InSAR Observations And Strain Partitioning Pattern In The Fault Transfer Zones Of The Central-western Qilian-Haiyuan Fault System

The Qilian-Haiyuan fault(QL – HYF),the main boundary fault in the northeastern margin of the Tibetan Plateau,is usually considered as a large left-lateral strike-slipping structure with an ESE trending continuously transferred deformation along fault segments from west to east.However,the recent filed geological investigations suggest that there does not exist an end-to-end structural connection between fault segments in the Gulang-Haiyuan triple junction at the middle of QL–HYF zone.The shere strain on the middle Lenglongling fault(LLLF)tends to be transferred northeast forward to the Haxi fault,rather than transmitted eastward to the Jinqianghe fault(JQHF).It remains controversial whether the fault deformation in this transfer zone is continuously transmitted.Geodetic observations present slip partitioning and segmented movement for the 1,000-km QL – HYF system.Therefore,it is significant to investigate the tectonic transformation model and slip partitioning in the transfer zones along the QL – HYF system.High-resolution regional crustal deformation filed is a key factor to improve understanding of the deformation partitioning patterns and tectonic transformation model in the middle-western section of the northeastern edge of the Tibetan Plateau.Concentrating on the 2022 Mw6.7 Menyuan earthquake and the Gulang-Haiyuan triple zone,we have conducted InSAR deformation extraction,inversion of co-seismic slip models and inter-seismic fault slip rates,and calculation of strain rate maps.The main works and conclusions are as follows:The ascending and descending Sentinel-1 satellite data are used to investigate the co-seismic slip model of the 2022 Mw6.7 Menyuan earthquake in Qinghai,China.The optimal slip model indicates that this event ruptured two fault segments.The main rupture(～20km)concentrated on the western LLLF with a purely left-lateral striking-slip motion.A small part of the eastern Tuolaishan fault(TLSF)section(～5km)was also ruptured,and the motion on it is mainly oblique slip at depth,with an obvious thrust component.Combined with the rupture characteristics of historical events,GPS velocity map,leveling profile and slip rate studies,we suggest that the TLSF – LLLF junction is a demarcation point where the deformation partitioning pattern has changed.Along the whole LLLF segment,the oblique convergence has completely partitioned into slip on the purely strike-slipping LLLF and thrusting faults in the north.The TLSF segment accommodates a fraction of compressional shortening,which compensates for the discrepancy in the left-lateral slip rate between the LLLF and TLSF.Such transformation in the strain partitioning pattern is likely to be determined by the geometric relationship between the fault strike and the direction of regional block movement.The time-series Sentinal-1 satellite data are used to extract the high-resolution InSAR deformation velocity map of the Gulang-Haiyuan triple junction area by applying the SBAS-InSAR technology,which is verified to be with a good quality by comparing with the published InSAR data.The screw dislocation model is used to invert the InSAR profile data for slip rates on different segments of the fault.The results show that there is a rate-slowing zone between the LLLF and the JQHF,and the slip is not transmitted continuously between the faults.InSAR deformation is projected onto the direction perpendicular to the strike of the Jinqianghe and Haxi faults.The cross-fault profiles show an obvious tensile deformation on the Jinqianghe and Haxi faults,which is consistent with the latest geological results.InSAR and GPS data were used to calculate the strain rate maps in the triple junction zone.A positive dilation rate at the location of the Haxi Fault and Jinqianghe Fault can be seen in the dilation rate map,which indicates a normal fault movement.The maximum shear strain rate map shows that the sinistral shear strain on the LLLF does not directly transition to the JQHF,but terminates at the Haxi fault in the northeast.The shear does not cut through the triple junction zone,which is consistent with the Geological investigation.In combination with high-resolution InSAR observation and latest geological results,we suggest that the tectonic deformation transformation in the Gulang-Haiyuan triple junction zone is not characterized by "horizontal transmission" in a continuous style as known previously,and the sinistral shear at the eastern end of the LLLF does not directly transition to the JQHF,instead,a tensile-shearing deformation is shown in it.The formation of this transtentional zone is likely to be caused by the difference of the tectonic movement between its eastern and western neighboring area.