Abstract: In this paper, self-potential and geoelectric field anomalies were analyzed, which were recorded at Changli station of Hebei province before M8.7 Earthquake in Indonesia on Dec. 26, 2006. The results show that the half-daily and half-monthly tidal wave periodicity changed abnormally about one month before the earthquake, with periodicity disappearing and amplitude reducing, but they recovered after it with clearer state than that at normal time. The observation data of earth resistivity at this station exhibits that it dropped slowly at the beginning, but changed intensely in one week before the earthquake with many times of step-variations. It is found that the formation mechanism might be as follows: the creep-sliding of faults before the strong earthquakes caused local variation of pore pressure, which produced a kind of strain wave propagating in the rock and overlapped with tidal waves, then disturbing the normal records at stations. Due to the better amplification in rock and soil layer beneath Changli station for small stress and strain signals, this wave with small amplitude, i.e. the remote static strain wave, can be recorded here. With the impending of the earthquake and strengthened stress, the remote strain waves resulted in the change beneath the station such as water level, water chemical property and rock porosity, permeability and so on, leading to the large amplitude step-variation in resistivity and geoelectric field, which reflect the dynamic effects of remote strain waves. Of course, the anomaly amplitude of resistivity, the effects of stress wave is not enough, so there may be overlapped anomalous electromagnetic wave from focal area on the electric anomalies recorded at Changli station.

Key words: M8.7 Earthquake in Indonesia, Static strain wave, Dynamic strain wave, Geoelectric field, Earth resistivity