The ionosphere is highly responsive to various geophysical processes, including solar activity, geomagnetic disturbances, and lithosphere–atmosphere–ionosphere (LAI) coupling related to major earthquakes. This study investigated variations in the Total Electron Content (TEC) of the ionosphere associated with the 30 September 2009 West Sumatra earthquake (Mw 7.6) using observations from 14 continuous Global Positioning System (GPS) stations of the Sumatran GPS Array (SuGAr). The data from GPS stations were processed using the GPS-TEC Analysis Applications (Gopi). The findings reveal significant TEC anomalies in the days leading up to the earthquake, particularly notable during the six-day period just prior to the event. These anomalies, marked by VTEC fluctuations between 1 and 25 TEC Units (TECU), were also consistent with Global Ionosphere Maps (GIM), showing the presence of unusual ionospheric conditions over Sumatra. In contrast, the VTEC during September 2010 control analysis at ABGS and TIKU showed no persistent VTEC increase exceeding the three-consecutive-day threshold, suggesting that the 2009 anomaly was not typical of the region’s interannual variability. To explore the potential influence of space-based factors on these disturbances, we carefully examined space weather conditions and geomagnetic activity using several key parameters, including the Disturbance Storm Time (DST) index, the Kp index, the F10.7 solar radio flux, and extreme ultraviolet (EUV) radiation data. Our analysis indicated that space weather and geomagnetic conditions remained relatively stable throughout September 2009, especially during the timeframe of the most significant VTEC anomaly. This observation suggests that the TEC surge was likely not linked to solar activity, but rather connected to physical processes occurring during the foreshock phase or the preparatory stage of the 2009 West Sumatra earthquake. Our result presents a preliminary investigation of possible ionospheric VTEC anomalies prior to the mainshock where a multi-station analysis of GPS-derived VTEC data reveals a persistent anomaly pattern approximately six days before the main shock. While these findings are suggestive of a seismic-ionospheric coupling, the statistical basis is limited by the single-event design. The results should be interpreted with caution, and future multi-event studies are recommended to validate our results, and to fully uncover the physical processes behind ionospheric disturbances caused by seismic activity.
Earth rotation parameters (ERP), including the polar motion components in the X and Y directions (PMX and PMY) and the difference between universal time 1 and coordinated universal time (UT1-UTC), contain important geodynamic information, and their accurate short-term prediction is of great significance. In this study, an optimized hybrid forecasting method integrating successive variational mode decomposition (SVMD), support vector regression (SVR), and bidirectional long short-term memory (BiLSTM) is proposed for short-term ERP prediction. First, the original ERP series are decomposed by SVMD into several intrinsic mode functions and a residual term. Then, the augmented Dickey-Fuller test and multiscale fuzzy entropy are used to evaluate the stationarity and complexity of the decomposed components, respectively. Based on these results, non-stationary or low-complexity components are assigned to the SVR branch, whereas stationary components with higher complexity are assigned to the BiLSTM branch. The proposed method was trained and evaluated using IERS 20 C04 ERP data from 2016 to 2026. Compared with the best reported results of the 2nd Earth Orientation Parameters Prediction Comparison Campaign, the 30-day prediction results showed the mean absolute error (MAE) improvements reach 25
Airborne gravimetry is a key technique for regional gravity field recovery where high quality surface gravity data is sparse, yet its accuracy is fundamentally limited by high-frequency measurement noise and temporally correlated instrumental errors along flight trajectories. Conventional processing often relies on low-pass filters, such as recursive Gaussian filters used in the well-known Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project, to suppress the measurement noise, at the expense of signal attenuation and reduced spatial resolution. In this study, a state-space Kalman filtering and smoothing framework is designed for airborne gravity disturbance estimates that explicitly models colored measurement noise using an embedded autoregressive (AR) process. The proposed method is evaluated against a nominal triple-pass Gaussian filter commonly used to process scalar airborne gravity data in the GRAV-D project. Using scalar gravity data collected over the Colorado area that covers the Geoid Slope Validation Survey 2017 (GSVS-17) lines, this study demonstrates that gravity disturbances processed with the Kalman smoother yield a local gravity field model whose derived height anomalies achieve an accuracy of approximately 3.0 cm, improving upon the 3.8 cm accuracy obtained using Gaussian-filtered data alone. These results indicate that a mathematically rigorous, model-based state-space modeling with explicit representation of temporally correlated measurement noise provides a measurable improvement in local quasi-geoid models augmented solely by airborne gravity data (without the inclusion of local surface gravity data), while better preserving geophysical signal content across a broad range of wavelengths.
