Inertia Of Earth Research Articles

Most of the Long-Term Solar Radiation Forcing on the Climate Is Due to the Strong Feedback Mechanisms

The main interactions of the Total Solar Irradiance (TSI) variations with the climatic system are considered. This research introduced new physically based indexes for 11-year cycles: the integral relative energetic power of the cycles solar activity (SA) and TSI, as well as the cycle-weighted mean value both of the TSI and sunspot number. The observed cycle-average TSI value in cycles XXII–XXIV has decreased by more than 0.5 W/m2, indicating its quasi-bicentennial decline. The onset of the Maunder-type Grand minimum phase of the SA and TSI of the quasi-bicentennial cycle (BCC) is expected in the 27th or 28th 11-year cycle in 2042 or 2053. The Earth's thermal inertia will cause a temperature reduction of around 0.25 K due to the deficit in TSI during the declining phase of the bicentennial cycle due to its negative impact on Earth's energy imbalance. As a result, the Bond's albedo will increase, and the abundance of atmospheric water vapor and carbon dioxide will decrease as a response to a direct solar forcing. BCC feedback mechanisms will also continue operating during both its minimum and maximum phases. The feedback mechanisms will continue further in short periods (about 30 years) at the beginning of BCC's growth and decline phases. These long-term self-reinforcing feedback effects will act as a response to the TSI, forcing in such a way as to trigger a strong amplification of the initial direct input to the corresponding temperature changes. They will lead to a significant increasing cooling in 2070 or 2080. Variations of the galactic cosmic rays and cloud cover practically do not affect the climate. The quasi-bicentennial cyclicity of the TSI also provides simultaneous climate variations on all Solar system planets.

Passive heating in thermally compliant office building: Validated and simulated scenarios

This work aims at answering the question–to what extent thermally compliant buildings are involved in ensuring the winter indoor thermal comfort of occupants in cold semi-arid climates in Morocco. Real-time monitoring of a new-built university building located in eastern Morocco was conducted. The good agreement between the measured and simulated temperatures of the case study building confirmed the accuracy of the developed model. Thereafter, the thermal performance of different external wall configurations was investigated. The results show that compliant buildings guarantee better comfort conditions in winter by passively increasing the indoor air temperature by up to 2 °C, resulting in a maximum percentage of dissatisfaction of less than 13%. The winter discomfort hours based on a temperature set point of 16 °C are reduced by 95% compared to typical buildings. Moreover, the thermal compliance of buildings significantly reduces the daily air temperature amplitude, leading to a periodic dynamic behaviour without peak loads, which could be effective for the building energy management. The efficacy of earthen walls in regulating indoor temperatures was also confirmed due to the excellent thermal inertia of earth. For optimum winter comfort (20 °C), the implementation of the Moroccan building energy code shows up to 72% of reduction in heating demand when applied to conventional buildings.

Parameter analysis of Earth-air heat exchangers over multi-layered soils in South Brazil

Earth-air heat exchangers (EAHE) represent a sustainable way to explore the soil to heat or cool buildings. Due to the Earth's thermal inertia, its superficial layers can be used as a heat source or sink, respectively, in the winter or summer. Hence, the EAHE employ buried ducts where the air is forced to flow and exchange heat with the soil. Such devices demand little energy, as they can use low powered fans or passive techniques for natural convection. Recurrent issues in the literature of EAHE explore how to improve their performances by analyzing their operational parameters; soil, climate, and global economic prospects; the development and validation of complete or simplified simulation models. This paper presents parameter analysis of EAHE to increase their thermal potentials (which is also related to the efficiency and the prospects for thermal comfort) and the heat exchanges (linked with the economy) considering a Brazilian city in a subtropical climate. The research uses simulations based on a validated 1D model. It is compared with three others, including 3D ones, giving more accurate results. Unlike most works covering 1D models, the methodology proposed here allows the study of multi-layered soils. After assessing the soil temperature variations, the results point out that most of the local thermal potential is attainable at relatively low depths, with magnitude peaks of 5°C. The parameter analyses are focused on this target potential, addressing changes in the airspeed, length, and diameter of the ducts. For all cases, the goal was achieved, allowing to obtain annual efficiencies close to 90%. However, only the tests increasing the length also increased the quantity of heat exchanged (in about 20%) and, consequently, proved to be economically affordable.

