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1.
The C -response connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic variations and forms the basis for deriving the conductivitydepth profile of the Earth. Time-series of daily mean values at 42 observatories typically with 50 years of data are used to estimate C -responses for periods between 1 month and 1  yr. The Z : Y method is applied, which means that the vertical component is taken locally whereas the horizontal components are used globally by expansion in a series of spherical harmonics.
In combination with results from previous analyses, the method yields consistent results for European observatories in the entire period range from a few hours to 1  yr, corresponding to penetration depths between 300 and 1800  km.
1-D conductivity models derived from these results show an increase in conductivity with depth z to about 2  S  m-1 at z =800  km, and almost constant conductivity between z =800 and z =2000  km with values of 310  S  m-1, in good agreement with laboratory measurements of mantle material. Below 2000  km the conductivity is poorly resolved. However, the best-fitting models indicate a further increase in conductivity to values between 50 and 150  S  m-1.  相似文献   

2.
We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with   VSH > VSV   starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a   VSV > VSH   signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with   VSV > VSH   , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric   VSH > VSV   in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that   VSH > VSV   is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.  相似文献   

3.
Coercive force of single crystals of magnetite at low temperatures   总被引：1，自引：0，他引：1  
The temperature dependence of coercive force H c was studied on well-characterized and stoichiometric millimetre-sized single crystals of magnetite at a series of 16 temperatures from 300 to 10 K using a SQUID magnetometer. H c decreases gradually with cooling to the isotropic temperature, T i = 130 K, where the first magnetocrystalline anisotropy constant K 1 becomes zero. H c exhibits a sharp increase at the Verwey transition, T v = 120 K, where the structure changes from cubic to monoclinic. In crossing the Verwey transition, H c increases by more than two orders of magnitude, from 20 μT to 2.4 mT, and the shape of the hysteresis loops becomes wasp-waisted.
Observed coercivity between 300 K and 170 K varies with temperature as λ s / M s , where λ s is the magnetostriction constant and M s is the saturation magnetization, indicating that the coercivity in MD magnetite is controlled mainly by internal stress associated with dislocations or other crystal defects. It seems likely that the stable single-domain-like magnetic memory observed in large MD magnetite crystals is due to magnetoelastically pinned domain walls. The discontinuous change in H c at the Verwey transition is controlled by abrupt changes in magnetocrystalline and magnetostriction constants due to crystal deformation from cubic to monoclinic structure.  相似文献   

4.
Crust and upper mantle structure of the central Iberian Meseta (Spain)   总被引：2，自引：0，他引：2  
Summary. Quarry blasts recorded along three lines on the central Iberian Meseta are used in an attempt to interpret the crustal structure. The results of the interpretation of the data, together with published surface wave and earthquake data, suggest a layered structure of the crust having the following features: the basement, in some areas covered by up to 4 km of sediments, has a P -velocity of 6.1 km s−1; a low-velocity layer, between 7 and 11 km depth, seems to exist on the basis of both P and S interpretation of seismic data; a thick middle crust of 12 km has a P -velocity of 6.4 km s−1 and overlies a lower crust with a mean P -velocity of 6.9 km s−1 and a possible slight negative gradient; the mean v p/ v s ratio for the crust is about 1.75; the Moho is reached at about 31 km depth and consists of a transition zone at least 1.5 km thick. The P -velocity of the upper mantle is close to 8.1 km s−1 and the S -velocity about 4.5 km s−1, which gives a v p /v s ratio of 1.8 for the uppermost mantle. A tentative petrological interpretation of the velocities and composition of the layers is given.  相似文献   

5.
We modify the receiver-functions stacking technique known as velocity spectrum stacking (VSS) so as to estimate combinations of velocity model ( VP and VS ) and depth that stack the Ps conversion from upper-mantle discontinuities most coherently. We find that by estimating the differences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of P410s and P660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities. We present two examples indicating that the transition zone beneath Obninsk, Russia, is 252±6 km thick and that beneath Pasadena, California, is only 220±6 km thick.  相似文献   

6.
Summary. In order to separate the scattering effect from the intrinsic attenuation, we need a multiple scattering model for seismic wave propagation in random heterogeneous media. In this paper, we apply radiative transfer theory to seismic wave propagation and formulate in the frequency domain the energy density distribution in space for a point source. We consider the cases of isotropic scattering and strong forward scattering. Some numerical examples are shown. It is seen that the energy density–distance curves have quite different shapes depending on the values of medium seismic albedo B 0=ηs/(ηs+ηa) where ηs is the scattering coefficient and ηa is the absorption coefficient of the medium. For a high albedo ( B > 0.5) medium, the energy–distance curve is of arch shape and the position of the peak is a function of the extinction coefficient of the medium ηe=ηs+ηa. Therefore it is possible to separate the scattering effect and the absorption based on the measured energy density distribution curves.  相似文献   

