| 西藏当雄一带地震形变带发生年代确定的新方法——树木年轮计算法 |
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| 引用本文：韩同林.1983.西藏当雄一带地震形变带发生年代确定的新方法——树木年轮计算法[J].地球学报,5(2):95-106. |
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| 中文摘要:<正> 西藏地区地震活动强烈而频繁。历史地震在1950年以前记录不多,且不完整、准确性也差。自1950年开始才有较系统的地震记录。尽管西藏地区历史地震形成的地震形变带极为发育,但因地处高寒、人烟稀少、交通不便,对它的研究甚少。根据西藏地区地震形变带之间相互切割关系,保留程度及其地貌、植被等特征的不同,大致可以鉴别它们形成的先后顺序,但多无法确定各自发生的年代。由于地震形变带特征的研究及其年代的确定,对于探讨地震震源、地震破裂的性质与过程、地震成因与地质构造关系及其应力场的演变等,都是十分珍贵的资料,因此较精确地确定各期地震形变带发生的年代就显得十分珍贵的资料，因此较精准的确定各期地震变形带发生的年代就显得十分重要，但在西藏地区常感到束手无策，无从下手。 |
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| THE DENDROCHRONOLOGICAL METHOD——A NEW METHOD FOR DETERMINING THE AGES OF SEISMIC DEFORMATIONAL BELTS IN DAMXUNG OF XIZANG (TIBET) |
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| Abstract:So far as we know, the dendrochronological method has not been usedwidely in the determination and calculation of the ages of seismic deformational belts.Preliminary observations and studies of the few samples collectedfrom the district of Yangbajain and Damxung in Xizang (Tibet) show thatthe use of dendrochronological method in the determination of age of theactive faults occurred during the last century, or even a few centuries ago isof great significance. It is simple, economical and reliable as compared withthe new geochronological method. Furthermore, it does not require complicatedequipment and can provide accurate quantitative data. The fundamentals of the dendrochronological method used to determinethe age of seismic deformational belts are based on the correlation betweenannual growth rings of trees and environmental factors, that is to say, whenthe climate is favorable and the rainfall is sufficient, trees will grow fastand wide growth rings will be formed; when the climate is unfavorable andthe rainfall is insufficient, trees will grow slowly and narrow growth ringswill be formed. A large amount of research work has proved that within thesame district the changes in width of corresponding growth rings of treeshave similar charactristics. When an active fault in the seismic deformational belts cuts through the tree-covered earth surface, the ecological environment of trees on the opposite walls of the fault will change sharply to therelative ascending and descending of the walls. On the ascending wall, groundwater level will drop in response to the ascending of the earth surface. Thewater absorption by trees will be much less and much more difficult than before the occurrence of the faulting, and trees will grow slowly, and thusthe growth rings of trees will be much narrower than before the faulting(these are tentatively called "narrowed rings"). On the descending wall, ground water level will rise in response to the descending of the earth surface.The water absorption by trees will be much easier and much more in quantity than before the occurrence of the faulting. Trees will grow faster andthus the growth rings of trees will be muh wider than before the faulting(these are tentatively called "widened rings"). From above it can be seenthat the time of the initial formation of narrowed and widened rings of trees is the time of the occurrence of faulting, and the number of these ringsis the number of years starting from the occurrence of faulting down to thesample collection. Therefore the formed narrowed and widened annual growthrings are evidence of the occurrence of faulting as well as the "automaticrecorder"of the ages of faults. Studies made on the few samples collected in the Damxung district haveproved that the third phase faults in the Yangbajain, Damxung district formed 600 years ago, and the fourth phase faults 29-30 years before the yearof sampling (1981) i. e. during the years 1951-1952. |
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