肃南5.7级地震过程剪切波分裂特征
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摘要
剪切波分裂应用于地震预报中的3个主要困难是:(1)如何自动定量识别剪切波快慢波到时;(2)用以认识其基本特征的震例太少;(3)因各次地震的复杂性,对所给剪切波分裂结果难以给出恰当的解释。本文研究了1988年11月22日甘肃肃南5.7级地震过程剪切波分裂的特征。所用资料为1988年8月到1990年8月中法合作张掖数字台网的记录。通过对张掖～肃南地区地壳结构模型的研究,确认了所用各次地震的真实出射角基本上位于剪切窗附近。联合使用本文提出的最大特征值法、波形识别算子法和最大似然法,定量确定了剪切波快、慢波到时。结果表明:1988年甘肃肃南5.7级地震前后,快、慢波时间延迟呈现了减小(1988年10月至11月22日)～发震～恢复(主震后)的形态,其基本形态类似于1990年天祝—景泰地震过程。综合分析了地震剪切波分裂的时间进程和空间动态分布,用EDA模式和组合模式解释了出现的特征,认为此次地震出现了具有物理意义的剪切波分裂前兆。
There are three key points to use the shear wave splitting into earthquake prediction: (l)How to determine automatically and quantitatively the arrivals of fast and slow splitting shear wave; (2)There are few examples for understanding shear wave splitting; (3)The interpretation of shear wave splitting before and after some earthquakes is very difficult.
We present a new processing method combined with Maximum-Eigenvalue(ME), Wave Distinguish Factor(WDF) and Maximum-Likelihood(ML) to determine fast and slow shear wave arrivals.
The shear waves splitting characteristics before and after Sunan earthquake with M = 5.7, occurred on November 22, 1988, are studied. The data are from Zhangye seismic digital station established jointly by China and France from Aug. 1988 to Aug. 1990. After the research of crustal structure of this region, the true emergent angles of various earthquakes were determined and they are about 33 ?. By simultaneously using ME, WDF and ML mentioned above, the arrivals of fast and slow split shear wave were automatically and quantitatively defined. The features of shear waves splitting are as follows:
1) The time delay shows three stages: decrease(Aug. 1988-Nov. 22, 1988)-occurrence- recovery(after main shock).
2) The polarization direction of fast wave after the main shock is NW 55 ?70?, consistent with the principal compressive stress.
3) These characteristics can be interpreted by both EDA and Combination Model simultaneously. These features show that in this region the upper crust anisotropy exists, and it is likely to be an earthquake precursor.
There are three key points to use the shear wave splitting into earthquake prediction: (l)How to determine automatically and quantitatively the arrivals of fast and slow splitting shear wave; (2)There are few examples for understanding shear wave splitting; (3)The interpretation of shear wave splitting before and after some earthquakes is very difficult.
We present a new processing method combined with Maximum-Eigenvalue(ME), Wave Distinguish Factor(WDF) and Maximum-Likelihood(ML) to determine fast and slow shear wave arrivals.
The shear waves splitting characteristics before and after Sunan earthquake with M = 5.7, occurred on November 22, 1988, are studied. The data are from Zhangye seismic digital station established jointly by China and France from Aug. 1988 to Aug. 1990. After the research of crustal structure of this region, the true emergent angles of various earthquakes were determined and they are about 33 ?. By simultaneously using ME, WDF and ML mentioned above, the arrivals of fast and slow split shear wave were automatically and quantitatively defined. The features of shear waves splitting are as follows:
1) The time delay shows three stages: decrease(Aug. 1988-Nov. 22, 1988)-occurrence- recovery(after main shock).
2) The polarization direction of fast wave after the main shock is NW 55 ?70?, consistent with the principal compressive stress.
3) These characteristics can be interpreted by both EDA and Combination Model simultaneously. These features show that in this region the upper crust anisotropy exists, and it is likely to be an earthquake precursor.
