Non-tidal Variations Research Articles

Tidal and nontidal variations in the water level in the middle part of the Ob’ Bay

Tidal and nontidal variations in the water level in the middle part of the Ob’ Bay

Meteorological impact on gravimeter and seismometer readings

Background. The density of atmospheric air varies with temperature. Therefore, variations in the atmospheric layer temperature leads the emergence of local gravity anomalies. The amplitude of such anomalies can exceed ±0.1 mGal. The results obtained by the Arti Geophysical Observatory(Yekaterinburg, Russia) on the non-tidal variations of gravity confirm the influence of temperature variations on gravimeter readings, which may significantly exceed the calculated data. Aim. To assess the impact of weather changes on seismic and gravity fields.Materials and methods. At the RAS Bishkek Geodynamic Test Area (Bishkek, Kyrgyzstan), RAS Institute of Geophysics (Yekaterinburg, Russia) and Zapolye Observatory (Vladimir, Russia), gravity was measured by CG-5 AUTOGRAV gravimeters. At the Shults Cape (Vladivostok, Russia), Arti (Yekaterinburg, Russia) and BFO (Black Forest, Germany [10]) observatories, gravimetric measurements were conducted by La Coste & Romberg tidal gravimeters. Results. Variations in the atmospheric layer temperature were found to lead to the emergence of local gravity anomalies. The amplitude of such anomalies can exceed ±0.1 mGal. The results obtained by the Arti Geophysical Observatory (Yekaterinburg, Russia) on the non-tidal variations of gravity confirm the influence of temperature on gravimeter readings, which may significantly exceed the calculated data. Conclusion. Meteorological processes, such as liquid-water content and the Earth’s atmospheric temperature, affect the readings of gravimeters and seismometers. The atmospheric air density varies with temperature. Along with temperature, gravimeters and seismometers are affected by pressure, humidity and water content of the atmosphere. This is related to atmospheric deformations under the influence of the Moon and the Sun. Meteor streams and their impact on the Earth’s atmosphere have the greatest influence on the readings of gravimeters and seismometers.

A combination of tides and nontidal variations in ocean bottom pressure may generate interannual slip fluctuations in the transition zone along a subduction plate interface

The tidal triggering of earthquakes has been studied for many years. The discovery of slow earthquakes in the early 2000s, including slow slip, has urged scientists to investigate the tidal responses of these earthquakes due to their sensitivity to weak stress perturbations. Previous studies have shown that slow earthquakes correlate with diurnal and semidiurnal tides and seasonal variations in surface loads more clearly than ordinary earthquakes. However, little is known about long-term responses to external stresses. In this paper, based on a widely accepted frictional law for faults, a mechanism is proposed by which nontidal variations in ocean bottom pressure, when combined with tides, promote the occurrence of slow earthquakes. Because slow earthquakes accompany a slip on the plate interface, this mechanism allows one to estimate slip modulations. A one-degree-of-freedom slip model is constructed and applied to Ise Bay in the Tonankai region of southwestern Japan, where large-scale ocean mass redistributions have occurred. The model calculated with parameters determined from the observation of tectonic tremors is quantitatively consistent with the slip during 1997–2013 inferred from GNSS data, suggesting that the decrease in the sea-level change in approximately 2006 could cause the acceleration of a slip observed after that. This result implies that the decreases in sea level in approximately 1996 and 2014 could also cause subsequent slip accelerations. These three slip acceleration periods temporally coincide with the increases in background seismicity in a shallower portion of the plate interface. These changes in seismicity are common to shallow earthquakes in the Tokai area, and a similar model can reproduce them. Further studies are expected to reveal causality between shallow earthquakes and long-term slip fluctuations based on modeling that considers changes in the frictional property along the plate interface.

Combination of Tsoft and ET34-ANA-V80 software for the preprocessing and analysis of tide gauge data in French Polynesia

Since 2008 a network of five sea-level monitoring stations was progressively installed in French Polynesia. The stations are autonomous and data, collected at a sampling rate of 1 or 2 min, are not only recorded locally, but also transferred in real time by a radio-link to the NOAA through the GOES satellite. The new ET34-ANA-V80 version of ETERNA, initially developed for Earth Tides analysis, is now able to analyze ocean tides records. Through a two-step validation scheme, we took advantage of the flexibility of this new version, operated in conjunction with the preprocessing facilities of the Tsoft software, to recover corrected data series able to model sea-level variations after elimination of the ocean tides signal. We performed the tidal analysis of the tide gauge data with the highest possible selectivity (optimal wave grouping) and a maximum of additional terms (shallow water constituents). Our goal was to provide corrected data series and modelled ocean tides signal to compute tide-free sea-level variations as well as tidal prediction models with centimeter precision. We also present in this study the characteristics of the ocean tides in French Polynesia and preliminary results concerning the non-tidal variations of the sea level concerning the tide gauge setting.