ГЕОДИНАМІКА

This study aims to derive the Earth’s temporally varying Earth’s tensor of inertia based on the dynamical ellipticity , the coefficients , from UT/CSR data. They allow to find the time-varying Earth’s mechanical and geometrical parameters during the following periods: (a) from 1976 to 2020 based on monthly and weekly solutions of the coefficient ; (b) from 1992 to 2020 based on monthly and weekly solutions of the non zero coefficients , related to the principal axes of inertia, allowing to build models their long-term variations. Differences between and , given in various systems, represent the average value , which is smaller than time variations of or , characterizing a high quality of UT/CSR solutions. Two models for the time-dependent dynamical ellipticity were constructed using long-term variations for the zonal coefficient during the past 44 and 27.5 years. The approximate formulas for the time-dependent dynamical ellipticity were provided by the additional estimation of each parameter of the Taylor series, fixing at epoch =J2000 according to the IAU2000/2006 precession-nutation theory. The potential of the time-dependent gravitational quadrupole according to Maxwell theory was used to derive the new exact formulas for the orientation of the principal axes , , via location of the two quadrupole axes. Hence, the Earth’s time-dependent mechanical and geometrical parameters, including the gravitational quadrupole, the principal axes and the principal moments of inertia were computed at each moment during the past 27.5 years from 1992 to 2020. However, their linear change in all the considered parameters is rather unclear because of their various behavior on different time-intervals including variations of a sign of the considered effects due to a jump in the time-series during the time-period 1998 – 2002. The Earth’s 3D and 1D density models were constructed based on the restricted solution of the 3D Cartesian moments inside the ellipsoid of the revolution. They were derived with conditions to conserve the time-dependent gravitational potential from zero to second degree, the dynamical ellipticity, the polar flattening, basic radial jumps of density as sampled for the PREM model, and the long-term variations in space-time mass density distribution. It is important to note that in solving the inverse problem, the time dependence in the Earth's inertia tensor arises due to changes in the Earth's density, but does not depend on changes in its shape, which is confirmed by the corresponding equations where flattening is canceled.

New estimation of triaxial three-layered Earth's inertia tensor and solutions of Earth rotation normal modes

Until now, the calculation of the principal inertia moment of the triaxial three-layered Earth mainly adopts the scaling method. This method assumes that the corresponding principal inertia axes of the layers coincide each other, but this is not the case. In this paper, a rigorous tensor transformation rule is adopted to calculate the principal inertia moments (PIMs) of different layers. Appling the new estimated PIMs to the triaxial three-layered Earth rotation theory with considering various couplings, the numerical calculations show that the periods of the Chandler Wobble (CW), Free Core Nutation (FCN), Free Inner Core Nutation (FICN) and Inner Core Wobble (ICW) are respectively 433.0, 430.8, 943.9 and 2735.9 mean solar days, which are well comparable with the corresponding values accepted at present in geoscience community. Better estimates of the PIMs of different layers may provide better constrains on relevant physical parameters of the Earth's interior.

Measurement and validation of tsunami Eigen values for the various water wave conditions

