Variations In Length Of Day Research Articles

Application of superconductive gravity technique on the constraints of core-mantle coupling parameters

The parameters, i.e. the Period and the Quality factor, of the Earth’s free core nutation (FCN) are closely related to the dissipative coupling between the core and the mantle. Based on the FCN parameters obtained from the actual observations and theoretical simulation, significantly constrained in this study were several key parameters near the core-mantle boundary (CMB), related to the core and mantle coupling, including viscosity at the top of liquid core, conductivity at the bottom of the mantle, and dynamic ellipticity of the CMB. In order to choose high quality observations from global stations of the superconducting gravimeters (SG) on the Global Geodynamics Project (GGP) network, we adopted two criteria, the standard deviations of harmonic analysis on tidal observations and the quality of the FCN parameters calculated with the observations from single station. After the mean ocean tidal effects of the recent ocean tidal models were removed, the FCN parameters were retrieved by stacking the tidal gravity observations from the GGP network. The results were in a good agreement with those in the recent research by using the SG and/or the VLBI observations. Combined with an FCN theoretical model deduced by angular momentum method, the viscous and electromagnetic coupling parameters near the CMB were evaluated. Numerical results indicated that the viscosity at the top of the liquid core was in the range from 6.6×102 to 2.6×103 Pa·s, which was in good agreement with those obtained from the Earth’s nutation, the FCN and variations in the length of day (LOD). The conductivity at the bottom of the mantle should be as large as 2.6×106–1.0×107 S m−1 to match the FCN quality factors from the actual observations. The dissipative coupling had a little influence of 1–2 sidereal days for the FCN period.

Lander radio science experiment with a direct link between Mars and the Earth

Mars' Orientation and rotation Parameters (called here MOP): precession, nutation, polar motion and length of day (LOD) variations, are related to the interior of the planet as well as to the dynamics of its atmosphere. The MOP can be determined using the Doppler shift on radio signal data due to the motion of a probe landed on Mars relative to tracking stations on Earth. In this paper we perform numerical simulations for assessing the precision on the determination of the MOP using Direct-To-Earth (DTE) X-band Doppler measurements for a nearly equatorial lander. We then discuss how a better knowledge of the MOP could improve our understanding on the interior structure. The sensitivity of such a DTE radio link to these rotation parameters is first investigated. This shows that the latitude of the landing site must be higher than 20° to detect the Chandler Wobble component of the polar motion in DTE Doppler data and must be at least 40° to get tight constraints on it. It is found that the precision in the determination of the seasonal LOD variations will be significantly improved after about 350 days of operation, reaching the 5% level after 550 days, thereby better constraining the CO2 mass budget in the Martian atmosphere and ice caps. The current precision in the precession rate (25milliarcsecond (mas) per year) will be matched after 150 days of mission. An uncertainty of less than 5mas/year will be reached after 700 days, improving the precision on the polar moment of inertia by a factor of five. The precision on the determination of the amplitudes of nutation is estimated at a few mas after one Martian year of mission (about 12mas on the prograde/retrograde semi and terannual amplitudes) allowing for the detection of the contribution expected from the liquid core. Considering Mars with a liquid core in accordance with recent geodesic measurements, the Free-Core-Nutation (FCN) period is estimated with a precision of less than 10 days after 550 days of mission in case of an actual FCN period equal to −240 days and with a precision of 1 day after 300 days of observation for an FCN period at −230 days, which is close to the forcing terannual period of 228.9 days (resonance). It is shown that only 100 days of mission duration are needed to detect the liquid core contribution to the nutation near the resonance from the estimates of the amplitudes of nutation. Finally, the precisions on core physical parameters are inferred from the FCN period estimate. We show that a precision of 30% for the core moment of inertia and 5% for the dynamical flattening can be obtained after one Martian year of mission duration.

