芦山7.0级地震前地震活动的临界点特征

The Critical Point Behavior of Seismic Activities Prior to Lushan M_S7.0 Earthquake in 2013

直接检测大地震之前空间相关长度的增长是一种从观测的地震活动中检测临界点特征的独立的方法。本文研究了2013年4月20日四川芦山7.0级地震前5年震中周围地区中等地震活动的空间相关长度变化。结果表明,从地震前近两年开始,中等地震以上地震的空间相关长度出现明显增长,变化趋势可以用幂次率进行很好的拟合。用该地区没有7级以上强震发生的时段的地震目录采用相同方法进行了计算,没有发现明显的幂次率增长现象,说明强震前中等地震以上地震的空间相关长度的幂次率增长是地震孕育过程中临界点物理特征的表现。

The seismic correlation length prior to the Lushan earthquake（April.20,2013） has been studied by use of single-link cluster（SLC） analysis.The catalogs used were downloaded from China Earthquake Networks Center（CENC,http：//www.csndmc.ac.cn）.The earthquakes were located in an area centered in the mainshock（30.3N°,103.0°E） and ranging 27°~33° to 100°~106° from 5 years before the mainshock to its origin time.We selected 3.5 as the minimum magnitude and checked the completeness of the catalogs used prior to the mainshock using the Gudengberg formula,which shows good completeness above magnitude 3.5.The results showed that the growing correlation length of earthquakes for moderate-size earthquakes,which had been observed prior to the mainshock nearly two years ago to the origin time,can be fitted well by the power law.At the same time,we have developed a method to test whether or not the result obtained is simply caused by data selection to produce the desired pattern.We selected a time interval before the mainshock in which no great earthquake occurred and several years later when the greater earthquake occurred.The catalogs in the time interval will be analyzed using the same method as the catalogs prior to the mainshock study to check whether the correlation length shows an increase with the power law.We cannot find an obvious increasing process by the power law.For the preparation and occurrence stages of the earthquake process,characterized as a self-organized critical phenomenon,to test directly for growing spatial correlation lengths prior to large earthquakes is an independent approach to detect critical point behavior in observed seismicity.The results from this paper showed that the self-organized critical point characteristic was observed prior to the Lushan 7.0 earthquake.The studies in this paper illuminate the notion that the growing seismic correlation length by the power law represents a physical process of critical-point characteristics in the source area preparing for a great earthquake.In future work,the predictive power of growing correlation length should be tested.In an area,the grid search technique can be applied to extract regions of growing correlation length systematically from a data set.The estimates for occurrence time,epicenter,and magnitude of expected mainshock may then be obtained from the fitting parameter,critical region,and value of correlation length.For a further reduction of uncertainties in a predictive approach,it also appears promising to combine the concepts of growing correlation length and other predictive methods.