| The East Kunlun fault is situated in the southernside of the Kunlun Mountains, which divides the north and south of China tectonics. The East Kunlun fault,Altyn Tagh fault, and Haiyuan faults constitute margin of the northern Tibetan Plateau.They paly a major role in accommodating Indo-Asian convergence as the great lateral extents and spatial relationship to high-angle shortening with large thrust. The accurate slip rates along these faults provide significant evidence in the field of research on the Tibetan Plateau deformation and evolution, determining horizontal slip and deformation degree and establishing plateau deformation and evolution models.The eastern segment of the East Kunlun fault (Maqin-Maqu), that is the eastern border between Kunlun-Chaidamu block and Bayankala block, lies in the northeast of the Tibetan Plateau where Gansu, Sichuan and Qinghai Provinces adjoin.As the most inhabited area of the East Kunlun fault,the research of its geometric structure and present motion features is very meaningful in reality to determining the safety distance for the Maqin and Maqu Town, seismic risk analysis and earthquake prevention.It is also meaningful in theory to test the various controversial models of the uplift mechanism of Tibet and research of tectonic transformation of the regional faults. Based on analysis, collection and summary of previous achievements, this thesis aims to analyze geometric structure of this segment through interpretation of satellite images and field test, to obtain its large earthquake sequence by connection of the tectonic geomorphology and paleoseismology.Finally,this work gets the geometric and movement features of the eastern segment of East Kunlun fault so as to research the lateral slip rate which diminishes sharply from the west segment to the east segment.Based on the analysis and coordination of the existing data,interpretation of ETM、Quickbird and Worldview satellite images, field geological survey as well as tests of age samples and calculation of slip rates on the East Kunlun fault (Maqin-Maqu), this dissertation comes to the following conclusions:1. Geometric structureIn the west it starts from the north pediment of the Animaqing mountain, passes through north of Dongqinggou, Dawutan, pediment of Zhamuer, Dawumuchang, Kendingna, Dagongka, north side of Namucakeer, Xigongzhou, Mohatang, Xikeheyangchang, Tangdi, Oula, crossing over the Yellow River in Keshegntuoluo, spreading on the face of northern Yellow River, then extending to Ruoergai everglade by Maqu and intersecting with the Tazang fault after coming out from northern wetland of Luocha. The total length is around 330 km, it generally strikes in 295°; mainly dips to south west at angles 70°-80°, locally close to upright. It is located in the eastern edge of the Tibetan plateau, which is the middle part of the north-south seismic belt with frequent earthquakes. It belongs to the cross section of east-west and south-north structures, which is quite complicated with so many ruptures crossing each other. On the west side of the Animaqing Mountain,the East Kunlun fault intersects with the Zhongtie fault.On the West side of Xigongzhou and south side of Mohatang, the East Kunlun fault intersects with the west and east strands of the Awancang fault,which form the fault intersection zone at Xigongzhou. The Diebu-Wudu fault is 15km to the North side of the East Kunlun fault, which form a broad dilational log with the East Kunlun fault in left-step manner.2. SegmentationA strike-slip fault has many structure features, such as en echelon secondary farctures, faults, folds and basins, which are combined into a narrow and long band in a certain law. Its geometrical segmentation indicators mainly include step-over,bend,discontinuity, bifurcation, intersection or the width variation and active transformation. Through interpretation of ETM and QuickBird satellite images and utilization of the segmental signs of the strike slip fault, such as the geometrical shape, the combination of the geomorphic features and the fault zone materials, the Kunlun fault (Maqin-Maqu) can be geometrically divided into eight segments, which are named Dongqinggou, Dawutan, Kendingna, Xikehe, Tangdi, Maqu,Moxi and Luocha by sequence from west to east. These eight segments form a left-stepping echelon, except the Tangdi and Maqu segments, which are a right-stepping echelon. The space between segments is small which are well-connected. The evidence of the extensive landform and paleoseismic rupture indicates that the Maqin-Maqu segment has experienced numerous movements. The longest step-over with length around 10km and width 1.3km is the spindle-shaped conjoining connection of the Xikehe segment and Tangdi segment. The smallest step-over is between the Maqu segment and Moxi segment, which form a small pull-apart basin with length 300m and width 200m at south side of Maqu Town. There is a 34°extrusion bend between the Dongqinggou and Animaqing segment,which form the Animaqing uplift. The Moxi segment is mainly buried in the swampland of the Ruoergai basin and intersects with the Tazang fault at north of Luocha,which forms an 10°extrusion bend. The East Kunlun fault intersects with Awancang fault at the south side of Mohatang, which is the segmentation indicators for the Xikehe segment. The extensively distributed landform and rapture signs caused by earthquakes in ancient times show that there have happened many movements.Suface rupture segmentation:The Animaqing mountain is a double restraining bend, long 40km, wide 10km, with the mountain uplift as a barrier and stress concentration.The Xigongzhou intersection zone is a structural discontinued point