In this paper we investigate the characteristics of earth-resistivity before and after the Haiyuan earthquake of magnitude 5.5 on April 14, 1982. The results are as follows:
(1) Before the shock the descending in ρs-changes appeared universally in certain areas. But the imminent-term ρs anomalies dropped or rose. The large amplitude and the high rate were responsible for imminent-term anomalies. The stages of ρs-changes before and after the earthquake were:descending-levelling-sudden jitter for impending earthquake-(quaking)-recovered.
(2) A feature of ρs-trend anomalies before the shock appeared to be large area and a long time. On the other hand, the direction of the maximum anomalous value of every station corresponded with the strike of the local large fault (or the earthquake fault). These characteristics showed that the ρs-trend anomalies reflected the rapid increase of intensity in the regional stress field.
(3) The ρs-anomalous amplitude of every station is closely related to Quaternary system overburden around a station. The thicker the overburden, the smaller its anomalous value will be. Therefore, it needs to have the better underground conditions of stations for forecasting the intermediate magnitude earthquake.