ALBERT

Description: Regeneration of the Earth's magnetic field by convection in the liquid core produces a broad spectrum of time variation. Relative palaeointensity measurements in marine sediments provide a detailed record over the past 2 Myr, but an explicit reconstruction of the underlying dynamics is not feasible. A more practical alternative is to construct a stochastic model from estimates of the virtual axial dipole moment. The deterministic part of the model (drift term) describes time-averaged behaviour, whereas the random part (diffusion term) characterizes complex interactions over convective timescales. We recover estimates of the drift and diffusion terms from the SINT2000 model of Valet et al. and the PADM2M model of Ziegler et al. The results are used in numerical solutions of the Fokker–Planck equation to predict statistical properties of the palaeomagnetic field, including the average rates of magnetic reversals and excursions. A physical interpretation of the stochastic model suggests that the timescale for adjustments in the axial dipole moment is set by the dipole decay time d . We obtain d = 29 kyr from the stochastic models, which falls within the expected range for the Earth's core. We also predict the amplitude of convective fluctuations in the core, and establish a physical connection to the rates of magnetic reversals and excursions. Chrons lasting longer than 10 Myr are unlikely under present-day conditions. However, long chrons become more likely if the diffusion term is reduced by a factor of 2. Such a change is accomplished by reducing the velocity fluctuations in the core by a factor of 2, which could be attributed to a shift in the spatial pattern of heat flux from the core or a reduction in the total core heat flow.

Description: We compute the gravimetric factor at the Chandler wobble (CW) frequency using time-series from superconducting gravimeters (SG) longer than a decade. We first individually process the polar motion and data at each individual gravity station to estimate the gravimetric factor amplitude and phase, then we make a global analysis by applying a stacking method to different subsets of up to seven SG stations. The stacking is an efficient way of getting rid of local effects and improving the signal-to-noise ratio of the combined data sets. Using the stacking method, we find a gravimetric factor amplitude and phase of 1.118 ± 0.016 and –0.45 ± 0.66 deg, respectively, which is smaller in amplitude than expected. The sources of error are then carefully considered. For both local and global analyses, the uncertainties on our results are reliably constrained by computing the standard deviation of the estimates of the gravimetric factor amplitude and phase for increasing length of the time-series. Constraints on the CW anelastic dissipation can be set since any departure of the gravimetric factor from its elastic value may provide some insights into the dissipative processes that occur at the CW period. In particular, assuming given rheological models for the Earth's mantle enables us to make the link between the gravimetric factor phase and the CW quality factor.

Description: SUMMARY We present a new time-varying model for palaeomagnetic axial dipole moment (PADM) for the past 2 Myr and compare it with earlier virtual axial dipole moment (VADM) reconstructions which have been based on stacking and averaging scaled relative palaeointensity records. The PADM is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood (PML) approach to recover a cubic B-spline representation of axial-dipole field variations on million year timescales. The PML method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average VADM reconstructions. We apply the PML method to a set of 96 032 published data (1800 palaeointensities from igneous rocks, 3300 archaeointensities and 86 relative palaeointensity time-series of variable lengths and resolutions). Two models are discussed: PADM2Mp is a trial model based on a subset of the nine longest available sedimentary records; PADM2M uses a comprehensive data set (76 records, 81 446 data; 10 records were eliminated) and is our preferred model. PADM2M has a lower mean than existing VADM reconstructions but shows similarities in long-period variability. Some differences in timing, amplitude and resolution of certain features can be attributed to variations in age assignments. Others result from our more comprehensive data set and a reduction in bias attributable to PML modelling. PADM2M has an average axial dipole moment over 0–2 Ma of 5.3 × 10 22  Am 2 with a standard deviation of 1.5 × 10 22  Am 2 . The Brunhes chron average ( 6.2 × 10 22  Am 2 ) is higher than for earlier epochs of Matuyama ( 4.8 × 10 22  Am 2 ), as seen in some previous studies. The power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 10 2  Myr −1 . We see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour.