This paper proposes the mathematical model to investigate how to compute and analyze the tsunami wave parameters and the retrieval of its early warning using remote sensing technique to improve the detection rate. The different methods available for the evaluation of the tsunami detection systems are also introduced. This research work is to compute and analyze the tsunami wave parameters such as Eigen functions in deep, intermediate and shallow water regions.The computational and algorithmic analysis of tsunami wave parameters are achieved in three steps: the first step is to calculate the earthquake fault parameters such as length, width, area and displacement from the epicenter, which corresponds to the effect of its variation with respect to the earthquake magnitude. The variation of each fault parameters have been measured and recorded out over the certain moment of magnitudes (M). This shows smaller values as compared to the earthquake magnitude M9.5 at which all the parameters shows very large response. The second step involves the measurements of water wave angular frequency in deep, intermediate and shallower region of ocean. The values of angular frequencies in deep water shows the lesser response as compared to other two water conditions because, in deep water the angular velocity of water particles travels with the lesser speed as compared to the shallower region as per Airy's wave theory. The simulation results show the angular velocity in Deep Ocean is very less, moderate in the intermediate water and very high on the coastal region. It indicates the higher impact on coastal region. The third step involves the simulation study of measurement of tsunami Eigen functions such as orbital velocity, acceleration, wave potential in deep, intermediate and shallower water regions. The simulation result shows the resultant orbital velocity and accelerations in deep, intermediate and shallow water regions provides the similar response as angular velocity due to same proportion of the water displacement from deep to coastal regions. Hence the dynamic forces are very high approximately 1 km from the beaches. Once the Tsunami waves approaches to the beach, wave height increases whereas particle acceleration decreases because near the coast, due to geological structure of the earth's surface, inertia and gravity forces increase to the extreme limits, while particle acceleration and velocity values reach close to zero. The wave potential Eigen function has also been plotted and measured with respect to the water depth condition and the similar response is obtained in the simulation result. The validation of each result has also been presented with the standard simulated data, which shows the results as less than 10% of the accuracy.

The Model of Dependence of the Temperature of the Surface Layer of Atmosphere from the Earth's Albedo and Thermal Inertia of the Hydrosphere

The assumption of M. Milankovich about the constancy of the Earth's albedo during the interglacial period was replaced with the alternative one. The model was developed where anomalies of the average annual temperature of the surface atmosphere were related with interannual changes in the planetary albedo and the thermal inertia of the hydrosphere. The surface temperature changes due to albedo actual and model changes were calculated. Possible external causes of albedo changes were considered.

What drives 20th century polar motion?

Astrometric and geodetic measurements show that the mean position of Earth's spin axis drifted through the solid crust toward Labrador, Canada at an average speed of 10.5±0.9 cm/yr during the 20th century. Understanding the origins of this secular polar motion (SPM) has significance for modeling the global climate, as it provides a link to ice mass balance and sea-level rise. A perplexing issue, however, is that while glacial isostatic adjustment (GIA) models satisfactorily explain the direction of SPM, the associated prediction of the amplitude is insufficient. Our Bayesian GIA analysis, with constraints from relative sea-level and vertical land motion data, reveals that this process only accounts for 33±18% of the observed SPM amplitude. This shortfall motivates a more broadly scoped reassessment of SPM drivers. To address this, we assemble a complete reconstruction of Earth's surface mass transport derived from recent advancements in modeling the global 20th century cryospheric, hydrologic, oceanic, and seismogenic mass exchange. The summed signals, nonetheless, cannot fully reconcile the observed SPM, even when considering the error statistics of each driver. We investigate an additional excitation source: changes in Earth's inertia tensor caused by mantle convection. Sophisticated models have recently been advanced in tectonic plate reconstructions, in conjunction with geoid and seismic tomographic models. Here we use these models to compute new estimates of SPM. While the convection-driven SPM has considerable uncertainty, the average direction of 283 recent models aligns with the residual SPM (within 2.7∘±14.8∘), significantly reducing the gap between observation and prediction. We assert that one key mechanism for driving 20th century SPM is long-term mass movement due to mantle convection.

Anomalous Late Jurassic motion of the Pacific Plate with implications for true polar wander

