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1.
Internal oscillations in the Earth's fluid core   总被引：4，自引：0，他引：4  
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2.
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3.
This article investigates the effects of a mushy inner core boundary on the eigenperiods of the Slichter modes for a simple, but realistic, earth model (rotating, spherical configuration, elastic inner core and mantle, neutrally stratified, inviscid, compressible liquid core). It is found that the influence of the mushy boundary layer is substantial compared with some other effects, such as those from elasticity of the mantle, non-neutral stratification of the liquid outer core and ellipticity of the Earth and centrifugal potential. The results obtained here may set a lower bound on the eigenperiods of the Slichter modes for a realistic earth model. For example, for a PREM model, the lower bound of the central period of the Slichter modes should be about 5.3 hr.  相似文献   

4.
Summary. The luni-solar forced nutations and body tide are believed to be resonant at frequencies near (1 + 1/460) cycle sidereal day−1 as seen from the rotating Earth. This resonance is due to the Earth's rotating, elliptical fluid core. We show here that tides in the open ocean and the Earth's response to those tides must also be resonant at (1 + 1/460) cycle day−1. We examine these resonant oceanic effects on the Earth's nutational motion and on the body tide. Effects on the forced nutations might be as large as 0.002 arcsec at 18.6 yr. The effects on the observed resonance in the body tide are more important. For tidal gravity, for example, the difference between K 1 and 0 1 which is usually used to determine the resonance, can be perturbed by 30 per cent or more due to the oceanic resonance effects.  相似文献   

5.
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   

6.
We have constructed a general first-order theory describing those small oscillations of a rotating elliptical earth that are affected by the presence of a liquid outer core. The theory is applicable to free core oscillations and earth tides. Care has been taken to include the effects of the wobble or nutation due to the rotation of the outer core relative to the solid earth. On the basis of the theory the free spheroidal modes of degree 2 and order 1 have been investigated. We have searched for and listed undertones with periods less than 28 hr. No upper limit to the eigenperiods has been detected. It is shown that stable, unstable and neutral polytropic cores are capable of free oscillation. At a period close to the sidereal day the spheroidal mode is accompanied by rigid rotation of the liquid outer core with respect to the solid earth. This is the well-known diurnal wobble of the Earth. It appears probable that the diurnal wobble is one of a class of similar wobbles that involve large toroidal motions in the outer core. Finally, the amplitudes of the 18·6-yr principal nutations has been computed. Excellent agreement is found with observed values.  相似文献   

7.
Summary. Small amplitude oscillations of a rotating, density-stratified fluid bounded by a spherical shell are examined. No restrictions are placed on the thickness of the shell. The internal mode spectrum is examined in the complete rotation-stratification parameter range including the regime that is appropriate for a plausible stratification distribution in the Earth's fluid core. A mathematical model is derived in terms of an eigenvalue PDE of mixed type. The existence of oscillatory solutions is exhibited in the limits of no rotation and no stratification. The frequency spectrum is extended asymptotically away from these limiting cases. A reduction in the complexity of the PDE for modes oscillating at the inertial frequency is exploited. A variational formulation is constructed in which the stratification parameter is treated as an eigenvalue of the system for fixed wave frequency. The spectral information is again extended asymptotically away from these 'accessible' points. Although the PDE reduces to Laplace's tidal equations (LTE) only under stringent parameter restrictions, it is observed that aspects of the behaviour of low frequency LTE modes are reproduced in the general model.  相似文献   

8.
This paper extends our earlier examinations of the utility of various approximations for treating the dynamics of the Earth's liquid core on time-scales of the order of 104 to 108 s. We discuss the effects of representing the response of the mantle and inner core by static (versus dynamic) Love numbers, and of invoking the subseismic approximation for treating core flow, used either only in the interior of the liquid core (SSA-1) or also at the boundaries (SSA-2). The success of each approximation (or combinations thereof) is measured by comparing the resulting surface gravity effects (computed for a given earthquake excitation), and (for the Slichter mode) the distribution of translational momentum, with reference calculations in which none of these approximations is made. We conclude that for calculations of the Slichter triplet, none of the approximations is satisfactory, i.e. a full solution (using dynamic Love numbers at elastic boundaries and no core flow approximation) is required in order to avoid spurious eigenfrequencies and to yield correct eigenfunctions (e.g. conserving translational momentum) and surface gravity. For core undertones, the use of static Love numbers at rigid boundaries is acceptable, along with SSA-1 (i.e. provided the subseismic approximation is not invoked at the core boundaries). Although the calculations presented here are for a non-rotating earth model, we argue that the principal conclusions should be applicable to the rotating Earth. Shortcomings of the subseismic approximation appear to arise because both SSA-1 and SSA-2 lower the order of the governing system of differential equations (giving rise to a singular perturbation problem), and because SSA-2 overdetermines the boundary conditions (making it impossible for solutions to satisfy all continuity requirements at core boundaries).  相似文献   

