Seismological Study On The Relationship Between The Xianshuihexiaojiang Fault System And The Honghe Fault Zone

The southeastern margin of the Qinghai-Tibet Plateau due to its special geographical location,internal stress field characteristics are complex,is the area of today's earthquakes and tectonic activity.Currently,there are two main types of dynamic models on the southeastern margin of the Qinghai-Tibet Plateau: First,under the right-handed action of the Honghe fault zone,Regional fracture activity and strike-slip fracture caused by lateral extrusion of blocks;Second,under the action of the Xianshuihe-Xiaojiang fault system,the Sichuan-Yunnan block rotates clockwise around the East Himalayan structural knot.The key to the two models is whether the XianshuiheXiaojiang fault system has crossed the Honghe fault zone and formed an eastern boundary that runs through the southeastern margin of the Qinghai-Tibet Plateau.The research area of this paper is located in the Yunnan-Burma region on the southeastern margin,it is a key area for understanding and studying the clockwise rotation pattern of the southeastern margin of the Tibetan Plateau.The Dien Bien fu fault zone,the Lancang-Gengma fault zone,the Honghe fault zone and the Xiaojiang fault zone are mainly developed in the area.According to GPS data and the established Sichuan-Yunnan knob configuration system: Xianshuihe-Xiaojiang fault system since the Pliocene(about 5.3 Ma),with the left-handed activity of the Dien Bien Fu fault zone began as an escape boundary for the division of the northern SichuanYunnan block and the southern Simao block,this time is about the same as the time when the Honghe fault zone begins to turn right.However,since the Quaternary,the Honghe fault zone and its right-handed shearing activity have been relatively weak,Its regulation is also reduced,the leftlateral rotation of the Xianshuihe-Xiaojiang fault system along the southeastern margin of the Qinghai-Tibet Plateau is relatively enhanced.Based on the results of previous studies,this paper attempts to explore the relationship between the Honghe fault zone and the Xianshuihe-Xiaojiang fault system from the perspective of seismology,whether the southern section of the Xiaojiang fault zone passes through the Honghe fault zone,it is the key to the two dynamic models of the southeastern margin of the Qinghai-Tibet Plateau.First of all,this paper collected 24543 earthquakes of ML?0.5 earthquake from January 1,2013 to October 31,2017 in the earthquake observation report of Yunnan Province.Due to the spatial distribution and seismic phase data of seismic stations and their errors during initial positioning,Monitoring capacity in the study area remains to be verified.The use of incomplete seismic catalogues has certain errors in the determination of seismic activity in the study area,therefore,the FMD method was used to calculate the minimum complete magnitude of the study area Mc=1.3,according to Mc,the complete earthquake catalogue in the study area is obtained.The determination of the minimum complete magnitude provides a reference for the selection of the lower limit of seismic precision.According to Mc screening 15208 earthquakes with ML?1.3 earthquakes in the study area.After removing the phase anomaly value,contains 86254 P waves and 81951 S wave phases recorded by 46 stations.During pretreatment,at least 4 phase records are recorded for the events participating in the positioning,the distance between events is less than 15 km and the distance between the event and the station is less than 400 km.Three different speed models(AK135,PREM and one regional velocity model)simultaneous positioning using the conjugate gradient method(LSQR).The final result shows that,using the PREM velocity model has better positioning results,a total of 11,531 relocated earthquakes were obtained,the positioning errors are 0.095 km in the E-W direction,0.092 km in the N-S direction,and 0.095 km in the vertical direction.After relocation,the earthquakes in the study area are relatively clustered,which is of great significance for studying the deep structure of faults.b value can reflect the stress state and uniformity of the medium.Divide the study area into a grid of 0.15° × 0.15°,and calculate the b value in the grid.The b value calculated after relocation is increased compared with that before positioning,this may be the result of discrete earthquakes tending to cluster during the positioning process.Seismic energy and density distribution,to a certain extent,it depicts the spatial distribution of seismic activity in recent years in the study area.Analysis of calculation and spatial distribution characteristics of seismic density and seismic energy,it can be clearly determined that there is an anomalous zone of small earthquake activity that is substantially perpendicular to the distribution of the Quaternary fault in the study area.mainly along the NE direction,across the fault of NW direction,from the middle section of the Xiaojiang fault zone to the south side of the Lancang-Gengma fault zone,cut through the Honghe fault zone.Known from the three seismic density profiles of the anomalous zone,the source depth of the anomaly is relatively uniform,and shows a deeper structural belt spanning three sections,most of the small earthquakes are concentrated in the shallow part,it shows that the shallow