True polar wander, or TPW, is the rotation of the entire mantle–crust system about an equatorial axis that results in a coherent velocity contribution for all lithospheric plates. One of the most recent candidate TPW events consists of a ∼30° rotation during Late Jurassic time (160–145 Ma). However, existing paleomagnetic documentation of this event derives exclusively from continents, which compose less than 50% of the Earth's surface area and may not reflect motion of the entire mantle–crust system. Additional paleopositional information from the Pacific Basin would significantly enhance coverage of the Earth's surface and allow more rigorous testing for the occurrence of TPW. We perform paleomagnetic analyses on core samples from Ocean Drilling Program (ODP) Site 801B, which were taken from the oldest available Pacific crust, to determine its paleolatitude during the Late Jurassic and Early Cretaceous (167–133 Ma). We find that the Pacific Plate underwent a steady southward drift of 0.49°–0.74° My−1 except for an interval between Kimmeridgian and Tithonian time (157–147 Ma), during which it underwent northward motion at 1.45° ± 0.76° My−1 (1σ). This trajectory indicates that the plates of the Pacific Basin participated in the same large-amplitude (∼30°) rotation as continental lithosphere in the 160–145 Ma interval. Such coherent motion of a large majority of the Earth's surface strongly supports the occurrence of TPW, suggesting that a combination of subducting slabs and rising mantle plumes was sufficient to significantly perturb the Earth's inertia tensor in the Late Jurassic.

Global geodynamic changes induced by all major earthquakes, 1976–2015

AbstractUsing normal‐mode summations we calculate the coseismic changes in a suite of global geodynamic quantities produced by all the 43,304 major earthquakes that occurred during 1976–2015 from the Global Centroid Moment Tensor catalogue. We calculate changes for the Earth's inertia tensor elements and the ensuing Earth's rotation variations, as well as the monopolar changes of the volume ΔV(r), the total moment of inertia ΔT(r), and the gravitational energy ΔEg(r) as functions of the radius inside the Earth. The cumulative geodynamic changes are found to have strong statistical trends that are continuing from the past and further strengthened in recent years. Statistical analyses indicate that the thrust‐faulting earthquakes are predominantly responsible for these trends, as they are the dominant type overshadowing the normal‐faulting earthquakes that tend to produce the opposite effects. In particular, we report that the polar motion excitations due to the few greatest earthquakes since 2004 have facilitated the abrupt turn of the pole path observed during these years. For the monopolar parameters, we find an overall shrinkage in Earth's volume V(r) internally but an expansion near Earth's surface. The ensuing gravitational instability cannot be sustained over time and will be reversed by aseismic and interseismic deformations. The ΔT(r) and ΔEg(r) also exhibit trends where the coseismic action of earthquakes collectively makes the Earth more compact and dissipate a great amount of gravitational energy. Although aseismic effects are not addressed here, these coseismic trends are in keeping with the long‐term physical evolution of the Earth.

Co-seismic change of length of day based on the point dislocation theory for a SNREI Earth

We developed a new approach for computing the changes in the length of day (LOD) due to earthquakes from both shear- and tensile-type faults. This approach was based on the point dislocation theory for a SNREI Earth, which was validated by comparing our results with the ones obtained by the normal mode method. We defined two numerical functions that vary with hypocenter depth. These functions allow us to compute the co-seismic change of the trace of the Earth's inertia tensor, and hence the co-seismic LOD change. For future applications, by adding a trace term, we corrected Lambeck's formulas (Lambeck, 1980), which are being commonly used for computing co-seismic LOD change. Finally, the new approach was used to compute the co-seismic LOD change from 1977 to 2011. The results show that the co-seismic trace and the co-seismic J2 changes contribute to the LOD change with the same magnitude (Gross and Chao, 2006). This means that, unlike other deformation mechanisms, the trace term cannot be neglected when modeling the co-seismic LOD change. The earthquakes from 1977 to 2011 decreased the LOD by about 12μs, and the rate of decrease was enhanced after the 2004 Sumatra earthquake due to several large earthquakes.