9.
The deglaciation history of Balsfjord, northern Norway, and post-glacial mass movement events were investigated. Radiocarbon dates indicate that the Balsfjord glacier retreated from the Tromsø–Lyngen moraines about 10.4 14C Ky BP. Between ca. 10.3 14C Ky BP and 9.9 14C Ky BP, deposition of a distinct end moraine–the Skjevelnes moraine–in the central part of Balsfjord occurred. The transition from glacimarine to open marine sedimentary environment took place before 9.6 14C Ky BP. Between ca. 9.5 14C Ky BP and 8.4 14C Ky BP, at least one local and three regional mass movement events occurred. After this period, no gravity flow activity is preserved in the cores. The high frequency of mass movements in the early post-glacial period is presumed to be due to fast sea level changes and/or tectonic activity induced by rapid isostatic uplift.  相似文献   

10.
Using the viscoelastic correspondence principle, we utilize the surface coseismic spheroidal deformation fields (i.e. vertical displacements, potential perturbations and gravity changes) of SNREI earth models caused by four typical types of point dislocation, derived by Sun & Okubo (1993 ), to deduce the fundamental formulas for spheroidal fields relevant to viscoelastic earth models. In computations, we employ a strike-slip dislocation on a vertical plane buried at the bottom of the lithosphere to estimate the maximal viscous relaxation responses to this kind of source that possibly exist on the surface of the earth. We take the seismic moment as 1022  N  m, which is characteristic of an average large earthquake. The numerical results demonstrate that, if we take the viscosity as 1019  Pa  s in the asthenosphere, and 1021  Pa  s in the other mantle layers, the rates of surface vertical displacements and gravity changes within about 2.5° for the 10 postseismic years are respectively 1.5–8.1  cm  yr−1 and 4.0–14.9  μgal  yr−1 : the viscous relaxation for this mantle viscosity profile proceeds much faster than for a constant mantle viscosity of 1021  Pa  s.  相似文献   

11.
We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion  18+4−3 mm yr−1  and opening of  7+3−2 mm yr−1  below a locking depth of  7+1−2 km  . The resulting deep motion of  20+4−3 mm yr−1  in the direction of  N(100+8−6)°E  agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula.  相似文献   

12.
207Pb/206Pb single-grain zircon, 40Ar/39Ar single-grain hornblende and biotite, and 40Ar/39Ar bulk-sample muscovite and biotite ages from the Nelshoogte trondhjemite pluton located in eastern Transvaal, South Africa, show that this granitoid had a protracted thermal history spanning 3213±4  Ma to about 3000  Ma. Whole-rock 40Ar/39Ar ages from cross-cutting dolerite dykes indicate that these were intruded at about 1900  Ma. There is no evidence of this or other, later events significantly affecting the argon systematics of the minerals from the pluton dated by the 40Ar/39Ar method.
  The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by 40Ar/39Ar dating of hornblende. This pole (18°N, 310°E, A 95=9°) yields a palaeolatitude of 0°, significantly different from other Archaean poles from the Kaapvaal Craton. The palaeolatitude difference implies that there was significant apparent polar wander during the Archaean. A second, overprinting magnetization seen in the pluton is also seen in the lower-Proterozoic dolerite dykes, and is consistent with other lower-Proterozoic (2150–1950  Ma) poles for southern Africa.  相似文献   

