利用小震分布和区域应力场确定临潭—宕昌断裂断层面参数

Determination of Fault Plane Parameter of Lintan-Tanchang Fault Using Distribution of Small Earthquakes and Regional Stress Field

首先结合Hyp02000单事件绝对定位方法和双差定位方法对临潭-宕昌断裂附近的小震进行精确定位;然后采用模拟退火全局搜索和高斯牛顿局部搜索相结合的方法分别估算了该断裂不同位置断层面的走向、倾角、位置及相应的误差,并基于局部应力场参数反演了各断层面上的滑动角.反演的断层面走向NW-NWW,倾向NE-NEE,倾角71°～86°,滑动角102°～ 132°.本文给出的断层面走向、倾向和倾角与已有研究结果相似,但断层错动类型与已有结果分别为右旋走滑和左旋走滑,存在区别.初步分析认为,小震震源机制解给区域应力场参数反演结果带来了误差,进而影响了断层面上滑动角的反演误差.

First,we precisely relocated small earthquakes near the Lintan-Tanchang fault using a combination of the Hypo2000 single-event absolute positioning method and the double difference location method.The biggest advantage of the first method is that we can use a multi-velocity model,which is in line with complex underground structures and crustal heterogeneity.Relative to using only one velocity model in the traditional positioning method,the use of the multi-velocity model can improve the near-field positioning precision in a large part.Further,the biggest advantage of the second method is that it can eliminate the influence of the underground velocity structure on the relative position of the earthquake; subsequently,we can obtain an accurate relative position between the two earthquakes.Then,we divided the small earthquake distributed in the northern part of the Lintan-Tanchang fault into two segments,which are the east and west segments,respectively,according to the distribution of the relocated small earthquake,location of the fault,and distribution map of a medium-strong earthquake＇ s intensity.Further,in consideration of three similar focal mechanisms of the Minxian M5.2 earthquake on November 13,2003,Minxian M5.0 earthquake on September 7,2004,and Minxian M6.6 earthquake on July 22,2013,we combined the two segments together into one segment.Then,we separately estimated the strikes,dips,location,and corresponding errors of fault planes at different locations by simulated annealing and Gauss-Newtonian nonlinear inversion algorithms.It can not only search the global optimal solution but also estimate the parameter errors.Further,we can estimate the slip angle on the fault by using the parameters of the regional tectonic stress field after a large earthquake fault plane.Further,we inverted the rakes of the different fault planes by using local stress field parameters.The strike,trend,dip angle,and rake angle of the inverted fault plane were found to be NW-NWW,NE-NEE,71°～86°,and 102°～132°,respectively.Because of the small number of earthquakes in the study area,the error values of the strike angle,dip angle,and distance from the origin of coordinates for the inverted fault plane are too large.The error of the inverted dip angle is larger than that of the inverted strike angle owing to greater dispersion of small earthquakes on the fault vertical.Further,the error of the slip angle is much larger.The strike,trend,and dip of the fault plane inverted in this study are similar to those in previous studies and also similar to the fault parameters displayed on focal mechanism of the three medium-strong earthquakes.However,the fault types differ between this study and previous studies,which are separately dextral and sinistral strike slip.Further,the fault-type considered in this study differs from that displayed on the focal mechanism of the three medium-strong earthquakes.It was primarily concluded that an inversion error of the regional stress field parameters exists because of focal-mechanism solutions of small earthquakes,and this error influences the inversion error of the rake angle on the fault plane.However,the inverted slip angle in this study is comparable to that reported in previous studies.