Associations between seasonal variations in day length (photoperiod), sleep timing, sleep quality and mood: a comparison between Ghana (5°) and Norway (69°)

The hypothesis of whether day length (photoperiod) is an important zeitgeber (time-giver) for keeping the circadian rhythm entrained to a 24-hour cycle was examined, as was its association with sleep patterns and mood problems. Data were collected prospectively from a site with very large differences in daylight duration across seasons (Tromsø in Norway, 69°39'N) and a site with very small seasonal differences in daylight duration (Ghana in Accra, 5°32'N). Two hundred subjects were recruited from both sites in January. At the follow-up in August, 180 and 150 subjects in Ghana and Norway participated, respectively. Use of a weekly sleep diary indicated low to moderately strong seasonal changes in rise- and bedtime, sleep efficiency and sleep onset latency only in the northern latitude. No seasonal changes in sleep duration or night awakenings were observed. The self-report measures indicated moderate to strong seasonal differences in insomnia and fatigue, and weaker differences in depressed mood in Norway, but small to non-existing seasonal differences in Ghana. Lack of daylight was related to phase-delayed rise- and bedtimes, increased problems falling asleep, daytime fatigue and depressive mood. However, total sleep duration and sleep quality appeared unaffected.

Rotation period of Venus estimated from Venus Express VIRTIS images and Magellan altimetry

The 1.02 μm wavelength thermal emission of the nightside of Venus is strongly anti-correlated to the elevation of the surface. The VIRTIS instrument on Venus Express has mapped this emission and therefore gives evidence for the orientation of Venus between 2006 and 2008. The Magellan mission provided a global altimetry data set recorded between 1990 and 1992. Comparison of these two data sets reveals a deviation in longitude indicating that the rotation of the planet is not fully described by the orientation model recommended by the IAU. This deviation is sufficiently large to affect estimates of surface emissivity from infrared imaging. A revised period of rotation of Venus of 243.023 ± 0.002 d aligns the two data sets. This period of rotation agrees with pre-Magellan estimates but is significantly different from the commonly accepted value of 243.0185 ± 0.0001 d estimated from Magellan radar images. It is possible that this discrepancy stems from a length of day variation with the value of 243.023 ± 0.002 d representing the average of the rotation period over 16 years.

Length of day variations due to mantle dynamics at geological timescale

SUMMARY The geological evolution of length of day (LOD) variations is mainly controlled by the frictional tidal torque responsible for the secular slowdown of the Earth’s rotation and for the receding of the Moon. Superimposed on this variation, which has existed since the early history of the planet, there are, at shorter timescales (less than 1 Myr), LOD perturbations induced by the glaciation–deglaciation cycles. In this paper, we investigate the influence of mantle dynamics on LOD at the geological timescale. We use the complete non-linear equations to compute the influence of mantle density heterogeneities on the angular velocity of the rotation, that is to say on the LOD. We discuss the degree zero coefficient of the spherical harmonic expansion of the mantle mass anomaly, which is strongly dependent on the conservation of mass of the Earth. We first compute the effects induced by upwelling domes and subducted plates sinking into the mantle, which are known to induce geological variations in the orientation of the rotation axis with respect to a fixed terrestrial frame (the so-called True Polar Wander). We find that the time-variable mantle density heterogeneities associated with the large-scale pattern of plate tectonic motions since 120 Ma and with the upwelling domes can perturb LOD by about per year, that is, with an order of magnitude smaller than the effects induced by the last deglaciation. Superimposed on this linear trend, we show that there are fluctuations around 0.1–0.2 s Myr−1 on a timescale of a few tens of millions of years. We then combine a spherical model of mantle circulation with solutions for the equations governing the rotation of a viscous planet, to improve the constraint of mantle mass conservation. Finally, we compare our results with other effects.

