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1.
Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ ts1-s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s-1 and the velocities of S -waves vary from 3.15 to 3.5 km s-1.
The delay times Δ tS1 - S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ tS1 - S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.  相似文献   

2.
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   

3.
The deformation at the core–mantle boundary produced by the 2004 Sumatra earthquake is investigated by means of a semi-analytic theoretical model of global coseismic and postseismic deformation, predicting a millimetric coseismic perturbation over a large portion of the core–mantle boundary. Spectral features of such deformations are analysed and discussed. The time-dependent postseismic evolution of the elliptical part of the gravity field ( J 2) is also computed for different asthenosphere viscosity models. Our results show that, for asthenospheric viscosities smaller than 1018 Pa s, the postseismic J 2 variation in the next years is expected to leave a detectable signal in geodetic observations.  相似文献   

4.
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   

5.
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   

6.
Summary. The potential function ø for a magnetic body of susceptibility μ in a medium of susceptibility μ* satisfies the integral equation
Here Φ* is the potential function for the region without the heterogeneity and R is the distance from the point of observation to the point on the surface, s , of the body. δΦ /δn is the normal derivative, in the direction of the outward normal. The equation allows for the effects of demagnetization. For numerical purposes the surfaces can be divided into N facets over which δΦ/δ n is a constant. The unknown quantities δΦ/δnj can be found from the system of equations defined by:
The prime on the summation sign denotes that the summation does not include the i th element. The magnetic field in the direction of the unit vector P( P 1, P 2, P3 ) is given by:   相似文献   

7.
We present an approximate method to estimate the resolution, covariance and correlation matrix for linear tomographic systems Ax = b that are too large to be solved by singular value decomposition. An explicit expression for the approximate inverse matrix A − is found using one-step backprojections on the Penrose condition AA −≈ I , from which we calculate the statistical properties of the solution. The computation of A − can easily be parallelized, each column being constructed independently.
The method is validated on small systems for which the exact covariance can still be computed with singular value decomposition. Though A − is not accurate enough to actually compute the solution x , the qualitative agreement obtained for resolution and covariance is sufficient for many purposes, such as rough assessment of model precision or the reparametrization of the model by the grouping of correlating parameters. We present an example for the computation of the complete covariance matrix of a very large (69 043 × 9610) system with 5.9 × 106 non-zero elements in A . Computation time is proportional to the number of non-zero elements in A . If the correlation matrix is computed for the purpose of reparametrization by combining highly correlating unknowns x i , a further gain in efficiency can be obtained by neglecting the small elements in A , but a more accurate estimation of the correlation requires a full treatment of even the smaller A ij . We finally develop a formalism to compute a damped version of A −.  相似文献   

8.
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   

9.
Some comments on the descriptions of the polarization states of waves   总被引：4，自引：0，他引：4  
Summary. Procedures are developed for specifying the polarization characteristics of n -dimensional waves, and in particular three-dimensional waves of geophysical interest. We show that when a wave is in a pure state or is totally polarized, all the polarization information can be represented by a single vector u in an n -dimensional unitary space. Simple measures of the degree of polarization of the wave are constructed from the characteristic equation of the spectral matrix S . These measures are functions only of the scalar invariants of S and consequently S need not be diagonalized. If S represents a purely polarized wave, the unitary vector which contains the polarization information about the wave can be obtained directly from S using any 2 n – 1 equations of n 2 possible equations. By multiplying by a phase-factor this unitary vector can be written in the form u = r 1+ i r 2 where r 1 and r 2 are orthogonal vectors in a real space. For an elliptically polarized wave, r 1 and r 2 locate the major and minor axes of the ellipse, and the ellipticity is given by the ratio of their magnitudes. The polarization parameters of ULF magnetic waves at the Earth's surface are computed from one set of five equations ( n = 3) and compared with parameters calculated using established techniques.  相似文献   

10.
The tidal dynamics of the Irish and Celtic Seas   总被引：1，自引：0，他引：1  
Summary. Current meter data collected over periods of more than 14 day from the Irish and Celtic Seas are harmonically analysed and presented in maps of tidal stream information. Making use of the analysed current data, and by constructing time series of frictional and inertial stresses which are also harmonically analysed, harmonic constituents of the surface tidal slopes at current meter stations are obtained. Using these with data collected from offshore tide gauges, and in conjunction with coastal tide data, cotidal maps are drawn with some confidence for M 2, S 2, O 1 and K 1, the M 2 chart resolving the discrepancy which exists between the different charts of the Celtic Sea already produced. Cotidal maps for M 3 and M 4 are also presented.
The mean over a tidal cycle of the energy flux for M 2, S 2 and O 1 is also presented in the form of the total energy flux in these constituents which crosses different sectional lines. A flux of 44 × 106 kW is observed to enter the Celtic Sea from the Atlantic and this is compared with previous estimates. An energy budget is also performed for M 2, including all the effects of astronomical forcing and Earth tides to enable comparison to be made between the true energy inflow and the estimated frictional dissipation. Finally, comparison is made between the mean of the instantaneous energy flux and the sum of the energy fluxes associated with the major harmonics.  相似文献   

