Source Process Of Strong Earthquakes In Eastern And Southern Borders Of Tibet By Joint Inversion Of Geodetic And Seismic Observations

Following the devastating 2008 Ms8.1 Wenchuan earthquake in Longmenshan,the 2013 Lushan Ms7.0 earthquake,the 2015 Gorkha Ms8.1 earthquake and the 2017Jiuzhaigou Ms7.0 earthquake are the three strong earthquakes with magnitude Ms?7.0occurred on the surrounding fault of the Tibet Plateau in recent ten years.Investigation on the source mechanism,source process and geometry of seismogenic fault would contribute to the present-day deformation and mountain building mechanism in Tibet.Base on the rupture process,source characteristics could be quantitated by the spatiotemporal source parameters.This thesis focuses on the inversion for source rupture process of these three earthquakes,and determination of spatiotemporal source parameters.Furthermore,some works have been done for the epicentral deformation analysis,seismogenic mechanism and stress change.In details,this paper mainly conscentrates on four aspects.First,inverts the geodetic and seismic data for the slip distribution by joint and separate approach.Second,investigates the source kinematic parameters by using high rate GPS?hrGPS?and seismic data.Third,discusses the relationship between the fault geometry and regional tectonics.And finally,analyses the slip deficits on the surrounding faults and associated seismic risks.Some parameters for the source characterization mentioned in this paper are introduced at first.The basic equation for rupture process is derieved from the law of displacement representation,and the key parameters for determining the rupture process are inferred.The multiple time window?MTW?approach is employed to form the source time function,and then the observation equations could be solved linearly by using non-negative least squares.Some constraints are involved in the inversion to keep the result stable.Not all the source parameters are inverted,some are determined by defferent strategies.In the study of 2013 Lushan earthquake,three fault models of one singe straight fault plane,a singe straight fault plane and a back thrust fault,a listric surface fault plane and a back thrust fault are constructed according to the distribution of aftershocks.The joint inversion of static GPS,hrGPS,strong motion and teleseismic waveform for slip model on three fault models are performed,and the results show that the main slip area is concentrated in depth of 5-20km with a size of 25km×25km.The slip maximum of 1.1m is located near the initial point.The static stress drop is estimated to be²Mpa from the average slip of 0.3m over a well-resolved area.The seismic moments are released largely by thrusting motions within duration of 25sec,and composed by two steps.The major slips occur in 0-10sec as the first step as well as the secondary slips are found in the second step of 10-25sec,resulting in a total seismic moment of¹.1×10¹⁹ Nm?equivalent to Mw 6.6?.Some low slips with thrusting and left-lateral strike motion are found on the back thrust fault,and act with slips on the main fault to form a clockwise rotation,which may infer the oblique motion along the southern segment of Longmenshan fault system.Some weak slips?<0.2m?deduced with high uncertainties are shown on the detachment below 25km depth.Slips on three fault models are constrained solely by hrGPS,to demonstrate the ability for hrGPS to infer a medium-sized?M6 class?earthquake source process and intrinsic resolution for asperities.The rupture patterns show stable slip features,but with insensitity for geometry of fault.The total seismic moment is better constrained by the data with permanent displacements than the sole time-varying waveforms.CoMpared to other measurements,hrGPS shows better resolution on the asperities on main fault,but worse resolution on the slips on the back thrust fault in this event.The 2015 Gorkha earthquake is investigated by a simple but fast inversion,and then a unform model after deeper study.Strong ground motions and permanent surface displacements induced by this event are observed unprecedentedly by continuous GPS networks in Nepal and southern Tibet,and these geodetic observations close to the rupture zone are important as such when a finite fault model of rupture is constructed to characterize rupture processes and source properties.In fast inversion,we focus on retrieving the slip distribution and temporal history of this earthquake through a joint inversion of teleseismic waveforms and some GPS located in the south Tibet.The joint inversion shows that the detachment fault fails unilaterally from the hypocenter with slip extending eastward over an area of 100km in along-strike length by 130 km in downdip width.The best-fitting model indicates that the seismic moments are released largely by thrusting motions within duration of 80 sec.The unlocked part of the detachment fault has yielded an averaged slip of 2.4 m with a total seismic moment of9.4×10²⁰ Nm,which gives Mw=7.9.If the asperity of this event corresponds to the rupture zone of the 1833 Mw7.7-7.8,its