Acta Geophysica最新文献

We present spectral and imaging LOFAR (LOw-Frequency ARray) observations in the 20 – 40 MHz range of solar radio bursts fine structures, such as flag-like, sail-like, and dot-like that appeared on 8 April 2019. These structures were associated with type III solar radio bursts that occurred in the 40 – 80 MHz band. The mean duration and spectral widths of the fine structures range from 1.0 to 3.4 s and from 0.3 to 0.9 MHz, respectively. Additionally, we investigated the radio images of eight fine structures – two flags, two sails and four dots. This allowed us to determine their emission source sizes, which ranged from 240 to 392 arcmin({}^2), and their frequencies from 25.58 to 39.25 MHz as well as their location. They occurred on the east side of the Sun and were most likely associated with an emerging active region NOAA AR 12738, where a weak B1.7 flare was observed.

The soil moisture drought is an intermediate event between meteorological and agricultural droughts. The information on soil moisture droughts provides an indication about the resilience of the agricultural systems. In the present study, a comparative assessment of the monthly soil moisture gridded data products of Modern Era Retrospective analysis for Research and Applications, i.e., MERRA-2 (0.5° × 0.5°), Climate Prediction Center, i.e., CPC (0.5° × 0.5°), Global Land Data Assimilation System, i.e., GLDAS (0.25° × 0.25°), and European Space Agency Climate Change Initiative, i.e., ESA CCI (0.25° × 0.25°) during 2000 to 2019 was carried out in terms of drought occurrence and severity. The long-term soil moisture information was transformed into standardized soil moisture index (SSMI) using nonparametric distribution, followed by computation of drought duration and magnitude using thresholding approach. The long-term trends of drought parameters, i.e., duration and magnitude, were extracted using Mann–Kendall test and Sen’s Slope method, respectively. It was interesting to note that irrespective of zones, the SSMI derived from MERRA-2 and CPC have maximum coherence in terms of both pattern and intensity, followed by GLDAS. The trends of drought duration and magnitude differ based on the data products; however, frequent droughts were observed over parts of peninsular India and Indo-Gangetic plains irrespective of data products. The increased drought duration and magnitude were found over major parts of central and peninsular India, western parts of north-eastern India and eastern parts of north-western India.

Throughout history, several large-magnitude earthquakes have caused damage to the Himalayan region and humanity. To understand the present-day strain rate distribution and associated seismic moment budget, a high-resolution velocity field is an essential component. The present study estimates the contemporary seismic moment budget along three spatial sections over the Nepal Himalaya using the state-of-the-art high-resolution velocity field. For this, (1) we integrate 5 years of InSAR data with 77 available GPS observations over the Nepal Himalaya; (2) we then calculate strain rate distribution (dilatational and maximum shear strain rates) from this integrated velocity field, and (3) at last, we compare the geodetic moment accumulation rate estimated from strain rate tensors with the seismic moment release rate based on an earthquake database of 500 years. The results reveal that: (1) the geodetic strain rate is not homogeneous over the Nepal Himalaya, rather along the main central thrust, a relatively higher strain rate is observed; (2) the geodetic moment rate from west to east across three sections ranges from (23.39times 10^{18}) to (16.59times 10^{18}) Nm/yr, with the minimum of (8.05times 10^{18}) Nm/yr in central Nepal, whereas the seismic moment rate varies between (5.02times 10^{18}) and (11.41times 10^{18}) Nm/yr, with the minimum of (3.69times 10^{18}) Nm/yr in central Nepal; (3) the difference between geodetic and seismic moment rates from west to east provides a moment deficit rate of (18.37times 10^{18}) to (5.18times 10^{18}) Nm/yr, with the minimum of (4.36times 10^{18}) Nm/yr in central Nepal, and more importantly, (4) the inferred moment deficit rate suggests that the western and eastern Nepal have an earthquake potential of magnitude (M_w) 8.5 and (M_w) 8.1, respectively, whereas the central Nepal has energy budget equivalent to an (M_w) 7.9 event. In summary, the present study provides spatial distribution of earthquake potential in Nepal Himalaya using the most updated high-resolution InSAR and GPS velocity field, and the findings inevitably contribute to the time-dependent earthquake hazard analysis of the study region.

Different types of rocks have different content and distribution characteristics of radioactive uranium, potassium, and thorium elements. Even for rocks of the same lithology, the content of radioactive elements varies due to different ages of formation, different origins, and different geological evolution processes. However, in various rocks and formations, the distribution of radioactive elements follows the general rule that the content of radioactive elements in igneous rocks is higher than that in sedimentary rocks, and increases with the acidity of the rocks. The younger the age of the rocks formed by the same type, the higher the content of radioactive elements. Based on this characteristic of radioactive elements in rocks, the author takes a region in the Qinling Mountains of China as an example. By using high-precision airborne gamma-ray spectrometry measurement data and existing geological information, the total content, K, eU, and eTh content, variation coefficient, ternary colour image, and other parameters of airborne gamma-ray spectrometry measurement are statistically analysed. The distribution characteristics of radioactive elements in various lithologies and formations in the area are studied, and the distribution rules are summarized. The rocks and formations in the research area are reclassified, and the typical fault structures in the area are analysed. The well-developed structural zone is mainly oriented in the northeast–southwest or north–northeast direction in the northern part and in the northwest–southeast direction in the southern part, which controls the favourable uranium mineralization zones in the research area.

