1975年海城地震与1976年唐山地震前后的重力变化 |
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引用本文: | 陈运泰,顾浩鼎,卢造勋. 1975年海城地震与1976年唐山地震前后的重力变化[J]. 地震学报, 1980, 2(1): 21-31 |
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作者姓名: | 陈运泰 顾浩鼎 卢造勋 |
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作者单位: | 1. 中国科学院地球物理研究所2. 辽宁省地震局 |
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摘 要: | 1975年2月4日海城7.3级地震前后在震中以西不远的一条长约250公里的北西—南东向剖面上进行了五次重力测量,震前三次,震后两次.剖面上相邻两个测点的重力差的测量均方误差小于40微伽.自1972年6月至1973年5月的一年期间的三次观测结果表明,剖面东南段重力值显著下降,最大达352微伽.地震以后,1975年3月的第四次重力测量发现,剖面东南段的重力值回升到第一次测量时的水平.1975年7月的第五次测量则表明剖面东南段的重力值继续上升.1976年7月28日唐山7.8级地震前后也观测到重力的变化,不过地震前重力是增加而不是减少.震前和震后,在震中以北不远的一条长约270公里的东西向剖面上各进行过两次重力测量.结果表明,主震后整个剖面,特别是靠近唐山的那些测点的重力也有逐渐恢复到震前第一次测量时的数值的趋势.由这些结果可以看到,重力的变化与地震的发生似有密切的关系.根据重复大地水准测量资料估计的地面高程变化所能引起的重力变化远比所观测到的变化为小.因此,我们推测某些大地震可能与地壳和上地幔内的质量迁移有关,认为所观测到的重力变化大部份是质量迁移引起的.我们对质量迁移的重力效应作了理论分析,但是,对迁移的物理过程我们还很不清楚.
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VARIATIONS OF GRAVITY BEFORE AND AFTER THE HAICHENG EARTHQUAKE, 1975 AND THE TANGSHAN EARTHQUAKE, 1976 |
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Affiliation: | 1. Institute of Geophysics, Aoademia Sinica;2. The Seismological Bureau, of Liaoning Province |
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Abstract: | Before and after the Haicheng earthquake of magnitude 7.3 which occurred on Feb. 4, 1975, five repeated gravimeter surveys were carried out, three before and two after the earthquake, along a northwest-southeast profile of about 250 km in length not far on the west of the epicenter. The mean-square error of the measurements of the gravity differences between two consecutive points on the profile is less than 40 Gal. From June, 1972 to May, 1973, within a period of about one year, the results of three surveys indicated a clear decrease of gravity values at points on the southeastern portion of the profile, amounting to as much as 352 Gal. After the earthquake, the fourth survey which was carried out on March, 1975, revealed that the gravity values had recovered to the levels at the first survey and continued to increase as was shown by a fifth survey carried out in July of the same year.Variations of gravity were also observed before and after the Tangshan earthquake of magnitude 7.8 which occurred on July 28, 1976, but in this case, gravity was increasing before the earthquake instead of decreasing. Along an east-west profile of about 270km in length and not far on the north of the epicenter, two gravity surveys were made before and two after the earthquake. The results showed that after the main shock, the gravity values of the whole profile, especially at those points closer to Tangshan, tended to return gradually to their values of the first survey before the earthquake.Prom these results, there seems to be a close relationship between these gravity variations and the occurrences of earthquakes. Based on results of repeated levelling work done in these regions, the estimated amount of gravity change caused by the change of elevation of the ground surface is far too small to account for the observed value. We therefore speculate that some big earthquakes might be associated with some sort of mass migration under the ground, within the crust or in the upper mantle. This migration would cause a large part of the gravity variation observed. We have made a theoretical analysis of this effect and attempted to get some estimate of the rate of this mass migration, even though we are not yet clear about the physics of it. |
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