Inversion Of The Activity Characteristics Of The Red River Fault Zone Using GPS Data

As a large strike-slip fault zone across Yunnan Province,the Red River fault zone plays an important role in the geological structure of Asia.However,the Red River fault zone is weak in seismicity currently,and whether it is locked or releasing energy through creeping at present is a concern of the academic circles.In order to study the activity characteristics of the Red River fault zone in recent years,the GPS measurements from 1999 to 2014 in the surrounding blocks of the Red River fault zone are modeled by block model,and the rigid block rotations,internal strain rates,fault slip rates and three-dimensional locking degree are obtained.The inversion results show that the block model considering internal strain can significantly improve the fitting effect to the GPS velocities,the inverted slip rates of the Red River fault are in good agreement with the results of the velocity profiles.The locking degree of the Red River fault is roughly symmetrical that the middle section is relatively strong,and the southern and northern segments are weak.The locking degree of the southern,middle and northern sections are about 0.34?0.52,0.67?0.75 and 0.26-0.53 respectively from the surface to 15km depth,and the dextral slip rate deficit were 1.7 mm/a,3.0 mm/a and 1.4 mm/a,respectively.The checkerboard test is used to analyze the sensitivity of the inversion results to the GPS distribution.The results show that the GPS data can resolve the locking signal of each node in the fault structure of this paper.In order to improve the data resolution,new GPS stations should be added to the north side of the middle segment where GPS station districted sparsely.The distribution of strain field in the study area is calculated by the least squares collocation,it is found that the Red River fault zone is converted from compression in the south to tension in the north.The middle section is in the transfer zone of compression and tension,and its shear strain is lower than the northern and southern sections,which may be due to the stronger locking degree of the middle,so that the fault dislocation is smaller.Earthquakes larger than Mw 5.0 in the same period are mainly distributed near the boundary of high and low values of the strain,indicating that the inhomogeneity of strain accumulation may lead to the preparation and occurrence of earthquakes.Based on the inversion results of slip rate deficit and historical strong earthquake catalog of the Red River zone,the accumulation and releasing of seismic moment are calculated.The results show that,the accumulated and released amount of moments in the northern section are almost in balance.The moment magnitude corresponding to the seismic moments accumulated in the southern and middle sections since 1839 have reached 6.98 and 7.12 respectively.We have collected earthquakes of M?2.0 in the study area since 1965,the b-value map of the study area was calculated according to the G-R relationship.The results showed that the distribution of b-value in the Red River fault zone is quite different.In the southern section of the Red River fault zone,there is a low b value zone of about 25km,and the rest of the southern segment has a high b value.In the middle section,there is a large low b value zone in the southern part of Chuxiong,which coincides with the location of the high-locking zone derived by block model,indicating that the stress and strain accumulation of the middle section is relatively large,and the seismic hazard is relatively high.The b value in the northern section is relatively high,indicating that the stress accumulation near the northern section of the Red River fault zone is not obvious.