Study Of Characteristics Of Block Movement And Fault Activity Of The Liupan Shan Fault And Its Adjacent Area Using Geodetic Data

The Liupan Shan fault is a major tectonic unit of the binding site of Lanzhou block and Ordos block. East of the fault is the Ordos Plateau covered by loess, which topographic relief is not large and Cenozoic tectonic movement is weak; west of the fault is northeastern margin of the Tibetan Plateau, which topographic relief is large and both Cenozoic tectonic deformation and strong earthquake activity of it are very strong. The Liupan Shan fault, which located in the northern section of the North-South seismic belt, many strong earthquakes happened surrounding the fault area in history and modern times, but the Liupan Shan fault zone has not more than magnitude 7 earthquake records, so the seismic risk of it should not be underestimated.This article synthetically use geodetic data of the Liupan Shan region to construct three dimensional crustal deformation field. Basing the deformation field and Block-Negative Dislocation model, we study the interseismic locking properties、slip deficit distribution in space of the Liupan Shan fault and the relative motion between Lanzhou block and Ordos block, and combined with regional seismicity data, we estimate the potential maximum earthquake magnitude and the recurrence period of historical maximum magnitude earthquake of the Liupan Shan fault, and then to comprehensive analysis the seismic hazard of the Liupanshan fault. The use of geodetic data include regional GPS horizontal velocity field consist of 1999-2015 China tectonic environment monitoring network GPS velocity field and data of 10 GPS continuous observation station cross the Liupan Shan fault, and level data of the study area in 1970-2012. The basic conclusions are as follows:1) Using GPS data, this article studies characteristics of block movement and fault activity of the Liupan Shan fault and its adjacent area. The results show that Lanzhou block rotate clockwise with respect to the Ordos block, and the rotation rate is about to 0.6498°/Ma. Besides it, the interseismic locking of the Liupan Shan fault has obvious segmentation characteristics: the southern segment of the fault has the strongest degree of locking with fully locking depth of 13-15km; the northern segment of the fault has weaker locking degree than the southern segment with fully locking depth of 6 km; the degree of locking in the middle is the weakest, only on the ground to the underground 2km or so is fullly locked. The largest overall average of slip deficit rate is the southern segment of the fault, then is the northern segment, and the minimum is the middle segment. The maximum value of slip deficit rate appears in the northern tip of the fault, and the average value of the shallow part of the fault in the northern segment is slightly larger than the southern, then the middle segment is minimum.2) Combining GPS and leveling data, this article studies characteristics of block movement and fault activity of the Liupan Shan fault and its adjacent area. The results show that the movement trend and speed of the Lanzhou block relative to the Ordos block has no change. Overall distribution trend of locking degree in various segments remains unchanged, still is the southern section of the strongest, the north of weaker, the middle of the weakest. But the locking depth decreases slightly with the southern of the fault completely locking to underground 10-12 km, the northern part to 5km, and the middle only about surface to 2km unground is completely closed. Both the overall average value in each section and the mean value in different depth of slip deficit rate are decreased.3) By comparing the slip deficit model only with GPS data and joint GPS and leveling data,we find that although the model in the spatial distribution has some changes, but the seismic hazard characteristic common directed by the two models unchanged: Overall, the southern segment of the Liupan Shan fault has the strongest strain accumulation ability and the biggest strain accumulation speed, so its seismic hazard is highest; the northern section of the fault has weaker strain accumulation ability than the southern section, but its strain accumulation speed in shallow part of the fault is fast, so it also has some possibility of earthquake occurrence; both the locking degree and the slip deficit rate of the middle segment are small, so it has lower ability of earthquake occurrence than the southern segment and northern segment of the Liupan Shan fault.4) Combining the slip deficit model obtained by inversion and seismic activity data in the Liupan Shan area, this article estimate that the potential maximum earthquake magnitude is Mw6.4 and the average recurrence period of 6.5 magnitude earthquake which is the maximum magnitude earthquake in historical records is 116~126 years of the Liupan Shan fault.