Research On Earthquake Cycle Deformation Modeling Of The Lenglongling Fault And The Kunlun Pass-Jiangcuo Fault

The key to predicting the seismic potential of fault zones and mitigating earthquake hazards depends on our ability to characterize the current slip deficit of the fault zone as it accumulates along block-specific boundaries and is directly related to the earthquake history of the fault zone,quasi-periodic recurrence rates,and the pattern of co-and post-seismic slip distribution.With the rapid development of observation technology,observations covering all phases of the earthquake cycle（e.g.,GPS and InSAR）have become increasingly abundant,allowing complete observational coverage of thousands of kilometers of faults,resulting in the rapid accumulation of observed deformation data at all phases of the cross-fault earthquake cycle,providing an unprecedented opportunity for a deep understanding of the slip deficit evolution patterns at all phases of faults.However,due to the differences in data processing methods and model inversion strategies,it is usually difficult to obtain a more consistent and stable self-consistent result when we study the deformation patterns at each stage of the earthquake cycle.Therefore,the main focus of this paper is to develop a set of self-consistent methods to study the deformation patterns in each phase of the earthquake cycle（interseismic,coseismic,and postseismic）with the constraints of high accuracy and high spatio-temporal resolution of GNSS and InSAR geodetic data to better constrain the spatio-temporal evolution characteristics of slip deficit in each phase of the earthquake cycle in the corresponding fault segment,so as to better serve the seismic hazard analysis.In this paper,two representative strike-slip fault zones（one intra-plate and one inter-plate）on the Tibetan Plateau are studied,and the complexity of the fault geometry and deformation mechanisms are fully considered in the research methodology.The main work and contributions of the paper are summarized as follows.Taking the Qilian-Haiyuan fault zone（QHF）Lenglongling segment as an example,a viscoelastic earthquake cycle model considering the fault transient slip disturbance is established in this paper.Understanding the variability in the size and location of moderate to large earthquakes along strike-slip fault zones and the controlling role of tectonics and geometry is essential for assessing the earthquake hazard.The viscoelastic interseismic model suggests that the rate in the middle section of the QL-HY rupture may be stable at 3.6-3.8 mm/yr,consistent with model two postulated by Liu et al.（2022）,unlike the significantly higher slip rates in the local fault section required for the elastic model results,for example,up to 5-6 mm/yr in the LLL section.Coseismic and postseismic analyses demonstrate the 2022 event re-ruptured the rupture terminus of the historical earthquake occurring on the Lenglongling Fault,releasing an under-released slip deficit.The rupture site and energy level released suggest that the 2022Menyuan earthquake may end a 100-yr-long stress equalization/release process starting with the 1927 Gulang earthquake and signal the beginning of a new re-equilibration process for the LLLF.The rupture of the three Menyuan（1986,2016 and 2022）,1954Minqin and 1927 Gulang earthquakes,together with the seismic imaging,conductivity,and wave velocity structure across the QHF suggest that the Haiyuan fault,as the northern boundary of the Tibetan Plateau,still maintains a northward expanding stress transfer due to continued lateral compression from the central Tibet.This may have led to a strain partitioning pattern coupling the main strike-slip mechanism of the high dip Haiyuan fault and the main thrusting mechanism of the low dip thrust-wedge system northern the QHF.Taking the 2021 Maduo earthquake occurring on the Kunlun Pass-Jiangcuo Fault zone（KPJF）as an example,a co-seismic model considering complex multi-segment ruptures is established.A nearly 70-year hiatus of major seismic activity in the central eastern Bayan Har Block（BKB）ended on 22 May 2021 when a multi-slip-peak sinistral strike-slip earthquake struck western Maduo County,Qinghai.This earthquake,which ruptured the nearly 170-km long Kunlun Pass-Jiangcuo fault（KPJF）,is a rather unique event and offer a rare opportunity to probe the mechanical properties of the intraplate lithosphere of the central eastern BKB.Here,we inferred the fault geometry associated with the Maduo earthquake by using interferometric synthetic aperture radar（InSAR）and relocated aftershocks and inverted the slip distribution through InSAR radar phases and range offsets.Our analysis revealed that the geometry of the fault varies along the strike:the southeastern end of the fault dips steeply to the northeast,whereas the northwestern end dips southwestward.Using the combined datasets to constrain a coseismic slip,we found that the 2021 Maduo event was dominated by sinistral strike slip movement,with a slight normal slip component at a shallow depth,rupturing the steep-dipping fault for nearly 170 km in length.Five asperities were detected along the fault strike in the shallow crust（0～12 km）with a peak slip of～4.2 m corresponding mostly to simple structures,namely,continuous and straight rupture segments,suggesting that the rupture propagated across geometrical barriers in a multi-asperity way.Based on an analysis of the strain field and the focal mechanisms of both the 2021Maduo earthquake and historical earthquakes that have occurred in the BKB,we propose that the fault zones within the BKB can also generate large earthquakes and have the ability to accommodate the ongoing eastward and northeastward penetration of the Indian plate into the Eurasian plate.Taking the observation of postseismic deformation following the 2021 Maduo earthquake on the KPJF as an example,the combined viscoelastic relaxation and stress-driven afterslip model proposed by Diao et al.（2018）is developed.Postseismic studies following the Maduo earthquake provide a unique opportunity to resolve the deep rheology within the Bayan Har Block（BHB）.Here,we inferred the rheology associated with the lithosphere across the Kunlun Pass-Jiangcuo fault（KPJF）through a combined model of viscoelastic relaxation and stress-driven afterslip with a～160 days of GPS and InSAR observations as constraint.The optimal result features a 25-km-thick brittle upper crust with afterslip mainly above 15 km overlying a ductile lower crust with a viscosity of 2×10¹⁸.This result combined with the broad lateral interseismic shear motion and the low shear velocity and high conductivity in the lower crust suggest that intra-block and boundary faults may jointly accommodate the across-BHB differential motion by mainly lateral shear.