Holocene Rupture Pattern,seismic Recurrence Feature Of The Lenglongling Fault Zone And Its Tectonic Implication For The Northeast Tibetan Plateau

The Lenglongling fault zone?LLLFZ?is situated in the eastern margin of the giant arcuate tectonic belt of the NE Tibetan Plateau?NETP?,and it is an important left-lateral strike-slip tectonic belt on the NETP.Meanwhile,the LLLFZ also compose the important part of the Qilian-Haiyuan tectonic zone?QHTZ?,whose tectonic deformation and seismic activity also reveals the tectonic accommodation of the QHTZ to the NETP.However,we know little of the LLLFZ compared with a series of fault zones with strong tectonic setting surrounding the LLLFZ.The slip rates of the LLLFZ since late Pleistocene were constrained to a large range.And also the active behavior and recurrence model of the LLLFZ need to be studied further,which has led to lack of understanding of the Holocene activity.In addition,the LLLFZ,Jinqiang River fault,Maomao Mountain fault zone,Laohu Mountain fault and the Haiyuan fault zone in the east all belong to the?Tianzhu seismic gap?.The two earthquakes occurred in in 2016 and 1986 on the LLLFZ made it very urgent to study its seismology and geology of the LLLFZ.It is very important to study the Holocene rupture pattern,the seismic recurrence features of LLLFZ and meanwhile to clarify its deep structural features and crustal deformation patterns and finally to construct the seismologic structural model.All the likely results will of benefit to understanding the influence of the LLLFZ on the regional seismicity and will provide significant information earthquake disaster prevention in this area.In addition,the results will also promote the understanding of inheritance relationship of tectonic deformation along the QHTZ and enhance the cognition of the regional seismic tectonic environment and tectonic dynamics,as well as the tectonic implications of the LLLFZ for the NETP.In view of the above scientific problems,this dissertation will use the terrestrial LiDAR and remote sensing data to develop a quantitative method for research of active faults based on high-precision survey of tectonic geomorphology.Firstly,interpretation markers of different types of active faults and tectonic micro-topography will be established based on RS in order to guide the application of high-resolution RS in the quantitative analysis of active faults and tectonic landforms.Based on the detailed structure and geometric features of geomorphic markers,the deformation features of active faults and micro-topographic structures will be analyzed.And the relations between different types of active faults,interpretation elements and RS images will be analyzed.Secondly,this paper will focus on the construction of the detailed structural models of typical faults and offset landforms,the pre-earthquake morphologic markers will be matched to reconstruct the fault rupture processes.The cumulative slips of offset landforms will be estimated and the cumulative offset probability density method will be used to study the seismic slip distribution and fault rupture patterns.Combined with the determination of fault slip rate,the slips single seismic events in different sections will be determined and the seismic recurrence intervals since the Holocene will be estimated.Finally,the regional geology,geodesy and geophysical data will be combined to analyze the regulatory mechanism and tectonic significance of the LLLFZ for the NETP\,and to discuss the tectonic dynamics of this area.Many achievements are made through our research,which include five aspects.?1?On the basis of tectonic geomorphology theory and the remote sensing method,this paper analyzes the current situation of the high-resolution RS on application of active fault survey,especially the differences between the active fault and traditional geological survey.The complex and diverse tectonic micro-topography models and their manifestations formed under the three different fault modes of tension,compression and shear are summarized according to analysis of fault formation mechanics.In addition,the identification markers and analysis methods of different active faults and related micro-topography are supplied based on analysis of typical topographic features on active faults.We divided the RS images related to the active faults into four group,which are RS features of the offset landforms by active fault,the derived tectonic geomorphologies by active faults,the indirect RS features related to active faults and the RS features of inactive fault.Through analysis of a large number of typical high-resolution RS images,we construct the relationship among the RS interpretation markers,tectonic geomorphologies and active faults.We should pay more attention to the similarities and superposition of tectonic landforms of different active faults.And the tectonic geomorphology models and fault mechanics environment should be comprehensively analyzed according to the RS interpretation markers.?2?A series of offset streams,offset terraces and pluvial fans are found in the eastern and middle part of the LLLFZ by both field survey and RS interpretation.Therefore,we measured the typical offset landforms in high-precision by the terrestrial LiDAR technology.And more attentions are paid to the Taola,Chailong gully and Niutou gully where we have found series of offset streams and offset terraces.Through the analysis of detailed structures of four phases of terraces in the Chailong gullies,the relationships among fault activity,aggradation sedimentation and landform degradation were studied.The rupture history of fault was recovered and four paleoearthquakes