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1.
Summary. Rotational remanent magnetizations and partial rotational remanent magnetizations have been induced in four specimens using alternating magnetic fields of 55 mT maximum peak strength and 128 Hz, and speeds of rotation between 0.0016 and 0.4 rev s−1. Each partial rotational remanent magnetization ( PRRM ), was produced by rotating the specimen only at the maximum setting of the alternating field. The variation of PRRM with (a) speed of rotation, ω, and (b) total angle of rotation, θ, was investigated. In (a), PRRM fell slowly but steadily as ω increased; for (b) it rose sharply as θ increased up to 60° and reached a maximum for θ between 90° and 120°. Alternating field demagnetizations of PRRMs were performed with the specimen (a) at rest, and (b) rotating about an axis perpendicular to the field. Rotation significantly enhanced the demagnetization process. Variation of the time T , taken to remove the inducing alternating field produced no detectable effect in the case of PRRM , but affected the value of ω at which a given feature of the RRM —ω curve appeared, and the product θF(=ω T ) appears to be more important than either ω or T separately. Current theories on RRM can be used to explain some of the new experimental data on PRRM .  相似文献   

2.
Summary. A new asymptotic formula is obtained for the spectrum of an isolated normal mode multiplet nSl or nTl , with n ≪ l , on a laterally heterogeneous Earth. The principal feature of this formula is that it is uniformly valid on the Earth's surface, including near the epicentre and its antipode. The formal conditions for its validity are that | δm / m 0|≪ 1 and s max≪ l ≪ s min| δm / m 0|–1, where | δm / m 0| is the relative magnitude of the lateral heterogeneity, and s min and s max are the minimum and maximum significant degrees in its spherical harmonic expansion. As well as providing a basis for the geographical interpretation of near-epicentral or near-antipodal long-period recordings, the new formula also unifies the asymptotic theory and adds insight into the phenomena which govern the details of multiplet spectra in general.  相似文献   

3.
Summary The magnetotelluric (MT) response is studied of a uniformly stratified earth which contains a magnetized layer. The impedance as a function of the layer parameters (resistivity, ρ permeability, μ and thickness h ) is discussed. The MT response from a layer (μ, ρ, h ) is equivalent to that from a layer (μ/μr=μ0, μrρ, μr h ) where μr is the relative permeability of the layer. Thus the effect of a magnetized layer is to make it apppear μr times more resistive and μr times thicker than an unmagnetized equivalent layer. Master curves of apparent resistivity and phase are computed for three-layer models with varying permeability associated with varying resistivity in each layer. An example of MT field data is presented in which the most reasonable interpretation is that a magnetized layer exists beneath the observatory site.  相似文献   

4.
A study of geomagnetic storms   总被引：4，自引：0，他引：4  
Summary. An attempt is made to find interplanetary magnetic field and solar-wind parameters which control the development of geomagnetic storms. For this purpose, the interplanetary energy flux is estimated in terms of the Poynting flux ( E × B /4π), and its time variations are compared with the rate of energy dissipation in terms of the ring-current particle injection u i( t ), Joule dissipation in the ionosphere uj ( t ) and auroral particle injection up ( t ) for 15 major geomagnetic storms.
It is shown that the growth of geomagnetic storms, namely the time variations of the rate of the total energy dissipation, u ( t ) = u i( t ) + u j( t ) + u p( t ), is closely related to the Poynting flux by the following relation:
where l 0≅ 7 R E and θ' is a measure of the angle between the interplanetary magnetic field vector and the magnetospheric field vector at the front of the magnetosphere in the equatorial plane. Further, it is shown that within a factor of 2 for each storm period.
A large increase of u ( t ) is associated with substorm activity. Thus, the energy flux ɛ( t ) entering the magnetosphere is dissipated through magneto-spheric substorm processes within the magnetosphere, and their accumulated effects can be understood as geomagnetic storm phenomena.  相似文献   

