Earth Tides Research Articles

Temporal and Spatial Evolution of Load/Unload Response Ratio Before the M7.0 Jiuzhaigou Earthquake of Aug. 8, 2017 in Sichuan Province

The load/unload response ratio (LURR) method has been proposed to evaluate earthquake potential. This method can be used to investigate the regional stress–strain field change and fault instability associated with the preparation of a large earthquake. In this study, we explore the temporal and spatial evolution of the LURR anomalies before the M7.0 Jiuzhaigou earthquake that occurred on Aug. 8, 2017 in Sichuan Province. We calculate the LURR values by using Benioff strain of the small earthquakes within 100, 200, 300 and 500 km from the epicenter and 12 years prior to the main shock. The loading and unloading phases are determined by calculating the change of Coulomb failure stress caused by earth tide in the tectonically preferred slip direction on the fault surface of the major event. Results show that the LURR values remain at a low level for years until 2014 when the LURR values increase to an anomalously high level. Meanwhile, the magnitude of the anomalies decrease with the distance from the epicenter, and can hardly be observed within the 500-km radius. Combining with the spatial distribution of the LURR, we find that the development of anomalies before the mainshock can be divided into two stages: during the periods of 2014–2015, the significant anomalies were observed in the epicenter and its northwest region; and then in 2016, the anomalies were detected in the southeast area of the earthquake, which expanded around the Huya fault and finally reached the epicenter months before the mainshock. The earthquake is located at the intersection area of LURR anomalies of the two time periods, suggesting the generation of the Jiuzhaigou earthquake.

Earth as a Gravitational-Wave Interferometr

Based on the principle of Equivalence of Gravitating Masses (EGM) and tectonostratigraphic model of the Earth outer shell structure (the Earth crust and upper mantle), the average depth of the lunar mass gravitational influence on the Earth was calculated as ~1600 km. The developed model is based on the mechanism of rocks tectonic layering of the Earth crust-mantle shell as an oscillatory system with dynamic conditions of a standing wave, regularly excited by the lunar tide and immediately passing into the damping mode. After comparing the average depth of solid lunar tide impact of ~1600 km with the height of the solid lunar tide “hump” on the Earth surface of 0.5 m, a “tensile strain” was calculated with an amplitude only one order of magnitude larger than the amplitude of the gravitational wave recorded by the Advanced LIGO interferometer: A≈10-18 m (the merger result of a black holes pair ca 1.3 Ga ago). The results of the present study suggest that the crust-mantle shell of the Earth may be used as a gravitational-wave interferometer.

Well Water Level Analysis Based on Barometric Pressure Effects and Earth Tides

Barometric pressure coefficient and tidal factor are used to study the porosity, the solid skeleton volume compressibility coefficient and the water volume compressibility coefficient of the Dahuichang well, Banqiao well, Huanghua well, Dadianzi well, Fengzhen well and Sanhaodi well in the northern region of North China under undrained condition. The results show that there is power function relation between the porosity and the volume compressibility coefficient(the solid skeleton and the water)in the aquifer. In the first quadrant, the solid skeleton volume compressibility coefficient of each well increases with the increase of the porosity, the volume compressibility coefficient of the water decreases with the increase of porosity. Between the volume compressibility coefficient of the solid skeleton and the water exist unary quadratic polynomial relationship, and the volume compressibility coefficient of water is larger than that of solid skeleton, the water is easier to compress. In addition, according to the step barometric pressure response function in the regression deconvolution method, the groundwater type identifying results of the six wells aquifer system are shown that there is an e based exponential function between the lag time and the step barometric pressure response function of each well water level to barometric pressure. The coefficient before the base e is positive or negative to determine the groundwater type of the well aquifer system. For confined wells, the step barometric pressure response function increases with lag time of well water level to barometric pressure, while the unconfined wells and semi-confined wells are opposite.