Effects of Huge Earthquakes on Earth Rotation and the length of Day

We calculated the co-seismic Earth rotation changes for several typical great earthquakes since 1960 based on Dahlen’s analytical expression of Earth inertia moment change, the excitation functions of polar motion and, variation in the length of a day (ΔLOD). Then, we derived a mathematical relation between polar motion and earthquake parameters, to prove that the amplitude of polar motion is independent of longitude. Because the analytical expression of Dahlen’s theory is useful to theoretically estimate rotation changes by earthquakes having different seismic parameters, we show results for polar motion and ΔLOD for various types of earthquakes in a comprehensive manner. The modeled results show that the seismic effect on the Earth’s rotation decreases gradually with increased latitude if other parameters are unchanged. The Earth’s rotational change is symmetrical for a 45° dip angle and the maximum changes appear at the equator and poles. Earthquakes at a medium dip angle and low latitudes produce large rotation changes. As an example, we calculate the polar motion and ΔLOD caused by the 2011 Tohoku-Oki Earthquake using two different fault models. Results show that a fine slip fault model is useful to compute co-seismic Earth rotation change. The obtained results indicate Dahlen’s method gives good approximations for computation of co-seismic rotation changes, but there are some differences if one considers detailed fault slip distributions. Finally we analyze and discuss the co-seismic Earth rotation change signal using GRACE data, showing that such a signal is hard to be detected at present, but it might be detected under some conditions. Numerical results of this study will serve as a good indicator to check if satellite observations such as GRACE can detect a seismic rotation change when a great earthquake occur.

Contribution of ground surface altitude difference to thermal anomaly detection using satellite images: Application to volcanic/geothermal complexes in the Andes of Central Chile

Detection of thermal anomalies in volcanic-geothermal areas using remote sensing methodologies requires the subtraction of temperatures, not provided by geothermal manifestations (e.g. hot springs, fumaroles, active craters), from satellite image kinetic temperature, which is assumed to correspond to the ground surface temperature. Temperatures that have been subtracted in current models include those derived from the atmospheric transmittance, reflectance of the Earth's surface (albedo), topography effect, thermal inertia and geographic position effect. We propose a model that includes a new parameter (K) that accounts for the variation of temperature with ground surface altitude difference in areas where steep relief exists. The proposed model was developed and applied, using ASTER satellite images, in two Andean volcanic/geothermal complexes (Descabezado Grande-Cerro Azul Volcanic Complex and Planchón-Peteroa-Azufre Volcanic Complex) where field data of atmosphere and ground surface temperature as well as radiation for albedo calibration were obtained in 10 selected sites. The study area was divided into three zones (Northern, Central and Southern zones) where the thermal anomalies were obtained independently. K value calculated for night images of the three zones are better constrained and resulted to be very similar to the Environmental Lapse Rate (ELR) determined for a stable atmosphere (ELR>7°C/km). Using the proposed model, numerous thermal anomalies in areas of ≥90m×90m were identified that were successfully cross-checked in the field. Night images provide more reliable information for thermal anomaly detection than day images because they record higher temperature contrast between geothermal areas and its surroundings and correspond to more stable atmospheric condition at the time of image acquisition.

New estimates of the inertia tensor and rotation of the triaxial nonrigid Earth

The Earth's rotation is perturbed by mass redistributions and relative motions within the Earth system, as well as by the torques from both the internal Earth and celestial bodies. The present study aims to establish a theory to incorporate all these factors perturbing the rotation state of the triaxial Earth, just like the traditional rotation theory of the axial‐symmetric Earth. First of all, we reestimate the Earth's inertia tensor on the basis of two new gravity models, EIGEN‐GL05C and EGM2008. Then we formulate the dynamic equations and obtain their normal modes for an Earth model with a triaxial anelastic mantle, a triaxial fluid core, and dissipative oceans. The periods of the Chandler wobble and the free core nutation are successfully recovered, being ∼433 and ∼430 mean solar days, respectively. Further, the Liouville equations and their general solutions for that triaxial nonrigid Earth are deduced. The Liouville equations are characterized by the complex frequency‐dependent transfer functions, which incorporate the effects of triaxialities and deformations of both the mantle and the core, as well as the effects of the mantle anelasticity, the equilibrium, and dissipative ocean tides. Complex transfer functions just reflect the fact that decays and phase lags exist in the Earth's response to the periodic forcing. Our theory reduces to the traditional rotation theory of the axial‐symmetric Earth when assuming rotational symmetry of the inertia tensor. Finally, the present theory is applied to the case of atmospheric‐oceanic excitation. The effective atmospheric‐oceanic angular momentum function (AMF)χeff=χeff1+iχeff2for the present theory is compared with the AMFχeffsym=χeff1sym+iχeff2symfor the traditional theory and the observed AMFχobs=χ1obs+iχ2obs; we find that the difference betweenχeffandχeffsymis of a few milliseconds of arc (mas) and can sometimes exceed 10 mas. In addition, spectrum analyses indicate thatχeffis in good agreement withχeffsymand, further, show an increase of coherency withχobsespecially in the low‐frequency band. The obvious advantage ofχeffin the low‐frequency band with respect toχeffsymis the critical support of the present theory. However, still better performance of our theory can be expected if the models of the mantle anelasticity and oceanic dynamics were improved. Thus we conclude that the traditional Earth rotation theory should be revised and upgraded to include the effects of the Earth's triaxiality, the mantle anelasticity, and oceanic dynamics. The theory presented in this study might be more appropriate to describe the rotation of the triaxial Earth (or other triaxial celestial bodies such as Mars), though further studies are needed to incorporate the effects of the solid inner core and other possible influences.

