Characteristics Of Present-day Crustal Deformation And Seismic Hazard Analysis In The Western And Central Tian Shan

With the remote influence of the far field from intense collision and successiveextrusion of the India-Eurasian plate, the Tian Shan orgenic system has been activeand uplifting again since Cenozoic, then it becomes the largest orogenic system incentral Eurasia. Results of GPS observations, spanning from74°to76°, show that thecrust has been shortening with a rate of20mm/a, accounting for almost half of thetotal shortening rate between the India plate and the Eurasia plate. According to theNational Earthquake Monitoring Network System(ANSS) and Xinjiang earthquakecatalog, from1889to2012,50events,which is larger than Ms6.0,6events of Ms7.0~Ms7.9, and3events of Ms8have occurred in western and central Tian Shan (70°E~80°E). Similar to Earthquake Wenchuan Ms8of2008due to intense coillison, slowdeformation and local structural setting of Eastern Tibet, the western and central TianShan is a typical inland tectonic belt and earthquake-prone region and will get moreand more attention with great reason.Since the1990s, the GPS, which is an important technology of geodesy withhigh-accuracy, high-efficiency, low-expense and all-weather survey, provides arevolutionary method for measuring crustal movement, tectonic deformation of variedscales. Recently some research teams have made a lot of GPS measurements incentral and western Tian Shan. Besides，previous studies have probed the hierarchicalstructure of the Earth’s lithosphere by deep seismic wide-angle reflection/refractionprofiles，mobile seismic arrays，deep seismic reflection profiling，and inversions ofgravity and magnetic data. They are very important constraints data to makequantitative research about crustal deformation characteristics (such as shorteningdeformation distribution across the Tian Shan and tectonic changes) and seismichazard analysis. Focusing on these scientific issues，this thesis presents research workin five aspects as follows:1. Collection and processing of geodetic data in the western and central TianShan.GPS observations in western and central Tian Shan began in1992, and a numberof relevant projects provided a wealth of GPS data in this region afterwards. Thiswork has collected data of396stations in the western and central Tian Shan. Amongthem, the Chinese Tian Shan, funded by many scientific and technological projects,China Institute of Seismology and Earthquake Administration of Xinjian have madejoint efforts to carried out multi-period GPS observations, such as China Crustal Movement Observation Network, Continental Tectonics Environment MonitoringNetwork of China，the GPS observation project about the modern tectonic movementand crustal deformation of the Jiashi earthquakes and the movement of thenortheastern Pamir，which is96-913-07-03project of the Ninth-Five Year PlanNational Key Scientific and Technological Plans. In the northern Tian Shan(Kyrgyzstan/Kazakhstan)，funded by the National Science Foundation，MassachusettsInstitute of Technology (MIT) and several universities have made a lot of joint GPSobservations with Germany, Russia, Kyrgyzstan, and Kazakhstan since1992.As it is impossible to collect the original GPS data from324sites of the Tian Shanoutside China and54sites of South Tian Shan inside China, this thesis only uses theGPS velocity data under the ITRF2005framework and stable Eurasia framework.For data of34stations from China Crustal Movement Observation Network andChina Crustal Movement Observation Network, Continental Tectonics EnvironmentMonitoring Network of China, this work adopts the GPS data processing softwareGIPSY and post-processing software QOCA which were developed by NationalAeronautics and Space Administration (NASA) and Jet Propulsion Laboratory (JPL)to get the newly velocity of western Tian Shan, Finally using the common station asbridge, this work computes the Euler rotation parameter between them, then convertsthe velocity of Tian Shan to the frame of this work by adopting the Euler parameterand gets the dense, consistent and delicate velocity of Tian Shan, which providesimportant material for constructing the dynamic model for research of crustalmovement and tectonic deformation.2. Quantitative analysis for characteristics of present-day crustal deformation inthe western and central Tian Shan.Based on the high-accuracy and high-density GPS velocity field in the westernand central Tian Shan，this work analyzes quantitatively the features of crustal motionand tectonic deformation in this study region. The results are as follows:(1) Relative to stable Eurasia plate，the NE-directed horizontal movement velocityacross the western and central Tian Shan decreases from20mm/a in the southern TianShan to2mm/a in the northern Tian Shan (Kazakhstan), implying the Tian Shan isintensively shortening and uplifting.