The study investigates computing of ionospheric delay using the Klobuchar model, based on assumed elevations and azimuths corresponding to Navigation with Indian Constellation (NavIC) positions alongside the Global Positioning System (GPS) over the Indian subcontinent during the year 2024. The analysis includes an examination of Klobuchar-modeled slant total electron content (STEC) during Solar Cycle 25 focusing on the geomagnetic storm (with peak negative Dst index of − 412 nT) that occurred during May 10–13, 2024. Six locations from the International Global Navigation Satellite System Service (IGS) network in India were selected, covering different regions of the Indian subcontinent. The diurnal and annual variations in NavIC STEC indicate that, overall, NavIC STEC magnitudes are lower compared to GPS STEC in all scenarios. These fundamental differences between GPS- and NavIC-based observations arise because NavIC satellite signals originate from geostationary Earth Orbit (GEO) or geosynchronous orbit (GSO) satellites, whereas GPS signals are transmitted from Medium Earth Orbit (MEO) satellites. An ionospheric spatial gradient estimation of NavIC total electron content (TEC) during the storm day was conducted and applied to baselines between two IGS stations. The results indicate that the TEC gradient varies throughout the day, with the most significant enhancements and reductions occurring before and after midnight during the selected storm day.
Accurate and high-resolution modelling of the regional gravity field using wavelets requires the integration of heterogeneous datasets. In this paper a wavelet-based multi-resolution fusion scheme is used in which each dataset is decomposed into different scale-dependent components. This decomposition enables a selective combination of the long-wavelength signals from satellite altimetry and marine geoid data with the high-frequency fine-scale details from terrestrial gravity data, reconciling spectral differences and reducing noise between various sources. Four datasets were assessed: marine geoid, satellite altimetry-derived gravity anomalies (Sandwell and SDUST2021), and land gravity measurements. Statistical analysis of the control point residuals shows that the most accurate and stable regional gravity model is obtained by combining the marine geoid data, satellite altimetry data of SDUST2021 and ground gravity data. The model obtained 10 mGal for the standard deviation of gravity anomalies, 13 cm for geoid heights and 8 cm for GPS levelling control. The results demonstrate that the SDUST2021 satellite altimetry data outperformed Sandwell data in terms of consistency and accuracy within the study area, emphasizing the significance of marine geoid data for coastal and marine regions.
Analysis of the travel-time ratio of S- and P- body waves (TS/TP) reveals significant temporal variations prior to moderate earthquakes in the Koyna–Warna reservoir-triggered seismicity (RTS) region of western India. An anomalous decrease in the TS/TP ratio, relative to its mean value, was consistently observed approximately one to three days before the occurrence of M 5 earthquakes in 2008, 2009, and 2012. These variations were further validated using statistical analyses which confirmed its significance beyond normal fluctuations. This phenomenon of decrease in TS/TP may reflect changes in the physical properties of rocks within the hypocentral zone, possibly associated with the earthquake preparation process and suggests its potential as a valuable earthquake precursor.
The North China Craton experienced multiple phases of deformation and reworking from the Mesozoic to the Cenozoic, but its tectonic evolution and its underlying dynamics have remained unclear. Based on teleseismic SKS waveform data from 52 permanent seismic stations in the craton, 452 valid shear-wave splitting parameters were obtained at 37 stations, while null splitting results were recorded at 14 stations. The results show that the delay time between fast and slow waves ranges from 0.5 to 1.3 s, with an average of 0.92 ± 0.11 s, indicating that the average thickness of the anisotropic layer in the North China Craton is approximately 138 km, mainly contributed by the asthenosphere. The fast-wave polarizations range from 71.6° to 148.9°, with an average of 99.1° ± 15.3°, consistent with the WNW direction of absolute plate motion, indicating that the anisotropy is caused by the relative motion between the lithosphere and asthenosphere. This suggests that the upper mantle anisotropy in the North China Craton is primarily induced by the underlying mantle flow. However, the anisotropy parameters obtained for each tectonic block show that the Western Block has a more complex anisotropy than the Eastern Block and Trans-North China Orogen, which may be collectively affected by the lithospheric and asthenospheric mantle. In addition, only null splitting results were observed around the Bohai Sea area and near the Datong volcanic group, possibly due to the upwelling of mantle thermal material.
By integrating cross-fault observations, first-order leveling data, and seismicity catalogs across the Bayan Har block and its periphery, we derived three-dimensional fault kinematic parameters and performed rigorous leveling adjustments. This integrated analysis delineates the relationship between regional crustal deformation, the coherent motion of the Bayan Har block, and the 2017 Jiuzhaigou MW6.5 earthquake. The main conclusions are as follows: (1) Prior to the Jiuzhaigou MW6.5 earthquake, the epicentral region exhibited pronounced anomalous uplift, while distal tectonic belts displayed varying degrees of anomalous activity. Owing to heterogeneities in geodynamic settings and structural frameworks, monitoring sites across the West Qinling, Liupanshan, Longmenshan, and Xianshuihe belts exhibited differential responses to the impending rupture. (2) The earthquake was triggered by rapid stress accumulation in the northeastern Bayan Har block, driven by the block’s coherent southeastward movement. Furthermore, preceding major earthquakes on the block periphery (e.g., Wenchuan and Lushan events) exerted a notable promoting effect—potentially via Coulomb stress transfer—on the initiation of this event. This mechanism provides key insights for predicting intraplate earthquakes and periphery earthquakes of continental plateaus. (3) While the Jiuzhaigou rupture effectively released accumulated stress in the northeastern block, tectonic stress appears to be increasing in the southeastern Triple Junction region. Given its potential for generating future moderate-to-strong earthquakes, this area merits heightened monitoring.