An approximate but accurate calculation of seismic wavefields in smooth (without interfaces) isotropic or anisotropic, inhomogeneous, attenuating media using ray theory is possible with the aid of the so-called weak-attenuation concept (WAC). In WAC, the attenuating medium is regarded as a perturbation of an elastic reference medium. The extension of this concept to layered media was prevented for a long time by problems with evaluation of the reflection and transmission (R/T) coefficients of plane waves in attenuating media, and by the role of inhomogeneous waves in this process. The R/T coefficients represent a fundamental part of ray calculations in layered media. It turned out that the problems are related to a limited applicability of the correspondence principle to reflection and transmission. Recently, this problem was solved by the use of WAC. It opened the way to the calculation of seismic wave fields in layered, attenuating media. Before such a calculation, it is necessary to solve several important problems. It is well known that the incidence of a wave at an interface separating attenuative media generates inhomogeneous body waves. These waves are not implicitly included in WAC. Therefore, it is important to find out how their absence affects the results of ray calculations with WAC. In this paper, we check the accuracy of such calculations by comparing synthetic ray seismograms with full-wave seismograms, which are considered to be an accurate reference. For simplicity and transparency, we focus on the problem of reflection and transmission of SH-waves in 2D horizontally layered isotropic attenuating media, since SH-waves are decoupled from P–SV waves and thus allow a simpler treatment. Another reason for the use of SH-waves is the availability of the full-wave code designed for SH-waves. Use of more complicated structures and other types of waves would lead to artefacts, which would mask the effects of attenuation. The results of the comparison show that outside singular regions, such as the critical region or the vicinity of the Brewster angle, ray theory with WAC provides results that closely approximate those obtained using full-wave modeling. Applicability of WAC to other types of waves and more realistic structures is thus open.
The deflection of the vertical (DOV), defined by the angle between the plumb line and the normal to the reference ellipsoid, plays a crucial role in geodesy and geophysics, particularly in geodetic data processing, geoid determination, and gravity field studies. Compared with traditional astro-geodetic methods, the determination of the DOV from gravity observations is more efficient and practically feasible. Moreover, gravity gradients contain richer high-frequency (short-wavelength) information than gravity anomalies. Therefore, they provide a promising data source for high-resolution DOV determination. In this context, this study employs three methods to compute the DOV from the vertical component of gravity gradients: the spherical numerical integration formulation, the planar numerical integration formulation, and the planar closed-form solution. Starting from the spherical formulation, a planar approximation is derived, and a corresponding closed-form solution under this approximation is further developed. Two test regions are selected: one bounded by [0°E, 2°E] in longitude and [0°S, 2°S] in latitude, and another in [150°W, 152°W] [0°, 2°N]. Closed‑loop tests based on the EGM2008 model confirm the theoretical consistency of the three methods. In a separate validation, the results were compared with DOV data from the Scripps Institution of Oceanography (SIO) model. The strong agreement between the computed DOV and the SIO model confirms the accuracy of the algorithms proposed in this study. A comparison of computational times with implementations based on the spherical formulation further demonstrates the high efficiency of the planar approximation. In addition, Graphics Processing Unit (GPU) parallel computing was applied to accelerate the computations. This efficient approach provides a valuable alternative for high-resolution DOV determination.
