Geodesy and Geodynamics最新文献

This study reports the morphological characteristics of anomalous variations in Global Navigation Satellite System Total Electron Content (GNSS-TEC) prior to the strong local earthquakes (EQ) that occurred during the period of 2011–2015. We have analyzed 20 earthquakes of magnitude M ≥ 5.6. A statistical technique is implemented on the data of six GNSS stations located in Tashkent, Kitab, and Maidanak in Uzbekistan, and Islamabad, Multan, Quetta in Pakistan. The results show continuous anomalous variations in TEC during 24 h before the occupancy of local earthquakes. It is shown that the precursors before the occurrence of strong earthquakes, in particular of magnitude 5.7, 7.7, 7.5, 7.8 and 7.3 are detected near Eastern Uzbekistan (26 May 2013), Southwestern Pakistan (24 September 2013), Hindukush region of Afghanistan (26 October 2015), and Central Nepal (25 April 2015) and (12 May 2015), respectively. The ionospheric anomalies appearing before the strong earthquakes at six GNSS stations are registered in 14 cases (70%) out of 20 selected EQs. It is depicted that anomalies referred to as ionospheric precursors appeared about 1–7 days prior to the occurrence of strong earthquakes.

For the reduction of atmospheric effects, observed gravity has initially been corrected by using the computed barometric admittance k of the in situ measured pressure, expressed in nms⁻²/hPa units and estimated by least squares method. However, the local pressure changes alone cannot account for the atmospheric mass attraction and loading when the coherent pressure field exceeds a specific size, i.e., with increasing periodicities. To overcome this difficulty, it is necessary to compute the total atmospheric effect at each station using the global pressure field. However, the direct subtraction of the total gravity effect, provided by the models of pressure correction, is not yet satisfactory for S2 and other tidal components, such as K2 and P1, which include solar heating pressure tides. This paper identifies the origin of the problem and presents strategies to obtain a satisfactory solution.

First, we set up a difference vector between the tidal factors of M2 and S2 after correction of the pressure and ocean tides effects. This vector, hereafter denoted as RES, presents the advantage of being practically insensitive to calibration errors. The minimum discrepancy between the tidal parameters of M2 and S2 corresponds to the minimum of the RES vector norm d.

Secondly we adopt the hybrid pressure correction method, separating the local and the global pressure contribution of the models and replacing the local contribution by the pressure measured at the station multiplied by an admittance k_ATM.

We tested this procedure on 8 stations from the IGETS superconducting gravimeters network (former GGP network). For stations at an altitude lower than 1000 m, the value of d_opt is always smaller than 0.0005. The discrepancy between the tidal parameters of the M2 and S2 waves is always lower than 0.05% on the amplitude factors and 0.025° on the phases. For these stations, a correlation exists between the altitude and the value k_opt. The results at the three Central European stations Conrad, Pecny and Vienna are in excellent agreement (0.05%) with the DDW99NH model for all the main tidal waves.

Earthquake prediction remains a challenging and difficult task for scientists all over the world. The tidal triggering of earthquakes is being proven by an increasing number of investigations, most of which have shown that earthquakes are positively correlated with tides, and thus, tides provide a potential tool for earthquake prediction, especially for imminent earthquakes. In this study, publications concerning the tidal triggering of earthquakes were compiled and analyzed with regard to global earthquakes, which were classified into three main types: tectonic, volcanic, and slow earthquakes. The results reveal a high correlation between tectonic earthquakes and tides (mainly for semidiurnal and diurnal tides; 14-day tides) before and after the occurrence of significant earthquakes. For volcanic earthquakes, observations of volcanoes on the seafloor and land indicate that volcanic earthquakes in near-shore volcanic areas and mid-ocean ridges have a strong correlation with tidal forces, mostly those with semidiurnal and diurnal periods. For slow earthquakes, the periodicity of the tremor duration is highly correlated with semidiurnal and diurnal tides. In conclusion, the tidal triggering of these three types of earthquakes makes a positive contribution to earthquake preparation and understanding the triggering mechanism, and thus, the prediction of these types of earthquakes should be investigated. However, there are still several inadequacies on this topic that need to be resolved to gain a definitiveanswer regarding the tidal triggering of all earthquakes. The main inadequacies are discussed in this paper from our point of view.

This study focuses on analyzing the time series of DORIS beacon stations and plate motion of the Eurasian plate by applying Singular Spectrum Analysis (SSA) and Fast Fourier Transform (FFT). First, the rend terms and periodic signals are accurately separated by SSA, then, the periodic seasonal signals are detected using SSA, and finally, the main components of the time series are reconstructed successfully. The test results show that the nonlinear trends and seasonal signals of DORIS stations are detected successfully. The periods of the seasonal signals detected are year, half-year, and 59 days, etc. The contribution rates and slopes in E, N, and U directions of the trend items of each beacon station after reconstruction are obtained by least-square fitting. The velocities of these stations are compared with those provided by the GEODVEL2010 model, and it is found that they are in good agreement except the DIOB, MANB, and PDMB stations. Based on the DORIS coordinate time series, the velocity field on the Eurasian plate is constructed, and the test shows that the Eurasian plate moves eastward as a whole with an average velocity of 24.19 ± 0.11 mm/y in the horizontal direction, and the average velocity of it is 1.74 ± 0.07 mm/y in the vertical direction.