A Regional Model for Predicting Tropospheric Delay and Weighted Mean Temperature in China Based on GRAPES_MESO Forecasting Products

Accurate tropospheric delay (TD) and weighted mean temperature (Tm) are important for Global Navigation Satellite System (GNSS) positioning and GNSS meteorology. For this purpose, plenty of empirical models have been built to provide estimates of TD and Tm. However, these models cannot resolve TD and Tm variations at synoptic timescales since they only model the average annual, semi-annual, and/or daily variations. As a result, the existed empirical models cannot perform well under extreme weather conditions. To address this limitation, we propose to estimate Zenith Hydrostatic Delay (ZHD), Zenith Wet Delay (ZWD), and Tm directly from the stratified numerical weather forecasting products of the mesoscale version of the Global/Regional Assimilation and PrEdiction System (GRAPES_MESO) of China. The GRAPES_MESO forecasting data has a temporal resolution of 3 h, which provides the opportunity to resolve the synoptic variation. However, it is found that the estimated ZWD and Tm exhibit apparent systematic deviation from in situ observation-based estimates, which is due to the inherent biases in the GRAPES_MESO data. To solve this problem, we propose to correct these biases using a linear model and a spherical cap harmonic model. The estimates after correction are termed as the “CTropGrid” products. When validated by the radiosonde data, the CTropGrid product has biases of 1.5 mm, −0.7 mm, and −0.1 K, and Root Mean Square (RMS) error of 8.9 mm, 20.2 mm, and 1.5 K for ZHD, ZWD, and Tm. Compared to the widely used GPT2w model, the CTropGrid products have improved the accuracies of ZHD, ZWD, and Tm by 11.9%, 55.6%, and 60.5% in terms of RMS. When validating the Zenith Tropospheric Delay (ZTD) products (the sum of ZHD and ZWD) using the IGS ZTD data, the CTropGrid ZTD has a bias of −0.7 mm and an RMS of 35.8 mm, which is 22.7% better than the GPT2w model in terms of RMS. Besides the accuracy improvements, the CTropGrid products well model the synoptic-scale variations of ZHD, ZWD, and Tm. Compared to the existing empirical models that only capture the tidal (seasonal and/or diurnal) variations, the CTropGrid products capture well the non-tidal variations of ZHD, ZWD, and Tm, which enhances the tropospheric delay corrections and GNSS water vapor monitoring at synoptic timescales. Therefore, the CTropGrid product is an important progress in GNSS positioning and GNSS meteorology.

Water Depth Dependence of Long‐Range Correlation in Nontidal Variations in Seafloor Pressure

AbstractIsolating the source of nontidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the nontidal components, as these are largely common‐mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, installed in the Hikurangi subduction zone offshore New Zealand. We find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms the result of previous studies from Cascadia that nontidal components are more similar along isobaths even if the distance between sites is large. Furthermore, in order to reduce noises, the required depth difference between site pairs also varies with site depths.

The Influence of Sea-Level Changes on Geodetic Datums along the East Coast of China

ABSTRSCT Non-tidal variation on sea level generates immediate changes in gravity field and deformation of the Earth's crust, thus influencing the stability of geodetic datums. To achieve higher accuracy of geodetic datums in coastal and island areas, sea-level changes should be adequately considered when conducting the maintenance of datums. Based on the data of sea-level anomalies from 2014 to 2017 and the theory of load-deformation of elastic spherical earth, this paper demonstrates that impacts of non-tidal ocean load can be quantitatively calculated on geodetic datums along the east coast of China, by using remove-restore technique and combining spherical harmonic analysis of gravity field and Green’s functions to calculate loads. Temporal and spatial analyses on the impacts are conducted with Continuously Operating Reference Stations (CORS) sites along the coast and on the islands. This paper has drawn some conclusions as follows: (1) The impact of non-tidal ocean loads on the east coastal geoid ranges from −4.16 to 4.17 mm, that on ground gravity ranges from −15.44 to 17.02 some conclusions as follows: (1) The impact oranges from −7.55 to 6.69 mm. (2) The impacts are dominated by annual and semi-annual cycles. (3) The impacts appear to be more prominent in coastal areas and on islands than in inland areas.

