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There have been reports about anomalies in mobile geomagnetic data before earthquakes; however, whether it can be used as an indicator for identifying potential earthquake areas was not be explored. In this study, we propose two parameters for earthquake forecasting based on annual mobile geomagnetic observation data. The spatial horizontal and three components' changes are calculated in each year and then used to forecast moderate–large earthquakes (M ≥ 5.0) in southwest China in the subsequent period. It is found that earthquakes are more likely to occur in low H- or F-value regions. We statistically assess their forecasting performance by using Molchan's error diagram, and the results indicate that there is considerable precursory information in the spatial H and F values. It is concluded that mobile geomagnetic observations might be useful in middle-term earthquake forecasts in the study area. We discuss the physical mechanisms of H and F values to explain their reasonability. The methodology proposed in this study could be helpful in finding out the optimal solution for annual mobile geomagnetic measurements for middle-term earthquake forecasting. [ABSTRACT FROM AUTHOR]

The magnetotelluric method has been used to fully study regional electrical conductivity structures in different areas in mainland China; however, there is a lack of overall understanding of the electrical structure distribution. A novel insight for the study of continental-scale underlying conductivity structures was proposed in this work via geomagnetic data recorded by permanent stations. To study the underlying electrical structure distribution in mainland China, we mapped the conductors and resistors at a depth range of 4–100 km beneath mainland China using Parkinson vectors through magnetic transfer function. Three-component geomagnetic data within a low artificial disturbance period (local time 23:00–05:00) from 98 stations in 2019 were collected and processed to derive Parkinson vectors in the frequency band of 0.001–0.5 Hz. The distribution of subsurface electrical structures at distinct depths was constructed using corresponding frequency through the skin depth. We compare the consistent results herein with previous magnetotelluric studies, which indicated the reliability of our method. Combining previous multiple geophysical inversion results, we found that large-scale plastic bodies are distributed along the east of the Qinghai-Tibet Plateau and extend to the west of Yunnan. In central mainland China, the areas are mainly highly resistive, indicating that the structures are overall rigid. In north China, there exist high-low-high-low conductive structures from west to east. The separate high- and low-conductive electrical bodies in the North China Craton provide geophysical evidence that the Craton is composed of multiple blocks. The distributions of the underlying electrical structures in this work can provide an overall perspective for studying tectonic evolution and geodynamics in mainland China. [ABSTRACT FROM AUTHOR]

To understand the deep tectonic environment at the origin of strong earthquakes along the northern section of the North-South seismic belt and the central continental block of Mongolia, large-scale gravity and geomagnetic surveys and rock sample collection were carried out along the Suhbaatar-Ulaanbaatar-Dalandzadgad profile in Mongolia. The spatial distribution characteristics of the lithospheric magnetic field at 160 geomagnetic measuring points along the profile were then systematically analyzed and summarized. Based on the composition of the lithospheric magnetic field from different sources at different depths combined with the regional geological structure and collected rock sample lithology, the lithospheric magnetic field was further decomposed into different components—the basement, middle layer, and shallow surface—by the upward continuation method. The results show that these components have obvious zoning characteristics and that the lithospheric magnetic field distribution of the Suhbaatar-Ulaanbaatar-Dalandzadgad profile is related to the geological structure, rock lithology, seismic activity, natural remanent magnetization, and magnetic susceptibility. Many destructive earthquakes have occurred in the area where the lithospheric magnetic field changes dramatically, the lithologies are dominantly igneous, and the magnetic susceptibility and natural remanent magnetization are high. In contrast, the lithologies in the area with moderate variation in the lithospheric magnetic field are mostly sedimentary, the magnetic susceptibility and natural remanent magnetization are low, and destructive earthquakes are rare. Since the lithospheric magnetic field reflects the magnetic characteristics of the rocks above the Curie interface, its spatial distribution not only corresponds to the geological structure but also predicts the depth and location of faults. Therefore, it is significant to carry out geomagnetic surveys and rock sampling to analyze the characteristics of geological structures and research regional seismicity. [ABSTRACT FROM AUTHOR]

Featured Application: We believe the paper may be of particular interest to the readers of your journal because in this paper, a simple but high-precision geomagnetic field modeling method is proposed. We also present Chinese Geomagnetic Reference Field 2020 (CGRF2020) as an example of this modeling method. Using CGRF2020, one can calculate high-precision geomagnetic field values in most of China and surrounding areas. We improved the surface spline method to make the centers of the base functions fixed. This improvement makes it meaningful to analyze the time series of coefficients. Based on the revised surface spline (RSS) method and 2382 geomagnetic data measured in mainland China, we built Chinese Geomagnetic Reference Field 2020 (CGRF2020) to describe the spatial distribution of the geomagnetic field in most of China and surrounding areas. The root mean square residual is 0.1318 degrees for D, 0.0857 degrees for I, 93.70 nT for F, 73.94 nT for X, 65.02 nT for Y, and 97.82 nT for Z. The values of F, X, Y, and Z are about 20 nT less than those by the SCH method and IGRF13. We also preliminarily analyzed the correlation between geomagnetic anomalies and geological structures. [ABSTRACT FROM AUTHOR]

Featured Application: This work is mainly applied to the analysis of relevant geophysical field changes caused by earthquakes. The tectonic processes leading up to an earthquake and the occurrence of the earthquake itself will cause local changes in the geophysical field (geomagnetic field, stress field, etc.). In this paper, the variation characteristics of the tectonic stress field (TSF) and local geomagnetic field (LGF) before and after the Yangbi Ms 6.4 earthquake are studied. The regional stress tensor damping inversion method was used to invert the TSF using focal mechanism solutions (FMSs). The change characteristics of the TSF before and after the earthquake were analyzed. An annual variation model of the LGF was constructed, and the variation of the horizontal vector was analyzed. The azimuth and plunge of the maximum principal compressive stress axis of the TSF in the epicentral region before and after the earthquake were −4.4° and 2.7°, 172.7° and 6.6°, respectively. The variations in the declination, inclination and total intensity of the epicenter one year before and one month after the earthquake were −0.20′ (0.07′), 0.29′ (−0.12′), and −1.7 nT (−1.9 nT), respectively. The epicenter is located at the boundary of the "weak variation region" of the horizontal vector. This research is of great significance concerning the TSF background and incubation mechanism of earthquakes. [ABSTRACT FROM AUTHOR]