Atmospheric tides and their consequences on the rotational dynamics of terrestrial planets

Atmospheric tides can have a strong impact on the rotational dynamics of planets. They are of most importance for terrestrial planets located in the habitable zone of their host star, where their competition with solid tides is likely to drive the body towards non-synchronized rotation states of equilibrium, as observed in the case of Venus. Contrary to other planetary layers, the atmosphere is sensitive to both gravitational and thermal forcings, through a complex dynamical coupling involving the effects of Coriolis acceleration and characteristics of the atmospheric structure. These key physics are usually not taken into account in modelings used to compute the evolution of planetary systems, where tides are described with parametrised prescriptions. In this work, we present a new ab initio modeling of atmospheric tides adapting the theory of the Earth’s atmospheric tides (Chapman & Lindzen 1970) to other terrestrial planets. We derive analytic expressions of the tidal torque, as a function of the tidal frequency and parameters characterizing the internal structure (e.g. the Brunt-Väisälä frequency, the radiative frequency, the pressure heigh scale). We show that stratification plays a key role, the tidal torque being strong in the case of convective atmospheres (i.e. with a neutral stratification) and weak in case of atmosphere convectively stable. In a second step, the model is used to determine the non-synchronized rotation states of equilibrium of Venus-like planets as functions of the physical parameters of the system. These results are detailed in Auclair-Desrotour et al. (2016a) and Auclair-Desrotour et al. (2016b).

On the question of the relationship of variations in geophysical fields, lunar-solar tidal effects and seismic events

Based on the results of instrumental observations made in the Tien Shan, the interrelation of time variations of a number of geophysical fields and seismic events is established. According to the results of the correlation analysis, it was concluded that the fault zones are characterized by significantly higher variations of geophysical fields, an intense response to weak external influences in the form of a solid moon-solar tide.

A Quantitative Estimate of the Effects of Sea Tides on Aftershock Activity: Kamchatka

The issue of whether tidal forces really affect seismicity has been raised many times in the literature. Nevertheless, even though there seems to be a kind of consensus that such effects do exist, no quantitative estimates are available to relate tide parameters to changes in the level of seismic activity. Such estimation for aftershocks of large earthquakes near Kamchatka is the goal of the present study. We consider the influence on seismicity due to ocean tides only, because their effects are stronger than those of solid earth tides. Accordingly, we only consider earthquakes that occurred in the ocean. One important feature that distinguishes the present study from most other such research consists in the fact that we study the height of ocean tides and its derivative rather than tidal phases as the decisive factors. We considered 16 aftershock sequences of earthquakes near Kamchatka with magnitudes of 6 or greater. We also examined shallow background earthquakes along the coast of Kamchatka. Our basic model of aftershock rate was the Omori–Utsu law. The background seismicity distribution was assumed to be uniform over time. In both of these cases we used the actual distributions in space. The heights of sea tides were estimated using the FES 2004 model (Lyard et al., 2006). The variation in activity from what the basic model assumes in relation to tidal wave height and its time derivative was estimated by the method of differential probability gain. The main practical result of this study consists in estimates of averaged differential probability gain functions for aftershock rate with respect to both of the considered factors. These estimates can be used for earthquake hazard assessment from aftershocks with ocean tides incorporated. The results of our analysis show a persistent tendency of aftershock rate increasing during periods when the ocean tide decreased at a high rate. For the background events, we found a typical tendency of events rate increasing when the ocean tide decreased with high tidal amplitudes. The difference in the main factors that affect aftershocks and background seismicity suggest the inference that the effects of tides on aftershocks are more likely to be direct dynamic initiation of events during high strain rates, while the effects on the background events were static in character.

Anelasticity from seismic to tidal timescales: Theory and observations

We examine strain energy dissipation of Earth's normal modes and body tides in order to compare these observations with predictions from an experimental model of intrinsic dissipation. For this comparison we employ a recently developed self-consistent treatment of modes and tides, that includes the separation of dynamical processes (self-gravity and inertia) from intrinsic attenuation. We select a set of normal modes and tides with similar depth sampling centered on the lower mantle, without contributions from the core. This dataset ranges from 7 min to 18.6 yr, or about 6 decades in frequency. The results show that the attenuation of the higher-frequency modes defines a broad plateau in dissipation (inferred experimentally to occur at the transition from elastic to anelastic behavior) whereas the attenuation of lower-frequency tides falls on a frequency-dependent absorption band. This reconciles published, seemingly contradictory models of dissipation for modes versus tides: the absorption band for modes terminates at periods where tidal models infer its onset. Microphysically, the experimental data can be explained by elastically accommodated grain boundary sliding, producing a broad plateau at short periods, transitioning to diffusionally assisted grain boundary sliding at longer periods, corresponding to an absorption band. Extrapolation of the fit to the experimental data with parameters adopted for the lower mantle can reconcile the seismological and geodetic observations.