Relationship between daylength and suicide in Finland

BackgroundMany previous studies have documented seasonal variation in suicides globally. We re-assessed the seasonal variation of suicides in Finland and tried to relate it to the seasonal variation in daylength and ambient temperature and in the discrepancy between local time and solar time.MethodsThe daily data of all suicides from 1969 to 2003 in Finland (N = 43,393) were available. The calendar year was divided into twelve periods according to the length of daylight and the routinely changing time difference between sun time and official time. The daily mean of suicide mortality was calculated for each of these periods and the 95% confidence intervals of the daily means were used to evaluate the statistical significance of the means. In addition, daily changes in sunshine hours and mean temperature were compared to the daily means of suicide mortality in two locations during these afore mentioned periods.ResultsA significant peak of the daily mean value of suicide mortality occurred in Finland between May 15th and July 25th, a period that lies symmetrically around the solstice. Concerning the suicide mortality among men in the northern location (Oulu), the peak was postponed as compared with the southern location (Helsinki). The daily variation in temperature or in sunshine did not have significant association with suicide mortality in these two locations.ConclusionsThe period with the longest length of the day associated with the increased suicide mortality. Furthermore, since the peak of suicide mortality seems to manifest later during the year in the north, some other physical or biological signals, besides the variation in daylight, may be involved. In order to have novel means for suicide prevention, the assessment of susceptibility to the circadian misalignment might help.

Immune responses to lipopolysaccharide challenge in a tropical rodent (Funambulus pennanti): Photoperiod entrainment and sex differences

Annual variation in day length (photoperiod) triggers changes in the immune system of seasonal breeders. The rationale behind this study was to delineate any sex differences in immune responses of photoperiodically entrained animals challenged against lipopolysaccharide (LPS)-induced inflammatory stress. We observed that photoperiodically entrained [short day, SD, 10 h light (L):14 h dark (D); long day, LD, 16 h L:8 h D; and natural day length, NDL, 12 h L:12 h D] male and female Indian palm squirrels, Funambulus pennanti, presented sexual dimorphism in immune status after LPS-induced stress. Females presented high humoral (anti-keyhole limpet hemocyanin immunoglobulin) and cellular immunity (lymphocyte proliferation) compared with the males of all photoperiodic conditions. Female squirrels showed reduced pro-inflammatory cytokine levels (interleukin-1β, interleukin-6, and tumor necrosis factor-α) than the males suggesting their high efficiency to recover from LPS-induced inflammatory stress. Increased duration of melatonin secretion and corticosterone concentration in squirrels experiencing SD evidently supported survival of squirrels as compared with control (NDL) and LD squirrels of both sexes. Decreased immune status in both sexes under LD condition might be due to a short melatonin signal mimicking the LDs of summer. Thus, we infer that photoperiodic entrainment via the levels of melatonin and corticosterone synergistically supported more the survival of female squirrels under LPS-induced stress.

Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod

Virtually all species have developed cellular oscillations and mechanisms that synchronize these cellular oscillations to environmental cycles. Such environmental cycles in biotic (e.g. food availability and predation risk) or abiotic (e.g. temperature and light) factors may occur on a daily, annual or tidal time scale. Internal timing mechanisms may facilitate behavioural or physiological adaptation to such changes in environmental conditions. These timing mechanisms commonly involve an internal molecular oscillator (a 'clock') that is synchronized ('entrained') to the environmental cycle by receptor mechanisms responding to relevant environmental signals ('Zeitgeber', i.e. German for time-giver). To understand the evolution of such timing mechanisms, we have to understand the mechanisms leading to selective advantage. Although major advances have been made in our understanding of the physiological and molecular mechanisms driving internal cycles (proximate questions), studies identifying mechanisms of natural selection on clock systems (ultimate questions) are rather limited. Here, we discuss the selective advantage of a circadian system and how its adaptation to day length variation may have a functional role in optimizing seasonal timing. We discuss various cases where selective advantages of circadian timing mechanisms have been shown and cases where temporarily loss of circadian timing may cause selective advantage. We suggest an explanation for why a circadian timing system has emerged in primitive life forms like cyanobacteria and we evaluate a possible molecular mechanism that enabled these bacteria to adapt to seasonal variation in day length. We further discuss how the role of the circadian system in photoperiodic time measurement may explain differential selection pressures on circadian period when species are exposed to changing climatic conditions (e.g. global warming) or when they expand their geographical range to different latitudes or altitudes.

