利用重力资料反演京津唐张地区震质中
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摘要
流动重力观测结果显示 ,地震孕育过程中重力变化最大处往往不与震中重合 ,美国科学家J .T .Kuo(郭宗汾 )提出了孕震体质量变化中心震质源 (hypocentroid)和其在地表的投影震质中 (epicentroid)的概念 .本文采用球、椭球和圆柱体等 3种旋转体模型模拟孕震体 ,分别计算孕震体均匀膨胀引起的表面重力变化模型 ,并利用最小二乘法对 1981～ 2 0 0 0年发生在京津唐张测网内 16次M >4 .0地震进行震质中迭代反演 .计算精度表明 ,圆柱体模型为三者中的最隹模型 .研究了震中、震质中与断层之间的位置关系 ,发现京津唐张地区M 4～ 5地震震质中与震中的距离在 0～ 4 0km之间 ,震中一般发生在断层末端 ,或断层交汇处 ;而震质中大都发生在断层间的块体内 ,且成丛分布 .这意味着区域内存在小范围的构造地震孵化区 ,一次次地震在孵化区内孵化 ,但却在其周围发生 .其间 ,断层的存在起着重要的控制作用
Gravity variation data observed in the process of seismogenesis and occurrences of earthquakes show that the location with the greatest gravity changes does not necessarily coincide with the epicenter. To explain this we defined the center of effective mass of stress volume as ″hypocentroid″, and the vertical projection of which on the earth′s surface as ″epicentroid″. Here we adopt three rotating models, including spheroid, ellipsoid and cylinder, to represent the region of an impending earthquake. Based on the models of gravity variations induced by uniform dilatancy, epicentroids associated with sixteen earthquakes of magnitude M 4～5 occurred in 1981 to 2000 in the Beijing Tianjin Tangshan Zhangjiakou region are determined by means of a proposed least squares iterative inversion method. The results indicate that cylinder model is preferable to the other two, and epicentroids obtained by the cylinder model separate from the epicenters by a range of 0～40 km. Epicentroids are inevitably located within intact tectonic blocks, and usually cluster in groups; while the epicenters are generally located at the terminations of faults or at the intersections of faults. It seems that there exist earthquake hatching areas in the block among faults. Earthquakes hatch in these areas, but occur around these areas, meanwhile the existence of faults may play an important role in controlling the processes.
Gravity variation data observed in the process of seismogenesis and occurrences of earthquakes show that the location with the greatest gravity changes does not necessarily coincide with the epicenter. To explain this we defined the center of effective mass of stress volume as ″hypocentroid″, and the vertical projection of which on the earth′s surface as ″epicentroid″. Here we adopt three rotating models, including spheroid, ellipsoid and cylinder, to represent the region of an impending earthquake. Based on the models of gravity variations induced by uniform dilatancy, epicentroids associated with sixteen earthquakes of magnitude M 4～5 occurred in 1981 to 2000 in the Beijing Tianjin Tangshan Zhangjiakou region are determined by means of a proposed least squares iterative inversion method. The results indicate that cylinder model is preferable to the other two, and epicentroids obtained by the cylinder model separate from the epicenters by a range of 0～40 km. Epicentroids are inevitably located within intact tectonic blocks, and usually cluster in groups; while the epicenters are generally located at the terminations of faults or at the intersections of faults. It seems that there exist earthquake hatching areas in the block among faults. Earthquakes hatch in these areas, but occur around these areas, meanwhile the existence of faults may play an important role in controlling the processes.
引文
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RichterCF .1958.ElementarySeismology[M ].SanFrancisco:Freeman,768
SinghSK .1977.Gravitationalattractionofaverticalrightcircularcylinder[J].GeophysJRastrSoc,50:243～246
顾功叙,KuoJT ,刘克人,等.1997.中国京津唐张地区时间上连续的重力变化与地震的孕育和发生[J].科学通报,42(7):1～12
贾民育,邢灿飞,孙少安.1995.滇西重力变化的二维图象及其与5级以上地震的关系[J].地壳形变与地震,15(3):9～19
李瑞浩.1988.重力学引论[M ].北京:地震出版社,14～15
刘端法,宋胜合,郑金涵,等.2000.1999年浑源地震前重力变化研究及2000年首都圈地震预测[J].地震地磁观测与研究,21(1A):89～94
梅世蓉.1993.地震科学研究论文选集[M ].北京:地震出版社,151～152,381～388
宋胜合,郑金涵,刘端法.2001.2000年首都圈重力场变化分析[J].地震地磁观测与研究,22(1A):82～85
ChenYT ,GuHD ,LuZX .1979.VariationofgravitybeforeandaftertheHaichengearthquake1975andtheTangshanearthquake1976[J].PhysEarthPlanetInter,18:330～338
GolubG .1965.Numericalmethodsforsolvinglinearleastsquaresproblems[J].NumerischeMathematik,7:206～216
KuoJT ,SunYF .1993.Modelinggravityvariationscausedbydilatancies[J].Tectonophysics,227:127～143
KuoJT ,ZhengJH ,SongSH ,etal.1999.DeterminationofearthquakeepicentroidsbyinversionofgravityvariationdataintheBTTZregion,China[J].Tectonophysics,312:267～281
LambertT ,LairdTO .1981.Asearchforlong termearthquakeprecursorsingravitydataintheChalevoixregion[J].Earth quakePrediction,Internationalreview,4:473～483
LiRH ,FuZZ .1983.LocalgravityvariationbeforeandaftertheTangshanearthquake(M =7.8)andthedilationprocess[J].Tectonophysics,97:159～183
RichterCF .1958.ElementarySeismology[M ].SanFrancisco:Freeman,768
SinghSK .1977.Gravitationalattractionofaverticalrightcircularcylinder[J].GeophysJRastrSoc,50:243～246
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