用滑动方向拟合法反演唐山余震区的平均应力场
MEAN STRESS FIELD IN TANGSHAN AFTERSHOCK AREA OBTAINED FROM FOCAL MECHANISM DATA BY FITTING SLIP DIRECTIONS
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摘要: 将每个震源机制解的两个节面交替取作尝试的断层面,使计算的断层面上的剪切力方向与观测的断层面的滑动方向夹角最小,从而根据29个震源机制解结果反演了唐山余震区三个分区的平均主应力方向和中等主应力相对大小。本文结果说明,唐山余震区的最大主应力轴水平,取近东西方向,较之1976年唐山大震前的方向可能水平地顺时针转动了大约30。对同样地区用平均P,B,T轴推断主应力方向的结果与用上述方法所得结果基本一致。所作的数值试验说明,在特定情况下,用平均P,B,T轴推断构造应力主轴方向时有可能出现系统偏差。Abstract: Alternatively taking each of the two nodal planes of focal mechanism solutions as the possible fault plane,the mean directions of principal stress axes and the relative magnitudes of intermediate principal stress in 3 sub-regions of the Tangshan aftershock area are estimated from 29 focal mechanism solutions by minimizing the angle between the calculated shear stress and the observed slip vector on every fault plane. The result indicates that the mean principal compressional stress axis in the Tangshan aftershock area is nearly horizontal,taking approximately the E-W direction. Comparing with the situation before the great 1976 Tangshan earthquake,this axis may have horizontally rotated about 30?clockwise. The stress axis directions inferred from the mean P,B and T axes of the composite fault plane solutions of aftershocks in the same 3 sub-regions are generally in agreement with the above result. A numerical test shows that,in some special case,there may appear a systematic deviation between the directions of the mean P,B and T axes and that of real principal stress axes in the crust.
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[2] 许忠淮,阎明,赵仲和,由多个小地震推断的华北地区构造应力场的方向,地震学报,5,268——279, 1983.
[3] Etchecopar, A., G. Vasseur, and M. Daignieres, An inverse problem in microtectonics Eor the Determination of stress tensors from fault striation analysis, J. Struct. Geol., 3, 51——55, 1981.
[4] Angelier, J., A. Tarantola, B. Valette and S. Manoussis, Inversion of field data in fault tectonics to obtaian the regional stress ——I. single phase fault populations: A new method of computing the stress tensor, Geophys. J. R. astr. Soc.,‘,,607——621, 1982.
[5] 许忠淮,戈澎滇,用滑动方向拟合法反演富蕴地震断裂带应力场,地震学报,6, 395——404, 1984.
[6] Ellsworth, W. L., and Xu Zhonghuai, Determination of the stress tensor from focal mechanism Data,EOS. Trans. Amen Geophys. Union., 61, 1117, 1980.
[7] B.H.斯米尔诺夫著,高等数学教程,第三卷第一分册,第二章39节,人民教育出版社,1954年9月新1版。
[8] 李钦祖,刁桂菩,戴英华,唐山地震序列的应力释放调整过程,地球物理学报,26, 224——236, 1983.
[9] Wilkinson, J. H., The Algebraic Eigenvalue Problem, Chapter 2, Clarendon Press, Oxford, 1965. -
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