7.
Summary An extension of the Love-Larmor theory to a low-loss unelastic earth model, leads to the surprisingly simple approximation
   
where τs= 447.4 sidereal day is the static wobble period, τR= 306 sidereal day is the rigid-earth wobble period and τw= 433 sidereal day is the observed Chandler period. Q W, Q μ are the respective average Q values of the wobble and the Earth's mantle at τW. The known numerical factor F is only slightly dependent on the Earth structure.  相似文献   

8.
The conductivity structure of the Earth's mantle was estimated using the induction method down to 2100  km depth for the Europe–Asia region. For this purpose, the responses obtained at seven geomagnetic observatories (IRT, KIV, MOS, NVS, HLP, WIT and NGK) were analysed, together with reliable published results for 11  yr variations. 1-D spherical modelling has shown that, beneath the mid-mantle conductive layer (600–800  km), the conductivity increases slowly from about 1  S  m−1 at 1000  km depth to 10  S  m−1 at 1900  km, while further down (1900–2100  km) this increase is faster. Published models of the lower mantle conductivity obtained using the secular, 30–60  yr variations were also considered, in order to estimate the conductivity at depths down to the core. The new regional model of the lower mantle conductivity does not contradict most modern geoelectrical sounding results. This model supports the idea that the mantle base, situated below 2100  km depth, has a very high conductivity.  相似文献   

9.
Approaches to Modelling the Surface Albedo of a High Arctic Glacier   总被引：1，自引：0，他引：1  
Broadband surface albedo measurements, made during the summer melt season at three weather stations on John Evans Glacier (79°40 ' N, 74°00 ' W), varied strongly with the solar zenith angle ( θ z ). Tests were carried out to assess the impact of diurnal variations in surface albedo on seasonal net shortwave radiation ( K * ) totals. Removing the diurnal signal from albedo measurements by daily averaging of hourly measurements, or by applying midday measurements to all hours of the day, changed K * by up to 16%. Ignoring measurements made at θ z & 75°, to account for measurement (cosine) error at high θ z , decreased K * by between 5 and 18%. Given the sensitivity of K * to diurnal patterns in surface albedo, experiments were carried out with two albedo models. One model accounted for albedo variations with θ z and one did not. The model driven by θ z , when implemented within a surface energy balance model for John Evans Glacier, produced better melt estimates. This suggests that diurnal variations in surface albedo should be accounted for in energy balance models of glacier melt.  相似文献   

10.
Absolute S-velocity estimation from receiver functions   总被引：2，自引：0，他引：2  
We present a novel method to recover absolute S velocities from receiver functions.
For a homogeneous half-space the S velocity can be calculated from the horizontal slowness and the angle of surface particle motion for an incident P wave. Generally, the calculated S velocity is an apparent half-space value which depends on model inhomogeneity and P -waveform. We therefore, suggest to calculate such apparent half-space S velocities from low-pass filtered (smoothed) receiver functions using a suite of filter-parameters, T . The use of receiver functions neutralize the influence of the P -waveform, and the successive low-pass filterings emphasize the variation of S velocity with depth.
We apply this   V S ,app.( T )  technique to teleseismic data from three stations: FUR, BFO and SUM, situated on thick sediments, bedrock and the Greenland ice cap, respectively. The observed   V S ,app.( T )  curves indicate the absolute S velocities from the near surface to the uppermost mantle beneath each station, clearly revealing the different geological environments. Application of linearized, iterative inversion quantify these observations into   V S ( z )  models, practically independent of the S -velocity starting model. The obtained models show high consistency with independent geoscientific results. These cases provide also a general validation of the   V S ,app.( T )  method.
We propose the computation of   V S ,app.( T )  curves for individual three-component broad-band stations, both for direct indication of the S velocities and for inverse modelling.  相似文献   

11.
Magnetic susceptibility ( χ ) variations and the behaviour of the ratio of susceptibility to saturation magnetization ( χ/J s ) along the loess/palaeosol section at Koriten (NE Bulgaria) are used to deduce climatic changes during the Pleistocene in southeastern Europe. A good correlation of susceptibility variations with the astronomically tuned oxygen isotope record from ODP site 677 enables us to propose a more precise dating of the upper part of the Bulgarian loess complex. Close correspondence between susceptibility and δ 1 8 O records demonstrates the global significance of the palaeoclimatic signal recorded, although differences in relative amplitudes of χ and χ/J s and δ 1 8 O create difficulties in making quantitative estimates of the climatic humidity in the past. The role of local factors affecting the palaeoclimatic mineral magnetic record deduced from the profile studied in Bulgaria is discussed.  相似文献   

