摘要:

Long‐period (100–330 s) fundamental‐mode Love and Rayleigh waves have been processed to measure the great circle phase velocities for about 200 and 250 paths, respectively. The observations are inverted for regionalized phase velocities and for an even‐order harmonic expansion of the lateral velocity heterogeneity. The regionalized inversions achieve a maximum variance reduction of about 65% and 85% for the Love and the Rayleigh wave data, respectively. Thelmax= 2 inversions give a maximum variance reduction of about 60% and 90% for Love and Rayleigh waves, respectively. Thelmax= 8 inversion does not make a large improvement in the fit. The Love wave phase velocities have more power inl= 4 and 6, relative tol= 2, than the Rayleigh waves. For both Love and Rayleigh wave data the sectoral component dominates thel= 2 harmonics, and this component is stable if we increaselmax from 2 to 6. Heat flow also has strong sectoral components (lm= 22), which are approximately in phase with those of the phase velocities. Thel= 2 harmonics of the nonhydrostatic geoid are dominated by large zonal (lm= 20) and moderate sectoral components. The sectoral components are in phase with those of the phase velocities. The sectoral pattern of heat flow and phase velocity is controlled by high heat flow‐low velocity of the East Pacific Rise and western North America, which is reinforced by low velocities in the antipodal region (Red Sea‐Gulf of Aden‐East African Rift). By contrast the geoidl= 2 pattern is dominated by geoid highs over the western Pacific subduction zones. A spherical harmonic expansion of regionalized phase velocities shows that they havel= 2 variations similar to those of thelmax = 2 nonregionalized inversions. This means that the regionalization approach is appropriate as a first step for studying lateral heterogeneity of the earth. However, the great circle phase velocities are not sufficient by themselves to uniquely locate the lateral heterogeneity. The same is true for free oscillation data. Regions of convergence have the interesting property of being slow for short‐period waves and fast, faster than shields, for lo

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10267

年代:1983

数据来源: WILEY

摘要:

In low‐frequency seismic spectra the Coriolis force theoretically causes quasi‐degenerate coupling of spheroidal and toroidal modes whose spherical harmonic degrees differ by unity and whose frequencies are close. It has been found that the coupling causes clearly observable effects on seismic spectra below 3.0 mHz. Many fundamental toroidal modes have a spheroidal component and vice versa. For example, the0Slmodes forl= 8–22 are significantly coupled to the0Tlmodes forl= 9–23. The mean frequencies of pairs of coupled multiplets are repelled, and the mean attenuations are averaged. The shifting of complex frequencies must be adequately encompassed in the construction of spherically averaged and aspherical models of the density and elastic and anelastic structure of the earth. The coupling also causes variations in multiplet amplitudes which result in spectral peaks at toroidal mode frequencies on vertical and radial components and at spheroidal mode frequencies on transverse components. The variations must be taken into account in constructing Green's functions for the study of low‐frequency source m

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10285

年代:1983

数据来源: WILEY

摘要:

The homogeneous set of 80‐year‐long (1900–1979) International Latitude Service (ILS) polar motion data is analyzed using the autoregressive method (Chao and Gilbert, 1980), which resolves and produces estimates for the complex frequency (or frequency andQ) and complex amplitude (or amplitude and phase) of each harmonic component in the data. Principal conclusions of this analysis are that (1) the ILS data support the multiple‐component hypothesis of the Chandler wobble (it is found that the Chandler wobble can be adequately modeled as a linear combination of four (coherent) harmonic components, each of which represents a steady, nearly circular, prograde motion, a behavior that is inconsistent with the hypothesis of a single Chandler period excited in a temporally and/or spatially random fashion), (2) the four‐component Chandler wobble model “explains” the apparent phase reversal during 1920–1940 and the pre‐1950 empirical period‐amplitude relation, (3) the annual wobble is shown to be rather stationary over the years both in amplitude and in phase, and no evidence is found to support the large variations reported by earlier investigations, (4) the Markowitz wobble is found to be marginally retro‐grade and appears to have a complicated behavior which cannot be resolved because of the shor

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10299

年代:1983

数据来源: WILEY

摘要:

An efficient method is developed to compute gravitational harmonics from low‐low satellite‐satellite range rate measurements. The satellites are assumed to be in nearly the same low eccentricity orbits. The residual range rate signal is modeled with frequencies derived from linear perturbation theory to an accuracy of about 99%. Significant nonlinear effects involving J2, not currently modeled, require both J2and J3to be known in the reference trajectories. Each harmonic (l,m) generatesl+ 1 principal frequencies, but they are not unique. Yet it appears possible to design a low‐altitude mission which keeps the pair at nearly constant separation and where the frequencies for all terms to (180,180) are separable after only about 4 weeks. A simple demonstration of the method is shown to recover (in two iterations) a complete (4,4) model (less J2and J3) from 1 day of “perfect” measurements (every 7 min) generated by numerical integration. In this result, the effects of orbit determination are included in a crude way, but no other gravitational effects (of higher degree or from luni‐solar attraction) are present. Nevertheless, the method is easily extended to high degree with rapid new techniques (which are described) for calculating the required inclination functions of

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10309

年代:1983

数据来源: WILEY

摘要:

