Study On The New Method Of Determining The Types Of Small Earthquake Swarms

The small earthquake swarms often occur , judging the types quickly or determining whether there will be larger earthquakes following is very important both in the hazard mitigation and social stability. In recent decades, much progress has been made in judging the types of the small earthquake swarms. Various methods were applied in practical efforts of earthquake prediction. But most of them are statistical. The uncertainties of judging the types of earthquake swarms are unavoidable. So it is necessary to broadly explore new methods that have clearer physical meaning and more effect results.The theoretical and experimental studies show that the focal mechanisms of foreshock sequence are more consistent than those of ordinary small earthquake swarm. Due to the intricacy of calculating the focal mechanism of small earthquakes, we use correlation coefficient of spectral amplitude to study the consistency of focal mechanism of small earthquakes, so as to analyze the possibility of determining the types of small earthquake swarms. The overall scientific thought is that after deducting the propagation path effects, the site effects and some other factors from the spectral of the observed displacement, we get the spectral amplitude of every earthquake in the swarms. We try to study whether the focal mechanisms are similar through analysing the correlation coefficients of source spectral amplitude. If the correlation coefficient is closer to 1.0, it indicates that the focal mechanisms of the small earthquakes are more consistent and the small earthquake swarms are more likely to be the foreshock sequences. On the contrary, the focal mechanisms are less consistent and the small earthquake swarms are more likely to be ordinary small earthquake swarms.As the cases study to the theoretical deduction, we choose three foreshock sequences and three ordinary small earthquake swarms to calculate their correlation coefficients of spectral amplitude respectively. The results show that the mean and minimal correlation coefficient of spectral amplitude of small earthquake swarms are all above 0.85, closer to 1.0 before the Xiuyan Ms5.4 earthquake in 1999, Shidian Ms5.9 earthquake in 2001 and the Yongsheng Ms6.0 earthquake in 2001; while the mean and maximal correlation coefficients of the ordinary small earthquake swarms are all below 0.8, significantly less than 1.0 in Yangbi swarm(2002), Eryuan swarm (2003) and Binchuan swarm (2005) after which no large earthquake took place. The theoretical deductions coincide with the actual observational results, which implies that the correlation analysis of spectral amplitude is a new method that has more clear physical implication and higher performance. Besides, in the computational process mentioned above, this study also gets the QP, QS of the Xiuyan region and the northwest area of Yunnan Province, and the site effects of the station used in the paper.We also study the six small earthquake swarms by means of the traditionally statistical parameter. The results show that most of them are miscarriages. It implies that the prediction performances of the new method put forward in this paper based on some earthquake physical models is more effective than the traditional method of statistical parameters in connection with this six earthquake swarms. It also suggests that seismic data has more seismogenic information than earthquake catalog. According to the study in the paper, it is very obvious that the correlation coefficients of spectral amplitude of the small earthquake swarms can be measured as long as there are some digital seismic stations around. So the correlation analysis of spectral amplitude is worthy of further research.