12.
Summary . Vertical component Rayleigh-wave amplitudes from 1461 shallow earthquakes recorded in the distance range 0–150° are analysed to separate the effects of earthquake size, epicentral distance (Δ) and recording station.
The estimated decay of amplitude with distance has the form of a theoretical curve for the decay of Rayleigh waves with distance if the assumption is made that the decay due to dispersion for the data analysed is that of an Airy phase. Writing the decay due to anelastic attenuation as exp (- k Δ), k is estimated to be 0.676/rad over the whole range of distance. If the distance effects are represented by a straight line of the form h log Δ+ constant, h is estimated to be 1.15. The calibration function for computing M s derived from the estimated distance effects is very similar to that of Marshall & Basham.
Station effects on Rayleigh-wave amplitudes though statistically significant are small, and can probably be ignored in the computation of M s.
Comparing the estimated surface-wave magnitudes (earthquake size) obtained in this study with the long and short period body-wave magnitudes ( m LPb and m SPb respectively) obtained by Booth, Marshall & Young for the same earthquake shows that m LPb is about equal to M s over the magnitude range of interest (˜4.0–7.0). The m LPb and Ms relationship shows that the greater the long-period energy radiated by an earthquake the smaller proportionately is the short-period energy.  相似文献   

13.
Summary. The statistical capability of the m b: M s discriminant for the discrimination of earthquake and explosion populations is examined by application of discriminant functions to a group of 83 explosions and 72 earthquakes in Eurasia. Equations are derived for the probability that an event is an earthquake or an explosion. The positive sign of DIS in the decision index equation, DIS i = 34.3383 – 11.9569 mb t + 7.1161 M si , indicates that the event i is an earthquake. Its negative sign indicates that event i is an explosion. The probability of correct classification for an event, P i , is related to its DIS i value, by P i = [1-exp (DIS i )]−1, where a large, positive DIS indicates a high probability that an event is an earthquake and a large, negative DIS indicates a high probability that an event is an explosion. The discrimination line M s = 1.680 m b– 4.825, or m b= 0.595 M s+ 2.872 very successfully separates the explosion population from the earthquake population. The points on this line have an equal chance of being an earthquake or an explosion; moreover, for any event, the distance parallel to the M s-axis from the point representing that event in the m b: M s plane to this line is a measure of the probability for the correct classification of that event.  相似文献   

14.
Summary. The asymptotic properties of spheroidal mode dispersion at high frequency for fixed phase velocity are related to the intercept times τβ( p ) for P and S waves. If the mode eigenfrequency and the ratio of horizontal to vertical displacement at the surface for the mode are known τα( p ) and τβ( p ) may be separately estimated. If discontinuities exist in the velocity model then 'solotone' effects occur, in frequency at fixed slowness, and in τα( p ), τβ( p ) estimated from the mode dispersion as a function of slowness. The coupling of P and S waves in the spheroidal modes means that the interaction of P waves with upper-mantle discontinuities affects also the estimates of the S wave τβ( p ) values for which the corresponding turning points lie in the lower mantle. The asymptotic formalism also shows that sharp pulses formed by superposition of spheroidal modes correspond to multiple PS reflections.
A study of τα( p ), τβ( p ) estimates derived from spheroidal modes with periods from 45–50s, calculated for model 1066B, shows that even in the presence of strong upper-mantle discontinuities the errors in intercept time are only about one-tenth of a period. The asymptotic properties may there-for provide a useful means of estimating intercept times from modes with a few seconds period as a supplement to travel-time methods.  相似文献   

15.
The   M w γ 9.0  2004 December 26 Sumatra-Andaman and   M w = 8.7  2005 March 28 Nias earthquakes, which collectively ruptured approximately 1800 km of the Andaman and Sunda subduction zones, are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle. Because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process. We present a stable method of computing relaxation of a spherically-stratified, compressible and self-gravitating viscoelastic Earth following an impulsive moment release event. The solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes. For simple layered viscoelastic models, which include a low-viscosity oceanic asthenosphere, we predict substantial post-seismic effects over a region several 100s of km wide surrounding the eastern Indian Ocean. We compare observed GPS time-series from ten regional sites (mostly in Thailand and Indonesia), beginning in 2004 December, with synthetic time-series that include the coseismic and post-seismic effects of the 2004 December 26 and 2005 March 28 earthquakes. A viscosity structure involving a biviscous (Burgers body) rheology in the asthenosphere explains the pattern and amplitude of post-seismic offsets remarkably well.  相似文献   

16.
The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition ( T v ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization ( M s ) is documented and the differences between the saturation magnetization and M s near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at T v , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through T v is likely to reduce the stability of closure domains.  相似文献   

