The ULF electromagnetic radiation excited by special typical minor clumped earthquakes

A group of more than 250 typical minor clumped earthquakes occurred from July 25 to August 17 in 1993 in Xianning, Hubei province, central China. Just 40 days before the first minor quake occurred, a series of intriguing electromagnetic radiation anomalies (EMRA) were observed at Wuhan electromagnetic seismic monitoring station (30/spl deg/ 37' 12" N, 114/spl deg/ 21' 52" E). Observations of the empirical phenomena showed that premonitory ULF EMRA hints preceding or during the Xianning typical minor clumped earthquakes could be distinctively spotted among the ubiquitous electromagnetic background by some sensitive receiving device. Further analysis indicated the ULF EMRA duration time had a negative relationship with the time interval from the ULF EMRA appearance to certain earthquake occurrence. The electromagnetic amplitude and wave mode of the ULF EMRA were complicated, sometimes it behaved like irregular moderate vibration appearing by a displaced dynamic zero point, at other times it appeared as an intensive electromagnetic impulse. Finally, the ULF EMRA information preceding or during the Xinning earthquakes was not observed on the right earthquakes' occurrence time.