The momentum equations applied to 5° block means are integrated from the observed surface plate velocities downward to a depth of 280 km, assuming no lateral heterogeneities in density or viscosity. It is assumed that 85% of the global heat productionQG= 0.85 × 4.0 ×1013W comes from below 280 km and that at this level the transfer is fully convective. A temperature field 7 is inferred at depth 280 km by minimizing the quantity∫|T− T0|ndS + nλ{QG−pC∫(T−T0)υrdS}, where the integrals are over the sphere, ρ is density,Cis heat capacity,υris radial velocity, T0 is a prescribed mean, and λ is a Lagrangian multiplier. Norms n, ranging from 1.5 to 2.5, are tried. The intervening temperature fields are then inferred, integrating the energy equation downward by using the previously calculated velocity field. This integration is subject to the limitations that the derivative of the temperature with respect to depth is everywhere sufficient to attain the fully convecting temperature, but never less than adiabatic. A surface heat flow based on observations plus age and tectonic setting is used. The principal inferences are: (1) the greatest lateral variations in temperature, ∼1000°C, occur with the top 20 km; (2) the greatest advection, ∼200°C/m.y., occurs within the top 20 km; (3) below 50 km, the greatest departures of temperature from the mean are negative “tongues,” reaching an extreme of about −800°C at depth 100 km; (4) below 50 km, heat transfer becomes more convective than conductive; (5) at the fully convecting level, 280 km, temperature variations are at least ±180°C about the mean. The principal defect in the entire calculation is unrealistically low temperatures arising from unrepresentatively low surface heat flows. The principal defect of the model probably arises from the assumption that all heat transfer at a depth of 280 km is representable by 5°

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10323

年代:1983

数据来源: WILEY

摘要:

Evidence for a martensitic type mechanism for the olivine‐spinel (α − γ) transition in Fe2SiO4has resulted from an investigation by rapid acquisition of diffraction data collected on fayalite subjected to high temperature and pressure in a diamond anvil cell using an intense X ray beam from the Cornell High Energy Synchrotron Source (CHESS). An energy‐dispersive technique employing a white X ray beam and a solid‐state detector made it possible to collect a complete diffraction pattern every 2 min as pressure was increased across the phase transition at 520°C. The series of patterns clearly showed that the γ phase peaks which appeared first were those also compatible with a structure whose unit cell dimensions are half those of the spinel unit cell. After the time of three to four spectrum acquisitions, all of the peaks of the spinel structure were present. This same effect was observed in two other series. An additional series was run at 400°C. As soon as the γ phase peaks appeared, the run was halted, and the sample was quenched and examined by the Debye‐Scherrer method. The only γ phase peaks observed were those also allowed for a structure with the smaller unit cell. These results are consistent with a restacking of the anions from the approximately hexagonal close‐packed (hcp) arrangement in the olivine structure to the nearly cubic close‐packed (ccp) arrangement in the spinel structure, followed later by a reord

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10333

年代:1983

数据来源: WILEY

摘要:

A theoretical derivation is made for the effective, frequency‐dependent thermoelastic bulk modulusK*=σ¯/θ¯of an isotropic composite subjected to hydrostatic stressσ¯eiωtproducing volumetric strainθ¯eiωt. No heat transfer is permitted to occur at the outer boundary of the composite, but as a consequence of differential heating, heat transfer among the constituents, and hence dissipation, occurs in the interior. Thus between the limits ω = 0 and ω = ∞K*(ω) is complex and the thermoelastic damping may be measured byQK−1= Im (K*)/Re(K*). Parametric studies show the influence of various elastic and thermal properties on thermoelastic damping. Numerical calculations are made for a hypothetical lower mantle assemblage of stishovite and magnesiowüstite. The results show that thermoelastic dissipation in the lower mantle can account for the observed attenuation of the fundamental radial normal mode and can also provide interesting constra

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10343

年代:1983

数据来源: WILEY

摘要:

We derive approximate quantitative criteria for the resolution and reconstruction of the acoustic impedance for horizontally stratified media calculated from band‐limited seismic data both without and with noise. The first criterion, for noise‐free data, relates the calculated acoustic impedance to the true acoustic impedance using only the DC (zero frequency) level of the seismic source or wavelet. The second criterion, for noisy data, predicts the band of uncertainty of the calculated impedance. The third criterion is the combination of the first two. One of the most important results of this paper is that the average shape of the acoustic impedance can be estimated from data without wavelet deconvolution, if the D.C. component of the wavelet function is not z

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10349

年代:1983

数据来源: WILEY

摘要:

The dependence of the friction force on slip history is described by an experimentally motivated constitutive law where the friction force is dependent on slip rate and state variables. The state variables are defined macroscopically by evolution equations for their rates of change in terms of their present values and slip rate. Experiments may strongly suggest that one state variable is adequate or prove that one is inadequate. Analysis of steady slip governed by a single state variable in a spring and (massless) slider predict oscillations at a critical spring stiffnessk=kcrit. The critical stiffnesskcritis given by a simple formula and steady slip is stable fork>kcritand unstable fork

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10359

年代:1983

数据来源: WILEY

摘要:

The St. Helens seismic zone (SHZ) is a crustal earthquake zone in southwestern Washington that is defined by small‐ to moderate‐magnitude (2.5 to 5.5) earthquakes and focal mechanisms. This zone, seismically active along a distance of 90 km, is interpreted as a fault zone capable of generating a moderate‐ to large‐magnitude shallow earthquake. At the northern end of this seismic zone (about 15 km north of the Cowlitz River) a large area of seismic quiescence (for earthquakes above magnitude 3.0) has predominated since 1977. This quiescence leaves open the question whether the SHZ continues northward into southern Puget Sound. Earthquake focal mechanisms along the SHZ indicating nearly vertical strike‐slip faulting with the preferred fault planes striking north–south. From these focal mechanisms we infer that the direction of maximum compression is northeast, approximately parallel with the direction of plate convergence between the North American and Juan de Fuca plates. We interpret the focal mechanism data as evidence for locked subduction. This tectonic model raises the possibility of a large‐magnitude subduction earthquake. Neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment f

ISSN:0148-0227    

DOI:10.1029/JB088iB12p10371

年代:1983

数据来源: WILEY