2020年河北唐山5.1级地震前震源参数变化

Source Parameter Changes before the 2020 Tangshan M5.1 Earthquake in Hebei Province

利用谱分析方法对河北省测震台网2016年1月—2020年7月唐山老震区(117.5°~119.5°E,39°~41°N)85次M_(L)≥2.5地震进行震源参数分析,发现该时段唐山老震区应力降范围在0.1~10.3MPa,主要集中在0.1~1.0MPa,其中应力降小于2MPa的地震占81%。当2.5≤M_(L)≤3.5时,应力降、拐角频率与震级之间无明显的对应关系;M_(L)≥4.0地震较少,但应力降有随震级增大而增大的趋势,拐角频率有随震级增大而减小的趋势。2019年12月5日河北丰南M 4.5地震和2020年7月12日唐山M 5.1地震之间发生了7次M_(L)≥2.5地震,震级范围为2.5≤M_(L)≤2.8,提取该震级范围的震源参数进行时空分析,发现唐山M 5.1地震前有2次明显的高应力降事件,2次高应力降事件呈现出应力降数值高、拐角频率高、震源尺度小的特点,表明单位体积所释放能量较多。2次高应力降事件均发生在唐山M 5.1地震震中17km范围内,可能是唐山M 5.1地震前震源区构造应力较高的一个标志。

The source parameters of 85 earthquakes(117.5°~119.5°E,39°~41°N)above M_(L)2.5 in Tangshan earthquake area from January 2016 to July 2020 were analyzed by using spectral analysis method.We found that the stress drop of Tangshan Earthquake region is 0.1~10.3MPa,mainly 0.1~1.0MPa,and 81%of the earthquakes with stress drop less than 2MPa.For the earthquakes with magnitude between M_(L)2.5 and 3.5,there is no obvious relationship between the stress drop and corner frequency and the magnitude.There are few earthquakes with magnitude above M_(L)4.0,but the stress drop increases with the magnitude increasing,and the corner frequency tends to decrease with the magnitude increasing.In between the M 4.5 earthquake in Fengnan district,Hebei on December 5,2019,and the M 5.1 earthquake in Tangshan on July 12,2020,there were seven earthquakes of M_(L)2.5 or greater,ranging in magnitude from M_(L)2.5 to 2.8.The analysis indicates that there are two obvious high stress drop events before the M 5.1 Tangshan earthquake.These two high stress drop events show high value of stress drop and high corner frequency,and the small focal size indicates that more energy is released per unit volume.Both events occurred within 17km distance from the M 5.1 Tangshan earthquake,which may be a sign of high tectonic stress in the source region before the 2020 Tangshan M 5.1 earthquake.