2020年7月12日唐山古冶MS 5.1地震前延庆地电场变化OA

Using the daily mean curve shape,the daily mean first-order difference,and the dominant azimuth method,the abnormal changes of the Beijing Yanqing station electric field before the July 12,2020 earthquake are analyzed.The daily mean values of the two sets of geoelectric fields in Yanqing have relatively clear and stable annual variation,with seasonal variation,and annual distortion or annual variation amplitude change since May 2020,indicating that the underground tectonic activities in the area where the stations are located have changed from stable to gradually intensified during the earthquake preparation process.The daily mean first-order difference showed high-frequency sudden jump or cluster anomaly 1 to 3 months before the earthquake and became gradually stable after the earthquake.The first-order difference of the daily mean value of the ground electric field can reflect the sudden,step,and rapid trend change of the daily mean value,but it is weak to reflect the slow gradual change.The median of the background value of the azimuth is about 70°,which is equivalent to the average value of the principal compressive stress axis in the Yanqing platform region of 74.45°,representing the main fracture direction in this region.The change of azimuth angle can reflect the underground stress-strain activity in a certain range during earthquake preparation.The range of azimuth jump has narrowed since May 2020,with a change of about 45°.It took three months from the beginning of the anomaly to the earthquake.Using the azimuth method,the geoelectric field short-term abnormal changes of MS 5 earthquakes within 300 km around the station can be extracted.