The Study Of The Typical Seismicity In Western Region Of Jilin Province

The western part of Jilin Province is the central depression of the central fault belt of Songliao Basin,and the geomorphology is a broad plain area.The boundary faults of the depression area are developed,there are Nenjiang fault in the west,Yilan-Yitong fault in the eastern boundary,and Chifeng-Kaiyuan fault in the southern boundary.There is a Neamohe fault in the northern boundary.The basement faults are well developed in the region.There are more than a dozen faults in the NE and NW directions.In recent years,the seismicity in the region is mainly distributed near the two faults.Songyuan-Zhaodong fault and the Di-erSonghuajiang fault,which is also the oil field exploitation area,has expoited for many years.Since the establishment of the Jilin Province Seismic Network in 1970,the seismic activity in this region has been relatively weak,but the seismic activity has increased significantly since 2003.Small scale earthquake swarms have occurred in Songyuan City,and in Gan'an County and Qianguo County,Several moderate earthquakes occurred in Tongyu County.In the history of this region,6?earthquake occurred on 1119.Songyuan City is the only seismic intensity?degree fortification area in Jilin Province.Therefore,the focal mechanism and future regional seismic risk of earthquakes have attracted great attention from seismologists.In this paper,we focus on the focal mechanism of two moderate earthquakes,the reorientation of earthquake sequence and the characteristics of local regional stress field,and discuss the tectonic mechanism of seismicity and the relationship between seismic activity and nearby faults.The b value of the earthquake sequence is studied,then the M_S 5.8 earthquake swarm in 2013 and the Songyuan M_S 4.9 earthquake in2017 are relocated by using the double difference method.At last,the moment Zhang Liang inversion of the larger earthquake and the inversion of the regional stress field by using the focal mechanism are carried out.Using the catalogue of earthquake sequence provided by Jilin Seismic Network Center and based on the magnitude frequency relationship of Gutenbau-Richter,the catalogue integrity of three significant earthquake sequences in western Jilin Province is analyzed,and the method of magnitude range of integrality is adopted.Based on the maximum likelihood estimation method,the complete catalogue and b value of the magnitude of different lower limits are roughly determined by fitting the values of seismic frequency and b,and the minimum complete magnitude M_C is considered to be M_L1.0,b value equal to 0.6⁰.8.The M_S 5.8 swarm sequence of Qianguo,Jilin Province,2013 and the MS 4.9earthquake sequence of Ningjiang River,Songyuan,Jilin Province on 2017 were relocated by using the program HypoDD.The sequence of M_S 5.8 earthquake in Qianguo,Jilin Province,provided by the Jilin Province Seismic Network Center was used to relocate the sequence.The observation report on the M_S 4.9 earthquake sequence of Ningjiang,Songyuan,Jilin,2017,The first arrival phase of Pg?Sg in the observation report is checked,and the seismic phase of the larger earthquake in the sequence is definitely located when the earthquakes with large travel time difference are picked up again.A more accurate spatial distribution image of earthquakes is obtained by relocating the earthquake swarm sequence by using the method of double difference location.Combined with the sequence characteristics,the seismogenic faults and tectonic mechanism of the earthquake swarm are analyzed.It is inferred that the thrust nappe in the middle and upper crust of Songliao Basin is the seismogenic structure of the M_S 5.8 earthquake swarm before 2013.There is a double-layer nappe structure on the lower slip surface in the epicentral area.Due to the typical shovel shape of the impact section,the depth is straight,the inclination angle near the surface increases,and the angle between the principal compressive stress P axis and the strike of the fault and the inclination space is relatively small.The rupture characteristics of all the strong earthquakes in the sequence are dominated by thrust.The focal mechanism of the 2006 M_S 5.0 earthquake is the same as that of the2013 earthquake.It can be seen that both of them are caused by the nappe thrust tectonics in the middle and upper crust of Songliao Basin,and the Fuyu Zhaodong near the epicentre area.There is no relationship between Chagan Bubble-Dao well fault.Due to the increase of fault inclination near the surface,the fault is staggered along the compressive stress axis,and the local stress can be released after the movement of M_S 5.8 earthquake swarm,but the regional