11.
Summary. This note presents an exact analytical formula for determining the magnitude of coseismic surface volume change (δ V ) of earthquake faults in a half-space. For a Poisson solid, the formula is remarkably simple; δ V = M zz |8μ, where M zz is one of the moment tensor elements of the source. Maximum δ V values derive from dip slip on faults plunging 45°. For these events, surface volume changes of 0.0001 and 4.3 km3 can be expected for magnitude 5 and 8 earthquakes respectively. All of the coseismic surface volume change is recovered in the interseismic period through relaxation of the Earth and rebound of the surface. A useful rule of thumb for estimating the magnitude of vertical rebound in 45° dip slip events is δ h p=Δ s /24, where Δ s is the coseismic slip on the fault.  相似文献   

12.
b
A method based on the coda attenuation law: Q = Q 0( f/f 0)v leads to the determination of the lateral variation of coda- Q in the southern part of the Iberian Peninsula using seismograms belonging to the seismological network of the Cartuja Observatory, located in Granada. The lateral variation of Q 0 ( Q value corresponding to a reference frequency f 0 of 1 Hz) and its frequency dependence for the 1 to 5 Hz frequency range are, in general, in agreement with coda- Q values for frequencies less than about 1 Hz, previously determined in the region under study.
To determine the coda- Q values analytical functions have been used to fit the magnification curves of the vertical component short-period seismographs belonging to the Cartuja network. The problem is solved by using least-squares techniques and non-linear inversion. The determined coda- Q 0 values and its frequency dependence correlate well with several known geophysical parameters in the southern part of the Iberian Peninsula.  相似文献   

13.
Summary. The geopotential is usually expressed as an infinite series of spherical harmonics, and the odd zonal harmonics are the terms independent of longitude and antisymmetric about the equator: they define the 'pear-shape' effect. The coefficients J 3, J 5, J 7, … of these harmonics have been evaluated by analysing the variations in eccentricity of 28 satellite orbits from near-equatorial to polar. Most of the orbits from our previous determination in 1974 are used again, but three new orbits are added, including two at inclinations between 62° and 63°, which have been specially observed for more than five years by the Hewitt cameras. With the help of the new orbits and revised theory, we have obtained sets of J -coefficients with standard deviations about 40 per cent lower than before. A 9-coefficient set is chosen as representative, and is as follows (all × 109): J 3=– 2530 ± 4, J 5=–245 ± 5, J 7=–336 ± 6, J 9=–90 ± 7, J 11= 159 ± 9, J 13=–158 ± 15, J 15=– 20 ± 15, J 17=– 236 ± 14, J 19=– 27 ± 19. With this set of values, the pear-shape asymmetry of the geoid (north polar minus south polar radius) amounts to 45.1 m instead of the previous 44.7 m. The accuracy of the longitude-averaged geoid profile is estimated as 50 cm, except at latitudes above 86°. The geoid profile and predicted amplitude of the oscillation in eccentricity are compared with those from other sources.  相似文献   

14.
Summary. Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity ( I ) with distance (Δ km) follows a simple relation of the type log I = log I 0 - m Δ, where I 0 is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I 0= exp [-(0.8999/ h 2+ 0.0014)Δ] where 16km ≤ h ≤ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States.  相似文献   

