利用人工标志线约束2022年门源MW6.6地震的同震离断层变形OA

[Objective]Accurate constraints on coseismic surface displacement are essential for revealing earthquake rupture processes,assessing regional seismic hazards,and understanding the partitioning of near-surface deformation.Conventional near-field displacement measurements generally capture only localized deformation along visible rupture zones,potentially underestimating total coseismic displacement by not considering off-fault deformation.Long linear anthropogenic markers crossing rupture zones,such as pasture fences,provide a larger measurement aperture and enable us to quantify total coseismic displacement and evaluate the contribution of off-fault deformation.The 8 January 2022 MW 6.6 Menyuan earthquake along the Haiyuan fault zone displaced multiple pasture fences across the well-preserved surface ruptures,providing an ideal opportunity to investigate total displacement and off-fault deformation.[Methods]In this study,we utilized unmanned aerial vehicle(UAV)photogrammetry to acquire high-resolution aerial images along the entire surface rupture zone of the 2022 Menyuan earthquake and to generate digital orthophoto maps(DOMs)and digital elevation models(DEMs)with spatial resolutions of 2-6 cm.Combined with detailed field investigations,we mapped the coseismic surface rupture at a fine scale and selected 12 groups of long,linear pasture fences crossing the rupture zone to conduct multi-aperture displacement measurements.[Results]The coseismic surface rupture extends for approximately 28 km and consists of two main branches:the southern branch along the Tuolaishan fault and the northern branch along the Lenglongling fault.The rupture zone was divided into four segments from west to east,namely S1 to S4,based on the geometric distribution and structural characteristics of the rupture traces,mainly NE-trending,right-stepping en echelon tensional-shear cracks,oblique compressional bulges,and mole-track-like deformation.The width of the surface rupture zone varies significantly along strike,reaching a maximum of approximately 160 m in the S3 segment.Except for the relatively wide S3,most rupture sections are mainly concentrated within a narrow width range of 10-30 m,indicating strong control by local fault geometry and rupture branching.Multi-aperture measurements using 12 groups of cross-fault pasture fences reveal that visible displacement within the mapped rupture zone ranges from 0 to 2.8 m,whereas the total coseismic displacement measured across the larger aperture of the fences ranges from 1.2 to 4.1 m.The corresponding proportion of off-fault deformation reaches 27%-76%,indicating that a substantial part of the coseismic deformation was accommodated outside the visible principal rupture traces.The maximum total coseismic displacement,approximately 4.1±0.8 m,occurs in the S3 segment of the Lenglongling Fault near the epicenter,where the mean proportion of off-fault deformation is relatively low at approximately 33%.From S3 to S1 along the Tuolaishan branch westward,the total coseismic displacement gradually decreases to a mean of 1.7±0.5 m,while the mean proportion of off-fault deformation increases significantly to about 55%,showing that deformation became progressively less localized and more widely distributed across the surrounding near-surface materials.[Conclusions]Compared with previous near-field measurements,the total coseismic displacements obtained in this study are generally larger.This discrepancy is mainly attributed to the larger measurement aperture provided by the long cross-fault fences,which enabled the capture of a more complete deformation field,including both localized displacement on the visible rupture and distributed off-fault deformation.The 2022 Menyuan earthquake produced a complex surface rupture system composed of the Tuolaishan and Lenglongling fault branches,with clear along-strike variations in rupture geometry,rupture-zone width,and displacement distribution.Multi-aperture measurements using long pasture fences indicate that off-fault deformation accounted for a considerable proportion of the total coseismic displacement,especially in the western branch rupture where deformation was more distributed.The comparison with previous near-field measurements demonstrates that relying only on localized rupture offsets may underestimate the total coseismic displacement.[Significance]This study highlights the critical importance of incorporating off-fault deformation into coseismic displacement measurements to prevent underestimating seismic slip in hazard assessments.Furthermore,it demonstrates that using long,linear,anthropogenic markers via high-resolution UAV photogrammetry is an effective,innovative approach for capturing complete near-surface deformation fields along complex strike-slip fault systems.