Abstract:The continual testing observation was carried out in Haiyuan, Ninxia from June to December 1982. Five observationary points along two observation lines were arranged. The current was about 70-100A, and the accuracy of observation for the current about 0.5%. We observed the man-made potential difference using a small instrument car once a day and twenty pulses of the potential were observed every time, and the accuracy of observation of the potential was about 3%.
The results have shown that in the resistivities observed at all the observational points occurred abnormalities of total change of 8-20% before the earthquake Ms=2.7 in Haiyuan on August 14,1982(epicentral distance was about 17km), meanwhile the abnormal change observed by the four-pole symmetric observation which is used to survey the resistivity of shallow layer was only about 1.5%.
There were other anomalies recorded at the observational site with the total change of 7-9% in geoelectric resistivity before another earthquake(Ms=3.3, Nov. 5,1982)in the north of Tongxin, Ninxia, which was 11okm from the observational site. It seems to the author that the anomalies could be explained on the basis of the hypothesis that there would exist a layer in the crust, where the precurssor is stronger than inother region of the crust in the preparing process of earthquakes and which is called the precursor advantageous layer proposed by Mr. Guo Zengjian.