13.
We examine quantitatively the ranges of applicability of the equation Ω= A+B [1− t/t f ] m for predicting 'system-sized' failure times t f in the Earth. In applications Ω is a proxy measure for strain or crack length, and A , B and the index m are model parameters determined by curve fitting. We consider constitutive rules derived from (a) Charles' law for subcritical crack growth; (b) Voight's equation; and (c) a simple percolation model, and show in each case that this equation holds only when m < 0. When m > 0, the general solution takes the form Ω = A + B [1 + t / T  ] m , where T   is a positive time constant, and no failure time can be defined. Reported values for volcanic precursors based on rate data are found to be within the range of applicability of time-to-failure analysis ( m < 0). The same applies to seismic moment release before earthquakes, at the expense of poor retrospective predictability of the time of the a posteriori -defined main shock. In contrast, reported values based on increasing cumulative Benioff strain occur in the region where a system-sized failure time cannot be defined ( m > 0; commonly m ≈ 0.3). We conclude on physical grounds that cumulative seismic moment is preferred as the most direct measure of seismic strain. If cumulative Benioff strain is to be retained on empirical grounds, then it is important that these data either be re-examined with the independent constraint m < 0, or that for the case 0 < m + 1 < 1, a specific correction for the time-integration of cumulative data be applied, of the form ΣΩ = At + B '{1 − [1 − t/t f ] m+1 }.  相似文献   

14.
Summary. Boundary layer techniques are used to examine the dissipative decay of an internal oscillation that is a member of the inviscid spectrum of normal modes for a rotating fluid shell stratified under a radially directed gravitational field. A formula is derived for the decay factor on the so-called homogeneous spin-down time-scale. Estimates are obtained for the size of the decay factor as a function of wavelength, a function of the frequency and a function of a parameter A which measures the ratio of the stratification strength to the rotation strength. It is shown that all modes decay on the spin-down time-scale. The results are interpreted in the context of a model for the Earth's fluid core. It is observed that the presence of regions of unstable stratification may increase the decay rate for oscillations at frequencies less than twice the rotation frequency.  相似文献   

15.
Inference of mantle viscosity from GRACE and relative sea level data   总被引：12，自引：0，他引：12  
Gravity Recovery And Climate Experiment (GRACE) satellite observations of secular changes in gravity near Hudson Bay, and geological measurements of relative sea level (RSL) changes over the last 10 000 yr in the same region, are used in a Monte Carlo inversion to infer-mantle viscosity structure. The GRACE secular change in gravity shows a significant positive anomaly over a broad region (>3000 km) near Hudson Bay with a maximum of ∼2.5 μGal yr−1 slightly west of Hudson Bay. The pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ICE-5G deglaciation history, strongly suggesting a postglacial rebound origin for the gravity change. We find that the GRACE and RSL data are insensitive to mantle viscosity below 1800 km depth, a conclusion similar to that from previous studies that used only RSL data. For a mantle with homogeneous viscosity, the GRACE and RSL data require a viscosity between  1.4 × 1021  and  2.3 × 1021  Pa s. An inversion for two mantle viscosity layers separated at a depth of 670 km, shows an ensemble of viscosity structures compatible with the data. While the lowest misfit occurs for upper- and lower-mantle viscosities of  5.3 × 1020  and  2.3 × 1021  Pa s, respectively, a weaker upper mantle may be compensated by a stronger lower mantle, such that there exist other models that also provide a reasonable fit to the data. We find that the GRACE and RSL data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle.  相似文献   

16.
Samples taken from the top surfaces of boulders on the Lake Misery moraines at Arthur's Pass, in the Southern Alps of New Zealand, were analysed for 10Be by accelerator mass spectrometry. Exposure ages calculated with the currently accepted production rate, along with scaling corrections for sample latitude and elevation (42°50'S, 960 m), are: 9300 ± 990, 11,000 ± 1360, 11,410 ± 1030, 12,050 ± 960, and 12,410 ± 1180 years. We consider the date of 9300 years to be an outlier, not included in our mean exposure age of 11,720 ± 320 years for the Lake Misery moraines. Based on exposure ages and geomorphologic similarities, we compare the Lake Misery moraines with an Egesen moraine complex at Julier Pass in the Swiss Alps (46°30'N, 2200 m). Based on the 10Be, 26Al, and 36Cl exposure ages of three boulders, we calculate a mean exposure age of 11,750 ± 140 years for the outer Egesen moraine at Julier Pass. Based solely on 10Be measurements, we obtain a mean exposure age of 11,860 ± 210 years for this outer moraine. Egesen moraines in the Swiss Alps represent glacier readvance during the Younger Dryas cold reversal, based on regional correlations and on basal radiocarbon dates from bogs located up-valley of Egesen moraines. The exposure dates from Arthur's Pass and Julier Pass show synchronous glacier advances both in the Southern Alps and in the European Alps during the European Younger Dryas chronozone of Mangerud et al .  相似文献   

