The Relationship Between Strain Accumulation Characteristics And Seismic Moment Release On The Southwestern Margin Of Ordos Block

The southwestern margin of Ordos block is located at a special tectonic region,which is the northeastern frontier of the Tibet Plateau.In order to analyze the spatial pattern of the strain accumulation and the associated seismic hazard in this area,we conducted a GPS profile across the Liupanshan fault.The GPS data of the region from 2009 to 2017 was collected,we can get the GPS velocity field with high spatial resolution by using science processing method,the values of extrusion and strike slip of Liupanshan fault are obtained by GPS profile.In addition,in this paper,the theory and procedure of calculate GPS strain rate field by multi-scale spherical wavelet method.The multi-scale spherical wavelet method was employed to solve for the strain rate field,meanwhile,the distribution of different strain rate field in the southwestern margin of Ordos block is analyzed.The Mw?5 historical earthquake catalogues from 1495 to 2015 in the southwestern margin of Ordos block were counted,and the integrity of the 520-year historical earthquake was analyzed.The seismic hazard through was quantificationally evaluated through the subtraction of the seismic moment release rate generated by the earthquakes from the seismic moment accumulation rate obtained from geodetic data.The value represents remaining seismic moment rate.The effects of different partition methods on results are also discussed.According to the formula,combined with the calculated seismic moment accumulation rate,the regional seismic recurrence period was analyzed,and the covariance propagation law was used to estimate the error of the seismic recurrence period.Based on the relationship between horizontal strain rate and vertical movement rate,the possible uplift mechanism of the southwest margin of Ordos was discussed.The results show that the velocity field in the southwestern margin of Ordos block is dominated by eastward and southeastward movements with respect to the stable Eurasia plate,with a value of 5-10 mm/yr.The average left-lateral strike slip of Liupanshan fault was 0.58 ± 0.33 mm/yr,and the extrusion shortening rate was 0.81 ± 0.34 mm/yr;The maximum shear strain rates were significant in the area around Minxian and Zhangxian,on both sides of the Haiyuan fault and in the area from Longxian to Tianshui.It can be concluded from the rotation rate field that the southwest margin of ordos has a trend of counterclockwise rotation.Since 1920,the remaining seismic moments on the southwestern margin of the Ordos block had continuously accumulated indicating the deficit status of the seismic moment,the value of it about 4.2755×1020 Nm.Thus,inferring the possibility of the Mw?7 earthquake.To be specific,the seismic moment of Haiyuan fault was in the surplus status,indicating lowseismic risk.While,the seismic moment in Liupanshan,the Longxian-Baoji fault,and the northern margin of the West Qinling fault were in the deficit status,indicating high seismic risk with the possibility of Mw ~7 earthquakes.Assuming that the crust is incompressible,in the Airy isostasy,the uplift rate of the GPS horizontal strain is-0.3-0.2 mm/yr,and the equilibrium uplift rate is much smaller than the vertical velocity(1-2 mm/yr)observed by the level data and GPS,which indicates that the horizontal extrusion has a small contribution during the vertical uplift of the southwestern margin of Ordos block.