| 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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| THE DENDROCHRONOLOGICAL METHOD——A NEW METHOD FOR DETERMINING THE AGES OF SEISMIC DEFORMATIONAL BELTS IN DAMXUNG OF XIZANG (TIBET) |
| 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. |
| HAN Tong-lin.1983.THE DENDROCHRONOLOGICAL METHOD——A NEW METHOD FOR DETERMINING THE AGES OF SEISMIC DEFORMATIONAL BELTS IN DAMXUNG OF XIZANG (TIBET)[J].Acta Geoscientica Sinica,5(2):95-106. |
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