地震空间分布非空网格数的统计分布及地震活动增强-平静的定量检测

Statistic distribution of non-empty grid numbers for earthquakes and quantitative test on enhanced and quiescent acivity

以华北地区1980—2010年资料为例,以单位边长的网格覆盖研究区,考察中小地震空间分布非空网格数的变化.网格尺度足够小则非空网格数趋于该时段的地震数,网格尺度足够大则非空网格数为1.实际资料显示,当空间网格尺寸大于0.5°之后,对结果稳定性的影响逐渐减弱.中小地震空间分布非空网格数的变化与"背景"地震活动的空间扩展(增强)或收缩(平静)有关.不同网格尺寸条件下的非空网格数有基本一致的变化趋势,可能间接反映了区域应力的短时扰动或起伏.小地震有更高的丛集特性,随着震级下限的提高,不同网格尺寸非空网格数之间差异逐渐变小.华北地区小震活动非空网格数的频次分布符合统计正态分布,因而给定置信概率、依据正态分布密度函数可计算非空网格数的"正常"分布范围,超出此范围的数据可视为异常.统计显示,就华北地区而言,中小地震非空网格数"平静"型异常的预测效率最低,"增强"型异常具有最高的报警对应率,而同时考虑"增强"及"平静"的异常判据则具有最高的预报评分.这也意味着,华北中强地震前以"增强"型的中小地震活动异常为主.研究结果还显示,小地震时空活动格局的改变与后续中强地震似乎具有更强的统计关联特性.

Based on the earthquake data during 1980-2010 in North China region, we use the grids with IXI unit size to cover the area and investigate the variation of non-empty box number of earthquakes. When grid size l is small enough, the number of non empty boxes tends to be equal to earthquake frequency. When grid size l is large enough, the number of non-empty boxes will be close to 1. This study shows that the influence of grid size on result stability is very weak when l is≥0.5°. The variation of non-empty boxes is related to the expanded （enhanced） or constricted （quiescence） patterns of earthquake spatial distribution. The numbers of non-empty boxes with different l roughly show a consistent changing tendency, potentially reflect the stress perturbation or undulation during a short time period indirectly. Small earthquakes are much more clustered, the difference between results with different threshold magnitude gradually tend to be small when threshold magnitude increases gradually. The non-empty grid numbers show a normal distribution, therefore the ＂normal＂ distribution range for non-empty grid numbers can be calculated with the given confident probability and the abnormal could be detected if the data is outside this ＂normal＂ range. The statistics show that in North China the forecasting efficiency is low for ＂quiescence＂ type abnormal, the ＂enhanced＂ type abnormal has a higher correct alarm rate, and the precursory criterion both by ＂enhanced＂ and ＂quiescence＂ type abnormal has a higher forecasting efficiency. This means that the precursory of small or moderate earthquakes before large earthquakes mainly displays as ＂enhanced＂ activity. The results also show that the pattern change of small or moderate earthquake spatial distribution seems to be related to coming large earthquakes more tightly.