Description: The aim of this paper is to determine the strain rate tensor (SRT) for the Iranian region. In this study, (1) we apply a method of computation of the SRT never used for the Iranian area and (2) we use a new GPS velocity field obtained from several previously published velocity fields. First, the method is described and tested on a synthetic case, which mimics the real Iranian case. The synthetic tests confirm that the method allows us to both retrieve high gradients of the strain rate field and reduce the effect of an erroneous velocity vector. Second, the method is applied to a real data set covering the Arabia-Eurasia collision zone in Iran. We particularly focus on the Zagros–Makran transition zone, the Central Iran region and the northernmost part of the Arabia-Eurasia collision zone (NW Iran–Caucasus–East Turkey). Whereas the main characteristics of the obtained SRT are consistent with known tectonic features, important new results are found in the Central Iran, with the strike-slip style along the Anar and Deshir faults, and the Zagros–Makran transition zone, with a north–south variation of the SRT along the Zendan–Minab–Palami fault system. We link these results to recent active tectonic studies.

Description: 〈span〉〈div〉SUMMARY〈/div〉The nutation of the Celestial Intermediate Pole can be considered as a retrograde diurnal polar motion. As the common polar motion, it presents a resonance, but with period 〈span〉T〈/span〉〈sub〉PM〈/sub〉 and quality factor 〈span〉Q〈/span〉〈sub〉PM〈/sub〉 differing from the ones characterizing the Chandler wobble (〈span〉T〈/span〉〈sub〉CW〈/sub〉 = 430.2−431.6 d, 〈span〉Q〈/span〉〈sub〉CW〈/sub〉 in the interval (56 255) according to Nastula & Gross): according to the nutation analysis presented in a separate paper, this period is about 〈span〉T〈/span〉〈sub〉PM〈/sub〉 = 380 d and the quality factor becomes −10. In this study, we aim to revisit the geophysical interpretation of this result. Two complementary factors account for the observed values: the non-equilibrium response of the ocean to the pole tide potential in the diurnal band, and the resonance of the solid Earth tide at the free core nutation period. This leads to a resonance of 〈span〉T〈/span〉〈sub〉PM〈/sub〉 in the vicinity of the free core nutation period, confirmed by estimates derived from nutation analysis.〈/span〉

Description: The rotational motions of the internal Earth layers induce resonances in the Earth nutations and tidal gravimetric response to external luni-solar gravitational forcings. The characterization of these resonances is a mean of investigating the deep Earth properties since their amplitudes and frequencies depend on a few fundamental geophysical parameters. In this work, we focus on the determination of the free core nutation and free inner core nutation periods and quality factors from the Bayesian inversion of VLBI and gravimetric data. We make a joint inversion of data from both techniques and show that, even if the results are only slightly different from the inversion of VLBI data alone, such approach may be valuable in the future if the accuracy of gravimetric data increases. We also briefly discuss the polar motion resonance, which is related to the Chandler Wobble as seen from the diurnal frequency band. Our overall estimates of the FCN period and quality factor, TFCN = (−430.2, −429.8) solar days and QFCN = (15 700, 16 700), respectively, are in good agreement with other studies, albeit slightly different for unclear reasons. Despite some concerns about the detection and characterization of the FICN, it seems that we could also successfully estimate its period, TFICN = (+600, +1300) solar days, and give a loose estimate of the upper bound on its quality factor.

Description: The nutation harmonic terms are commonly determined from celestial pole offset series produced from very long baseline interferometry (VLBI) time delay analysis. This approach is called an indirect approach. As VLBI observations are treated independently for every session, this approach has some deficiencies such as a lack of consistency in the geometry of the session. To tackle this problem, we propose to directly estimate nutation terms from the whole set of VLBI time delays, hereafter referred as a direct approach, in which the nutation amplitudes are taken as global parameters. This approach allows us to reduce the correlations and the formal errors and gives significant discrepancies for the amplitude of some nutation terms. This paper is also dedicated to the determination of the Earth resonance parameters, named polar motion, free core nutation, and free inner core nutation. No statistically significant difference has been found between the estimates of resonance parameters based upon ‘direct’ and ‘indirect’ nutation terms. The inclusion of a complete atmospheric-oceanic non-tidal correction to the nutation amplitudes significantly affected the estimates of the free core nutation and the free inner core nutation resonant frequencies. Finally, we analyzed the frequency sensitivity of polar motion resonance and found that this resonance is mostly determined by the prograde nutation terms of period smaller than 386 d.