Non-Tidal Mass Variations in the IGS Second Reprocessing Campaign: Interpretations and Noise Analysis from GRACE and Geophysical Models

Vertical deformations caused by non-tidal mass variations still remain in global navigation satellite system (GNSS) height time series, and can be computed from both Gravity Recovery and Climate Experiment (GRACE) and geophysical models. In this study, we provide a thorough evaluation of the relationships between these different techniques in the global scale by comparing non-tidal vertical deformations from IGS second reprocessing campaign (IG2), GRACE and Global Geophysical Fluid Center (GGFC) solutions, and investigate the noise properties of the GNSS corrected by GRACE solutions and GNSS corrected by GGFC solutions for global stations using optimal noise models. Our results demonstrate that the consistency between seasonal vertical deformations derived from GNSS, GRACE and GGFC is high. When correcting GNSS deformations with GRACE and GGFC solutions, 81% and 73% of the 186 stations have the weight root mean square (WRMS) reduction, respectively. The WRMS variations averaged over all stations are −12.3% and −5.6%, respectively for GNSS corrected by GRACE and GNSS corrected by GGFC solutions. The obvious difference occurs in the GNSS corrected by GGFC solutions WRMS increase, with the mean increase value up to 29%, mainly happening to stations located on islands or small peninsulas. In addition, noise properties of the GNSS corrected by GRACE solutions and GNSS corrected by GGFC solutions for global stations are investigated using optimal noise models. After correcting non-tidal loading effects, the solutions of GNSS corrected by GRACE solutions have the lowest noise level, and can occupy 5% of the noise behavior presenting in global stations, while the solutions of GNSS corrected by GGFC solutions can bring more than 5% of the noise into global stations, implying that GRACE correction solutions can present more favorable results when interpreting GNSS non-tidal loading deformations.

Моделирование неприливных вариаций гравитационного поля с использованием корреляционной модели прогноза

Моделирование неприливных вариаций гравитационного поля с использованием корреляционной модели прогноза

Спектральный анализ неприливных вариаций силы тяжести

Приведены результаты спектрального анализа неприливных вариаций силы тяжести в г. Бишкек (Кыргызстан) в год спокойного Солнца (2018 г.). Предварительно неприливные вариации были разделены на локальную и осредненную части. Анализ проведен отдельно для исходных значений вариаций, локальной и осредненной частей. Все выбросы значений вариаций за пределы средних значений колебаний исключены. Выбросы вариаций связаны преимущественно с землетрясениями. Исключение выбросов показало, что амплитудные спектры практически на протяжении всего года не меняются. Вероятно, остаточные вариации связаны с собственными колебаниями атмосферы, которые по большей части своей определяются метеорными потоками.

Seismic-induced gravitational pulsations on the Eurasian continent

For the first time, gravity pulsations were detected during the processing of stationary observations at the Geodynamic range of RAS in Bishkek (2014). Then, similar surges were detected in stationary seismic observations in the city of Voronezh, which simultaneously coincided with the gravimetric fluctuations in Bishkek. Later, gravity pulsations were detected at Artie, Perm and Vladivostok stations. Seismic ripples are also found in Southeast Asia. In coastal zones and in the ocean, additional pulsations occur due to atmospheric processes (typhoons, cyclones, etc.). Pulsations of non-tidal variations of gravity and seismic fields are global.

Seafloor Crustal Deformation on Ocean Bottom Pressure Records With Nontidal Variability Corrections: Application to Hikurangi Margin, New Zealand

AbstractOcean bottom pressure (OBP) observations are a powerful tool for determining vertical crustal displacements, especially due to earthquakes and slow earthquakes, with centimeter‐level resolution. In these studies, removal of oceanographic noise (tens of centimeters) is required to identify centimeter‐level crustal deformation. We undertake barotropic modeling to remove oceanographic signals from data from an OBP array deployed offshore New Zealand in 2014/2015. We show that removing the nontidal component calculated from a barotropic ocean model reduces the variance in the data by about 66% and provides a feasible means to resolve pressure changes due to crustal deformation during the slow slip events. We also discuss the vertical displacements from slow slip events that occurred in late September to mid‐October 2014, and we outline our procedure for processing OBP data.

