Study On The Deformation Process Of The Wenchuan And Lushan Earthquakes Based On GPS Data

1 OverviewThe Wenchuan 8.0 earthquake on May 12,2008 and the Lushan 7.0 earthquake on April 20,2013 occurred on the Longmenshan fault zone(LFZ)in the southeastern margin of the Qinghai-Tibet Plateau,which forced people to urgently ask the temporal and spatial evolution process of the structural deformation characteristics,fault coupling fraction and slip deficit rate of each fault segment in the LFZ before and after these two earthquakes.The quantitative explanation of this scientific problem is of great significance for us to have a deeper understanding of the influence of large earthquakes on the regional tectonic strain in the southeastern margin of the Qinghai-Tibet Plateau,the post-earthquake healing process,recovery of strain accumulation,the risk of future earthquakes of the LFZ,and the seismogenic mechanism,earthquake trigger and earthquake cycle of the continental thrust strong earthquakes.Constrained by high-precision and high-density GPS crustal deformation data and supplemented by short-leveling observation data across faults,this paper has carried out in-depth research and quantitative analysis on the above scientific problems in many aspects.We quantitatively studied the temporal and spatial evolution characteristics of surface deformation,deep coupling fraction and slip deficit rate of the LFZ and its surrounding area before and after the Wenchuan and Lushan earthquakes by analytical analysis and multiple fault system 3-dimensional inversion models.On this basis,the increasing of the slip deficit rate before the Wenchuan earthquake,the post-earthquake healing process of different segments of the LFZ and the influence of the Lushan earthquake on the healing process are dynamically identified.Finally,the healing time of different segments of the LFZ were estimated based on the dynamic inversion results of coupling fraction,which improves the effective application of GPS data in evaluation of the whole earthquake cycle process.2 Important understanding and major progress made in this paper(1)The slip deficit rate of the Longmenshan fault increased rapidly before the Wenchuan earthquakeBased on the GPS velocity field data of three periods(1999-2001,2001-2004,and 2004-2007),we analyzed the evolution process of coupling fraction and slip deficit rate of the LFZ.The inversion results of the DEFNODE negative dislocation show that the Longmenshan fault was in a strong locking state before the Wenchuan earthquake,but the slip deficit rate seems to have increased rapidly from 2004 to2007,that is,the deep fault slip rate may have accelerated.This discovery may provide an important clue for the exploration of short-imminent prediction of large earthquakes.(2)The Lushan earthquake did not significantly reduce the risk of large earthquakes in the southwest segment of the Longmenshan faultThe Wenchuan earthquake accelerated the stress and strain accumulation in the southwest segment of the LFZ and enhanced the fault coupling fraction.The Lushan earthquake did not break the main fault in the southwest segment of the LFZ,and the strain on the main fault was not significantly released.At present,the aftershock gap between the Wenchuan and Lushan earthquakes,and the aftershock gap southwest of the Lushan earthquake remain strongly coupled and continue to accumulate strain energy.As such,we should pay attention to the seismic risk in this segment of the LFZ.(3)The relocking and rapid healing of different segments of the Longmenshan fault after the Wenchuan earthquakeBased on the GPS velocity field data of five periods(2009-2011,2011-2013,2013-2015,2015-2018,and 2017-2020),we analyzed the relocking and healing process of the ruptured fault plane of the LFZ,the influence of the Wenchuan earthquake on the Lushan earthquake,and the influence of the Lushan earthquake on the healing process.The inversion results of the DEFNODE negative dislocation show that the post-earthquake healing process of the Longmenshan ruptured fault presents different characteristics and significant differences in different segments.The middle segment of the fault was in the creep state after the Wenchuan earthquake.However,after the Lushan earthquake,the coupling fraction was rapidly enhanced,and coupling area rapidly increased in the southwest direction of the Wenchuan earthquake,indicating that the fault is healing rapidly.The healing area expanded rapidly towards the Wenchuan epicenter.However,most of the ruptured fault located in the northeast direction of the epicenter has not healed yet and are still in the creep state.It is estimated that it will take more than a few years for this segment to heal.After the short-term creep caused by the Wenchuan earthquake,the northeast segment of the Longmenshan fault recovered the locking state quickly during 2011-2013,indicating this segment of fault plane has healed.At present,the range of strong locking has begun to expand towards the middle segment.