17.
The potential use of 40Ar/39Ar thermochronologic data from K-feldspars in reconstructing basin thermal history has been evaluated using the example of the Warburton/Cooper/Eromanga Basin, Australia's largest onshore oil- and gas-producing basin. Results from 40Ar/39Ar step-heating experiments reveal details of the evolution of the basin system, including the following: (1) the operation of high geothermal gradient regimes during the earliest basin evolution, suggesting that basin formation was active rather than passive; (2) slow cooling from a Permo-Triassic temperature peak of at least 250–300°C; (3) a rise in thermal gradients to contemporary bottom hole temperatures in the last 5–10 Myr; and (4) spatially variable recrystallization events between 100 and 50 Ma and at around 20 Ma. Initial microstructural observations serve as a useful predictor of the quality and nature of the obtainable age information. Data from 'pristine' K-feldspars may constrain the peak temperature conditions experienced in the basin, the basin's early thermal history and also any recent changes in thermal gradient. Contrasting data from texturally modified K-feldspars may constrain times of thermal transients and/or fluid flow, with the preferred interpretation that K-feldspars recrystallize in response to such events. The Warburton/Cooper/Eromanga Basin example suggests that the 40Ar/39Ar technique may serve as a useful adjunct to apatite and zircon fission track analysis and conventional organic maturation indices in basin thermal history analysis.  相似文献   

18.
We compare the present-day sediment discharge (solid phase) of some of the largest rivers in Asia to the average discharge deduced from the mass accumulated in several sedimentary basins during the Quaternary. There is a very good correlation, especially for the largest rivers: the Ganges–Brahmaputra, the Changjiang, the Huanghe and, to a lesser extent, the Indus and the Zhujiang. This suggests that present-day average discharge at the outlet has remained constant throughout the Quaternary at least for very large rivers (drainage area of the order of 105–106 km2). This, in turn, suggests either that continental denudation of large Asian catchments has remained on average constant, implying a strong tectonic control on erosion during the Quaternary, or that the river network has the ability to buffer changes in hillslope erosion or in sea-level in order to conserve the total discharge at the outlet. We show how this buffering capacity relies on the characteristic reaction time-scale of Asian alluvial plains (of the order of 105–6 years), that is, much higher than the time-scales of the Quaternary climate oscillations (of the order of 104 years). A short-term perturbation originating in hillslopes will be diluted by the floodplain. At the outlet the signal should have a longer time span and a smaller amplitude. In the same manner, an alluvial plain should not instantaneously react to a 104-year sea-level drop because of its inertia. Along with long-term tectonic control we infer this buffering to be the main cause for the average constancy of sediment yield of large Asian rivers during the Quaternary.  相似文献   

19.
Gravity changes are presented from a series of field microgravity surveys conducted at Mt Etna between August 1994 and November 1996, a period including the 1995–1996 explosive summit activity. Data were collected along a microgravity network of 69 stations at a monthly to annual sampling rate, depending on each subarray of the network.
  Results show that seasonal changes in water level within the volcano may induce gravity changes of up to 20  μgal on Etna's southern slope, and indicate that significant magma movement occurred within and below Etna's edifice between 1994 and 1996. In particular, between September 1994 and October 1995, a mass increase of 2 × 1010  kg occurred 2000  m beneath the summit craters. Between October 1995 and July 1996 this mass was lost, while another 2 × 1010  kg was injected at about 1000  m  a.s.l. into the 1989 fracture system. From the gravity data alone, it is not possible to distinguish whether the first shallow intrusion (1994–1995) was then injected laterally into the 1989 fracture, or summit activity was fed by the first shallow intrusion, while new magma entered the 1989 fracture system.
  While magma was being redistributed within the volcanic edifice, measurements along an E–W-trending profile on the southern slope of the volcano detected some 1.5 × 1011  kg of magma accumulating 2–3  km below sea level between October 1995 and November 1996.  相似文献   

20.
Summary The linearized equation of motion for the slightly elliptical rotating earth is obtained and using Phinney & Burridge's generalized spherical harmonics, the variational principle is derived for the normal mode oscillations of the Earth. The numerical solutions of two earth models 1066B and B1S6 are searched by minimizing the energy functional for the terrestrial spectral range longer than the lowest order free oscillation. The periods of core modes computed for the earth model B1S6, with stably stratified outer core, ranges from about 4 to 13hr and the periods for the 1066B are much more spread without clustering around the periods of 6 and 12 hr as in B1S6. The results for the earth model 1066B indicate that an outer core can support long-period oscillations even when it is not stably stratified. The Chandler wobble periods obtained are 402.3 day for B1S6 and 402.7 day for 1066B.  相似文献   

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