Does Mood Affect Trading Behavior?

The assumption that mood affects investors’ behavior in the field is gaining acceptance due to experimental studies and papers linking stock returns with environmental variables, such as weather and length of day. To identify mood effects this paper utilizes account level stock trading data from all investors in Finland, a country with significant variation in weather and length of day. While some weather-related mood variables and calendar effects are individually significant, little of the day-to-day variation in trading is collectively explained by all such factors. In contrast, we find strong seasonal lower frequency patterns that seem connected to vacations.

27.3-day and average 13.6-day periodic oscillations in the Earth’s rotation rate and atmospheric pressure fields due to celestial gravitation forcing

Variation in length of day of the Earth (LOD, equivalent to the Earth’s rotation rate) versus change in atmospheric geopotential height fields and astronomical parameters were analyzed for the years 1962–2006. This revealed that there is a 27.3-day and an average 13.6-day periodic oscillation in LOD and atmospheric pressure fields following lunar revolution around the Earth. Accompanying the alternating change in celestial gravitation forcing on the Earth and its atmosphere, the Earth’s LOD changes from minimum to maximum, then to minimum, and the atmospheric geopotential height fields in the tropics oscillate from low to high, then to low. The 27.3-day and average 13.6-day periodic atmospheric oscillation in the tropics is proposed to be a type of strong atmospheric tide, excited by celestial gravitation forcing. A formula for a Tidal Index was derived to estimate the strength of the celestial gravitation forcing, and a high degree of correlation was found between the Tidal Index determined by astronomical parameters, LOD, and atmospheric geopotential height. The reason for the atmospheric tide is periodic departure of the lunar orbit from the celestial equator during lunar revolution around the Earth. The alternating asymmetric change in celestial gravitation forcing on the Earth and its atmosphere produces a “modulation” to the change in the Earth’s LOD and atmospheric pressure fields.

Environmental control of the annual erect phase of <i>Nemalion helminthoides</i> (Rhodophyta) in the field

In temperate areas, lack of nutrients during summer, particularly N, is the main limiting factor of macroalgal growth. However, Nemalion helminthoides (Velley) Batters in northern Spain is conspicuous in the field during this time (from mid-May to late-July). Therefore, we assumed that its nutrient requirements are low enough to be sustained by transient nutrient inputs and we hypothesized that the physiological condition of the thalli was transiently improved when nutrient pulses occurred. A range of proxies for physiological condition (internal N, C, proteins and phycobilins), growth and phenological status of N. helminthoides were measured over time and related to temporal variations in nutrient availability, irradiance, temperature and daylength. Data were analyzed using a multivariate approach (redundancy analysis). Transient nutrient inputs were mainly due to freshwater runoff and wind-driven upwelling events; however, these pulses did not lead to any short-term improvement in the physiological condition of the algae because in such dominant nutrient limiting conditions plants divert transient available resources directly to growth and reproduction. Probably because of the strong endogenous nature of the N. helminthoides life-history, only daylength and temperature were found to be major environmental factors: increasing daylength was associated with growth, sexual maturation, fertilization and the increment of internal N and C, the amount of proteins and phycobilins. Decreasing daylength together with increasing temperature were correlated with spore release and senescence. This research suggests that N. helminthoides requires a high light dose to sustain growth and reproduction, and therefore it must grow and reproduce in summer even though it has to overcome nutrient deprivation during this period.