that becomes another barrier. They can divide the Eastern Kunlun fault into two surface rupture segments, Maqin segment and Maqu segment.3. Slip rate from tectonic geomorphologyHorizontal slip rate:Based on of geomorphic measurements, dating and calculation of long term horizontal slip rates of 30 investigated sites, the long term horizontal slip rates of the three rupture segments since late Pleistocene are obtained. The horizontal slip rate of the fault is 9.3± 2.5mm/a in the Maqin segment,7.4±1mm/a in Xigongzhou intersection zone, and 4.9±1.3mm/a in the Maqu segment since late Pleistocene,respectively.It can be seen that the horizontal slip rate decreases gradiently,by about 4mm/a from Maqin to Maqu segment.The discontinuity is just the Xigongzhou intersection zone, which is the intersection of the East Kunlun fault and Awancang fault and may be the structural transform to the Awancang fault with crustal shortening.The slip rate gradients are corresponding to the geometrical structures.Vertical slip rate:Based on of surveying of geomorphic survey, dating and calculation of long term vertical slip rates of 30 investigated sites, the long term vertical slip rates of the three rupture segment since late Pleistocene are obtained. They are 0.7±0.2mm/a in the Maqin segment,1.6±0.4mm/a in the Xigongzhou intersection zone,and 0.25±0.05mm/a in the Maqu segment,respectively. It is biggest in Xigongzhou segment with ratio is 1:5 to the horizontal slip rate.It reflects the fault is mainly of strike slip with oblique components,which is reverse strike slip at the Dongqinggou,the west side of the Maqin fault, and is normal strike slip at the east of Maqin town.4. Large earthquake sequenceMaqin segment:Seven events are revealed by the trenches on the Maqin segment. The ages of these thirteen events are as follows:(1) 358～430Cal a BP; (2) 977-1090 Cal a BP;(3) (1689-1736) Cal a BP～(2.0±0.3) ka; (4) (3058-3211) Cal a BP～(3342～3454) Cal a BP (5) (6.6±0.7)～(7.2±0.8) ka; (6) (7971～8050) Cal a BP-(8451～8632) Cal a BP; (7) (9.9±1.0)ka～(10.1±1.0)ka. The recurrence interval is about 500-1000a since Holocene. The average interval is 600±100a from event 1 to event 3,whereas the intervals vary largely between 500～1000a from event 4 to event7 since Holocene. The event 1061A.D can be confirmed by the epic of Gesar. The elapsed time of the latest event is 120a,and the another event will come only 180a later to the minimum of the interval, so the hazard analysis should be paid attention to.Maqu segment:Eight events are determined through the analysis of the trenches on the Maqu segment. The ages of these eight events are as follows:(1) 1055～1524a Cal BP;(2)(1210±50)～(1730±50)a BP;(3) (1730±50)～(2530±40)a BP;(4) (3736±57)a～(4586±124); (5) (4850±40)～(7460±60)a BP; (6)(7460±60)～(8690±40)a BP; (7) 9000～10000a Cal BP;(8) (15800±2500)～(24100±2900)a BP. The recurrence interval is about 500-2000a since late Pleistocene.The average interval is 1000a from event 1 to event 3. The elapsed time of the latest event is～1000a,beyond the maximum of latest interval, so the seismic hazard analysis should be made strenuously.The Wenchuan M8.0 earthquake of 12 May 2008 has imposed the stress load to the eastern Kunlun fault and increased the seismic hazard.So we should pay attention to the seismic hazard of the Maqin-Maqu fault and strengthen the dissemination of earthquake prevention and disaster reduction to enhance the consciousness of quakeproof.5 Slip rate from paleoseismology Maqin segment:The latest horizontal slip of the Maqin fault is 4±0.5m.The interval of the paleoearthquakes is 600±100a since 2000a.Using the formula S=D/Rx, and calculated the horizontal slip rate 7±1mm/a, similar to the slip rate 9.4±2.5mm/a from the tectonic geomorphology.Maqu segment:The latest horizontal slip of the Maqu segment is 3m.The interval of the paleoearthquakes is1000a since the latest three events and the calculated slip rate is 3mm/a, similar to the slip rate 4.9±1.3mm/a from the tectonic geomorphology.6 Tectonics transformationBoth the tectonic geomorphology and the paleoseismology suggest that the slip rate gradient decreases. The slip rate reduces sharply by almost 4mm/a when the East Kunlun fault passes the Xigongzhou intersection zone.The reduction is centered at the bend and transverse structural intersection, corresponding to the geometric change and the decreased slip rate transform to the transverse structure,the Awancang fault.By the transformation vector partitioning, the horizontal slip rate of the west strand of the Awancang fault is 2.4mm/a, the horizontal slip rate of the east strand is 1.4mm/a,and the shortening rate is 2.3mm/a.The west strand and the east compose of the slip partitioning mode, by the vectorial resultant, the horizontal slip rate between the bilateral block is 4.6 mm/a relative motion at the direction of 112.1°.7. Formation times of terraceThe terraces primarily formed in 7 periods, i.e. 1-2ka,3-5ka,7-10ka,12.5-15ka,28-35ka,40-45ka,65-70ka, respectively corresponding to this area’s climate characters. The time 1-2ka mainly corresponds to the interglacial epoch after neoglaciation,3-5ka corresponds to the warm humid phase of the Holocene thermal maximum,7-l0ka corresponds to the humid phase of early big warm period in Holocene,12.5-15ka corresponds to the last deglaciation,28-35ka, 40-45corresponds to the abnormal warm period of the last glacial maximum and abnormal megathermal period of the Interstadial of the last glacial age,65-70ka corresponds to the early Interstadial of the last glacial age. So we can see the terrace formation is mainly influenced by the humid and warm climate. |
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