Study Of Activity Characteristics Of Main Faults In The Northeast Margin Of The Tibetan Plateau Using GPS

This paper researched interseismic locking characteristics and slip deficit rates of partial main faults in the northeast margin of the Tibetan plateau using GPS data over 1999–2015.The seismic risk of the dangerous faults was evaluated quantitatively.The primary results are drawn as follows:(1)Using GPS velocity field data,based on rigidity block model,the slip rate of Haiyuan fault,Liupanshan fault,Longxian-Baoji fault and West Qinling North faults was calculated,including components parallel and vertical to fault.Haiyuan fault's strike slip rate was about 6mm/a with a thrust extrusion rate 1.0 ± 0.4 mm/a–2.6 ± 0.2mm/a,showing that Haiyuan fault is dominated by left-lateral strike slip.Liupanshan fault is dominated by thrust extrusion with extrusion rate at 0.6 mm/a–3.0 mm/a and left-lateral strike slip at 0.8 mm/a–1.4 mm/a.Longxian-Baoji fault is dominated by leftlateral strike slip with tension rate at 0.4 mm/a and left-lateral strike slip at 1.2 mm/a.West Qinling North fault is dominated by left-lateral strike slip with extrusion rate at 1mm/a and left-lateral strike slip at 3.5 mm/a.Simultaneously the activity characteristics of Haiyuan fault,Liupanshan fault,Longxian-Baoji fault and West Qinling North fault were researched using velocity profile.Results show that the slip rate parallel to Haiyuan fault is about 5 mm/a,1mm/a parallel to Liupanshan fault,2 mm/a vertical to Haiyuan fault,2.5 mm/a vertical to Liupanshan fault,1mm/a vertical to West Qinling North fault.The slip rates of main faults calculated by GPS velocity profile are in accord with those calculated by rigid block model.As for Longxian-Baoji fault,the slip rates calculated by two methods are different,which may be caused by sparsity of GPS stations and the unreasonableness of part of block boundary.(2)Using GPS velocity field,the locking degree and slip deficit rate of Haiyuan fault,Liupanshan fault,Longxian-Baoji fault and West Qinling North faults,blocks' Euler vector relative to reference block and strain tensor of blocks were inverted,the strain accumulation and risk of the faults were discussed qualitatively.The results show: Lanzhou block,Alashan block,West Qinling North block moves clockwise,anticlockwise and clockwise relative to the reference block with angular rotation rates at 0.5224°/Ma,0.1935°/Ma and 0.2609°/Ma respectively.The homogeneous strain within Alashan block is relatively small.The locking degree of the southern segment of Liupanshan fault,the northern segment of Longxian-Baoji fault and the eastern segment of West Qingling is strongest with locking depth up to 24 km.The slip deficit rate of the eastern segment of the West Qinling North fault is largest with average up to 3mm/a.The southern segment of Liupanshan fault and the northern segment of Longxian-Baoji fault are slightly smaller than that of the northern segment of LongxianBaoji fault with average up to 1.9 mm/a.Both the locking degree and slip deficit rate of Haiyuan fault are relatively small.The earthquake risk of the southern segment of Liupanshan fault,the northern segment of Longxian-Baoji fault and the eastern segment of West Qinling North fault is larger than other segments of these faults.(3)Using energy-magnitude empirical formula and historical earthquake catalogue data in study area,the possible maximum earthquake magnitudes in Haiyuan fault,West Qinling North fault,Longxian-Baoji fault and Liupanshan fault were calculated.The results show that the maximum magnitude in east segment of Haiyuan fault is 6.4, 6.3 in central segment of Haiyuan fault,6.0 in west segment of Haiyuan fault,7.4 in west segment of West Qinling North fault,7.4 in eastern segment of West Qinling North fault, 6.9 in Longxian-Baoji fault, 6.9 in Liupanshan fault.According to seismicity b-value,the earthquake recurrence intervals for West Qinling North fault and Liupanshan fault were calculated.The results show that the average periods in in west segment of West Qinling North fault are 602-672 years,265-302 years for the eastern segment of West Qinling North fault,102-107 years for Liupanshan fault,respectively.