Aliased Tidal Variability in Mesoscale Sea Level Anomaly Maps

Sea level anomaly (SLA) maps are routinely produced by objective analysis of data from the constellation of satellite altimeter missions in operation since 1992. Beginning in 2014, changes in the Data Unification and Altimeter Combination System (DUACS) used to create the SLA maps resulted in improved spatial resolution of mesoscale variability, but it also increased the levels of aliased tidal variability compared to the methodology employed prior to 2014. The present work investigates the magnitude and spatial distribution of these tidal signals, which are typically smaller than 1 cm in the open ocean but can reach tens of centimeters in the coastal ocean. In the open ocean, the signals are caused by a combination of phase-locked and phase-variable baroclinic tides. In the coastal ocean, the signals are a combination of aliased high-frequency nontidal variability and aliased variability caused by erroneous tidal corrections applied to the along-track altimetry prior to objective analysis. Several low-pass and bandpass filters are implemented to reduce the tidal signals in the mapped SLA, and independent tide gauge data are used to provide an objective assessment of the performance of the filters. The filter that attenuates both the small-scale (less than 200 km) and the high-frequency (period shorter than 108 days) components of SLA removes aliased baroclinic tidal variability and improves the accuracy of tidal analysis in the open ocean while also performing acceptably in the coastal ocean.

The nodal dependence of long-period ocean tides in the Drake Passage

Abstract. Almost three decades of bottom pressure recorder (BPR) measurements at the Drake Passage, and 31 years of hourly tide gauge data from the Vernadsky Research Base on the Antarctic Peninsula, have been used to investigate the temporal and spatial variations in this region of the three main long-period tides Mf, Mm and Mt (in order of decreasing amplitude, with periods of a fortnight, a month and one-third of a month, respectively). The amplitudes of Mf and Mt, and the phase lags for all three constituents, vary over the nodal cycle (18.61 years) in essentially the same way as in the equilibrium tide, so confirming the validity of Doodson's “nodal factors” for these constituents. The amplitude of Mm is found to be essentially constant, and so inconsistent at the 3σ level from the ±13 % (or ∼±0.15 mbar) anticipated variation over the nodal cycle, which can probably be explained by energetic non-tidal variability in the records at monthly timescales and longer. The north–south differences in amplitude for all three constituents are consistent with those in a modern ocean tide model (FES2014), as are those in phase lag for Mf and Mt, while the phase difference for Mm is smaller than in the model. BPR measurements are shown to be considerably superior to coastal tide gauge data in such studies, due to the larger proportion of non-tidal variability in the latter. However, correction of the tide gauge records for non-tidal variability results in the uncertainties in nodal parameters being reduced by a factor of 2 (for Mf at least) to a magnitude comparable (approximately twice) to those obtained from the BPR data.

Prediction of nontidal sea level variations in the Persian Gulf using data assimilation techniques

ABSTRACTThe regular pattern of tidal water level fluctuation in the open seas is often deviated by the effect of weather systems. The stochastic behavior of these systems makes it difficult to predict the nontidal residuals (NTR) precisely. The hydrodynamic models as efficient tools for the simulation of NTR may suffer from lack of accuracy due to approximate boundary conditions and forcings. Data Assimilation (DA) techniques combine hydrodynamic model predictions with available field observations to improve the predictions. In this study, a number of DA methods were employed to improve the NTR predictions in a semienclosed basin. The results have shown the studied DA techniques, which are originally developed for weather prediction problems, can be improved by considering the physical mechanisms of NTR generation. A new formula was introduced for the calculation of weight factors of the existing methods. Application of the proposed method increased the accuracy of NTR predictions significantly so that the correlation coefficient of the NTR predictions increased from 0.660 to 0.896 and the root mean square error decreased from 5.22 cm to 2.51 cm. In addition, so it is concluded among the studied DA schemes, Barnes method has better efficiency for prediction of NTR in the region.