引文
1 Crampin S, Lovell J H. A decade of shear-wave splitting in the earth's crust: what does it mean? what use can we make of it? and what should use do next? Geophys. J. Int, 1991, 107: 387～408．
2 Crampin S, Evans R, Ucer S B, et al. Observations of dilatancy-induced polarization anomalies and earthquake prediction. Nature, 1980, (286) : 874～877．
3 Crampin S, Booth D C. Shear-wave polarization near the North Anatolian Fault Ⅱ, Interpretation in terms of crack-induced anisotropy. Geophys. J.R. Astron. Soc, 1985, 83: 75～92．
4 Crampin S. The basis for earthquake prediction, Geophys. J.R. Astron. Soc, 1987, 91: 331～347．
5 Chen T C, Booth D C, Crampin S. Shear-wave polarizations near the North Anatolian Fault Ⅲ, Observations of temporal changes. Geophys. J.R. Astron. Soc, 1987, 91: 287～311．
6 Peacock S, Crampin S, Booth D C, et al. Shear-wave splitting in the Anza seismic gap, southern California; Temporal variations as possible precursors. Geophys. J. Res. 1988, 93: 3339～3356．
7 Crampin S, Booth D C, Krasnova M A, et al.. Shear-wave polarizations in the Peter the First Range indicating crack-induced anisotropy in a thrust-fault regime. Geophys. J. R. Astron. Soc, 1986, 84: 401～411．
8 Kaneshima S, Ando M, Crampin S. Shear-wave splitting above small earthquakes in the Kinki District of Japan. Phys. Planet. Int, 1987, 45: 45～58．
9 姚陈,王培德,陈运泰.卢龙地区S波偏振与上地壳裂隙各向异性.地球物理学报,1992,35(3) .
10 郑治真.S波分裂研究.地球物理学进展,1990,5(1) .
11 李清河,刘希强,张范民,等.剪切波分裂的某些理论问题及天祝-景泰6． 2级地震剪切波分裂特征研究.中国地震,1994,10(增刊):9～21．
12 刘希强.剪切波分裂中的快、慢波识别方法.西北地震学报,1992,14(4) .
13 高元勤,杨玉衡.1988年11月22日甘肃肃南5． 7级地震.西北地震学报,1989,11(3) .
14 张范民.利用最大似然法进行波场分解和震相识别.西北地震学报,1993,15(3) .
15 姚立询,温增平,虞雪君,等.民乐盆地及其周围地区的介质特征和小震震源参数.河西祁连山中段地震危险基础研究.北京:地震出版社,1994．
16 荣代潞,李亚荣,金铭.张掖民乐盆地中强地震前后微震尾波Q值的变化特征及其短临预报意义.河西祁连山中段地震危险基础研究.北京:地震出版社,1994．
2 Crampin S, Evans R, Ucer S B, et al. Observations of dilatancy-induced polarization anomalies and earthquake prediction. Nature, 1980, (286) : 874～877．
3 Crampin S, Booth D C. Shear-wave polarization near the North Anatolian Fault Ⅱ, Interpretation in terms of crack-induced anisotropy. Geophys. J.R. Astron. Soc, 1985, 83: 75～92．
4 Crampin S. The basis for earthquake prediction, Geophys. J.R. Astron. Soc, 1987, 91: 331～347．
5 Chen T C, Booth D C, Crampin S. Shear-wave polarizations near the North Anatolian Fault Ⅲ, Observations of temporal changes. Geophys. J.R. Astron. Soc, 1987, 91: 287～311．
6 Peacock S, Crampin S, Booth D C, et al. Shear-wave splitting in the Anza seismic gap, southern California; Temporal variations as possible precursors. Geophys. J. Res. 1988, 93: 3339～3356．
7 Crampin S, Booth D C, Krasnova M A, et al.. Shear-wave polarizations in the Peter the First Range indicating crack-induced anisotropy in a thrust-fault regime. Geophys. J. R. Astron. Soc, 1986, 84: 401～411．
8 Kaneshima S, Ando M, Crampin S. Shear-wave splitting above small earthquakes in the Kinki District of Japan. Phys. Planet. Int, 1987, 45: 45～58．
9 姚陈,王培德,陈运泰.卢龙地区S波偏振与上地壳裂隙各向异性.地球物理学报,1992,35(3) .
10 郑治真.S波分裂研究.地球物理学进展,1990,5(1) .
11 李清河,刘希强,张范民,等.剪切波分裂的某些理论问题及天祝-景泰6． 2级地震剪切波分裂特征研究.中国地震,1994,10(增刊):9～21．
12 刘希强.剪切波分裂中的快、慢波识别方法.西北地震学报,1992,14(4) .
13 高元勤,杨玉衡.1988年11月22日甘肃肃南5． 7级地震.西北地震学报,1989,11(3) .
14 张范民.利用最大似然法进行波场分解和震相识别.西北地震学报,1993,15(3) .
15 姚立询,温增平,虞雪君,等.民乐盆地及其周围地区的介质特征和小震震源参数.河西祁连山中段地震危险基础研究.北京:地震出版社,1994．
16 荣代潞,李亚荣,金铭.张掖民乐盆地中强地震前后微震尾波Q值的变化特征及其短临预报意义.河西祁连山中段地震危险基础研究.北京:地震出版社,1994．
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