Using Cors Network To Study Crustal Deformation And Earthquake Risk In The Sichuan Area

Besides the function of navigating and positioning,the Global Navigation Satellite System(GNSS)has also been widely used to monitor the crustal deformation,to study the geodynamics and to inverse the mechanism of earthquake.In recent years,the national wide dense network of the Continually Operating Reference Station(CORS)and the data processing of the GNSS monitoring network have developed quickly.Benefited from the rapid development of CORS and GNSS technologies,higher accuracy and reliability can be achieved in monitoring the global plate motion,the regional crustal deformation,the active blocks as well as the fault zones.The Sichuan area has been characterized with active crustal motion.Especially,the frequent movement of the tectonic plates in the Longmenshan fault zone,the Xianshuihe fault zone as well as the Anninghe-Zemuhe fault zone have resulted in frequent occurrence of geological hazard(e.g.earthquake and mudslide).In this study,we processed the data collected from a number of CORS stations.We solved the coordinate time series for all stations,and successfully inversed the velocity field and the stress-strain field for the Sichuan area.And further analysis on mechanism and trend of crustal deformation in Sichuan area was also carried out.Additionally,taking the velocity field result as its input data,we investigated the locking characteristics,the spatial distribution of slip loss rate as well as the direction of relative movement for adjacent blocks in the Xianshuihe fault zone by applying the Block Negative Dislocation Model DEFNODE.Finally,we succeeded in evaluating the strain accumulation characteristics and the possibility of earthquake occurrence for the Xianshuihe fault zone and also quantifying the motion of adjacent blocks.This study is divided into two parts: crustal deformation in Sichuan area;seismogenic characteristics of the Xianhuihe fault zone and the potential risk of earthquake occurrence.For the first part,we focused on obtaining the velocity field and strain-stress field for the study site by processing the collected GPS data.And different velocity field models for different reference frames were generated,and movement characteristics for both crust and fault zones in the Sichuan area were analyzed.Furthermore,we also studied the spatial distribution and the accumulation characteristics of the strain stress.For the second part,we studied the activity characteristics of the Xianhuihe fault zone with application of the Negative Dislocation Model of Block.Then we generated the locking characteristics of this fault zone,and obtained the information regarding the fault slip loss,the direction of relative activity for adjacent blocks as well as the fault activities.Our findings contribute to evaluate the potential occurrence of longterm large earthquake in this region.The main work and the corresponding conclusions are listed as follows:(1)So as to extract the time series record about the coordinates for more than 50 CORS stations in Sichuan area,we discussed the data processing scheme for the observation network and also the adjustment scheme for the CORS network.Data processing flow based on software GAMIT/GLOBK was designed,which includes baseline calculation,parameter setting as well as model correction for adjustment procedure.As a result,we obtained the results at the single day scale for all CORS stations and extracted the coordinate time series for the reference station.(2)According to the coordinate time series for the observation stations,we generated the horizontal and vertical velocity field on the basis of ITRF08 frame for Sichuan area.In order to eliminate the effect of plate motion,we converted the generated horizontal velocity field from ITRF08 frame to Eurasian frame using Global Plate Motion Model NNR-NUVEL-1A.Results suggested that the mean ITRF08 horizontal velocity is 38.72mm/a with dominant direction S73.9°E,while the mean Eurasian horizontal velocity is 14.40mm/a with dominant direction S77.78°E.In addition,for ITRF08 frame,the mean vertical velocity is 3.43mm/a.An obvious uplifting tendency was discovered in Sichuan area,although limited number of CORS stations distributed in the eastern part of this area revealed a sinking tendency.It was speculated that the subsidence might be induced by city construction,and the overdraft of natural resources,e.g.,the underground water,coal mine and petroleum.(3)Based on the generated horizontal velocity field,we discussed how to obtain the strain characteristic parameters by applying the Delaunay Triangulation method.On one hand,we found that the principal strain rate is relatively large in the western part of Sichuan area,around and on the Longmenshan fault zone,in the northeastern segment of Xianhuihe fault zone,and in the southern segment of Anninghe-Zemuhe fault zone.And the Xianshuihe and Longmenshan fault zones were dominated by principal pressure strain.On the other hand,account for superficial expansivity and maximal shear strain rate,we discovered that the crustal swelling deformation generally existed in most parts of western Sichuan,especially obvious in southwestern segment of Xianhuihe fault zone.Besides,the rate of swelling in the Longmenshan fault zone,the Xianshuihe faults zone as well as the Anninghe-Zemuhe fault zone present low values.And this phenomenon reveals the compression deformation in three fault zones.The shear strain rate increased to the high point in the Xianhuihe fault zone and its neighboring areas.It means that this region is heavily influenced by the potential occurrence of large earthquake,as much energy was accumulated.(4)We inversed the tectonic motion characteristics for the Xianhuihe fault zone based on the Negative Dislocation Model DEFNODE.The southeastern segment of Xianhuihe fault zone is strongly locked with a locking depth of nearly 20 km.The mean locking rate is 0.74,while the mean slip loss rate is 7.64 mm/a for the locking depth no more than 15 km.The results revealed the strong stress/strain accumulation in this region,and this segment is judged as seismogenic fault with large earthquake potential.Additionally,the northwestern segment of Xianhuihe fault zone is relatively weakly locked with the mean locking rate ranging from 0.4 to 0.9.And the mean slip loss rate(locking depth no more than 15 km)is 3.66mm/a.And thus,this segment is also evaluated with large earthquake potential.Lastly,for the middle segment of the Xianhuihe fault zone,the minimum slip loss rate and the weakest locking are discovered.The mean locking rate is merely 0.2,while the mean slip loss rate is just 1.6 mm/a.And it means that the possibility of large earthquake is small in the medium segment of Xianhuihe fault zone.