Present tectonic stress field in the Circum-Ordos region deduced from composite focal mechanism method

With more and more densely seismic stations deployed,a large number of P wave firstmotion data are observed,which provides us an opportunity to further study the tectonic stress field.In this paper,by using the 49844 Pwave first motion polarity data from 8499 earthquakes,which occurred in and around the Ordos block from August 1,2007 to July 21,2013,we obtained the precise and detailed stress field in the study area.The composite focal mechanism method was used in this study.By fitting the focal mechanism radiation pattern with P wave polarity data from a large number of earthquakes,this method can obtain the composite focal mechanism in specific grid,and P,B,Taxes can be regarded as the principal stress axes of the tectonic stress field.The grid search method is used to fit the focal mechanism radiation pattern,the focal mechanism which has the minimum contradictory ratio was regarded as the optimal solution.The inconsistent ratio is the ratio of the number of P wave first motion polarity which is inconsistent with the focal mechanism and the total number of P wave first motion polarity data,which is weighted by the distance of the earthquake epicenter to the grid point.The study results show that the stress field has the following characteristics:First,in the rift zone of the Yinchuan-Jilantai,Hetao,Daihai,Shanxi,and Weihe around the Ordos block,the main types of composite focal mechanism are normal types,and the strike of the nodal planes is consistent with the strike of the main fault which controls the boundary of the rift zone,that is,in accord with the extension regime in the rift zone around the Ordos block.Second,the main types of the composite focal mechanism are thrust,thrust-strike-slip and strike-slip types in the southwest boundary of the Ordos block,which reflects that the Ordos block is subjected to the northeastward squeezing of the Qinghai-Tibetan plateau.The principal compressive stress direction is eastward in the distance from the southwest boundary of Ordos block,that may derived from the northeastward squeezing of the Qinghai-Tibetan plateau,the principal compressive stress direction turns to NE-SW direction when near the Ordos block.Third,the Paxis azimuth shows generally regular changes except in some local areas.From south to north,the P-axis azimuth becomes more to the north direction in the west boundary of the Ordos block.From west to east,the P-axis turns more to the east direction in the north boundary of the Ordos block.In the south and east boundary of the Ordos block,the P-axis azimuth has small change and is parallel to the main fault′s strike which controls the boundary of the rift zone.The dip angle of the P-axis is nearly horizontal in the southwest boundary of the Ordos block,and nearly vertical in the basins around the Ordos block.Fourth,the T-axis azimuth is NW-SE overall.The T-axis is generally perpendicular to the strike of the main fault which controls the boundary of the rift zone and the strike of the basin in the basins around the Ordos block.Fifth,under the NE pushing force from the Qinghai-Tibetan plateau and the NW-SE extension caused by the uplifting of the deep substance,the regions around the Ordos block are shear zones with extensional components,except for the compressional region in the southwest of Ordos-block;these results are consistent with the previous studies which suggest that the Circum-Ordos block is in extensional stress field,also better explain the fault basins surrounding the Ordos block,also accord with the formation of right-lateral shear zones with extension components in its western and eastern boundary and left-lateral shear zones with extensional components in its southern and northern boundary.Overall,we obtained the stress field around the Ordos block,and its general characteristics are consistent with the results of previous study,our results are more consistent with the basin structure around the Ordos block.In this study we focus on the general characteristics of our results.In the further study,we will pay more attention on the inconsistency of our study with the previous study,try to find out the reason and give a reasonable explanation;and pay more attention on the phenomenon of sudden changes in the direction of the stress axis,try to find whether it caused by the inhomogeneity of stress field,or the complex geological structure,or other reasons.