5.
Summary. We investigate one-dimensional waves in a standard linear solid for geophysically relevant ranges of the parameters. The critical parameters are shown to be T*= tu/Qm where t u is the travel time and Qm the quality factor in the absorption band, and τ−1 m , the high-frequency cut-off of the relaxation spectrum. The visual onset time, rise time, peak time, and peak amplitude are studied as functions of T* and τ m. For very small τ m , this model is shown to be very similar to previously proposed attenuation models. As τ m grows past a critical value which depends on T* , the character of the attenuated pulse changes. Seismological implications of this model may be inferred by comparing body wave travel times with a'one second'earth model derived from long-period observations and corrected for attenuation effects assuming a frequency independent Q over the seismic band. From such a comparison we speculate that there may be a gap in the relaxation spectrum of the Earth's mantle for relaxation times shorter than about one second. However, observational constraints from the attenuation of body waves suggest that such a gap might in fact occur at higher frequencies. Such a hypothesis would imply a frequency dependence of Q in the Earth's mantle for short-period body waves.  相似文献   

6.
We construct a catalogue of all the possible elementary point sources of seismic waves. There are three general classes of sources, two spheroidal and one toroidal. We consider excitation functions for these point-like sources as well as for sources of finite size in far-, intermediate- and near-field for an infinite homogeneous isotropic medium. The sources corresponding to seismic-moment tensors for the second-, third- and fourth-ranks are considered in more detail; we identify 10 different seismic sources in this range: one monopole, two or three dipoles, three quadrupoles, etc. For the step-function of the scalar seismic-moment release, the amplitude spectrum for the third-rank sources is proportional to the angular frequency ω in the region below the corner frequency ω cr. The fourth-rank sources have an ω 2 spectrum in the same range. The possibility of separate and simultaneous inversion of seismic body-wave data and static deformation data for sources of different order is discussed. Some equivalent-force moment higher-rank sources are 'shielded' by lower-rank sources of the same order; the former sources cannot be inverted from seismic data without additional assumptions. Because of their simple radiation pattern, the lower order multipoles, i.e. the monopole and dipoles, are the first sources other than the double-couple which should be considered for inversion.  相似文献   

7.
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   

8.
Summary. Remanent acquisition curves, remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well-defined grain-size fractions between 5 and 250 μm of natural magnetite, titanomagnetite and hematite. From these curves, the remanent acquisition coercive force H 'cr, the remanent coercive force H cr and the median destructive field of IRM H ½I were determined. Theoretically these parameters should be the same for an assembly of non-interacting, homogeneously distributed, randomly oriented single-domain grains. For a given hematite specimen H 'cr, H cr and H ½I have about the same value in spite of the strong grain-size dependence of these parameters. For each specimen of magnetite and titanomagnetite the value of H 'cr is larger than H cr which again is larger than H ½I. However, the ratios H 'cr/ H cr and H ½I/ H cr appear to have a (different) constant value. An interesting relationship which appears to hold for dispersed magnetite, titanomagnetite or hematite grains between 5 and 250 μm, independently of grain-size, quantity and packing density of the magnetic material, is:   相似文献   

9.
Summary. It is shown that at any orientation of a dipole of direct current at the surface of a uniform anisotropic halfspace there exists a unique critical value ηcr of the "vertical" electric anisotropy η. For η > ηcr not all field lines are closed, and three straight field lines pass through the centre of the dipole. Also there exists a unique value ηeq > ηcr, where equatorial η is equal to zero. If η > ηeq then equatorial ρapp is negative since the electric vector is turned more than through 90° at the equator of dipole, and it is shown that ηcr= 1.79, ηeq= 1.93 if the axis of dipole is directed symmetrically with respect to the axes of anisotropy. Routine observations of variations of ρapp with the aim of earthquake prediction should be carried out taking into account the possibility of great fluctuations on η far from the epicentre of an earthquake.  相似文献   

