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M4.5地震和2020年7月12日唐山M5.1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M5.1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M5.1地震震中17km范围内，可能是唐山M5.1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_L2.5 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_L4.0,but the stress drop increases with the magnitude increasing,and the corner frequency tends to decrease with the magnitude increasing. In between the M4.5 earthquake in Fengnan district,Hebei on December 5,2019,and the M5.1 earthquake in Tangshan on July 12,2020,there were seven earthquakes of M_L2.5 or greater,ranging in magnitude from M_L2.5 to 2.8. The analysis indicates that there are two obvious high stress drop events before the M5.1 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 M5.1 Tangshan earthquake,which may be a sign of high tectonic stress in the source region before the 2020 Tangshan M5.1 earthquake.

P315

河北省地震局测震专业仪器系统检测评估与观测场地遴选创新团队项目资助