Detection of free core nutation resonance variation in Earth tide from global superconducting gravimeter observations

In order to verify the time variability of free core nutation (FCN) period, global superconducting gravimeter (SG) observations were analyzed based on synthetic test data. The gravity data series were synthesized to check the detectability of resonance variation caused by FCN period change. The tests indicate that the discrepancy between the FCN periods determined by SG and VLBI observations is caused by the high correlation between the FCN parameter and the amplitude factor of the ψ1 wave. The K1 wave is more sensitive to the FCN period change than other diurnal waves. The limit of the standard deviation of the K1 wave is found for more precisely observing the FCN period change. Tidal parameters of diurnal waves estimated from long series of 20 global SG stations were analyzed. A common variation trend is found in the amplitude factor of both K1 and ψ1 waves in all 8 stations above the limit, which indicates the FCN period may be not so stable in time. Furthermore, the variation in the K1 and ψ1 waves constrains the FCN period change to between 2.5 and 4 sidereal days, which also agrees with the possible variation from the current VLBI and SG observations.

The Atlantida 3.1_2014 Program for Earth Tide Prediction: New Version

The main distinctive features of ATLANTIDA3.1._2014, the first domestic program for the prediction of Earth tides, are discussed in comparison to similar programs of international authors, and some examples of the practical application of the program by different researchers are presented. The main functional features of the program are listed. Among them, the features for calculating amplitude delta factors of tidal waves for an oceanless Earth, as well as amplitude delta factors and phase shifts for an Earth with an ocean, tidal time series, and amplitudes and phases of the oceanic gravimetric effect, are especially distinguished. The theoretical developments included in the program are briefly described. The main differences of the latest version of the program are identified. The theoretical values of the amplitude delta factors for an inelastic self-gravitating rotating Earth, calculated with respect to the action of relative and Coriolis accelerations, as well as dissipation, are considered in detail. A total of 12 model variants are considered, which differ from each other by the inclusion or exclusion of separate factors affecting the result. The closeness of the results yielded by the program and the observations using the European superconducting gravimeters of the Global Geodynamic Project (GGP) is evaluated. It is shown that according to this criterion, our program surpasses the world’s most well-known counterparts. Tasks are listed that after their solution will expand the functional features of the program and clarify the results obtained so far.

Tidal Love Numbers of Degrees 2 and 3

In the coming years, the high-precision processing of large arrays of modern observational data from the Global Satellite Navigation System (GNSS) will require knowledge of the theoretical values of the tidal numbers h and l of degrees 2 and 3 with a relative error no worse than 10–4. This will make it possible to predict vertical and horizontal tidal displacements of the earth’s surface at the modern level. The paper presents the values of Love numbers h, k, and l of degrees 2 and 3 calculated for an inelastic ellipsoidal self-gravitating rotating earth without an ocean. Twelve different models are considered that differ from each other by the inclusion or exclusion of individual factors that affect the result. In particular, two different versions of the earth’s structure model are used, corrections for the relative and Coriolis accelerations are considered, and the values of the Love numbers are determined considering their latitude dependence. Lame elastic parameters were recalculated for the periods of semidiurnal and diurnal tidal waves using the logarithmic creep function to account for dissipation. The normalization of the obtained values of the Love numbers corresponds to International Earth Rotation and Reference Systems Service (IERS) Conventions regarding the calculation of tidal displacements of the earth’s surface. The values are compared with the results of widely known works of other authors. Currently, the results are used to develop a new version of the ATLANTIDA3.1 software for predicting earth tides.