Modern geodetic technologies such as high-precision ground gravity measurements, satellite gravity measurements, the global navigation satellite system, remote sensing methods, etc. provide rich observation data for monitoring various geodynamic processes of the global Earth and its surface. The 19th International Symposium on Geodynamics and Earth Tides brought together scientific researchers from 26 countries around the world, shared the application of various measurements in different geoscience issues, covering Earth tidal deformation, oceanic and atmospheric loading effects, earthquake cycle, hydrology, Earth rotation changes, etc., and provided a precious exchange platform for global peers.

The geodynamic hazards and risk assessment at the Karachaganak oil, gas, and condensate field (KOGCF) were explored on the northern board of the Pre-Caspian Basin to predict the consequences of the long-term exploitation of this field. We integrate multiple measurements, including repeated accurate leveling, Global Positioning System (GPS) measurements, and high precision gravimetric and seismological monitoring at the KOGCF. The results of geodynamic monitoring at the KOGCF for the first time made it possible to prove noticeable seismic deformation processes in the sedimentary cover under the influence of hydrocarbon production. The vertical displacements and horizontal movement along faults, changes in local gravity anomalies, and earthquake sources at depths comparable to hydrocarbon production intervals at the KOGCF have been identified. The maximum amplitudes of modern vertical movement of the earth's surface and the minimum values of the differently oriented horizontal movement were revealed within the projection on the ground surface of the crest of the carbonate massif (Upper Devonian-Lower Permian age). The results suggest the expansion of uneven compression in the crest of the KOGCF while tension processes occur on its periphery. There is a decrease in gravity variations in relation to the slopes of this massif in areas with active hydrocarbon production. An extended zone of high-gradient steps of ΔG_a anomalies, spatially coinciding with the position of fault zones, is mapped along the periphery of the contour of production wells. In the northeastern part of the KOGCF, seismic events were registered practically in the depth intervals of the productive horizons from which hydrocarbons are produced. A spatial relationship between the seismic events and the anomalous deformation activity in the northeast KOGCF has been revealed. Consequently, the field development has provoked both intense deformation of the earth's surface and weak local seismicity.

In this paper, we present observations of free oscillations of the Earth after major earthquakes in Chile (February 27, 2010, M_W8.8) and Japan (March 11, 2011, M_W9.1) using data from the dual-sphere superconducting gravimeter (SG - 055), installed at Badargadh (23°0.47 N, 70°0.62 E), Kutch, Gujarat, India in March 2009. To see the noise characteristics, we calculated the power spectral density of the gravity time series of 5 quiet days in the frequency band 0.05–20 mHz using the new low noise model (NLNM) as a reference. We compared the noise level of the Badargadh site to other SG sites around the world. This shows that the Badargadh SG is in a low noise state. We find that the noise increases at frequencies below 1 mHz. Such a characteristic is also observed in Djougou (Afrique, Benin) and Strasbourg (France). Using theoretical tides for Gujarat, we estimated a scale factor of about −814 nm/s²/V for Grav1 (lower-sphere) and about −775 nm/s²/V for Grav2 (upper-sphere). We corrected the influence of atmospheric pressure from the one-second gravity data before switching to the frequency domain. We extracted a total of 53 Earth's Free Oscillations (EFO) modes during the earthquake in Japan and about 47 EFO modes during the earthquake in Chile. We are able to extract the lowest ₀S₂ spheroidal mode (0.30945 mHz or 54 min) and ₀S₀ radial mode (0.81439 mHz or 20 min). The longer time series shows individual ₀S₂ singlets and ₀S₃ (0.46855 mHz) singlets due to the Coriolis splitting effect. We cross-referenced the frequencies of these modes using the PREM model and previous global observations. The correlation coefficient between the observed and the PREM model for these two events are 0.999 for Japan earthquake and 0.993 for Chile earthquake. This validates the quality of the data useful for low-frequency studies in seismology. We also calculated the relative deviations of our observed fundamental modes with previously determined observed and theoretical values. We found that the relative deviations of our observed free oscillations do not exceed 0.5%, indicating good correlations.

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.

Using Global Positioning System (GPS) coordinate time series, we detect three transient slow slip events (SSEs) offshore the Boso Peninsula in central Japan during 2011–2019. To extract the tiny SSE signals obscured by the significant post-seismic deformation after the 2011 M_W9.0 Tohoku earthquake, we develop a new GPS coordinate time series processing software to obtain these SSE-induced deformations from high-noise GPS data. In addition, we apply the principal component analysis-based inversion method (PCAIM) to get the spatio-temporal slip distribution of the three SSEs. The spatio-temporal evolutions of these slips reveal that the nucleation styles are different. Compared to the 2011 and 2018 SSEs, the 2013–2014 SSE displays faster slip spatio-temporal variation, deeper slip, shorter slip duration, minor seismic moment, and lower maximum slip rate. The 2018 SSE exhibits the most significant seismic moment, the maximum slip, and the maximum slip rate of these three SSEs. The spatio-temporal variations of the 2011 SSE are the most complex, containing two acceleration and deceleration phases. The slip zone expanded along the eastern side of the Boso Peninsula in the acceleration phase and shrank back in the deceleration phase. Furthermore, the recurrence interval of SSEs spans from 2.2 to 4 years during 2011–2019, suggesting that the recurrence interval might become shorter and non-periodic due to the enormous earthquake. After the 2013–2014 SSE, the recurrence interval of the SSE gradually returns to normal. Thus, we can infer that the SSE may occur every 4–7 years after the 2018 SSE if there is no large earthquake.