云南地区强震的短临中强前震序列特征初探

对1965￣2003年云南地区强震前3个月内发生过的短临中强前震进行整理,将其按距离主震发生时间分为直接前震、临震前震与短期前震三类,然后通过地震序列动态分析地震活动,发现各类前震有以下显著特征:(1)直接前震:3、4级地震时间间隔临震时骤减;将地震序列从时间上分成三等分后,第二时段的地震频次没有明显衰减。(2)临震前震:震后第一天地震的频次较高而第二天相对过低,同时同一台站P波初动符号基本上一致或振幅比稳定在某一范围。(3)短期前震:震后10天前震频度衰减至零;20-30天左右地震频度骤增,同时h、p值也反应出类似的特征,30天h值迅速减小接近1或小于1,而b值基本稳定且均低于区域背景值。

Preliminary Research on the Sequence Characteristic of the Short Impending Medium Strong Foreshocks before Strong Earthquake in Yunnan Region

Through analyzing and sorting out the short-impending medium-strong foreshocks occurred from 1965 to 2003 in three months before the strong earthquakes in Yunnan region,short-impending foreshocks are classified as three kinds according to their durations,the direct foreshocks,the impending foreshocks and the short-term foreshocks.By studying on seismic activity for earthquake sequence variation,the features of the three kinds of foreshocks are obtained as following:(1)Common anomalies for the direct foreshocks appeared a sudden decrease of time intervals of the M3 earthquakes and M4 earthquakes and obvious attenuation of earthquake frequency in the second time interval after dividing seismic sequence into the three equal part on time.(2)Common characteristics for impending foreshocks manifested high frequency of events in the first day and low frequency of events in the second day after the foreshocks,with roughly identical signs or linear distribution of P wave first motion.(3)Common characteristics for short-term foreshocks showed attenuation of foreshocks frequency to zero in 10 days after foreshocks and seismic frequency showed a sudden increase in about 20 to 30 days,at the same time,h-value and p-value also showed similar to characteristics.Rapidly decreased of h-value towards 1 or less than 1 and stable b-value lower than regional background value in 30 days after the foresshocks.The results provide the basis for short-impending prediction of the strong earthquakes in Yunnan region.