Accelerated lambing achieved by a photoperiod regimen consisting of alternating 4-month sequences of long and short days applied year-round1

The objective of this work was to evaluate the reproductive performance of ewes exposed to a photoperiodic regimen consisting of continuous alternating 4-mo periods of long days (LD: 16 h of light/d) and short days (SHD: 8 h of light/d) in an accelerated lambing program of 3 lambings in 2 yr. A total of 211 prolific Rideau Arcott ewes were assigned to the photoperiodic treatment, whereas 37 ewes were maintained under natural annual variation in day length (control group). Ewes under the photoperiod regimen were divided into 4 subgroups (A, B, C, D). All these groups of ewes were exposed to the same light regimen, but the LD and SHD light sequences were staggered by 2 mo to permit the evaluation of the effect of time and season of mating on performance of the ewes treated with the photoperiod. The control ewes were treated with intravaginal sponges in the out-of-season breeding periods (conventional approach). Each group of ewes was studied over 3 reproductive cycles. Two groups of rams exposed to alternating 2-mo sequences of LD and SHD were used for mating. The short mean interval between ram introduction and conception for the groups exposed to artificial photoperiod (9.4 d) confirmed the effectiveness of the treatment to induce intense sexual activity. For the 12 breeding periods studied (8 in out-of-season and 4 in sexual season), fertility rate of the ewes treated with photoperiod, mated at various times of year, was 91.6%, which is comparable with the fertility normally seen in the natural breeding season. The number of lambs born/ewe remained constant across reproductive cycles and was greater in photoperiod-treated groups (2.81 vs. 2.27 for photoperiod and control groups, respectively; P = 0.0002). Groups exposed to photoperiod treatment obtained better fertility rate than the control group in out-of-season breeding (91.1 vs. 76.3%; P = 0.016). Ewes managed under the photoperiod regimen produced 1.38 lambings/yr and 69% of them lambed 3 times in 2 yr. Overall, the ewes in the photoperiodic treatment produced annually 3.78lambs/ewe. The reproductive performances achieved throughout the years indicate that the photoperiodic program tested, consisting of continuous alternating 4-mo periods of LD and SHD, allows control of the annual reproductive cycles in ewes.

Seasonal variation in daily insolation induces synchronous bud break and flowering in the tropics

In many temperate plants seasonal variation in day length induces flowering at species-specific times each year. Here we report synchronous bud break and flowering of tropical perennials that cannot be explained by seasonal changes in day length. We recorded flushing and flowering of more than 100 tropical trees, succulents and understory herbs over several years. We observed the following phenological patterns throughout the northern Neotropics: wide-ranging trees flush or flower twice a year at the Equator, but annually further north; many trees leaf out in February; in autumn, wide-ranging perennials flower 4 months earlier in Mexico than at the Equator. This latitudinal variation of phenology parallels that of the annual cycle of daily insolation, a function of day length and solar irradiation. Insolation has two annual maxima at the Equator, it rapidly increases in February at all latitudes, and between Mexico and the Equator its maximum shifts from the summer solstice to the autumn equinox. These unique, manifold correlations suggest that throughout the tropics insolation, rather than day length, may control the phenology of many perennials. Our observations significantly extend current knowledge of environmental signals involved in photoperiodic control of plant development.

Closure in the Earth's angular momentum budget observed from subseasonal periods down to four days: No core effects needed

Short period variations in the Earth's rotation rate, length‐of‐day (LOD), are driven mainly by the atmosphere with smaller contributions by the oceans. Previous studies have noted a lag of atmospheric angular momentum (AAM) with LOD that would imply another source. We examine AAM from the European Centre for Medium‐Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) reanalysis series, along with oceanic angular momentum (OAM) from the ECCO consortium; land hydrological effects made no discernible impact. The NCEP reanalysis together with OAM produces a significant lag with LOD, while the ECMWF reanalysis AAM with OAM shows no phase lag. We find significant coherence with LOD variations down to periods of 4 days; coherence losses at shorter periods likely arise from the inverted barometer assumption and unmodeled dynamical processes. Thus the inclusion of core effects is not needed to balance the axial angular momentum budget on sub‐seasonal time scales.