3-D Positioning and Target Association for Medium-Resolution SAR Sensors

Associating a radar scatterer to a physical object is crucial for the correct interpretation of interferometric synthetic aperture radar measurements. Yet, especially for medium-resolution imagery, this is notoriously difficult and dependent on the accurate 3-D positioning of the scatterers. Here, we investigate the 3-D positioning capabilities of ENVISAT medium-resolution data. We find that the data are perturbed by range-and-epoch-dependent timing errors and calibration offsets. Calibration offsets are estimated to be about 1.58 m in azimuth and 2.84 m in range and should be added to ASAR products to improve geometric calibration. The timing errors involve a bistatic offset, atmospheric path delay, solid earth tides, and local oscillator drift. This way, we achieve an unbiased positioning capability in 2-D, while in 3-D, a scatterer was located at a distance of 28 cm from the true location. 3-D precision is now expressed as an error ellipsoid in local coordinates. Using the Bhattacharyya metric, we associate radar scatterers to real-world objects. Interpreting deformation of individual infrastructure is shown to be feasible for this type of medium-resolution data.

Thermal Effects of Magma Intrusion on Ocean Temperatures Through Ice Ages, with Particular Reference to Interglacial Events

Recent observations are used to crudely quantify and scale the direct heating of the global ocean surface by submarine volcanism. These suggest a current submarine magma budget that is substantially greater than the prevailing consensus. The current ocean heating effect is estimated at c. 0.7W/m2. Interglacial events are geologically sudden and predictably cyclical, overtly caused by the combined effect of Milankovitch cycles. Several possible causes for interglacial warmings arising from these cycles have been proposed, and rejected as inadequate. These are summarily reviewed. It is concluded, in the absence of other capable causes, and in particular given the evidence of continued ocean warming throughout the Dryas cooling events, that ice age interglacial events are probably caused by a sustained increase in the rate of submarine volcanic heating of the oceans. The primary cause of increasing emissions is suggested as the rising levels of solid gravitational tides caused by the combined orbital forcing of all three main Milankovitch cycles. These tides are at their largest during interglacial warmings. How the dominant climate control system of oceanic feedback responds to this internal perturbation change is also discussed. In particular how the upper equilibrium temperature of an interglacial period is imposed by this control.

Seasonal Permeability Change of the Shallow Crust Inferred From Deep Well Monitoring

AbstractPermeability is a fundamental rock property that controls several important transport processes in Earth's crust. Here we use 10 years of continuous data to analyze the tidal response of water level in a well in SW China to Earth tides and show that permeability of the shallow crust may change seasonally. The relationship between permeability and the groundwater level suggests that the vertical permeability may increase ~6 folds in response to seasonal increases of water level over a threshold equivalent to ~0.1 MPa of pore pressure. The specific storage does not show corresponding change, suggesting that the change in permeability may be related to the clogging and unclogging of existing fractures in low‐porosity rock.

Evidence for diurnal periodicity of earthquakes from midnight to daybreak.

Previously, inadequate earthquake catalogs and the lack of method made it challenging, if not impossible, to determine the dominant period of earthquake occurrence. With the advent of densely distributed seismic stations in Japan since 2002, 410 642 M1.0+ earthquakes have been cataloged under the mainland within 30-km depth, which provide a superb dataset to identify the periodicity of earthquakes. We processed this catalog using time-frequency analyses and daily stacking, which are powerful in extracting weak periodical signal from strong random noises. We concentrated on the time sector from 0:00 to 6:00 (i.e. from midnight to daybreak), which is a reliable time window for much higher detectability of weak earthquakes, since it has the lowest affects from cultural noises. We successfully observed two apparent periodicities of 12- and 24-hour, which are much smaller than the fortnightly periodicity presented previously in the literature. Synthetic earth tides, after intentionally ignoring the contribution from the Moon, present similar dominant periods as the earthquakes. This may indicate that the dominant period of earthquakes is statistically associated with the Sun rather than the Moon. The daily stacking number of earthquakes using a 15-minute or 1-hour interval shows a peak around 1:30, rather than usually expected 3:00 to 4:00. In addition, bigger earthquakes show more evident variations in the stacking results, and the trend is very consistent for various lower limits of earthquake magnitude from M1.0 to M4.0. These discoveries settled the disputes on the existence of the periodicity of earthquakes since 1886 and may open a window to unravel the mystery of earthquakes.