(2)Through the selection of the whole fixed framework of Tian Shan orgenicsystem，removing the “rigid rotation” of the Tian Shan orgenic system and itssurrounding GPS stations，the results further highlight the difference of the horizontal movement inside the Tian Shan orgenic system.An obvious extrusion-type “flow slide” movement exists in the mid-westernTian Shan. The west side of it flows along the Talas-Fergana fault zone，andeast side moves eastward along the northern Kemin fault zone.In addition to the6~8mm/a north-south extrusion shortening of the Kalpinthrust system，the whole has an obvious eastward “flow slip” relative to thethe Tian Shan orgenic system. This work also speculates that the Maidan faulthas a left-lateral slip of1~3mm/a. As to the combination of the Kashi sag andKalpin thrust system，the north-south extrusion shortening is only about1~2mm/a.The activity of Talas-fergana fault today has an obvious feature ofsegmentation. The northwest segment has2~3mm/a right-lateral slip rate，themiddle segment has no obvious movement，and the southeast segment notonly has3mm/a right-lateral slip rate but also has a large tensile component.Both the Chon-Kemin fault and Chilik fault have obvious left-lateral slip of2~3mm/a.(3) Based on the GPS velocity field，this thesis calculates and analyzes thecrustal strain rate and shear strain rate of the mid-western Tian Shan.The entire Tian Shan orgenic system was strongly squeezed. The max strainrate of the junction area of Pamir and Tian Shan is221.2nanostrain/a. There issome tension existing in the Kalpin thrust system，north part of Kashi sag andsome other places.Limited by the Piqiang fault-west Issyk lake，the strain rates of eastern TianShan orgenic system is higher than the western Tian Shan. To the west inKyrgyz, the Naryn basin and3other compressive basins are of low strainrates lower than20nanostrain/a. Inside the orogenic belt near the Natila fault，there is a low strain rate area called”strips”.The strain rates are relatively low in the basin-range transition north of theTian Shan orgenic system. In addition to the junction area between the end ofthe eastern Alamtu fault and Yili basin where the strain rates are relativelyhigh (more the40nanostrain)，the strain rates of the other areas in the northernKyrgyz Mountains are lower than20nanostrain/a. While in the transition zonebetween the basin and southern Tian Shan orgenic system, the strain rates aremore than40nanostrain/a，excluding the eastern Kashi sag and Kalpin thrustsystem where the strain rates are lower than20nanostrain/a. (4)Based on the GPS strain rate field，this work calculates the seismic momentaccumulation rate which is directly related to seismic risk. By the overlay analysis ofthe max shear strain rates，seismic moment accumulation rates and earthquakedistribution，it finds that the area with higher shear strain rates and earthquakemoment accumulation rates has relatively frequent seismic activity. However in someareas where the crustal deformation is relatively strong，the seismic activity is notfrequent like the western Naryn basin and the eastern Kalpin thrust system. Thisshows there is a relatively complex relationship between the strength of crustaldeformation and seismic activity.3. Tectonic kinematic model interpretation of crustal deformation of the westernand central Tian Shan.Restrained by the horizontal velocity field derived from data of the396GPSstations and the long-term average rates of movement of some active faults，using athree-dimensional deep-fault dislocation model，this work establishes an kinematicmodel of crustal deformation and active faults for the western and central Tian Shan,and inverts long-term slip rates of major faults. Then, based on this model andforward calculation, this thesis acquires the three-dimensional continuous distributionof crustal motion and the strain rate field for the whole region, which are statedbelow:(1) GPS crustal movement velocity field in the western Tian shan area can besimulated well by the fault dislocation model of tectonic kinematics. Most residuals ofthe GPS displacement rates are