rupture is relatively active.The complex stress field characteristics in the study area are the focus of long-term research.Early research results mainly reveal the characteristics of stress field in this area from moderate to strong earthquakes,there are a large number of small earthquakes in the study area,the rupture may be affected by local or regional stress field characteristics.Under the leadership of large-scale stress characteristics,there are also some differences within it.Therefore,this paper is based on the results of seismic relocation,continuous waveform recording of fixed stations in Yunnan Province from January 2013 to June 2017 recorded by China Earthquake Network Center,interception of waveforms 10 s before the earthquake and 50 s after the earthquake,a total of 5,508 event waveforms were obtained,relocation results(latitude,longitude,magnitude,and time)are written into the waveform data using SAC.On the premise of removing the instrument response,according to the gCAP method of large earthquakes in the study area,the results of the focal mechanism solution are compared with other institutions,continuously optimize sampling frequency,filtering range and Pnl wave weight,determining the inversion parameters of small earthquakes.Finally,the focal mechanism solution of 5466 small earthquakes was obtained.Comparison of the inversion results of the same magnitude(ML?2.0)in the same study area(23°~25°N,101°~103°E)as the predecessors,have good consistency,the statistical results can determine that the overall stress field in the area is mainly tension.Combine the results of previous earthquakes with other institutions,it shows that the results obtained in the inversion of small and medium earthquakes are relatively reliable,the obtained focal mechanism solution data can basically explain the local stress field characteristics of the region.To further understand the overall stress field characteristics of the study area,calculation of focal mechanism solution data in the study area using damping stress inversion method(MSATSI).The damping stress inversion method should first select the appropriate damping coefficient,the optimal damping coefficient of the study area obtained by the MSATSI program is 0.9.Then divide the study area into a grid of 0.15° × 0.15°,calculating the tectonic stress tensor.The calculation results of the regional overall stress field indicate: the non-uniform characteristics of the stress field are more obvious,may be related to the magnitude of the earthquake used in this paper.Due to the small earthquake focal mechanism solution presents a very large diversity,therefore,the earthquake may evenly rupture at each orientation level.The focal mechanism solution of a single small-tosmall earthquake has relatively few data to solve during inversion,and interference is greater,but the small and medium earthquakes are far more than the strong earthquakes.The 1970 Tonghai M7.7 earthquake and the 2014 Jinggu Ms6.6 earthquake are strong earthquakes that occurred on both sides of the Honghe fault zone in recent years,its fracture properties are affected by the regional stress field.Description of seismic stress field characteristics and fault plane parameters of Tonghai earthquake and Jinggu,two earthquake-induced faults--NW to the right-handed Qujiang fault and the NW-to-new right-handed fault,the current stress field characteristics are dominated by tensile shear.Combining the above calculation results,the following inferences are made about the overall relationship between the study area and the Xianshuihe-Xiaojiang fault system and the Honghe fault zone:(1)Xianshuihe-Xiaojiang fault system intersects with the Honghe fault zone,earthquake activity is concentrated and seismic energy is concentrated,after crossing the Honghe fault zone,the seismic density and energy distribution only weakly decrease.The statistical results of the focal mechanism solution are similar,both are normal-slip-slip,and their stress field characteristics are consistent.It is indicated that the southern section of the Xiaojiang fault zone has not been affected by the Honghe fault zone,and it has continued for a length after crossing the Honghe fault zone.(2)The seismic density distribution on both sides of the Dien Bien Fu fault zone is similar,the stress field features are consistent and the sliding rate is small,the seismic activity caused by the fault extension after the Xiaojiang fault zone passes the Honghe fault zone is quite different.Therefore,the Dien Bien Fu fault zone is not an extension fault of the Xiaojiang fault zone.The Dien Bien Fu fault zone and the two sides of the zone should be included as a whole into the outer boundary block in the clockwise rotation dynamics model of the southeastern margin of the Tibetan Plateau.(3)along the NE direction,starting from the middle section of the Xiaojiang fault,it trans-qujiang fault,the Shiping-Jianshui fault zone,the Honghe fault zone,the Ailaoshan fault zone,the Wuliangshan fault zone and the southern section of the Zhiyu-Puma fault zone,the seismic density and energy distribution are similar,and the R value is stable,the profile density distribution shows that there is a structural belt with deep focal depth inside.This should be used as the eastern boundary of the clockwise rotational dynamics model of the Qinghai-Tibet Plateau in this study area.