The relationship between atmospheric potential gradient descent along with negative potential gradient anomalies and earthquake precursors

Earthquake prediction is based on identifiable and reliable earthquake precursors. The existence of reliable seismic precursors, the methods to identify precursors, and the definition of abnormal threshold are still controversial in the seismological academia. We have examined the atmospheric potential gradient over many years and found that the atmospheric potential gradient does not conform to the traditional default normal distribution. Therefore, the abnormal threshold of atmospheric potential gradient is redefined by using the box plot with non-normal distribution. On March 27, 2017, in Yangbi County, China, 5.1 earthquake occurred. There are 11 surface atmospheric electrostatic observation stations in the earthquake preparation area. In this paper, the anomaly of the atmospheric potential gradient before the Yangbi earthquake is studied by the box plot, and the change trend of the atmospheric potential gradient before the earthquake is calculated by the least square method. The results show that, after eliminating the influence of the main meteorological factors on the atmospheric potential gradient, with the approach of the main earthquake, the atmospheric potential gradient of 11 stations in the earthquake preparation area all presents a downward trend, and most of the atmospheric potential gradient presents obvious negative anomaly. This phenomenon can be explained by the mechanism of radon emission enhancement before earthquakes. Therefore, the general downward trend of atmospheric potential gradient and the large proportion of negative anomalies of atmospheric potential gradient may be precursors of earthquakes.