VARIATION OF THE GRAVITY FIELD AND THE HAICHENG EARTHQUAKE
LU ZAO-XUN PANG CHANQ-LIU SHI ZUO-TING ZHANG MAO-SHULI RUN-JIANG YANG JUN-ZHEN (Seismological Bureau of the Liaoning Province)
Repeated measurements of the gravity field around the epicentral region, before and after the Haicheng Earthquake of February 2, 1975, revealed the fact that the gravity values systematically varied.In June, 1972, a gravity profile was established from the point Beizhen (北镇) in the north and Zhuanghe (庄河) in the south and actually surveyed. In November of the same year and in May of 1973 the profile was again surveyed twice. A maximum decrease of the relative gravity values, amounting to -352 μ gals was detected. After the Haicheng earthquake of magnitude 7.3, a gravity survey in March, 1975 was carried out along the same profile. It was found that the gravity profile essentially returned to the level of their starting values of June, 1972. In July, 1975, another repeated survey was done, the results showed a further increase of the relative gravity values, reaching a maximum of +382 μ gals. After that, until December, 1976, several surveys were made in succession, but the profile maintained essentially the same level as that of July, 1975. Hence, it can be presumed that the gravity field within the epicentral region of the main shock decreased before and increased after the earthquake.By analysing the field method of observation and of its accuracy, the gravity data so obtained could be considered as reliable. It seems that the variation of the gravity field is closely related to the earthquake occurrence of the Haichengarea.