(4)The evolution process of coupling fraction of the middle and southern segment of the Longmenshan fault before and after the Lushan earthquakeBy using time series of continuous GPS stations from 2010.30 to 2013.30 between the Wenchuan and Lushan earthquakes,and time series from 2014.0 to2015.0 after the Lushan earthquake,the dynamic evolution of coupling fraction and slip deficit rate in the middle-southern segment of the LFZ were analyzed,and the healing process of the middle segment were discussed.The inversion results of TDEFNODE show that,the coupling fraction of the ruptured area around the Wenchuan epicenter rapidly increased,which was basically in creep state in 2010,and then in strong locking state in 2013.The locking area also rapidly increased from the southwest of the Wenchuan epicenter to the epicenter.After the Lushan earthquake,the coupling fraction and locking range near the Wenchuan epicenter is further enhanced,which indicates that this part of the fault has been healing rapidly.Most of the ruptured fault plane in the northeast of the epicenter is still in creep state and the fault has not yet begun healing.The fault near and southwest of the epicenter of the Lushan earthquake has been in strong locking state,and the slip deficit rate of the complete locking area has been decreasing year by year,probably indicating that the healing region shares part of the compression effect of the Bayan Har block on the Sichuan Basin.In addition,the inversion profile results of GPS time series show that before the Lushan earthquake,the strike-slip movement of parallel faults in the southwest segment of the LFZ shows the left-lateral characteristics near the fault,while the right-lateral movement characteristics between blocks in the far-field,which are consistent with the results of thrust and left-lateral rupture during the Lushan earthquake and far-field right-lateral movement of GPS velocity profile respectively.(5)Surface three-dimensional deformation evolution around the LFZThe GPS strain rates field and velocity profiles show that,the Wenchuan earthquake completely unlocked the middle segment of the LFZ,and there is an obvious step change in the thrust and strike-slip components of the middle segment of the LFZ,and the thrust component is greater than the strike-slip component.The internal deformation of the Bayan Har block changed from compression deformation before earthquake to tensile deformation after earthquake.At present,although the creep rate of the middle segment has decreased,it is still in the state of post-earthquake creep,and the post-earthquake tensile deformation in Bayan Har block is also gradually weakened.After the Wenchuan earthquake,the southwest segment of the LFZ still shows a large range of continuous deformation,and the east-west compression deformation is strengthened after the Lushan earthquake,indicating that the fault is in a continuous state of strong locking.At present,the horizontal shortening rate perpendicular to faults and the dextral torsion parallel to faults are gradually decreasing,but the horizontal shortening rate is still significantly higher than that before the Wenchuan earthquake.The GPS baseline time series of long time scales across the whole southwest segment of the LFZ show that,LS01?LS03 and LS04?LS08 still showed the rapid shortening characteristics after the Lushan earthquake,and shortening rate increases slightly than that before the earthquake,which may indicate after the earthquake,the area around the epicenter shows the characteristics of enhanced compression;after the Lushan earthquake,the baseline of LS04?LS10 continues the annual attenuation of the rate before the earthquake,and the shortening rate gradually approaches 0,which may indicate that the fast eastward movement of Bayan Har block was stopped by the blocking effect of the Sichuan Basin in the southern segment of the LFZ.The average annual vertical rate of cross-fault short-leveling sites show that,the rates of Qipangou site on the hanging wall,Guanxian and Shuanghe sites on the footwall increased obviously due to the Wenchuan earthquake,which mainly reflected the influence of coseismic.The vertical motion rates have been very small in 2010 and remain stable in the following years,indicating that the Wenchuan earthquake has weak influence on the vertical deformation near the LFZ.