were identified.According to analysis of multi-seismic events cumulated slips and the single-seismic slip in both Taola and Chailong gullies,we concluded that the LLLFZ obeys the quasi-periodic rupture mode.Dislocations produced by the single-seismic event is about 9 m.Based on the geological sample dating in the Tata gullies,the slip rate of the Holocene of the east LLLFZ is estimated which is about 6.6±0.3 mm/yr.?3?We studied the extracting of offset landforms based on high-precision relief data and high-resolution RS image data.And we also analyzed the recovery and reconstruction of fault rupture processes based on offset geomorphologic markers and influential factors of the offset landforms.Then,more than 300 left-lateral offset landforms were extracted within 120 km of the LLLFZ based on the RS images and the terrestrial LiDAR data,including the deflected gullies,terraces,pluvial fans and moraine ridges,and the cumulative slips were estimated for each Holocene offset geomorphologic markers.The cumulative offsets in different segments of LLLFZ are statistically analyzed based on cumulative offset probability density method.Offset density peaks in the eastern section of the fault obey an exponential function,which also reveals at least seven paleoearthquakes event occurred during the Holocene in the eastern segment,and the average single seismic slip is about 8 m.The cumulative offset in the western segment of the fault increases gradually from the west to the east.The slip of the single-seismic is about 2 to 3 m in the west end and increase to about 5to 7 m in the east end.The distributions of seismic displacement of both the eastern and western sections of the LLLFZ reflect that the fault conforms to the quasi-periodic rupture pattern.The eastern segment is typical left-lateral strike-slip movement and the western segment behaves a weaker left-lateral strike-slip movement.According to the empirical statistical formula among the different fault rupture parameters,the magnitude of 8.0 was estimated of the LLLFZ during the Holocene era.We recalculated the recurrence interval of the large earthquake which is estimated to be 1218±132 yr.The LLLFZ is similar to the Jinqianghe fault,Maomaoshan fault,Laohushan fault and Haiyuan fault in the eastern part of the fault zone,all of which belong to the millennium recurrence intervals.?4?The InSAR co-seismic deformation field of the Menyuan earthquake is measured by the Sentinel-1A SAR data.The results incidated that the co-seismic deformation map is distributed around the secondary fault in the northwest side of the Lenglongling fault zone,where the earth surface uplifts as a tent during the seismic event.Besides,the best-fit slip model results indicate that the epicenter of the 2016earthquake is located in the depth of about 8 to 11 km,with the maximum slip of 0.45m in the depth of 9.5 km.The cumulative seismic moment is up to 9.9×10¹⁷ N?m.The1986 and 2016 events occurred on the western and eastern curving areas of the northern secondary branch fault,and its two ends of the branch fault bend gradually and at last join together with the Lenglongling main fault.Both two fault branches compose the left-order alignment and right-order alignment in the surface geometry in the two ends,and these structures give birth to the releasing bend and restraining bend in the two ends due to the left-lateral strike-slip movement of the LLLFZ.Therefore,the normal and thrust faults are produced in the two bending areas between the two fault branches.Therefore,the focal mechanism solutions of the 1986 and 2016seismic events present the tensional and extrusive movements and both events all present strike-slip movement.?5?This paper studies the detailed crustal structures and tectonic deformation pattern of the LLLFZ and its surrounding regions constrained by the geophysical and geodetic data,and the crustal structure and seismic structural model of this area which can reveal the regional earthquake structure background were built.The gravity field inversion results show that the QHTZ is one important geophysical boundary in the NETP.The field depth of gravity anomalies extends to the lower crust and deeper,with the distribution of gravity anomalies closely related to the distribution of geological bodies.Both the Gobi-Alashan block and the Ordos block behave a wide range of high gravity anomalies,reflecting the shallow Moho interface and the higher density of lithosphere,which also are much more intact and rigid blocks.The NETP shows a wide range of low gravity anomalies with high-and low-anomalies paralleling with each other,reflecting the deeper Moho interface and the lower lithosphere density,which also reveal this region is squeezed intensively by the Tibetan Plateau under north-east compressive stress.Therefore,the crust of the NETP was strongly shortened,thickened and folding deformed.According to GPS velocity field,the deformation field of the NETP rotates clockwise due to the blocking effect of the Gobi-Alashan block and the Ordos block.This variation of the GPS velocity field reflects the complication of tectonic deformation under the influences of the three blocks,which are Gobi-Alashan rigid block in the north,the North China Craton block in the east and Tibetan Plateau in the south.The three blocks are deformed by the Eurasian Plate in the north,the Pacific Plate in the east,and the Indian Plate in the south,respectively,which together give rise to the complexity and diversity of tectonic deformation of the NETP.At last,we constructed the seismic tectonic model and the three-dimensional crustal structure model of the LLLFZ and the surrounding area.