Microseismic Detection And Seismicity In The Middle Section Of The Northern Tianshan Region

The North Tianshan Seismic Belt in China has complex seismogenic structures and typical thrust-fold structures with high risks of strong earthquakes,which is an ideal place to study inland earthquake.In recent years,the Shawan Ms 5.0 earthquake in 2015,the Hutubi Ms 6.2 earthquake in 2016 and the Jinghe Ms 6.6 earthquake in 2017 occurred successively in this region,causing a number of people injuried and buildings destructed.However,the seismic network in North Tianshan is relatively sparse and lack of the ability to monitoring micro-seismicity;hence the completeness magnitude in earthquake catalogue is relatively high.In the region of the middle section of the North Tianshan Mountains(about 43.5 N ~ 44.5 N;85 E ~ 87.5 E),1D velocity model was refined,and earthquakes from January 2014 to September 2018 listed in the seismic catalog were relocated with hypo DD software.Next,event waveform templates were constructed based on relocated earthquakes and applied to scan continuous waveforms using waveform match filter technique.Then on the basis of newly obtained catalog which is more complete and better relocated,the seismicity,seismogenic structures,and earthquake trigger and migration characteristics in North Tianshan region were discussed.The main results of this study are as follows:1.The number of earthquakes detected by the match filter technique is 10 times larger than the CENC(China Earthquake Networks Center)catalog,and the complete magnitude drops significantly from 1.2 to 0.5.2.The seismic events occurred in recent years in the study area mainly distributed in the shallow parts of the Huoerguosi-Manasi-Tugulu anticlinal belt and deep parts in South Manasi-Qigu anticlinal belt,and the their distributions along the strike of anticlinal tectonic belt are unevenly.The seismicity in the western Huoereguosi-Manasi-Tugulu anticlinal belt is more active than that in the eastern part.3.There are many seismic events in the Nannanasi-Qigu tectonic anticline belt occurred on the "blind faults" within the folds.The fault system consisting of the southern margin faults of Junggar,blind faults in the thrust fold zone,slip planes and shallow fault slopes,have bred active seismicity in the piedmont of the North Tianshan Mountains,and may have the potentials for large earthquakes in future.4.The Hutubi earthquake in 2016 has a significant impact on the seismicity in the surrounding area.The simultaneous enhancement of seismicity in Liuhuanggou area is presumed to be caused by the dynamic triggering of Hutubi earthquake.The swarm activity in Shitizi area during May 2018 may be the delayed trigger of the stress state change caused by Hutubi earthquake.