10.
The VLF-R (very low frequency-resistivity) data, i.e. the apparent resistivity ( ρ a ) and phase ( φ ) data, were inverted individually and jointly using the VFSA (very fast simulated annealing) global inversion approach. Global inversion results for synthetic data without and with various amounts of random and normally distributed Gaussian noise reveal that the inversion of neither the ρ a nor φ data alone yields the true parameters of the structures. However, the joint inversion of the ρ a and φ data yields very good estimates of the model parameters. Five models, representing typical subsurface structures in the shield areas, are studied here. Various models achieved after 10 VFSA runs were used to compute the mean model and the corresponding covariance and correlation matrices, which were used to estimate the uncertainties in the mean model parameters and correlations between the model parameters. We observe that these correlations follow the physics associated with the problem. VLF-R field data due to a nearly vertical contact structure and a very thick dyke-like structure were also inverted to demonstrate the efficacy of the approach in the delineation of the parameters of 2-D structures.  相似文献   

11.
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   

12.
  相似文献   

13.
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   

14.
In an attempt to improve the ground motion modelling, the characteristics of the slip velocity functions (SVF) generated using the kinematic k −2 source are investigated and compared to the dynamic solutions proposed in the literature. Several numerical simulations were performed to test the influence of the model parameters on the SVF modelling. Overall, the shapes of SVF are very complex and exhibit a large variability in time and space. However, we found out that the mean SVF is a simple boxcar with duration equal to the largest rise time value. In the areas of weak slip, the SVFs are characterized by the existence of negative values, whereas in large slip areas, the SVF is more impulsive. Overall, on the examples investigated, the SVFs modelled with this k −2 source model are different from a typical Kostrov's solution. The critical analysis of the kinematic k −2 source led us to identify the Fourier decomposition of the slip to be responsible for these difficulties, and to propose a new recombination scheme. It consists of adding a positive correction to the Fourier slip components. The slip is described as the sum of positive contributions at various scales. The SVFs modelled using this new scheme are greatly improved. Moreover, through several parametrical analyses performed to qualify this new approach, we show that the SVF are corrected while preserving the essential quality of the k −2 modelling, that is, the ω2 spectral shape and C d apparent directivity of the synthetic accelerograms. Strong ground motion modelling in the near-fault region was made and numerical ground motion parameters were compared to the empirical relationships. We show that predicted peak ground motion is consistent with near-source attenuation laws.  相似文献   

15.
The traveltime perturbation equations for the quasi-compressional and the two quasi-shear waves propagating in a factorized anisotropic inhomogeneous (FAI) media are derived. The concept of FAI media simplifies considerably these equations. In the FAI medium, the density normalized elastic parameters a ijkl ( X i ) can be described by the relation a ijkl ( X i) = f 2( x i ) A ijkl, where A ijkl are constants, independent of coordinates x i and f 2( x i) is a continuous smooth function of x i . The types of anisotropy ( A ijkl ) and inhomogeneity [ f ( x i)] are not restricted. The traveltime perturbations of individual seismic body waves ( q P , qS 1 and qS 2) propagating in the FAI medium depend, of course, both on the structural pertubations [δ f 2( x i)] and on the anisotropy perturbations (δ A ijkl ), but both these effects are fully separated. The perturbation equations for the time delay between the two qS -waves propagating in the FAI medium are simplified even more. If the unperturbed (background) medium is isotropic, the perturbation of the time delay does not depend on the structural perturbations (δ f 2( x i) at all. This striking result, valid of course only in the framework of first-order perturbation theory, will simplify considerably the interpretation of the time delay between the two split qS -waves in inhomogeneous anisotropic media. Numerical examples are presented.  相似文献   

16.
Summary. Asymptotic expressions for components of the electromagnetic field of a grounded electric dipole are considered for the model consisting of a thin surface-layer overlapping a stratified medium with a highly resistive screen on the roof. It is shown that the method of spatial derivatives makes it possible to obtain proper estimates of the impedance at distances of r ≥|λ0| from the nearest edge of the surface anomaly (|λ0| being the effective depth of the field penetration in the underlying section). the magnetotelluric methods allow one to obtain the true values of impedance, provided r ≥ max {|λ0|, |/( S −1+ Z −0|1/2} where S is the integrated conductivity of the surface layer, is the transverse resistance of the screen, and Z −0 is the Tikhonov—Cagniard impedance for the medium underlying the surface layer.  相似文献   