Current knowledge on the photoneuroendocrine regulation of reproduction in temperate fish species

Seasonality is an important adaptive trait in temperate fish species as it entrains or regulates most physiological events such as reproductive cycle, growth profile, locomotor activity and key life-stage transitions. Photoperiod is undoubtedly one of the most predictable environmental signals that can be used by most living organisms including fishes in temperate areas. This said, however, understanding of how such a simple signal can dictate the time of gonadal recruitment and spawning, for example, is a complex task. Over the past few decades, many scientists attempted to unravel the roots of photoperiodic signalling in teleosts by investigating the role of melatonin in reproduction, but without great success. In fact, the hormone melatonin is recognized as the biological time-keeping hormone in fishes mainly due to the fact that it reflects the seasonal variation in daylength across the whole animal kingdom rather than the existence of direct evidences of its role in the entrainment of reproduction in fishes. Recently, however, some new studies clearly suggested that melatonin interacts with the reproductive cascade at a number of key steps such as through the dopaminergic system in the brain or the synchronization of the final oocyte maturation in the gonad. Interestingly, in the past few years, additional pathways have become apparent in the search for a fish photoneuroendocrine system including the clock-gene network and kisspeptin signalling and although research on these topics are still in their infancy, it is moving at great pace. This review thus aims to bring together the current knowledge on the photic control of reproduction mainly focusing on seasonal temperate fish species and shape the current working hypotheses supported by recent findings obtained in teleosts or based on knowledge gathered in mammalian and avian species. Four of the main potential regulatory systems (light perception, melatonin, clock genes and kisspeptin) in fish reproduction are reviewed.

Effects of transient water mass redistribution associated with a tsunami wave on Earth’s pole path

We have quantified the effects of a water mass redistribution associated with the propagation of a tsunami wave on the Earth’s pole path and on the Length-Of-Day (LOD) and applied our modeling results to the tsunami following the 2004 giant Sumatra earthquake. We compared the result of our simulations on the instantaneous rotational axis variations with the preliminary instrumental evidence on the pole path perturbation (which has not been confirmed) registered just after the occurrence of the earthquake. The detected perturbation in the pole path showed a step-like discontinuity that cannot be attributed to the effect of a seismic dislocation. Our results show that the tsunami induced instantaneous rotational pole perturbation is indeed characterized by a step-like discontinuity compatible with the observations but its magnitude is almost one hundred times smaller than the detected one. The LOD variation induced by the water mass redistribution turns out to be not significant because the total effect is smaller than current measurements uncertainties.

Photoperiod History Differentially Impacts Reproduction and Immune Function in Adult Siberian Hamsters

Seasonal changes in numerous aspects of mammalian immune function arise as a result of the annual variation in environmental day length (photoperiod), but it is not known if absolute photoperiod or relative change in photoperiod drives these changes. This experiment tested the hypothesis that an individual's history of exposure to day length determines immune responses to ambiguous, intermediate-duration day lengths. Immunological (blood leukocytes, delayed-type hypersensitivity reactions [DTH]), reproductive, and adrenocortical responses were assessed in adult Siberian hamsters (Phodopus sungorus) that had been raised initially in categorically long (15-h light/day; 15L) or short (9L) photoperiods and were subsequently transferred to 1 of 7 cardinal experimental photoperiods between 9L and 15L, inclusive. Initial photoperiod history interacted with contemporary experimental photoperiods to determine reproductive responses: 11L, 12L, and 13L caused gonadal regression in hamsters previously exposed to 15L, but elicited growth in hamsters previously in 9L. In hamsters with a 15L photoperiod history, photoperiods < or = 11L elicited sustained enhancement of DTH responses, whereas in hamsters with a 9L photoperiod history, DTH responses were largely unaffected by increases in day length. Enhancement and suppression of blood leukocyte concentrations occurred at 13L in hamsters with photoperiod histories of 15L and 9L, respectively; however, prior exposure to 9L imparted marked hysteresis effects, which suppressed baseline leukocyte concentrations. Cortisol concentrations were only enhanced in 15L hamsters transferred to 9L and, in common with DTH, were unaffected by photoperiod treatments in hamsters with a 9L photoperiod history. Photoperiod history acquired in adulthood impacts immune responses to photoperiod, but manifests in a markedly dissimilar fashion as compared to the reproductive system. Prior photoperiod exposure has an enduring impact on the ability of the immune system to respond to subsequent changes in day length.

