Tectonic Deformation And Major Earthquakes On The Boundary Faults Of Bayan Har Block

That a series of major earthquakes occurred around the Bayan Har (BYHR)block since1997provides an unprecedented opportunity tostudytectonicdeformationand seismicityon its boundary faults.Since1980s, some short-leveling,short-baseline and creepmeter sites have been operated on the Xianshuihe fault zone(XFZ) and the Longmen Shan fault zone (LFZ), which are the southwest and eastboundaries of the BYHR block, respectively. This thesisfocuses on thetemporal-spatial evolution processes of these cross-fault deformation and the majorearthquakes on basis these data, and combining GPS, focal mechanism solutions, andother relevant data.The temporal variation and the spatial distribution of cross-fault deformation areequally important in the analysis of tectonic activity.Measurements at single sites canonly reflect local variations with time on a fault. To reveal the active pattern of theentire fault requires analysis of the temporal-spatial evolution of cross-faultdeformation by data of multiple sites. In this line of thought,this workcombinesmeasurements at individual and multiple sites along the fault, and long-termand the short-term variations of the cross-fault deformation.And it pays muchattention on the temporal-spatial evolution processes of the cross-fault deformationand their consistency with major earthquakes as well as block motion derived fromthem.The research contents and primary results of this thesis are summarized as thefollowing five aspects:1.Dextral strike-slipopposite to the long-term left-lateral geologic slip alongtheXFZThe sinistral strike-slip rates are in5~15mm/a as revealed by field geologicalinvestigations. During seismically active1973~1981, the maximum strike-slip rate onthe Lushan fault is about10mm/a according to the short-baseline observation.But inthe recent20years, the slip rates at different sites along the XFZ decrease to0~2.49mm/a, suggesting that the XFZ tend to healorbe relocked gradually. Bounded by the Daofu County,many times of right sliphave occurred along the southeastsegment of the XFZ in some periods. Specially, before the2008Wenchuan Ms8.0earthquake (by the end of2007), dextral strike-slip on the southeast segment of theXFZ was observed, implying a strengthened southeastward movement of the BYHRblock before the Wenchuan earthquake. The reversal strike-slip of the XFZ alsoimplies the non-synchronous motion between the BYHR block and theSichuan-Yunnan block, and the dynamic variation of velocity difference betweenthem is the primary cause for the retrograde strike-slip, rather than postseismicoverthrust or earthquake-induced stress perturbation.2. Whole movementof the BYHR blockThe fault slip vectors and temporal-spatial evolution show that:(1) The majorearthquakes around the BYHR block since1997have spawned various changes orresponses in the cross-fault deformation on the XFZ, LFZ and Minjiang fault.(2) Theeffects inferred theoretically from the major earthquakes accord with the observedresults from cross-fault deformation observations, in other words, they are consistentin kinematics.(3) The compression of the LFZ intensifies when the XFZ is in dextralstrike-slip, while it is weakened to some extent when sinistral strike-slip isstrengthened along the XFZ. In view of above three aspects, this work infers thatthese phenomenareflect the wholeness and unity of the movement of the BYHRblock.3. Segmental activity of the XFZBounded by Daofu County, the segmental activities between the northwest andsoutheast segments of the XFZ is outstanding, evident in following aspects:(1) Thecumulative slip of most sites on the northwest segment is larger than10mm, and themaximum value is55.35mm, However, the cumulative slip of sites on the southeastsegment is less than3mm.(2) The stable sinistral strike-slip characterizes thenorthwest segment, whereas the complex strike-slip is notable on the southeastsegment, where dextral strike-slip happens sometimes.(3) The Daofu fault on thenorthwest segment is tensional in nature,while the Qianning fault on the southeastsegment is dominated by compressive activity.(4) Before and after the Kunlun,Wenchuan, and Lushan earthquakes, the Daofu fault and Qianning fault exhibitedcontrary variations, whilearound the Mani and Yushu earthquakes, their changeswererelatively consistent. Because the Daofu County is located on the junction of the XFZand the Longriba fault zone, the segmental activity of the XFZ illustrates that the its northwest segment belongs to the boundary ofthe Aba sub-block, whereas itssoutheast segmentis part of the boundary ofthe Longmen Shan sub-block. Meanwhile,such differential activities of the both segments may indicate that the motion of Abasub-block is relative stable, and the motion of the Longmen Shan sub-block iscomplicated and changeable. Furthermore, the Daofu fault and Qianning fault havedifferent dynamic responses to the different major earthquakes on the blockboundaries, suggesting the common motionor decoupling between the Aba andLongmen Shan sub-blocks during varied times.4. Strike-slip rates of the XFZ observed by different methods and the spatialdistribution features of shear deformation zones.Different strike-slip rates of the XFZ are observed bydifferentmethods.Creepmeters (18.7m to65.1m across the fault) recorded left sliprates of0.08~0.74mm/a, short-baselines(72m to288m across the fault)saw0.04~2.4mm/a, and GPS observations (dozens of kilometers across fault) yielded6~14.4mm/a.These data all show dominant left slip of the XFZ. In contrast, dextralstrike-slip on the XFZ was recorded by all the three tools after the2008Wenchuangreat earthquake. These results indicate that the near-field and cross-fault deformationobservations can reflect the relative motion between the blocks. The fitting analysissuggests that the observed left-lateral slip rates of the XFZ increase with distancefrom the fault following a logarithm function, and the approximate shearing strainrates attenuate with distance following a power function.Andboth tended to be steadybeyond the dynamic influence width of theXFZ.5. Enormous contrast of cross-fault deformation on the XFZ before andafterthe2008Wenchuan and2013Lushan earthquakesIt is surprising that the cross-fault deformation responses of the XFZ to the2013Ms7.0Lushan earthquake are more notable than that the2008Ms8.0Wenchuanevent.In details, the XFZ all experienced dextral strike-slip before these twoearthquakes, and strong extensional movement during the postseismic period. But, thedeformation rates before and after the Lushan earthquake is2~4folds that theWenchuan earthquake. This studysuggests that the Longmen Shan sub-block can bedividedfurther into the Danba micro-block in the west and the Songpan micro-blockin eastby the NW-striking Ma’erkang and Miyaluo fault zones, and the tectonicactivity of these micro-blocks might influence the strain partition of the LongmenShan region and the characteristics of crustal deformation anomalies. The seimogenic structure of the Wenchuan earthquake could be the Songpan micro-block, however,the seimogenic structure of the Lushan earthquake (that is southwest segment of theLFZ) is related closely with the southeast segment of the XFZ because they are all theboundaries of the Danba micro-block, and their tectonic associationis more direct andintense, so the responses of the cross-fault deformation on the XFZ to the Lushanearthquake are more remarkable.This workdeems that it is necessary to set up geodetic instruments of variedspans perpendicular to the fault as a new thought line to the layout of cross-faultdeformation sites.Comparing with the static results based on geological and seismicdata, the most important progress in this work is the discovery of the relation betweenthe dynamic processes of boundary fault activity and the whole movement of theblock. Meanwhile, this thesis points out that the differential movements between thesub-and micro-blocks in the interior of the BYHRblockis not negligible.