17.
Summary. A method is outlined to determine the dynamic behaviour of a phase boundary in the Earth when non-uniform time-varying pressure and temperature conditions are assumed at the Earth's surface. An integral equation describing the phase boundary motion is derived and it is solved under a linearizing assumption. The solution is obtained in the form of a double integral transform. Short and long time-expansions of the solution can be obtained from series expansion and integration of the Laplace transform along a branch cut. The method is illustrated by considering a stepwise change in surface pressure conditions.
For short times, the solution exhibits the same type of time dependence (i.e. the first-order term is in t 1/2) as the one obtained in the one-dimensional case (i.e. uniform pressure perturbation at the Earth's surface).
For long times, it is shown that the time dependence of the phase boundary motion is almost identical to the one derived for the one- dimensional case if the wavenumber k L of the surface excitation is such that κ k 2Lτ≤ 1 (where τ is the relaxation time associated with the one-dimensional phase boundary motion and κ is the thermal diffusivity). If κ k 2Lτ > 1, then the relaxation time for the phase boundary motion in two dimensions is of the order of κ−1 k −2L.
When considering parameters that would be appropriate for a basalt to eclogite phase transition at Moho depth, the latter situation is met only when the load wavelength is smaller than 35 km.  相似文献   

18.
We evaluate the seismic moment–frequency relation for the Harvard catalogue in the period 1977–1994. This catalogue is composed of about 12 000 earthquakes. After selection of events in terms of depth and energy, we retain about 8000 data points. We estimate two parameters of the seismic moment distribution: the power exponent β and the cut-off value M m . The method used is a least-squares linear fit on a log–log scale performed over a range selected on the basis of the standard deviation from the histogram. The analysis is carried out for different subdivisions of the Earth in square grids of different sizes. Neither parameter exhibits a dependence on cell size, suggesting the universality of their values and the interpretation of the existence of a cut-off as a finite size effect linked to a finite catalogue length. The variations of the parameters are investigated as a function of time (duration of the catalogue) and versus the number of events used for building up the distribution. Again, β and M m do not depend on time, but M m depends on the number of events, reaching a stable value for N ≈ 1000. The only significant change in the parameters is observed for different values of M 0upper in the catalogue, revealing the existence of universality classes.  相似文献   

19.
Summary. ARM and I sr stability with respect to alternating field and low-temperature treatment were measured for a set of artificial rock specimens. Each of the specimens contained a known amount of a natural magnetic mineral of a known grain size. The following minerals were used: a homogeneous natural magnetite, a natural titanomagnetite with submicroscopic Fe-rich inclusions, and a natural titanomagnetite and a natural titano-maghemite that both show exsolution phenomena. The grain size of the fractions varied from < 5 to 250 μm. It is found that the stability of ARM and I rs of multi-domain grains with respect to alternating fields and low-temperature treatment is similar, whereas for the smallest grains studied (< 5 μm) ARM is more stable than I sr. Multi-domain ARM data from this study are in agreement with current multi-domain ARM theory. Low-temperature behaviour of ARM and I sr can be explained either in terms of a model in which stable moments in the bulk of the grains are screened by the surrounding matrix or in terms of a model in which regeneration of the bulk remanence plays a role and in which stable moments are situated on the surfaces of the grains.  相似文献   

20.
Summary. In this paper some aspects of the deviations of frequency distributions of earthquakes from a linear trend are considered. For recurrence curves with inflexions, the formula
is proposed as a magnitude-frequency relation, with critical magnitude M cr. Comparison with actual data shows better agreement than other formulae. The slope of the frequency graph is considered in the most general case to be a function of M and some parameters p 1, p 2, …. A classification of the types of magnitude-frequency relation is given and all known types of relations N : M and N: K ( K = log E , E = seismic energy) are summarized.  相似文献   

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