less than3mm/a，close to the average observationalaccuracy of2~3mm/a. In the basin-range transition zones on either side of themid-western Tian Shan，the thrusts slip rates derived from inversion are different fromthat from geological research. This is mainly because the locked sections of thebottom detachments of the thrust faults go down deeply into the Tian Shan. Amongthem，the northern Tian Shan fault goes deeply into the Issyk-Kul basin and SusaMyer basin. The bottom detachment latching segment of the Kalpin thrust system inthe southern Tian Shan does not run along with the front fault of Kalpin-Tage thrustsfault，instead goes down into the Tian Shan, in agreement with the Maidan-Kalatiefault.(2)Because each of thrust folds of the Kalpin thrust system has a listric feature,the crustal deformation cannot be simply explained by a simple three-dimensionaldeep-fault dislocation model. This work establishes a geometrical model for theKalpin thrust system based on the geometric characteristics of each of thrust fold zone. The inversion results using this model match largely with the GPS observations,except some stations. The reasons for these discrepancies are:①The model of thisthesis is a simplification to a complex fault system，in which only two fold thrustfaults have been taken into consideration. In the real Kalpin thrust system，the KalpinTage thrust fold and the Yimugantawu thrust fold belt are most intensive in activity,and the others are relatively weak. While the Kalpin thrust system is made up by4~6rows of fold thrusts，the velocity from GPS observations is different from the velocitycalculated by the model in the rear faults.②Due to lack of sufficient geophysical dataof deep structure，it is impossible to determine accurate depth of the deep detachmentsurface of basement-involved thrusts.③Because of the limitations of field geologicalwork, reliable data cannot cover all fault segments and all faults.(3) The forward calculation using the kinematic model yields thethree-dimensional velocity field of present-day crustal movement of the study area. Itappears that the uplifting rate of the western and central Tian Shan is about2.0±1.5mm/yr relative to the Tarim basin. Taking into account the uplift history and theuplifting amount, this work infers that the uplift in the western and central Tian Shanis progressive process from weak to intensive, in which the long-term denudation rateis close to the long-term uplift rate from GPS, which is about1.0mm/yr.4. Seismic hazard analysis of the Kalpin-Aksu regionAs to the project “long-term prediction of seismic hazard of Ms7.0~Ms8.0earthquake in China”, which is based on the seismic gaps in second-order active blockboundary belts and crustal deformation with high gradients by GPS observations，theKalpin-Aksu focused earthquake-monitoring area was preliminarily delimited in thesouthwestern Tian Shan. Taking the long-term viscoelastic relaxation effects of lowercrust and upper mantle into consideration，this work uses a viscoelastic static stresstriggering model to calculate the loading/unloading conditions of Coulomb stress onmain active faults of this focused area in the past nearly100years (1898~2012),during which31major earthquakes had happened in the southwestern Tian Shan andits surrounding areas. At the same time，stress accumulation under long term tectonicmovement loadings from the GPS observations are added. The results are as follows:Among the joints of the western segment of the Kalpintage reverse fault and theAtushi-Bapan Watermill reverse fault，the western part of Maidan fault，the westernpart of Kalatieke fault and the divergence of the Kalpintage reverse fault，theCoulomb failure stresses are in a loading state (triggering zone), implying a big seismic risk. At the Dakoushan-Wujianfang section of the Kalpintage reverse fold beltand the Aozigeertawu fold belt-reverse fault，northern part of the Wushi reverse fault，South Tian Shan range-front reverse fault and the Yijianfang section of the Kalpintagereverse-fold belt nearby，the Coulomb failure stresses are in an unloading state(inhibitory region), meaning low seismic hazards. On many rows of reverse-fold belts,such as Piqiangshan, in the Kalpin thrust system, as well as the Qiulitage andGumubiezi thrust faults, and the Ayikule section of the Kalpintage thrust fold zonenorth of the Kalpin fork, the cumulative Coulomb stresses are relatively smaller,indicating these areas are little affected by seismicity of surroundings (uncertainregion).