2016年8月唐山震群视应力变化特征

2016年8月河北唐山发生震群活动，具有频次高、持续时间长等特点。利用首都圈数字化地震台网监测的2016年1月—2017年5月河北唐山地区M_L≥2.5地震，采用遗传算法反演震源参数，计算相应地震视应力值。分析结果如下：①此次地震序列为前震—主震—余震型；②视应力值在震群发生前明显增高，发生过程中出现高低相间现象，在M_L 4.3主震出现峰值，震源区应力状态不稳定，震后视应力恢复正常；③震群视应力与震级呈现一定正相关。

Apparent stress changes of Tangshan earthquake swarm in August, 2016

Earthquake swarm activity occurred in Tangshan, Hebei in August, 2016. The earthquake swarms have the characteristics of high frequency and long duration. From January 2016 to May 2017, a series of M_L ≥ 2.5 earthquakes recorded by digital seismic network of the capital region. After inversion of source parameters by genetic algorithm, the apparent stress in this region is calculated. The result shows that, Firstly, the earthquake sequence was of foreshock-main shock-after-shock type. Secondly, the apparent stress value increased obviously before earthquake swarm occurred and there is a phenomenon of occurrence of high and low phases in the occurrence of earthquake swarms. The highest value appears in main shock of M_L 4.3 earthquake. The stress state in the focal region is unstable, the apparent stress returned to normal after the earthquake. Finally, the apparent stress and earthquake magnitude of earthquake swarms are positively related in general.