stress will continue to increase.In the future,the seismic risk in this area and its vicinity will be further upgraded.Because the Qianguo 6 earthquake occurred within 100 km in the history of the epicentral area,whether the strong eruptive swarm activity on 2013 is a precursor to the recurrence of historical earthquakes,In the future,it is necessary to pay close attention to the triggering effect of tectonic events in the western Pacific,Japan and surrounding environments on earthquakes in the Songliao Basin.The repositioning results of the Ningjiang earthquake sequence in Songyuan Jilin Province on 2017 show that the aftershocks are distributed in a strip along the NW direction with focal depth concentrated at 810 km.One of the focal mechanisms is also NW.In line with the Fuyu section of the second Songhua River fault near the epicenter,it is obvious that the seismogenic structure of this earthquake should be the second Songhua River fault.The focal mechanism solutions of 5 earthquakes with M_S?5 or more in the front M_S 5.8 earthquake swarm are obtained by using the inversion study of the moment Zhang Liang.The prominent characteristics of the Samu_person moment solution are the large non-DC component and the shallow moment center depth.The high non-DC component indicates that the rupture mechanism of the earthquake contains more non-fault dislocation factors.The large ISO component and shallow focal depth in Moment tensor solution of seismic moment are the main characteristics of induced earthquakes in mining area.The induced earthquakes are mostly collapse or explosive focal mechanism,and the volume of focal area changes greatly.Usually,the non-DC component of the moment Zhang Liang result is used to judge.According to the published research results of the focal depth of the 2006 M_S 5.0 earthquake in Gan'an,Jilin Province,the focal depth of the five times M_S?5.0 earthquakes of Guo before 2013 is synthesised.It is considered that the M_S 5.8 earthquake swarm before 2013 is not a typical tectonic earthquake.In 2017,the inversion of M_S 4.9 seismic moment Zhang Liang in Songyuan,Jilin Province,was studied,and their focal mechanism solutions and uncertainty evaluation of parameters were obtained.The center of moment is a typical earthquake triggered by fault shear dislocation with a depth of 6 km,which is essentially different from the focal mechanism of the M_S 5.8 earthquake swarm in Qianguo,Jilin province on 2013.Based on the waveform data of regional seismic network and the mobile stations of the epicenter zone,the focal mechanism solutions of the 37 earthquakes from QianGuo M_S 5.8 earthquake swarm in Jilin Province were obtained by using the method of the first motion of P waves and P/S amplitude ratios.Firstly,the three stress axes?P,T,B?of the all earthquakes were projected to Frohlich triangle plot and the earthquake types are classified by their focal mechanism solutions,results show that in the earthquake sequence there are four earthquakes in the M_S?5.0 earthquakes fall in the thrust earthquake zone and the others mainly fall in the strike slip zone in the plot.Due to the earthquake focal mechanism solution reflect directly the stress state characteristics in the source region,in order to determine the detailed variation characteristics of stress state in focal region,all of two nodes of the all focal mechanism solutions were used as input,and multiple stress field inversion method is applied,namely,in the Euclidean space,the reduced stress tensors of different cluster centroid from nodal planes of the focal mechanisms was obtained by resampling method and k-mean algorithm of cluster analysis,and it is used to represent the stress state of clustering of focal mechanism solutions.According to the reduced stress tensor stress angular distance we can judge the similar degree between different stress state,thus a study is done to determine the characteristics of different stress state in the source area and the mechanism of earthquake formation.The results show that there are two significant stress states in the source area,their azimuth angle and plunge angle of the principal stress axes and stress ratio are:^?1:79.1^?/1.2^?,?₃:169.7?/22.9?,?37?=0.69;₁^?:80.4?/1.3?,₃^?:349.7?/31.1?,?37?=0.42,the stress angular distance=54.36°and their similar degree is very low.According to Venn diagram shows the former independent triggers the four M_S5.0 earthquakes,the latter play a role in the aftershock sequence.The larger earthquakes perturbed the dominative stress field of region and triggered the aftershock is different from the main shock in focal mechanism solution type,it lead to the inconsistency of focal mechanism solutions in whole earthquake sequence.