the 'nearly diurnal free-wobble'. We assessed the effect of systematic errors from the ocean correction to the tidal measurements employing a Monte-Carlo method. The observed eigenfrequency is 1 + 1/(434 ± 7) cycles per sidereal day, and is significantly higher than predicted by theories. The observed quality factor is (2.8 ± 0.5) × 103.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号