肃南5.7级地震过程剪切波分裂特征

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

THE CHARACTERISTICS OF SHEAR WAVE SPLITTING BEFORE AND AFTER SUNAN EARTHQUAKE WITH M = 5.7

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.