1975年2月4日海城地震(M=7.3)的前震系列/
THE FORESHOCK SEQUENCE OF THE FEBRUARY, 4,1975,HAICHENG EARTHQUAKE (M=7.3)
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摘要: 我们研究了1975年2月4日海城地震(M=7.3)前震的定位及其辐射图象.用六个区域台的到时,相对于一次参考地震,将前震及主震进行了定位.这组前震开始相互很接近,然后随时间及其分布有一定的延伸.最大前震前,前震均位于直径约两公里的小体积内,而在最大前震后,其活动则向北西和南东方向扩展,形成六公里长的在北西方向上伸展的分布带.初动及 P 波 S 波振幅比表明,在前震系列中有两种不同的断裂机制.我们推测这两种辐射图象可能和前震处于分布带的不同部位有关.可能主震震源不处于前震震源所决定的断层上,而是位于这组前震南面6公里、且较这组前震浅几公里处.我们认为,在垂直于主震破裂面及前震分布带走向的方向上,前震和主震相距这么大的距离,可能是由于主震时产生滑动的断层是以雁行排列的.分析了在前震期间断层上滑动所引起的应力变化,认为由前震引起的主震断层上剪应力的增加是很小的.因之由前震直接触发主震的可能性不大.Abstract: We have examined the locations and radiation patterns of the foreshocks of the February 4, 1975, Haicheng earthquake (M = 7.3). Using arrival times from six local seismic stations, the foreshocks and mainshock were located relative to a master event. The foreshocks occurred in a tight cluster that elongated with time, Before the largest foreshock, the activity was located within a small, approximately equidimensional volume with a diameter of about 2 km. After the largest foreshock, the activity spread northwest and southeast forming a 6-km-long, northwest trending zone. First motions and ratios of P to S amplitudes indicate that two different faulting mechanisms occurred during the foreshock sequence. The two radiation patterns can tentatively be correlated with different parts of the zone. The hypocenter of the mainshock was not located on the same fault as that defined by the foreshocks hypocenters but rather was located 6 km south of and several kilo meters shallower than the foreshock cluster. We think this large separation between foreshocks and mainshock in a direction perpendicular both to the plane of rupture of the mainshock and to the trend of the fore-shocks might be the result of an en echelon step in the fault that slipped during the mainshock. An analysis of the change in stress due to slip during the foreshocks shows that the increase in shear stress on the mainshock fault caused by the foreshocks is very small and that direct triggering of the mainshock by the foreshocks is unlikely.
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[2] L. M. Jones, and P. Molnar, Some characteristics of foreshocks and their possible relationship to earthquake prediction and premonitory slip on faults, J. Geophys. Rea., 84, 359E——3608, 1979.
[3] Y. Kagan, and L. linopoff, Statistical atndy of the occurrence of shallow earthqxxakes, Geophys.J. R. dstron. Soc., 55, 67——86, 1878.
[4] E. R. Engdahl, and C. giaslinger, Seismological preaursars to—— a magnitude 5 earthquake in the Central Aleutian Islands, J,Phys,Earth, 25, s243——a250, 1977.
[5] A. Lindh, G. Fuis, and C. Mantis, Seismic amplitude measurements suggest foreehoeke have different focal mechanisms than aftershocks, Science, 201, 56——59, 1978.
[6] S. Billington, E.R.Engdahl, and 8. Price, Changes in the seismieitp and focal mechanism of ——small earthquakes prior to an Ma=6.7 earthquake in the Central Aleutian Island Arc, Earrthquake Prediction, An International Review, Maxirice Ewing 8er. 3, 4, edited by D. W. Simpson and—— P. G. Richards, pp. 348——356, AGU, Washington, D. C., 1981.
[7] J. Cigar, Source processes of the Haicheng, China eartahquake from observations of P and S waves,Bull. Seiamod. Soc. dm., 69, 1903——1916, 1979.
[8] 吴开统、岳明生等,海城地晨序列的特征,地球物理学报,19, 2, 95——109, 1976,
[9] T. Fitch. and H. J. Muirhead, Depths to larger earthquakes associated with crustal loading, Geophya. J. R. datron. Soc., 37, 285——296, 1974.
[10] J. Jackeon, ands T. Fitch, 8eismoteetonic implications of relocated aftershock sequences in Iran and Turkey: an application of the master event technique, Geaphys. J. R. datron. .Soc., 57, 209——229, 1979.
[11] T. Fitch, and J. M. W. Ryan, Inversion for Vr/Y, in shallow source regions, Geophys. J. R. dstron. Soc., 44, 253——267, 1976.
[12] 金严、赵毅等,辽宁省海城地晨前晨震源错动方式的一个特点,地球物理学报,19, 3, 156——164,1976,
[13] L. M. Jones, Q. Deng, and P. Jiang, The role of conjugate faults in the development and oceurrence of earthquakes (in Chinese), Seiamiol. Geol., 2, 19——26, 1980.
[14] B. A. Bolt, J. Btifler, and R. Urhammer, The Briones Hills earthquake swarm of January 8, 1977. Contra Costa County, California, Bull. Se%smol. Soc. dm., 67, 1555——1564, 1977.
[15] 陈顺,用震源机制一致性作为描述地晨活动性的新参数,地球物理学报,21, 2, 142——159, 1978,
[16] A. Ben——Menahem, S.W.Smith and T. L. Teng, A procedure for s.'ource studies from. spectrums of long——period seismic body waves, Bull. Seismol. Soc, dm., 55, 203——235, 1965.
[17] 顾浩鼎、陈运泰等,197,年2月4日辽宁省海城地晨的震源机制,地球物理学报,19, 4, 270——285,1976.
[18] P. Paria, and G. Sih, 8tresa analysis of cracks, Symposium on Fracture Toughm,ess Testvng and Applications, ASTM, Spea. Teoh. Publ., 381, 30——81, 1965.
[19] Q. Deng, P. Jiang, L. Jones, and P. Molnar, A preliminary analysis of reported changes in ground water and anomalous animal behavior befoxe the 4 February 1975 Haicheng earthquake, Earth——quake Prediction, An International Review, Mancrice Ewing Ser. 3, 4, edited by D.W. Simgaon and P. G. Richarda, pg. 543——565, AGU, Washington, D. C., g. 543——565, 1981.
[20] C. B. Raleigh, et al., Prediction of the Haicheng earthquake, Eos. Trans. AGU, 58, 236——272, 1977.
[21] H. ganamori, and D. Anderson, Theoretical basis of some empirical relations in seismology, Bull. Seism.ol. Soc. din,65, 1073——109:5 1975. -
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