Non-normal distribution of radon and residual radon and short-term abnormal precursors of residual radon before major earthquakes

The basis of short-term earthquake prediction is the identification of reliable precursors. Anomaly of radon concentration escaped from groundwater (CRn) is often used as a traditional earthquake precursor. However, the thresholds of anomaly are still controversial. Using the data series of long-term CRn, the data series of residual radon concentration (CRr) was obtained to exclude periodic environmental interference, and the distribution of long-term CRn and CRr was tested. It is verified that the distribution of CRn and CRr does not conform with the traditional default normal distribution. Therefore, the box plot of non-normally distributed is used to redefine the anomaly threshold. The temporal and spatial analysis results of CRn and CRr show that before the Wenchuan, Lushan and Jiuzhaigou earthquakes, CRn and CRr show similar short-term anomaly patterns as follows: (1) With the approach of main shocks, the CRn and CRr near the epicenter/fault zone increase sharply and are accompanied by obvious positive anomalies. (2) The duration of anomalies is closely related to the location of the epicenter/fault zone and the observation station. (3) The anomaly of CRr which filtered out periodic environmental disturbance can reflect the abnormal precursor caused by tectonic geological activity before earthquake more accurately than the CRn anomaly. (4) The non-normal anomaly analysis method is more consistent with the mathematical definition than the normal distribution anomaly analysis method. Therefore, analyzing the abnormal threshold of residual radon which is non-normally distributed may be a reliable method for the analysis of radon anomalies before large earthquakes.