Ocean tidal effects on Earth rotation

Abstract Tidal forces due to the tide-raising potential deform the solid and fluid regions of the Earth, causing the Earth's inertia tensor to change, and hence causing the Earth's rate of rotation and length-of-day to change. Because both the tide-raising potential and the solid Earth's elastic response to the tidal forces caused by this potential are well-known, accurate models for the effects of the elastic solid body tides on the Earth's rotation are available. However, models for the effect of the ocean tides on the Earth's rotation are more problematic because of the need to model the dynamic response of the oceans to the tidal forces. Hydrodynamic ocean tide models that have recently become available are evaluated here for their ability to account for long-period ocean tidal signals in length-of-day observations. Of the models tested here, the older altimetric data-constrained model of Kantha et al. (1998) is shown to still do the best job of accounting for ocean tidal effects in length-of-day, particularly at the fortnightly tidal frequency. The model currently recommended by the IERS is shown to do the worst job.

Long-term polar motion excited by ocean thermal expansion

[1] Ocean warming is commonly considered unable to excite significant long-term trends in polar motion. Here, however, we argue that this assumption needs to be revised. We demonstrate that steric sea level rise leads to a distinct pattern of horizontal mass redistribution within ocean basins and hence to ocean bottom pressure changes that alter Earth's inertia tensor on decadal and longer time scales. Based on Earth system model simulations, we estimate that ocean warming leads to polar motion of 0.15 to 0.20 milli-arcseconds per one millimeter of thermal sea level rise. This is equivalent to a polar motion rate of about 0.47 milli-arcseconds per year towards 155°W to 160°W for current projections of steric sea level rise during the 21st century. The proposed polar motion signal is therefore not negligible in comparison to other decadal and secular signals, and should be accounted for in the interpretation of polar motion observations.

Gravitational effects from earthquakes

Two types of propagating gravitational effects, from the mass redistribution within the Earth due to a large earthquake, are investigated: (i) the velocity of the change of the Newtonian potential field; and (ii) the gravitational luminosity of the seismic source. The mass redistribution caused by an earthquake and the resulting change in the gravitational potential field is computed through application of geophysical dislocation theory. The global mass redistribution is postulated to be progressive, starting at the instant (and location) of the nucleation of the earthquake fault rupture, and then spreading globally at the velocities of various seismic waves. Information about the changes of the gravitational potential is postulated to travel at the velocity of light. Superconducting gravimeters (SG) can resolve changes of the order of 10 nGal, i.e., (10–9 cm/s2) (1 Gal = 0.001 0197g), sufficient to detect the changes in the potential field. The time difference between observation of the change of the potential field and the arrival of the primary seismic wave from the earthquake would allow a crude estimation of the velocity of the gravitational effect. A preliminary search for the preseismic gravitational signal using an SG has given inconclusive results, primarily due to the limitations of the spline curve fitting methods. Despite this, we suggest that the observation of preseismic gravitational potential changes should be feasible, with the existing array of SGs in the Global Geodynamics Project (GGP) network, and by detectors designed to observe gravitational radiation (e.g., the Laser Interferometer Gravitational-wave Observatory (LIGO)). We have used published values of the changes in the Earth's inertia tensor due to the Alaska earthquake of 1964 to estimate the magnitude of the metric perturbation of the gravitational wave produced by such an earthquake. The gravitational luminosity is estimated at 1.90 × 10–10erg/s (1 erg/s = 10–7 W = 1 J/s). PACS Nos.: 95.55.Ym, 04.80.Nn, 91.30.Px

