Gravity changes before and after the Tangshan earthquake of July 28, 1976, and possible interpretation

Abstract

The data of 10 years from 1971 to 1981 were calculated and analyzed for this paper. Significant gravity changes were observed before the Tangshan earthquake of July 28, 1976 (M = 7.8), in the epicentral area along two lines that were independent of each other and intersected at Tangshan. The latest repeated surveys before the shock in the two lines were finished on July 6 and 25, 1976, only 22 and 3 days before the event, respectively. Maximum gravity changes at Tangshan of 142 μGal occurred on April 24, 1976, in the line from Beijing to Shanhaiguan and of 194 μGal occurred on July 6, 1976, in the line from Sanhe to Leting. These changes are much larger than 3 times the standard deviation. Because all possible causes of errors and disturbances were considered and corresponding methods were adopted to correct them in the analysis, it is impossible to attribute such large anomalies entirely to any kind of error. After the earthquake, gravity values at most stations decreased sharply, but the anomalies persisted at two stations in Tangshan. Remarkable difference between the gravity changes before and after the earthquake shows strongly that the anomalous gravity changes were associated with the earthquake. The relationship between the changes in gravity and in elevation and groundwater table was studied. The result indicates that the gravity changes were not caused by either the elevation change or the groundwater table.