Study On High-precision Earthquake Location And Shallow-deep Geometry Of Faults In North China

The North China Plain is an area with strong tectonic activity and dense population.Over the past 300 years,there have been five earthquakes above magnitude 7,such as the modern great earthquakes in Tangshan,Luanxian,and Ninghe,which have caused serious economic losses and casualties.The area has always been a key region for China’s earthquake monitoring and defense.However,due to the coverage of thick sedimentary layers,the activity and deep structure of some concealed faults in North China are difficult to explore through traditional geological and geophysical exploration methods.Therefore,detecting and accurately locating small seismic events has become one of the feasible methods for characterizing the shallow crustal structures,including concealed faults.How to make full use of the data obtained since the digital seismic observation in North China,and use artificial intelligence technology and precise location technology to obtain more detailed spatio-temporal distribution characteristics of micro-seismicity in the region,investigate the seismic structure of North China,especially the historical strong earthquake areas,is an important topic with significant scientific value and seismic disaster reduction significance.Therefore,this paper focuses on the identification of shallow crustal active structures in North China with thick sedimentary layers.It systematically compiles seismic observation reports in North China from 2008 to 2022,forming a relatively complete seismic observation data to date.Using the double difference algorithm for relative location,the paper obtains the relocation results of small earthquakes in North China.Furthermore,the paper analyzes the spatio-temporal migration characteristics of seismic activity in North China and the geometric features of fault structures in the Tangshan 7.8 earthquake in 1976.In addition,the paper explores the hidden faults in the North China basin with thick sedimentary layers using an artificial intelligencebased micro-earthquake detection method.The continuous waveform records of the Hebei Seismic Network from August 2021 to July 2022 were processed,and the generated earthquake catalog was compared with the manual earthquake catalog.The main scientific insights include:Firstly,precise location results for 18,030 earthquakes in the North China region over the past 14 years are obtained.The average location errors in the eastwest,north-south,and vertical directions are 0.15km,0.15km,and 0.18km,respectively,with an average location residual of 0.096s.The focal depths are mainly concentrated between 6 and 15km,and 95%of the earthquake focal depths are less than 20km.The depth of the brittle-ductile transition zone in the North China region is approximately 20km.Secondly,the precise positioning results reveal that the Zhangjiakou-Bohai seismic belt,the North China Plain seismic belt and the Shanxi rift basin seismic belt have obvious stripe-like distributions,and the depth profile of the epicenter shows that the fault has a steep dip angle and is dominated by strike-slip.The dip angle of the faults in Tangshan area is generally steep in the deep part,and the activity level of each fault is different.The Tangshan-Guye fault has a significant change in trend,with the north section tilting southeastward and the south section tilting northwestward.The Lulong fault shows strong seismic activity,and there is a seismic sparse area between Tangshan-Guye fault and Luanxian-Leting fault.Thirdly,we explored the feasibility of using artificial intelligence to detect micro-earthquakes in areas with thick sedimentary layers,to determine the hidden fault structures and activity.By detecting one year of continuous seismic wave records in North China,a large number of seismic events were identified,which is 6 times the number identified by manual cataloging.The consistency with the manual catalog is relatively high in both horizontal and depth directions,and it presents a clear stripe-like distribution along active faults.The automatically generated catalog also identified a large number of events that were not processed by the human seismic catalog in areas such as Chengde and Taihang Mountain area,providing new clues for studying the tectonic earthquakes and non-natural seismic activity in this region.