The paper addresses the issue of estimating the coefficients of the sotolongo-costa and posadas (SCP) model in the presence of uncertain earthquake magnitude data. The SCP model offers a more accurate representation of regional seismicity compared to the traditional Gutenberg–Richter (G-R) law and has been integrated into the probabilistic seismic hazard analysis (PSHA) framework as NEPSHA. The study aims to develop a method to calculate the SCP coefficients in the presence of uncertain magnitude data, implement the process in R programming language, and validate its effectiveness through a case study. The methodology involves developing the mathematical relationship for estimating the SCP parameters using maximum likelihood estimation (MLE) and modifying the MLE approach to account for magnitude uncertainty. The method is tested using simulated earthquake catalogs with varying degrees of magnitude uncertainty. The results demonstrate that the proposed method can alter the estimated values of the SCP parameters, particularly the a value, by approximately 50% when magnitude uncertainty is considered. The q variable is found to be less affected by the estimation method.

The main reason that deteriorates air quality in mega cities is the increase in concentrations of air pollutant parameters. Meteorological parameters and atmospheric conditions play an important role in the increase of pollutant concentrations. This study provides insights into temperature inversions (TIs) during polluted days (PDs) and severe polluted days (SPDs) in Istanbul. Key findings include higher inversion frequencies during SPDs, particularly at 0000 UTC, along with a positive relationship between inversion frequencies and pollutant concentrations, notably with a 99% occurrence of inversions at 0000 UTC along SPDs. Analysis of inversion subgroups reveals surface-based inversions (SBIs) dominating at 0000 UTC, while elevated (EIs) and lower-troposphere inversions (LTIs) prevail at 1200 UTC. Winter months exhibit increased frequency and intensity of SBIs, aligning with expectations of subsidence motion under high-pressure systems. Inversion strengths and depths are higher during SPDs, with the highest strengths observed in winter at 0000 UTC and the deepest inversions occurring in winter for SPDs. Generally, the highest inversion strengths and shallowest inversion depths were observed in SBIs. EIs had the lowest frequency during the winter months, while LTIs occurred more often in the spring months. These findings underscore the importance of understanding TI patterns for effective air quality management in Istanbul.

Underground carbonate deposits are widespread worldwide and have considerable hydrocarbon potential. They are generally characterized by a complex microscopic structure that affects the properties of the macroscopic fluid flow and the relevant petrophysical behavior. In recent years, advances in digital technology have helped reveal the microstructures (i.e., pore connections, cracks, pore size and radius, etc.) of rocks in the subsurface. In this work, drill cores (cylinder) are taken from a deep carbonate deposit in the Sichuan Basin in western China to perform computed tomography (CT) scans, thin sections and mineral analysis. The characteristics of lithology and pore structure are investigated. Ultrasonic experiments with different fluid types and pressures are conducted to determine rock samples’ wave velocities, attenuation and crack porosity. The experimental data show that the rocks have low porosity/permeability and a complex pore/crack system, leading to significant pressure, crack and fluid type effects on the velocities, dispersion and attenuation. We develop a model of multiple pore-crack structures for carbonates by considering the complex structure and fluid properties. Digital cores are reconstructed based on CT scans, image processing and threshold segmentation. The aspect ratios of pores and cracks are extracted with their volume fractions to simulate the rock skeleton with the differential effective medium theory. The Biot–Rayleigh wave propagation equations are applied to simulate the effects of different pore and fluid types on the velocity and attenuation of P-waves. The agreement between the modeling results and the ultrasonic and log data confirms that the model can validly reproduce the wave responses.

The Tuolaishan–Lenglongling fault (TLSF–LLLF) is located in the middle-western segment of the Qilian–Haiyuan fault zone. The 2022 Menyuan Mw 6.7 earthquake that occurred in the TLSF–LLLF highlights the urgent need for understanding the mechanical property and seismicity over this fault segment. In this study, Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technique was used to process Sentinel-1 acquisitions covering the TLSF–LLLF fault from 2016 to 2022 to determine the interseismic velocity field along the satellite line-of-sight. The interseismic deformation field confirmed the absence of surface creep behavior across the whole TLSF–LLLF segment. Then, we utilized both the screw dislocation and block modeling strategies to invert the comprehensive spatial distribution of fault slip rate and locking depth across the TLSF–LLLF fault. The new fault locking model, constrained by all GNSS and InSAR measurements, suggests comparable fault slip rates between 4.7 and 5.6 mm/yr in the TLSF–LLLF segment, which is generally consistent with long-term geological slip rates. The locking depth increases gradually from 8 km in the western segment of the TLSF to 18 km in the eastern segment, while the locking depth for most sections of the LLLF is relatively deep (15–18 km), indicating existence of asperities on the locking along the TLSF–LLLF fault zone. In particular, a fault segment with obvious shallow locking depth was identified in the stepover region where the TLSF and LLLF intersect. The shallow locking section shows a good spatial correlation with the coseismic rupture of the 2022 Menyuan earthquake. The calculated moment rate deficit suggests that the TLSF is capable of producing an Mw 7.3 earthquake given the high seismic moment accumulation rate and a lack of small-to-moderate earthquakes.