Tidal effects on the LAGEOS–LARES satellites and the LARASE program

The effects of solid and ocean tides have been computed on the right ascension of the ascending node of the two LAGEOS and LARES satellites and on the argument of pericenter of LAGEOS II. Their effects—together with the possible mis-modeling related to systematic errors in the estimate of the tidal coefficients, especially in the case of ocean tides—are quite important to be well established for the key role of the LAGEOS satellites, as well as of the newly LARES, in space geodesy and geophysics as well as in fundamental physics measurements. In the case of the measurement of the Lense–Thirring effect, the mis-modeling of long-period tides may mimic a secular effect on the cited orbital elements, thus producing a degradation in the measurement of the relativistic precession. A suitable combination of the orbital elements of the three satellites can help in avoiding the effects of the long-period tides of degree $$\ell =2$$ (as for the Lunar solid tides with periods of 18.6 and 9.3 years) and $$\ell =4$$ , but other long-period tides, as the ocean $$K_1$$ tide, which has the same periodicities of the right ascension of the ascending node $$\varOmega $$ of the satellites, may strongly influence the measurement, especially if it is performed over a relatively short time span. These results are particularly important in the case of LARES, since they are new and because of the role that the orbit of LARES, and especially of its ascending node right ascension, will have in a new measurement of the Lense–Thirring effect by the joint analysis of its orbit with that of the two LAGEOS.

Diurnal and Semidiurnal Cyclicity of Radon (222Rn) in Groundwater, Giardino Spring, Central Apennines, Italy

Understanding natural variations of Rn (222Rn) concentrations is the fundamental prerequisite of using this radioactive gas as a tracer, or even precursor, of natural processes, including earthquakes. In this work, Rn concentrations in groundwater were continuously measured over a seven-month period, during 2017, in the Giardino Spring, Italy, together with groundwater levels in a nearby well installed into a fractured regional aquifer. Data were processed to reduce noise, and then analyzed to produce the Fourier spectra of Rn concentrations and groundwater levels. These spectra were compared with the spectrum of tidal forces. Results showed that diurnal and semidiurnal cycles of Rn concentrations, and filtered oscillations of groundwater levels, in the nearby well, are correlated with solar and luni-solar components of tidal forces, and suggested no correlation with the principal lunar components. Therefore, influencing factors linked to solar cycles, such as daily oscillations of temperature and atmospheric pressure, and related rock deformations, may have played a role in Rn concentrations and groundwater levels. An open question remains regarding the correlation, which is documented elsewhere, of Rn concentrations and groundwater levels with the lunar components of the solid Earth tides.

Performance analysis of a new baseline-based SS-Y extensometer

In order to solve the problems in the observation with the SS-Y extensometer, such as background noise and discontinuity of earth tide curve, this paper proposed to improve the stability of the instrument and the smoothness of the earth tide curve by improving the baseline structure of the instrument. In this study, a new φ20*1.2 invar tube was introduced in replacement of the existing φ6 invar rod as the baseline of the instrument and compared with the existing φ6 invar rod on the instrument's linearity, sensitivity and other indexes. Firstly, the SS-Y extensometers using φ6 invar rod and φ20*1.2 invar tube were tested. Test results were then calculated, and accuracy errors of the instruments based on the two baselines were obtained. Finally, their accuracy errors and earth tide curves were compared. Results showed that, compared with the extensometer based on the existing φ6 invar rod, the extensometer based on the new φ20*1.2 invar tube was superior in performance, with linearity error reducing to 0.495% from 0.937%, sensitivity increasing to 68.65 mV/μm from 65.46 mV/μm, and earth tide curve growing more stable and continuous.

Analysis of 90 day operation of the GINGERINO gyroscope.

