After the completion of projects as part of the ninth and tenth five-year plans, the geomagnetic observatories networks of Gansu and Qinghai provinces have become densely distributed. The Gansu network has four geomagnetic stations and a geomagnetic array, while there are eight geomagnetic stations in the Qinghai network. The two networks have produced a large amount of geomagnetic data since 2008.
The Gansu-Qinghai region (33°~43° N, 94°~107° E) was identified as the study area for this study. The geomagnetic data in the study area was analyzed using the harmonic amplitude ratio method, and the relationships between the results and the earthquakes in the Gansu-Qinghai region were investigated. Feng Zhi-sheng et al. analyzed the geomagnetic humorous wave amplitude ratios of Chongming, Sheshan, Kashi, and other geomagnetic stations and reported the existence of down-turning-recovery characteristics before and after medium-to-strong earthquakes, which are similar to the earthquake precursor anomaly variations of earth resistivity. We analyzed the geomagnetic harmonic amplitude ratios of Jiayuguan, Shandan, Dulan, Huangyuan, and Yingge geomagnetic stations and reached the same conclusions. The amplitude ratio method has been widely employed for earthquake prediction. The duration of geomagnetic humorous wave amplitude ratio curve abnormalities of earthquakes with M≥5 is 1~3 years. Earthquakes typically occur at the abnormal twist or recovery points. The anomaly amplitude varies between 0.03 and 1.30. Feng Zhi-sheng et al. reported the occurrence of migration phenomenon among the curves of different periods. Further, there may be some correlation between the distance of the phenomenon and the earthquake epicenter. In this study, the migration phenomenon was observed only in the curves corresponding to the Shandan station, and no obvious correlation was observed between the migration phenomenon and the earthquake epicenter.
The judgment standard must be determined when analyzing abnormal variations and summarizing earthquake case studies. Researchers analyze abnormal variations in geomagnetic humorous wave amplitude ratio curves using the earth resistivity anomaly judgment standard and the characteristics of different items or different periods during the same item. The starting times of inconsistent variations in the geomagnetic humorous wave amplitude ratio curves of Jiayuguan and Shandan geomagnetic stations are the same. Normally, if the geomagnetic humorous amplitude ratio curves of different item or different periods during the same item show a poor synchronization, it indicates an abnormal body near the distance station. In the future, we will focus on the variations in harmonic amplitude ratios of the two abovementioned geomagnetic stations.