Focal Mechanism Of Small And Medium Earthquakes And Its Dynamic Significance In Gansu "Gulang Window"

The “seismic window” is a special structural site that sensitively reflects the regional tectonic stress state in the form of a weak earthquake and can provide abnormal information for the occurrence of dangerous earthquakes around it.Since the concept was proposed,seismologists systematically studied the physical meaning,forecasting methods,and forecasting effectiveness of the “seismic window”,and have achieved good research results,providing reliable information for seismic research and seismic monitoring.After the Gulang M8.0 earthquake in 1927,the small and medium earthquakes around the epicenter were spatially concentrated and temporally continuous.The activity of small and medium earthquakes in this area has certain significance for earthquakes above moderate to strong in the Qilianshan seismic zone.Therefore,Gansu “Gulang Window” is one of the important earthquake windows in the northwest region and has received extensive attentio n.This paper uses the abundant data obtained from the digital seismic network to analyze the seismic features of Gulang and its adjacent areas,based on the relocation of earthquakes,the focal mechanism of small and medium earthquakes and inversion of tectonic stress fields and its dynamic significance through various digital seismology methods.Firstly,a multi-stage location method was used to relocate the small and medium earthquakes in the Gulang window and its vicinity,and accurate small and medium earthquake parameters were obtained.The three-dimensional spatial distribution characteristics of seismic activities were summarized.The relocating results show that,the earthquake distribution is more concentrated by multi-stage method.From the spatianl point of view,the earthquake in the window are concentrated on both sides of the imperial Huangcheng-Shuangta fault,it is distributed in the envelope area of the Wuwei-Tianzhu fault,the Lenglongling fault,and the Tianqiaogou-Huangyangchuan fault;from the depth distribution,the earthquake mainly occurs in the range of 5 to 15 km,and earthquakes on the depth profile are intermittently concentrated in the vicinity of the fault plane,the results of the four sections clearly show the positional relatio nship between the small and medium earthquakes and the structural faults in the area.Secondly,applying the CAP method and the Hash method,we obtained the focal mechanism solution of the Gulang window and its surrounding seven hundred small and medium earthquakes since 2009.The statistical results show that the rupture type of the medium and small earthquakes in the Gulang window is dominated by the strike-slip type,second is the thrust.Finally,based on a large number of small and medium earthquake foca l mechanism solutions,the regional stress field and R value of the northeastern Qinghai-Tibet Plateau were obtained using tectonic stress field damping inversion method.The results show that the maximum principal stress is the northeast direction,at the same time,explored the complicated dynamic interaction between the Gulang trust-body and surrounding secondary blocks.