Study On The Crustal Deformation Of Moderate-strong Earthquakes In Northeastern Tibetan Plateau Using Geodetic Data

Seismic activity is a window to shed light on the interior of the earth and a rare opportunity to reveal the properties of the lithosphere.The co-and post-seismic process will produce specific deformation response on the ground,and the analysis of deformation field caused by earthquakes is an important way to understand the mechanism of earthquake genesis.Space geodesy is a unique effective observation method to measure surface displacement precisely.In this study,based on the geodetic observations,we analyzed the co-and post-seismic deformation of earthquakes,including the 2016 and 2022 Menyuan earthquakes,the 2019 Changning earthquake sequence and the 2021 Maduo earthquake,as well as the relevant physical mechanisms.The results could provide new geodetic evidence for the investigations of seismic activities,geodynamics environments,seismic hazard assessments in the northeastern Tibet Plateau and the mechanisms of the human-induced earthquakes.This paper studied the coseismic slip models of the 2016 M_w5.9 and 2022 M_w6.7Menyuan earthquakes using InSAR data,and then discussed the characteristics of coseismic models of the two events,the change of cumulative Coulomb stress on the western segment of the Tianzhu seismic gap and the associated seismic hazard.The2022 Menyuan earthquake occurred on the steeply-dipping sinistral strike-slip Lenglongling fault.The bulk of the coseismic slip is concentrated on the shallow part of the fault（0-10 km）,with a maximum slip of 3.25 m.The 2022 event has probably caused the largest coseismic slip in well-recorded M_w<7 strike-slip events in Chinese Mainland.The 2016 Menyuan earthquake was a blind reverse event,for its rupture failed to reach the surface.The geodetic-derived rupture model showed that the slip is concentrated in a depth range of 5-15 km.The coseismic slip model detected the dip angle of the north Lenglongling fault（～40°）,which indicated that the north Lenglongling fault is possibly intersected with the Lenglongling fault,forming a multi-fault flower structure.The structure accommodates the sinistral shear and compression of the crust in the study area.The 2022 event occurred in the Coulomb stress loading segment on the western Tianzhu seismic gap,but only ruptured part of the segment.Considering the high locking depth and the stress accumulation on the western Tianzhu seismic gap,the seismic hazard has not been completely relieved by the 2022 Menyuan earthquake.The 2022 Har Lake earthquake sequence,which lasted for～70 days since January 2022,jolted the Har Lake area,which is located in the west Qilian Shan, northeastern Tibetan Plateau.Two M_w>5.5 earthquakes are involved in the earthquake sequence,in which the M_w 5.8 event occurred on March 25 is considered to be the largest event recorded in the area.We use Sentinel-1 SAR data to determine the coseismic deformation field,the possible ruptured faults and associated fault geometries of the two M_w>5.5 earthquakes,and further estimated the detailed slip models.The results show that the two earthquakes are dominated by dextral-slip.The January 23 M_w5.6 earthquake occurred on a N12°W trending fault with a dip angle of～60°,while the March 25 M_w5.8 event occurred on a～N-S trending steeply-dipping fault.The seismogenic faults intersect with each other in the down-dip direction,forming a complex bookshelf faulting structure together with the Har Lake fault and the Danghe-Nanshan fault.We suggest that the right-lateral slips and earthquake ruptures on the～N-S trending faults are driven by the left-lateral shear and anticlockwise rotation in the study area,which originate from the widely-spread NE-SW trending compression near the Qilian Mountain.No clear evidence supports the Coulomb stress triggering relationship between the two M_w>5.5 events.The seismic sequence is possibly driven by a more complex mechanism.Basing on GPS and InSAR observations,this paper analyzed the coseismic rupture model and postseismic deformation mechanism of the 2021 M_w7.4 Maduo earthquake.The geodynamic mechanism of the northeasern Bayan Har block,the deformation pattern of the Kunlun Pass-Jiangcuo fault（KPJF）and the regional seismic hazard were also discussed.The 2021 M_w 7.4 Maduo earthquake produced a～165-km-long rupture on the KPJF with an estimated seismic moment release of 1.80×10²⁰ N·m,equivalent to an M_w 7.43 event.Four asperities were detected on the main fault with a maximum slip of 4.79 m at～3.0 km depth on the Dongcao Along Segment（～98.7°E）.The rupture on the Changmahe segment,which cuts through the pre-existing NW-SE trending faults,indicated a significant energy release during the rupture propagation and the unique tectonic environment on northern Bayan Har block.Afterslip and viscous relaxation process dominated the postseismic process of the Maduo earthquake.The seismic and aseismic slip distributions showed complementary patterns.Transient viscosity coefficient(1-1.2×10¹⁸Pa·s)of the lower crust in the northeastern Bayan Har block did not support the existence of large-scale middle-lower crustal flow in the study region.There is no significant difference between the lower crust viscosities of the southern margin of the Qaidam block and the Bayan Har block,indicating that the weaker material of the Bayan Har block may wedge into the lower crust of the Qaidam basin.The rupture of the Maduo earthquake was located in the stress shadow induced by historical earthquakes.The phenomenon pointed out the high accumulated stress level on the KPJF.The rupture of the 2021Maduo earthquake and the 2001 Kokoxili earthquake leaved a～300-km-long seismic gap on the KPJF,which is probably under relatively high potential seismic hazard.The activities of the faults in the interior of the Bayan Har block deserve more attention.