InSAR Coseismic Deformation And Earthquake Rupture Process Study

During the past twenty years or so,Interferometric Synthetic Aperture Radar(In SAR)has been developed rapidly as a relatively newborn geodetic technology and been widely used in surface deformation monitoring.However,In SAR measurements are along Line-Of-Sight(LOS)direction,which is the integrated component of the 3-D surface deformation based on certain satellite geometry and called LOS ambiguity.This will hurt the In SAR applications in studying the complex tectonic motion and inverting the source parameters.Thus,multi-angled In SAR observations arenecessary in acquiring 3-D coseismic displacement and obtaining the reliable source parameters at depth,as well.Earthquake rupture process are based on dislocation theory and can give us it's thekinematic characteristics about earthquake sources.Therefore,inverting the geodetic observation and seismic waveform data jointly to obtainearthquake rupture process has attracted intensive interest for seismologist.The high-space-resolution geodetic measurements can tightly constrain the source fault geometry and slip distribution at near-field.The seismic waveform data can be used to determine the epicenter location,the seismic moment release rate and the rupture direction.Therefore,joint inversion base on these two kinds of data is more reliable than either of these two separately.With the development of this technology,obtaining the magnitude and the rupture direction of strong earthquakes in real-time is becoming available,which can give significant technological support in earthquake early warning,instant response and rescue deployment of the government.In this study,the 2008/2009 Qaidam earthquake sequence and the 2008 Wenchuan Mw7.8 earthquake were studied.Following scientific questions are investigated: the 3-D coseismic displacement field calculation using multi-perspective SAR acquisitions;the slip distribution inversion on fault plane;the dynamic source rupture process inversion based on the geodetic data and seismic waveform data simultaneously;and quasi-real-time retrospective inversion.The results and conclusions are as follows:(1)We calculated the 3-D coseismic displacement of 2008 and 2009 Qaidam earthquakes based on the ENVISAT/ASAR descending and ascending data and Scan SAR data.The results indicatethat the vertical displacements dominated the coseismic deformation in two events,with magnitude of 10 cm and 27 cm.It is consistent with the movement features of reversal fault and the focal mechanism of two events given by USGS and GCMT.The result also shows that the source fault moved eastward with slight magnitude during coseismic rupture.We concluded that the 2008 and 2009 events are characterized by thrusting and slightly left-lateral strike slip,which is consistent with the tectonic settings of the uplifting and eastward extrusion of Tibetan plateau caused by the northeastern subduction of the Indian plate.(2)The static slip distribution on the fault plane of the 2008 and 2009 Qaidam earthquake were inverted using three kinds of In SAR datasets with different incidence and azimuth angles.The slip of the 2008 event was mainly distributed at the depth range of 9.2-23 km.The peak slip of 0.5 m was at the depth of 15 km;and the mean rake angle was 89°.While the slip of the 2009 event was mainly distributed at the depth of 1.7-10 km,the maximum slip was approximately 1.62 m located at the depth of 4.6-6 km in the middle segment of the source fault.The mean rake angle of the western,the middle and the eastern segment is about 61.5°,85.5° and 75.5°,respectively.(3)We conducted joint inversion of descending and ascending In SAR,and teleseismic data for the rupture process of the Qaidam earthquake sequence.Results show that the duration of the 2008 Qaidam earthquake was about 9 sec.Rupture propagated from deep to the surface,with the main slip patch concentering between 8.2 and 18.8 km and a maximum slip of nearly 0.94 m.The average rake angle was 81°,indicating dominantly thrusting characteristics.The total seismic moment resulting from the joint inversion was 4.57×1018N·m,which is equivalent to Mw6.4.The duration of the 2009 Qaidam earthquake was about 21 sec.Rupture propagated unilaterallyfrom northwest to southeast,resulting in three patches with significant slip distribution.The largest slip patch located at depths between 2.2 and 8.2 km.Joint inversion results indicated that the total seismic moment was 3.82×1018N·m,which corresponds moment magnitude(Mw)6.35.(4)Analysis of the source faults and rupture mechanisms of the 2008 and 2009 Qaidam earthquakes.By conducting a comprehensive analysis of the joint inversion results of static slip distribution and rupture process of the 2008 and 2009 Qaidam earthquakes,we proposed the southwest dipping Qaidam-Zongwulongshan fault as the source fault.The rupture of the two events did not reached the surface.Besides,results show that the source was dominated by thrusting,accompanied with a small amount of left lateral components.Mechanisms of the two events were consistent with the GPS velocity field and regional tectonics.The 2008 earthquake ruptured the lower half of the seismogenic layer,the 2009 eventthe upper half.The upper crust,about 23 km below the surface,was almost totally penetrated by the two earthquakes.Rupture propagations of the 2008 Qaidam earthquake might been blocked by the northeast dipping Southern Zongwulong fault or high-strength barrier,resulting in zones of coseismic slip deficit in the shallow depth.Coincidentally,the 2009 earthquake was located in the positive Coulomb stress triggering zones caused by the 2008 earthquake.As a consequence,we proposed that the 2009 earthquake was triggered by the 2008 event.The flower-shaped structure peculiar to the Qaidam area is conducive to the formation of shallow slip deficit,resulting in a high possibility of cascade destructive earthquakes in a short period of time.To sum up,the study of the relationships between the 2008 Qaidam earthquake and the 2009 Qaidam earthquake provides us with an important reference for future earthquake hazard assessment in such cases.(5)We investigated the rupture process of the 2008 Wenchuan earthquake by joint inversion of In SAR,GPS and strong motion data.Our results confirmthe rupture process previously determined,that is the rupture propagated unilaterally from southwest to northeast.Fault movement was mainly thrusting at the southwestern segment of the Longmenshan fault,while the middle segment was dominated by thrusting with certain right-lateral strike-slip components.To the northeastern segment of the fault,dextral strike slip movement was the main factor.Three coseismic slip concentrated patches were identified,with the maximum slip of more than 15 m.The total seismic moment resulting from the joint inversion was about 9.4 × 1020N·m,corresponding moment magnitude(Mw)7.9.Based on the details of the spatiotemporal evolution of the rupture process of the Wenchuan earthquake,we propose that the source kinetic model of the Wenchuan earthquake can be explained by a barrier-asperity model.The rupture of Guanxian-Jiangyou fault might be the result of dynamic stress triggering during the rupture process of the main fault.In addition,we conducted preliminary quasi-real-time retrospective inversions of the rupture process of the Wenchuan earthquake by utilizing strong motion data.Our results confirm that it is possible to obtain the direction of fault rupture rapidly during an earthquake.