Cycles in the scaling properties of length-of-day variations

Abstract Variations in the length-of-day (LOD) reflect the effects of several mechanisms in the Earth's rotation dynamics, including Earth–Sun and Earth–Moon line-up, geomagnetic effects and gravitational changes. Several studies showed that signatures of cycles occurring over a wide range of time scales are present in the LOD variations. The present work uses a fractal scaling study based on detrended fluctuation analysis (DFA) to study persistence of LOD variations and to provide insights in the different cycling mechanisms. The results showed that that the LOD variations are persistent over a wide range of time scales, meaning that an increment (resp., decrement) is more likely to be followed by an increment (resp., decrement). The temporal variation of the scaling exponent obtained from the DFA showed that several cycles already reported from the direct LOD variations analysis are inherited by the scaling properties. Inter-annual cycles, including 4.3 and 18.6 years cycles, are linked to the variations of the stochastic dynamics of LOD fluctuations. In this way, the 18.6 years cycle attains a period where variations are mostly affected by white noise effects, reducing the predictability of the LOD anomalies. The results are discussed in terms of the different lunar tidal and core–mantle mechanisms and related to recent results in the literature.

Geodetic aspects of seismological phenomena

Seismology is related to many problems of geodesy. The energy production of our planet is rather close to energy consumption of the Earth so that the energy balance can be disturbed significantly by minor processes acting on global scale. From this point of view the effect of tidal triggering of earthquakes is discussed by the study of tidal stress tensor components expressed in spherical system of coordinates. Tidal friction influences through the despinning of the axial rotation the geometrical flattening. This flattening variation causes stresses along the longitude and this phenomenon is closely related to the seismic energy release. Until now there is no unambiguous success to relate changes of the Earth orientation parameters with seismicity. Present-day accuracy of the length of day variations is not sufficient yet to detect spin variation generated by the greatest earthquakes. The polar motion is probably more sensitive to earthquakes and then there is a chance to detect the polar displacements generated by seismic events. In the last section of the present contribution, the strain rates derived from the static seismic moments and from space geodetic observations are compared. Future geodetic strain rate data will be useful in earthquake prediction.

Determination of gravimetric parameters of the gravity pole tide using observations recorded with superconducting gravimeters

In this study, the loading gravity effect of air mass changes calculated with the three-dimension (3D) meteorological data from the European Centre for Medium-range Weather Forecasts (ECMWF) are removed from superconducting gravimeter (SG) observations. The global hydrological gravity effect is computed and removed with hydrological data from the Global Land Data Assimilation System (GLDAS). Otherwise, the gravity influences induced by a theoretical self-consistent ocean pole tide and variations in length of day (LOD) are considered in the calculation. After removing the influences mentioned previously and also considering the long term trend in the data, a very nice linear relationship between the theoretical gravity pole tide and observed gravity residual (containing the observed gravity pole tide) for each of the selected 9 GGP stations we considered can be obtained. Therefore, the gravimetric factor of the gravity pole tide can be estimated with a simple linear regression. The results show that no clear phase lag is found between the theoretical gravity pole tide and observed gravity residuals from the nine SG stations.