Loading‐Induced Earth's Stress Change Over Time

AbstractWe establish a physical framework and build a preliminary database of stress changes on the Earth from 2000 to 2017 in a global scale and at various depths. We consider six loading forces that would generate stress changes on the Earth: hydrological loading, atmospheric pressure, ocean water (including tides and nontidal variation), solid lunisolar tides, pole tide, and postglacial rebound (PGR). The maximum amplitudes of normal deviatoric stress changes on the Earth's surface caused by hydrological loading, atmospheric pressure, and solid lunisolar tides reach 10−2 bar, those by pole tide 10−4–10−3 bar, and those by ocean tides 10−1 bar mostly in the coastal regions. The shear stress changes are about 1 order of magnitude smaller than the normal deviatoric stresses. The PGR‐induced stress rates are 10−3–10−2 bar/year. Stress variations caused by the different forces also exhibit different spatial patterns, with large hydro‐induced stresses mainly distributed in torrid and frigid zones, atmosphere‐induced stresses in temperate and frigid zones, and ocean‐induced stresses in coastlands and PGR‐induced stress rates in Greenland, north Europe, North America, and Antarctica. The hydro‐induced stresses exhibit significant seasonal variations in Amazon Basin, northern India, and central Africa and persistent increase/decrease in Antarctica, Greenland, and Alaska due to ice shelf melting. All loading‐induced stress changes exhibit significant depth variations. The stress database would provide a resource for better understanding the triggering mechanisms of various tectonic events, and the physical framework we establish to build the database could be useful for better studying properties and physical states of Earth's interior.

The influence of the atmospheric front on the readings of the gravimeters and seismographs

Non-tidal variations of gravity are the residual part of the monitoring of the variations after subtraction from them the lunar-solar gravity variations and the drift of the zero point of the gravimeter. Non-tidal variations have a complex morphology and structure. The sources of the non-tidal variations are the intracrustal processes, flows of the charged particles in space and meteor showers. Meteor showers interacting with the Earth’s atmosphere determine the occurrence, development and movement in the atmosphere of the large-scale eddies - cyclones and anticyclones. The atmospheric fronts are distinguished in isolated cyclones. These fronts can influence the measurements of the gravimeters and seismographs.

ABNORMAL CHANGES OF THE NON-TIDAL VARIATIONS OF GRAVITY

Non-tidal variations of gravity are the residual part of the monitoring of the variations after subtraction from them the lunar-solar gravity variations and the drift of the zero point of the gravimeter. Non-tidal variations are sometimes of complex morphology and structure. The sources of the non-tidal variations are the intracrustal processes and flows of the charged particles in space. The streams of the charged particles can affect the sensor of the gravimeter. The streams of the charged particles can create a powerful magnetic hydrodynamic (MHD) shocks that cause abnormal changes of gravity. It is necessary to consider the non-tidal variations when carrying out high-precision gravimetric measurements.

Analysis of decade-long time series of GPS-based polar motion estimates at 15-min temporal resolution

We present results from the generation of 10-year-long continuous time series of the Earth’s polar motion at 15-min temporal resolution using Global Positioning System ground data. From our results, we infer an overall noise level in our high-rate polar motion time series of 60 $$\upmu \hbox {as}$$ (RMS). However, a spectral decomposition of our estimates indicates a noise floor of 4 $$\upmu \hbox {as}$$ at periods shorter than 2 days, which enables recovery of diurnal and semidiurnal tidally induced polar motion. We deliberately place no constraints on retrograde diurnal polar motion despite its inherent ambiguity with long-period nutation. With this approach, we are able to resolve damped manifestations of the effects of the diurnal ocean tides on retrograde polar motion. As such, our approach is at least capable of discriminating between a historical background nutation model that excludes the effects of the diurnal ocean tides and modern models that include those effects. To assess the quality of our polar motion solution outside of the retrograde diurnal frequency band, we focus on its capability to recover tidally driven and non-tidal variations manifesting at the ultra-rapid (intra-daily) and rapid (characterized by periods ranging from 2 to 20 days) periods. We find that our best estimates of diurnal and semidiurnal tidally induced polar motion result from an approach that adopts, at the observation level, a reasonable background model of these effects. We also demonstrate that our high-rate polar motion estimates yield similar results to daily-resolved polar motion estimates, and therefore do not compromise the ability to resolve polar motion at periods of 2–20 days.

Synchronous ripples in non-tidal variations of gravity and seismic fields

Synchronous ripples in non-tidal variations of gravity and seismic fields