Gyroscopes IN GEneral Relativity (GINGER) is a proposed experiment with the aim of measuring in a ground laboratory the gravitoelectric and gravitomagnetic effects foreseen by general relativity through an array of ring laser gyroscopes. GINGERINO is a square ring-laser prototype that has been built to investigate the level of noise inside the Gran Sasso underground laboratory. GINGERINO has shown the advantage of the underground location. Now it provides suitable data for geophysics and seismology. Since May 2017, it has continuously acquired data. The analysis of the first 90 days shows that the duty cycle is higher than 95%, and the quantum shot noise limit is of the order of 10-10(rad/s)/Hz. It is located in a seismically active area, and it recorded part of the central Italy earthquakes. Its high sensitivity in the frequency band of fraction of hertz makes it suitable for seismology studies. The main purpose of the present analysis is to investigate the long-term response of the apparatus. Simple and fast routines to suppress the disturbances coming from the laser have been developed. The Allan deviation of the raw data reaches some 10-6 after about 106 s of integration time, while the processed data show an improvement of 1 order of magnitude. Disturbances at the daily time scale are present in the processed data, and the expected signal induced by polar motion and solid Earth tides is covered by those disturbances.

Earth tidal stress as an earthquake trigger prior to the Wenchuan earthquake, Sichuan, China

Cyclic stress variations in the crust of the earth are caused by the earth tide. Although these stress variations are small in comparison to tectonic stress, tidal stress rates are often larger than those of tectonic stress accumulation. Thus, tidal stress could be considered as one of the possible triggers for an earthquake. Many scientific studies have been conducted to examine the correlation between the earth tide and earthquake occurrence. In this study, the shear and normal stresses caused by the earth tide on the focal fault plane of the Wenchuan earthquake, Sichuan, China, are calculated using the focal mechanism solution published by the Global Centroid Moment Tensor catalog and the Preliminary Reference Earth Model. We analyzed how the tidal stresses on the fault plane changed when the Wenchuan earthquake occurred. The Schuster’s test was applied to define correlations between earthquakes occurring around the epicenter area before the Wenchuan earthquake and stresses caused by the daily earth tide. A P -value was calculated and used to quantify the statistical significance of the evidence. Earthquake data used in this study were obtained from the China Earthquake Networks Center, China Earthquake Administration. The temporal variation and spatial distribution of the P -value, indicating the significance of the test, were investigated. The obtained results show that the daily tidal stress was in favor of slip of the focal fault when the Wenchuan earthquake occurred. For earthquakes ( M L≥2.7) that occurred in the study area, the P -value began to drop in June 2007, approaching the lowest value (0.85%) very close to the occurrence time of the M S8.0 Wenchuan main shock. Most of low- P -value ( P -value situated in the near northeast of the epicenter can be found prior to the Wenchuan earthquake from the spatial distribution of P -value in the Longmenshan Fault. It can be concluded from the above-mentioned results that significant correlations between earthquakes occurring around the epicenter area and the daily tide existed prior to the Wenchuan earthquake. These tide-triggered earthquakes were in the south part of the rupture zone of the Wenchuan earthquake, northeast of the epicenter. Therefore, tidally triggered earthquakes occurred around the epicenter area prior to the Wenchuan earthquake.

Global Ocean Altimetry With GNSS Reflections From TechDemoSat-1

TechDemoSat-1 (TDS-1) is an experimental Global Navigation Satellite System Reflections (GNSS-R) satellite launched in 2014. The GNSS-R receiver onboard performs real-time navigation and generates delay–Doppler correlation maps for Earth-reflected Global Positioning System (GPS) L1 C/A ranging signals. This paper investigates the performance of the TDS-1 data for ocean surface altimetry retrievals. The analysis includes consideration of the transmitter and receiver orbits, time tag corrections, models for ionospheric and tropospheric delays, zenith to nadir antenna baseline offsets, ocean and solid Earth tides, and a comparison with mean sea surface topography. An error budget is compiled to account for each error source and compared with the experimentally derived surface height retrievals. By analyzing data sets covering global ocean surfaces over ±60° latitude, the current performance of spaceborne GNSS-R altimetry with the TDS-1 data set is experimentally established. In comparison with the mean sea surface topography, the surface height residuals are found to be 6.4 m, $1\sigma $ with a 1-s integration time. A discussion of the factors limiting this performance is presented, with implications for future GNSS-R altimetry missions designed for the observation of mesoscale ocean circulation.