Frequency spectral properties of an ULF electromagnetic signal around the 21 July 1995, M=5.7, Yong Deng (China) earthquake

The frequency temporal analysis of the electromagnetic field recorded from 30 May 1995 to 24 August 1995 at a site about a hundred kilometer North of the epicenter of the 21 July 1995 Yong Deng earthquake (China) has shown a clear systematic variation on an electric dipole. In the ultra low frequenc y range from 0.01 to 0.0005 Hz, a three-fold decrease of the frequency of t he maximal energy concentration was observed, on 30 May 1995 from 0.005 to 0.0010 Hz few hours at the time of the earthquake. This decrease was nearly monotonic, with superimposed small scale step-like fluctuations, and conti nued after the Yong Deng earthquake for about two weeks, when the minimum o f 0.001 Hz was observed just before its abrupt disappearance. The ULF signal is observed on one of the four electric dipoles. The instrum entation and the data acquisition system do not generate this signal. The s ignal could be generated by an electrokinetic effect depending on stress fi eld changes in the local environment of one or two probes. The results need further understanding and interpretation. (C) 2001 Elsevier Science B.V. A ll rights reserved.