15.
Seismic sources with observable glut moments of spatial degree two   总被引：1，自引：0，他引：1  
Let ζΛ and r Λ. be the hypocentral position and time of an extended indigenous seismic source. Backus showed that the force moment tensors of the source, Γ( m +1, n )(ζΛ, r Λ), determine and are determined by the motion which the source produces. For small m + n , only the long-period motion is relevant. The glut moment tensor Λ( m,n ) (ζΛ, r Λ.) can be calculated uniquely from γ( m +1, n )(ζΛ r Λ) only if m = 0 or m = 1. The tensor G =Λ(2,0)(ζΛ) gives the spatial variance tensor WΛ of the source, and WΛ. roughly describes the size, shape and orientation of the source region. Therefore the failure of the observed F =Γ(3,0)(ζΛ) to determine G uniquely is of seismological interest. In the present paper we show that F determines G uniquely if we assume the source to be a simple straight line source (SSLS) or an ideal fault in an isotropic medium with isotropic prestress (IFIMIP). We give tests on F which determine whether it can come from a SSLS, from an IFIMIP or from a simple plane surface source (SPSS). If we assume the source to be a SPSS then knowing F and the fault plane determines G to within an unknown scalar multiple of a certain tensor tangent to the fault plane. Moreover F determines the fault plane uniquely unless F can come from a SSLS. If it can, then F determines this virtual source line uniquely, and F permits the fault plane to be any plane containing the virtual source line.  相似文献   

16.
This paper presents a method to invert underside-reflection ( P d P or S d S arrivals) data for lateral depth variations of upper-mantle discontinuities, combining traveltime and amplitude data. The method greatly improves the resolution of small-scale undulations obtained by existing imaging methods and does not suffer from the long-wavelength biases that are likely to be present in currently available models. Existing inversion methods account for the large size of the Fresnel zone of underside reflections, but not for its complexity, arising from the mini-max traveltime nature of PP- and SS -related waves. This neglect results in long-wavelength artefacts from small-scale undulations of the discontinuities, obscuring true long-wavelength depth variations. The inversion method presented in this paper uses a complex-valued sensitivity kernel, derived from the representation of underside reflections through a Kirchhoff integral formulation. The sensitivity kernel accounts for the varying sensitivity of the waveforms to discontinuity structure over the Fresnel zone. The method is applied to a large, synthetic data set. The data set consists of P d P amplitudes and traveltimes. The results show that the new inversion method resolves depth variations on a lateral scale that is smaller than the size of the Fresnel zone of individual underside reflections (but larger than the dominant wavelength), retaining the resolution of large-scale variations. The results presented here suggest that the discontinuity depth variations induced by slab penetration of the 670 discontinuity could be resolved by current broad-band P 670 P data sets.  相似文献   

17.
Summary. Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of 0 S 0 and 1 S 0. The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for 0 S 0 gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of 0 S 0 for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q -1 is about 5 per cent for the minimum power method. For 1 S 0 a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q -1 of about 12 per cent for the minimum power method.  相似文献   

18.
Summary. Differences between estimated average heat flow values for the Mesozoic and Cenozoic formations ( Q 1) and estimated average heat flow values for the Palaeozoic formations below the erosional unconformity ( Q 2) are calculated for the Alberta part of the western Canadian sedimentary basin. Significant heat flow differences exist for these two intervals and the map of Δ Q = Q 1– Q 2 shows that Q 2 is generally greater than Q 1 in the western and south-western part of Alberta, while in the northern part of the province Q 2 is generally less than Q 1. The regional variations of Δ Q are large, with standard deviation of 26 mW m−2 and average value –13.5 mW m−2. A regional trend of Δ Q correlates with topographic relief and the hydraulic head variations in the basin. It is shown that there is a heat flow increase with depth in water recharge areas and a decrease in heat flow with depth in the low topographic elevation water discharge areas when comparing the average heat flow in Mesozoic + Cenozoic and Palaeozoic formations.  相似文献   

19.
In this article the interaction of plane waves with a weak-contrast interface between two weakly anisotropic half-spaces is investigated. The anisotropy dealt with is of a general type. The stress–displacement vectors of the plane waves are calculated by perturbation theory. By assuming that the jump in elastic parameters and density across the interface is small, one can derive a simple expression for the R qPqP coefficient. In cases in which the wave motion is restricted to a symmetry plane of an anisotropic medium, simple expressions for the R qSVqSV and R SHSH coefficients are also derived.  相似文献   

20.
We have studied the response of normal modes to perturbations in inner-core shear velocity and attenuation, using fully coupled mode synthetics. Our results indicate that (i) mode pairs   n S l – n ±1 S l   are strongly coupled by anelasticity, (ii) this coupling causes shear velocity perturbations to strongly affect the Q values of modes through exchange of inner-core characteristics, (iii) there is no evidence for a weakly attenuating inner core in shear, and (iv) the discrepancy between attenuation models returned from normal modes and body waves is small. These results suggest that inversions for inner-core attenuation and shear velocity should be performed simultaneously and should take account of the strong cross-coupling due to attenuation.  相似文献   

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