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.

This study addresses the problems and shortcomings of the traditional field geomagnetic diurnal variation reduction method, the single-observatory reference method, by applying the Taylor polynomial model to establish a spatial reference field for diurnal variation reduction based on geomagnetic network data and comparing the results of the two methods. The findings are as follows: 1. The Taylor polynomial spatial reference field method can effectively describe the spatial distribution pattern of the geomagnetic field, and its results are more accurate and reliable than those of the single-observatory reference method. 2. The results obtained by the proposed method are more uniform and stable than those obtained by the single-observatory reference method, and they are closer to the theoretical shape of Earth’s crustal magnetic field changes. 3. While applying the proposed method on earthquakes, it optimizes the display of seismomagnetic information and has good application values.

The superposed epoch analysis (SEA) method is a traditional extraction method for the solar quiet daily variation (Sq) and obtains the monthly Sq by the averages of the data on the international five quietest days (five Q-days) in one month. This study tried to optimize this method through two technical measures, increasing the number of calculation days and moving the calculation period day by day. To test the effectiveness of the new method, the observatory data were decomposed into Sq components and non-Sq components by the characteristic of Sq varying with local time (LT). The observatory data were reconstructed by correcting the Sq components for time differences. Then, the two Sq extraction methods were evaluated based on the correlation coefficient and mean square error (MSE) of the difference between the reconstructed data and the reference observatory data. Comparison with the traditional method showed that the correlation coefficients for the three factors H, D, and Z were improved by 0.0142, 0.0017, and 0.0235, and MSEs were improved by 0.24 nT, 0.03´, and 0.17 nT, respectively, indicating that the Sq results are more reasonable.

The correction of the secular variation (SV) of the main geomagnetic field is a key link of field seismogeomagnetic data processing, and the current method relies on the observatory data for the relevant technical processing. To optimize the data products and obtain more accurate and reliable seismomagnetic information, this study adopted a new technical idea, which uses the repeated survey data from field stations to obtain the SV of the main geomagnetic field over the survey area by the weighted least-squares method, and compared the results with those of the current methods. The results were as follows: 1. The SV results of the main geomagnetic field produced by the new method are closer to those of the International Geomagnetic Reference Field (IGRF)_SV model. The mean square error (MSE) of the difference of the three elements F, D, and I between the new method and the IGRF_SV model is 10.7%, 47.0%, and 14.5% of that of the original method, respectively. 2. By applying the new SV correction method, more stable and reasonable variations in Earth’s crustal magnetic field can be obtained. The average amplitude of the Earth’s crustal magnetic field variation in the three elements F, D, and I is 28.5%, 55.4%, and 34.4 of the original results, the MSE is 59.1%, 56.5%, and 40.3% of the original results, and the mean gradient is 93.6%, 91.9%, and 97.0%, respectively. 3. In the processed results of the new method, the seismomagnetic information is clearly optimized, and the location of the epicenter is more consistent with the 0 value line of the Earth’s crustal magnetic field. The processed results of the new method are significantly better than those of the original method and have a higher application value.