17.
Summary. The inverse gravity potential problem consists in the determination of the form and the density of the body by its exterior gravity potential. We describe two similar classes of bodies for which this problem has a unique constructive solution.
(1) The first class contains the cylindrical bodies with finite length, arbitrary form of section and ρ( R , ø, z) =ρ1( z )ρ2( R , ø) density distribution, where z is the cylindrical coordinate; R , ø are the polar coordinates in a section plane. This class is important for prospecting geophysics in that it allows us to determine in a unique and constructive way, the function ρ1( R , ø), the length, form and orientation of the cylinder if we know the function ρ1( z ) and the exterior potential. The classical moment problem of functions is the basis for the solution of this problem.
(2) The analogous problem for the class of the spherical cylinders, or bodies bounded by arbitrary similar sections of two different concentric spheres and the radial lateral surface, appears when bodies of planetary size are studied. (An example of these bodies would be the Moon mascons.) The density distribution of these cylinders is ρ(τ, θ, ø) =ρ1(τ)ρ2(θ, ø) where τ, θ, ø are the spherical coordinates. The function ρ1(θ, ø), length and form of spherical sections can be uniquely determined by exterior potential if we know the function ρ1(τ). We propose a new constructive method for harmonic continuation of the gravity potential into the region containing the perturbing masses for the solution of the problem.  相似文献   

18.
Summary. A technique based on ray asymptotics has been developed to propagate complex spectra of elastic normal mode surface waves in a waveguide with material and geometrical properties varying smoothly in the lateral directions. In the technique, the original problem defined in the unstretched coordinates has been transformed into an eiconal equation as well as into a certain number of transport equations defined in stretched coordinates.
The solution of the eiconal equation is equal to the solution of the eigenproblem of the eiconal operator A0. Due to the self-adjointness of A0, in each of the relevant local inner product spaces, LIPS, the solution of the eigenproblem, A0ψ= v ψ results in the set { v t} of real local eigenvalues and in the orthonormal system {ψt} of local eigenvectors.
As the Hamiltonian function of an initial value problem, each eigenvalues gives birth to a bicharacteristic curve as well as to the related ray. The introduction of the rays induces connections between the vertical cross-sections of the waveguide.
Finally, for each asymptotic order j , the LIPS-valued transport equations are reduced to a set of matricial propagation equations in the local spectral amplitude vectors, LSAVs. Consequently, a knowledge of the initial conditions at a vertical cross-section makes it possible to propagate the LSAVs along the rays of the relevant modes. However, to complete the propagation one needs, in addition to the initial values, information about certain additional quantities, non-diagonal terms of order j , diagonal terms of orders lower than j and the auxiliary boundary terms of orders from 1 to j . The treatment has been completed by the propagation of the modal phases along the relevant rays.  相似文献   

19.
According to recent estimates, the continental mid-crust contains 35–40 per cent amphibolites. Heating of the crust by an underlying mantle plume, for example beneath continental rifts, high plateaus, and areas of intraplate volcanic activity, releases water. Dehydration of amphibole-bearing rocks at depths of 20–40  km occurs mainly in the temperature range 650–700 °C, and this releases about 0.4  wt per cent of water.
  Seismic tomography studies of the crust in the Kirgyz Tien Shan Range, where the age of the tectonic activity is less than 30  Ma, revealed a low-velocity zone in the mid-crust. The velocity of P waves was 0.4  km  s−1 lower than in normal crust. MT sounding data in the region show the existence of a low-resistivity layer with an average resistivity of about 25  Ω  m at the depth of the low-velocity layer. The spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid-crust.  相似文献   

20.
Summary. We present the results of a systematic study of events with M s > 6 in northern Chile (20–33°S), for the period between 1963 and 1971. Medium to large earthquakes near the coast of this region are of three types: (1) Interplate events at the interface between the downgoing slab and the overriding South American plate. These events can be very large reaching magnitudes greater than 8. (2) Intra-plate earthquakes 20–30 km inside the downgoing slab. They have fault mechanisms indicating extension along the dip of the slab and may have magnitudes up to 7.5. (3) Less frequent, M s∼ 6 events that occur near the top of the downgoing slab and have thrust mechanisms with an almost horizontal E-W compressional axis. This type of mechanism is very different from that of the events of type 1 which are due to shallow dipping reverse faulting. There is a rotation of about 30° of the compressional axis in the vertical plane between events of types (1) and (3). Three groups of events near 32.5°, 25.5° and 21°s were studied in detail. Depth and mechanisms were redetermined by P -wave modelling and relative locations were obtained by a master event technique. Near 32.5°S, only events of types 1 and 2 were found in the time period of this study. At the two other sites, the three types of events were identified. This shows clearly that there are compressive stresses at the top of the slab and extension at the centre, a situation which is usually found in the areas where a double Benioff-zone has been identified in the seismicity.  相似文献   

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