| Shanxi rift zone,as an important active tectonic belt in North China,has frequent historical strong earthquakes,serious earthquake disasters,and modern small and medium-sized earthquakes are active,which is one of the important earthquake danger areas in China and is also a hot spot for many seismologists to pay attention to and study.The breeding of earthquakes is closely related to the occurrence and regional tectonic stress field,and by studying the focal mechanism and regional tectonic stress field in this area,it is helpful to understand the seismogenic background,seismogenic mechanism,and tectonic deformation of the region.Based on literature reading and data search,this paper collects and collates the focal mechanism of the Shanxi rift zone and its adjacent areas(34°-42°N,108°-115°E).The focal mechanism solutions of 697 earthquakes with M_L?2.5 in the Shanxi rift zone and its adjacent areas from January 1929 to October 2018 were obtained through careful analysis and screening of data.Based on the above data,the characteristics of focal mechanism solutions were analyzed.Based on the focal mechanism solutions of 367 earthquakes with M_L?3.0 and 94 earthquakes with M_L?4.0,the stress field in the study area was inversed by using the SLICK method and grid search algorithm commonly used by regional stress field inversion,and the following understanding was obtained:(1)The focal mechanism types of the Shanxi rift zone and its adjacent areas are mainly strike-slip and normal fault types,which are consistent with the tectonic activity characteristics of the Shanxi rift zone.Among 697 focal mechanism solutions of earthquakes with M_L?2.5,207 are strike-slip earthquakes,accounting for 29.7%of all seismic events;176 are normal fault earthquakes,accounting for 25.3%;50 are normal strike-slip earthquakes,accounting for 7.2%;72 are reverse fault earthquakes,accounting for 10.3%;24 are reverse strike-slip earthquakes,accounting for 3.4%.The seismicity is mainly related to the surrounding active structures.Most of the earthquakes are mainly distributed in the fault zones in the region,and most of them are concentrated in the intersection of multiple faults.The number of strong earthquakes is less,and the number of medium and small earthquakes is more.The shallowest focal depth is 1km,and the deepest focal depth is 35km.All of them belong to shallow earthquakes,and the seismic events with a focal depth of 5-20km account for the vast majority,accounting for 75.6%of all seismic events.(2)Based on the focal mechanism solution data of 367 earthquakes with M_L?3.0,the stress tension inversion was carried out by SLICK method.The results show that the optimal solution of the strike of the maximum principal compressive stress axis in the study area is towards NEE53.4°and the dip angle is 21.5°;The strike of the middle principal stress axis is towards SWW-120.1°and the dip angle is 68.3°;The strike of the maximum principal tensile stress axis is towards SSE144.2°and the dip angle is nearly horizontal(2.2°).(3)Based on the focal mechanism solution data of 94 earthquakes with M_L?4.0,the stress tension inversion was carried out by SLICK method.The results show that the optimal solution of the strike of the maximum principal compressive stress axis in the study area is towards NEE46.1°and the dip angle is 48°;The strike of the middle principal stress axis is towards SWW-132.2°and the dip angle is 41.9°;The strike of the maximum principal tensile stress axis is towards SSE136.9°and the dip angle is nearly horizontal(0.8°).Based on the focal mechanism solution data of 94 earthquakes with M_L?4.0,the stress tension inversion was carried out by the grid search algorithm.The results show that the optimal solution of the strike of the maximum principal compressive stress axis in the study area is towards NEE57.97°and the dip angle is 24.27°;The strike of the middle principal stress axis is towards SWW284.00°and the dip angle is 57.00°;The strike of the maximum principal tensile stress axis is towards SSE157.91°and the dip angle is 20.94°.The results of the SLICK method and the results of the grid search algorithm are basically the same.At the same time,there is a certain gap between the results and the inversion results based on earthquakes of magnitude 3 or above,but they are comparable.(4)The results of the comprehensive inversion of this paper show that the Shanxi rift zone is mainly affected by NNW-SSE tensile stress and NEE-SWW compressive stress,and the stress state is mainly tensile stress,which is consistent with GPS observation data.At the same time,the results show that the regional tectonic stress field of the Shanxi rift zone is basically consistent with that of North China,which further proves that the former is constrained by the latter.The research results of this paper can provide a theoretical basis and reference for the scientific and engineering problems such as the preparation and prediction of strong earthquakes,the evaluation and prevention of seismicity and earthquake disasters,and the seismic fortification of earthquake engineering in the Shanxi rift zone. |
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