recurrence in the same rupture area would be every 150-200 yr.In the further study,the inversion for the space-time history of fault slip during the mainshock and its largest aftershock?Mw=7.3?using separate and joint inversions of hrGPS,static GPS,InSAR data,teleseismic waveform and strong-motion data is performed to pursue a self-consistent and coMpatible rupture model.After obtaining the preferred rupture velocity and subfault rise time by using the tradeoff line between the cross correlation coefficients of observed and synthetic waveforms of HRGPS versus rupture velocity and subfault rise time,separate inversions of the individual datasets are performed.Separately inverted models present different slip patterns due to the intrinsic resolution of different datasets.Finally,two joint inversions of the near-field datasets and all datasets have been carried out.The near-field datasets joint model improves the resolution of GPS model,but no better than InSAR model.However,the scattered slip patches due to the non-coseismic deformation or observation errors has been just depressed in the joint models.The optimal joint model of all datasets,supplemented by far-field observation,can be regarded as a unified model for preserving the common features of all separate inversions and yielding good combined resolution of slip.The Checkerboard tests also show the unified model has the best spatial resolution for almost recovering the20km×20km slip patch and more stable to rupture velocity variance.The slip pattern mainly constrained by InSAR as well as the temporal process agrees well with teleseismic P waves model.The preferred unified model reveals the slip zone of mainshock extending¹40km along strike and⁸0km down dip.The large slip patch?slip>4m?shaping a belt area is located 15km deep and 30km north of Kathmandu,with a maximum slip of 7.4m.The slip process lasts 60sec with a rupture velocity of³.3km/s,and with the rise time of 10sec for each subfault.The slip of Mw7.3aftershock lies to the east of the boundary of mainshock slip zone within an area³0km along strike and²0km down dip,features in coMpact pattern with a maximum slip of 4.4m and total lasing time of 35 sec.The rupture of mainshock stopped at the location of the Mw7.3 aftershock,and left an slip deficit.The optimal strikes and dips of mainshock and Mw7.3 aftershock show little differences,which could be inferred that the termination of mainshock could be independent with the geometry varation of the fault.2017 Jiuzhaigou earthquake draws public attention on its source model,relationship with 2008 Wenchuan earthquake,and future seismic risk in the eastern Tibet.This event highlights the northward migration of major earthquakes along the single strike-slip Huya fault.The source characteristics are analyzed with GPS/InSAR static displacements and high-rate GPS/teleseismic dynamic motions.The preferred solution shows slip distribution of 25-30 km along-strike long and 18-20 km downdip wide on a two-segment fault.If the onset of rupture started at the assumed hypocenter,the rupture would have propagated simultaneously to the northwest and southeast,resulting in a total seismic moment of 9.0×10¹⁸ Nm?equivalent to Mw 6.6?.The rupture process occurs over a duration of 15 s and has a mean speed of 3.2 km/s.The outstanding features of the slip distribution are two major slip patches separated marginally by a low-slip zone?<0.6 m?if a weak to modest smoothing is imposed.Based on the slip model,the static stress drop on each of the two major patches are estimated to be 3.3 Mpa and 3.6 Mpa respectively.The earthquakes in 1960,1973 and1976-08-16 each has a potential to power the Jiuzhaigou earthquake,although the1976-08-22,1976-08-23 and 2008 earthquakes also contribute to stress increases on the two segments.The Coulomb stress increase of 0.82 bar caused by preceding earthquakes triggered the failure of the northern Huya fault in 2017.In particular,this event was partially promoted by the 2008 Wenchuan earthquake and its postseismic deformation.Both the coseismic and postseismic stress loading imposed by 2017Jiuzhaigou earthquake would promote the seismic risk on a 30 km-long section of the northern part of Minjiang fault,and a 30km-long section of the northwestern segment of Tazang fault which is presumed as a high seismic risk zone of the Eastern Kunlun fault system.Given a slip rate of 0.5 mm/yr derived from preseismic GPS observations,the mean slip of 0.6 m inferred from the slip model suggests a recurrence interval of¹000 years for this earthquake and alike on the Huya fault.This estimate and the fact that most segments of the fault have been unzipped in the recent decades indicate that the Huya fault is in the latest phase of one seismic cycle.Slip deficits on most parts of the Huya fault have been balanced largely by a sequence of strong earthquakes since the early 1970s,only leaving a seismic gap of 20-25 km long in proximity to the Jiuzhaigou National Natural Reserve.The ongoing postseismic relaxation will hasten further this section,completing one seismic cycle of the Huya fault.