Earthquake Recurrence Behavior And Seismic Hazards Of The Lenglongling Fault,Northern Qilian Shan

The northeastern margin of the Tibetan Plateau is where the Tibetan block interacts strongly with the Gobi-Alashan and Ordos blocks.It is also the leading edge of the expansion of the plateau into the interior of the continent,playing an important role in absorbing and accommodating the NNE trending convergence and extrusion from the Indian plate to the Eurasian plate.The late Quaternary tectonic deformation in the northeastern margin of the Tibetan Plateau is partitioned into localized left-lateral shearing along the Qilian-Haiyuan fault zone and distributed crustal shortening and thickening across the Qilian Shan fold-and-thrust zone,indicating that the Qilian-Haiyuan fault zone is subject to long-term and relatively fast stress accumulation and thus represents high seismic risk.The 1920 M8.5 Haiyuan earthquake that ruptured the Haiyuan fault,along the eastern section of the fault zone,is the largest historical earthquake in the northeastern margin of the Tibetan Plateau.The Lenglongling fault(LLLF)is located at the tectonic transition and pivotal position along the NE Tibetan Plateau.The tectonic deformation to the east and west is dominated with the left-lateral shearing and fold-and-thrust shortening,respectively.The fault is considered to be the highest slip rate strike-slip fault in the Qilian-Haiyuan fault zone,and is also an important part of the “Tianzhu seismic gap”.The 1927 M8.0 Gulang earthquake was located near the Dongqingding Shan on the northeast side of the LLLF,but its seismogenic fault has been controversial.Therefore,studying the late Quaternary tectonic activity and faulting behavior of the LLLF is of great significance for understanding the tectonic deformation mechanism,strain release process and seismic hazards of the NE Tibetan Plateau.However,little is known about the recurrence behavior of large earthquakes,including the timing of the latest event and the tectonic relationship between the fault and the Gulang earthquake.Using high-resolution satellite imagery,field investigations,and UAV-based photogrammetry,we mapped the surface trace of the fault in detail and measured the left-lateral offsets of geomorphic markers along the fault trace.A binned cumulative offset probability density(COPD)distribution was calculated to analyze single-event and multi-event cumulative offsets.Trench excavations and radiocarbon dating were used to analyse and study the latest activity age of the fault,paleoseismic history,earthquake recurrence behavior and the relationship between the most recent event and the Gulang earthquake.Based on the results of previous studies and the understandings of the LLLF in this study,we investigated the latest surface offset characteristics and slip properties of the main faults in the epicenter area of Gulang earthquake.The distribution characteristics of the surface rupture zones of the Gulang earthquake was comprehensively analyzed.We also analyzed the rupture style and the seismotectonic model of the earthquake,and the structural features of large earthquakes and strain partitioning pattern on the transpressional boundary zone of the NE Tibetan Plateau.The main findings and conclusions are as follows.(1)The LLLF has fresh seismic surface ruptures along the entire ?120 km length,indicating that the most recent event might have ruptured the full-segment of the fault.The analysis of the binned cumulative offset probability density(COPD)for the 192 offset geomorphic markers revealed that the coseismic offset of the most recent event changes along the fault trace and has an asymmetrical bell-shaped distribution.The maximum offset is 7.5±0.8 m,located in the middle-eastern section of the fault,and the offset decreases progressively towards both ends,with an average coseismic offset of ?4.8 m.The cumulative offsets of the four most recent paleoearthquakes are multiples of the coseismic offset of the most recent event,and show similar slip distributions,indicating that the fault ruptures with a characteristic slip.We also show that the binned COPD technique performs better to separate single-event and multi-event cumulative offsets than the full-segment COPD.(2)Six most recent surface rupturing paleoearthquakes of the LLLF were revealed and labeled E1–E6 from youngest to oldest,and their timings were constrained to the following time ranges: 438-350 to present,2951–1155,4016–3609,5325–4476,7284–6690,and 8483–7989 years BP,respectively.Combined with the latest geomorphic activity characteristics of the fault and historical earthquake records,the latest event E1 was most likely the 1927 M8.0 Gulang earthquake.The average recurrence interval of the six paleoearthquakes is 1640±570 years,and the coefficient of variation is 0.34,indicating that the LLLF follows a quasiperiodic recurrence model.The dominated structure,steady state process of strain accumulation and release,and simple and smooth fault trace at seismogenic depths may be additional indicators of a quasiperiodic recurrence pattern for the fault.(3)Due to the discontinuous surface traces between the LLLF and the Laohushan fault,it is less likely that the two faults have ruptured simultaneously.Based on the new understanding of the lastest event,the LLLF should not be a part of the Tianzhu seismic gap.However,the 160 km-long gap section composed of the Jinqianghe,Maomaoshan,and Laohushan faults is still the most hazardous area for major earthquakes in the future.(4)The surface rupture characteristics of the 1927 M8.0 Gulang earthquake were studied from a new perspective.We find that the Lenglongling fault(LLLF)and Southern Wuwei Basin fault(SWBF)might have both ruptured during the Gulang earthquake,but they exhibited different motions,demonstrating the complexity of surface ruptures in large earthquakes.The LLLF shows left-lateral strike-slip motion.The SWBF can be divided into two segments,with an surface rupture zone of ~42 km.The eastern segment shows thrust motion,while the western shows thrust motion with a left-lateral strike-slip component.Thus,the Gulang earthquake may be a multifault rupture event where strike-slip and thrust faults ruptured simultaneously.The seismic moment released when the two faults ruptured were added,and the magnitude of the Gulang earthquake was estimated to be Mw7.6-7.7 using the empirical relationship between the seismic moment and the moment magnitude.The proposed distribution of the surface rupture zones and the estimated magnitude of the Gulang earthquake in this study agree with the earthquake magnitude obtained by previous studies and the earthquake damage distribution with a length of up to 600 km and slow attenuation in the NWW direction.Through the comparison with paleoearthquakes of the adjacent faults,it is found that the combined rupture of the LLLF with the SWBF is not a unique event that occurred only in the 1927 Gulang earthquake but may have occurred at least one other time in the past.(5)Analysis of deep and shallow structures and three-dimensional finite-element modeling reveal that the north-dipping LLLF and the SWBF may converge downward to a low-angle decollement.The two faults released the accumulated strain energy under oblique compression as partitioned strike-slip and thrust motion during the earthquake,indicating that the combined rupture of both the strike-slip faults along the crest of the Qilian Shan and the frontal thrust faults is an important strain accommodation pattern in the northeastward extrusion of the Tibetan Plateau.Through the comparative analysis of other earthquakes,the pattern of deformation partitioning,where oblique block convergence is partitioned into strike-slip motion on steeply dipping faults and vertical motion on gently dipping frontal faults,may have some universality.It also shows that some large earthquakes may not only be the result of tectonic motion of a fault structure,but the behavior of energy accumulation and release of the active block as a whole.(6)The rupture property and style of 1927 Gulang earthquake is different with those of 1920 Haiyuan earthquake and 1709 Zhongwei earthquake to the east,indicating the seismotectonics and strain partitioning pattern in different parts of the NE Tibetan Plateau are quite different.It may be controlled by changes of the regional principal compressive stress directions and fault geometry at depth.The possibility of combined rupture of faults with different senses of motion(such as the 1927 Gulang earthquake)and the differences in rupture behaviors at different parts on the NE Tibetan Plateau need to be considered in future earthquake prediction and seismic hazards analysis.