Discussion on anomaly of atmospheric electrostatic field in Wenchuan Ms8.0 earthquake

Abstract On May 12, 2008, a surface wave magnitude 8.0 earthquake occurred in Wenchuan, China. In several hours to tens of days before the earthquake, a series of abnormal changes of the atmospheric vertical electrostatic field was found in Pixian and Wenjiang stations, located near the epicenter. After excluding potentially interference factors such as lightning, meteorological factors, aerosol, solar radiation and human activities, we found that the earthquake preparation process may cause abnormal synchronous changes in the atmosphere vertical electrostatic field, or equivalently, surface atmospheric potential gradient (PG), at different locations. About 40 days before the Wenchuan earthquake, the daily median of PG at Pixian and Wenjiang exhibited a synchronous downtrend, and the correlation was relatively high. With the main shock approaching, with the rapid increase of radon concentration in Guzan station, the daily mean value of PG decreased synchronously in the two stations. The PG anomaly in Wenjiang on May 9 corresponds to the ionospheric anomaly. On May 12, 6 h before the main shock, PG abnormally synchronously increased in Wenjiang and Pixian accompany with the decrease of gas radon concentration in Guzan station (PG correlation coefficient of the two stations was as high as 0.818). Two minutes after the main shock, there was a highly synchronized negative pulse lasting 225 s. These abnormal synchronous changes of PG can be explained by the mechanism of acceleration of radon emission before earthquake. The abnormal synchronic change of PG of Wenchuan earthquake was consistent with the phenomenon occurred in Japan in 2009. The results indicate that abnormal synchronous changes of atmospheric electrostatic field (or PG) may be an important earthquake precursory.