Possible influences of core processes on the Earth's rotation and the gravity field

In this work, we review the processes in the Earth's core that influence the Earth's rotation on the decadal time scale. While core–mantle coupling is likely to be responsible for the decadal length-of-day variations, this hypothesis is controversial with respect to polar-motion variations. The electromagnetic-coupling torques are strongly dependent on the assumed electrical conductivity of the lower mantle, while the topographic torques are influenced by the topography of the core–mantle boundary (CMB). Because no comprehensive theoretical framework for determining the topographic and material parameters of the CMB region is currently available, the modeled results about the coupling torques can only be verified by their consistency with the observed variations of the Earth's rotation and the geomagnetic field. A second path of investigation is to consider the relative angular momentum of the core. Recently, the axial angular-momentum balance has been found to coincide with observed variations in the geomagnetic field and the length of day. However, with respect to polar-motion variations, the angular-momentum balance is not yet closed. We also discuss the role of an irregular motion of the figure axis of the oblate inner core with respect to the outer core and mantle in the excitation of polar motion. In particular, we assume that the associated changes of the Earth's inertia tensor cause the observed decadal variations in polar motion. From this assumption, we can derive the temporal variation of the orientation of the figure axis of the inner core from polar-motion data. Finally, we calculate the gravity variations caused by this relative inner-core motion and compare them with the accuracy of current and planned satellite missions.

The effect of global seismicity on the polar motion of a viscoelastic Earth

We evaluate the polar drift of a stratified viscoelastic Earth, generated by 20 years of global seismicity (from the Centroid Moment Tensor catalog, 1977–1997). Our estimate is better than that of Soldati and Spada [1999] because it accounts for the effects, previously neglected, of earthquakes at depths larger than 80 km. In fact, we show that deep focus and intermediate‐depth focus seismic events tend to affect the Earth's inertia tensor more significantly than shallow ones; specifically, we find that the mean rate of polar drift (evaluated over a 6‐year time interval immediately following the seismic event) due to a vertical dip‐slip source at the bottom of the asthenosphere (280 km depth in our model) is 5 times larger than that generated by an analogous source, located at the boundary between lithosphere and asthenosphere (80 km depth). We conclude, nevertheless, that global seismic activity is not able to alter significantly the rotational parameters of the Earth: according to our computations, the changes that it induces in the inertia tensor result in an average polar drift that is still significantly smaller than the observed one.

Semidiurnal and long-period components in polar motion connected with the triaxiality of ellipsoids of inertia of the earth and planets

The influence of the Earth's inertia ellipsoid triaxiality, both on the motion of the Angular Velocity Vector with respect to Tisserand's reference frame (hereinafter – AVV,T), the motion of the same vector in Space (hereinafter – AVV,S) as well as on the motion of Tisserand's Reference Frame in Space (hereinafter – TRF,S) is considered. It is shown that the observable effects are as follows: (1) in all three types of motions (AVV,T), (AVV,S) and (TRF,S) there are significant (observable) semidiurnal components which may be detected by modern GPS- and VLBI-measurements. Additionally, there is a long-periodic perturbation of (TRF,S) motion which is rather small for the Earth, but may be significant for the other planets of the Solar System (if the triaxiality of the inertia ellipsoid is sufficiently large and whenever the orbital and diurnal rotations are synchronous and, at the points of equinoxes, the orientation of the planet's principal equatorial moments of inertia is invariable in space). For the case of the Earth, the results of our calculations are compared with the results of modern GPS-observations.