Boundary Features Of The Seismic Rupture Segments Along The Xianshuihe Fault Zone

As an important left strike-slip fault zone on the southeast margin of the Tibetan plateau which is in the strong deformation field of the Asian continent, there are abundant history earthquake records, distinct tectonic geomorphology and surface rupture feathers on the Xianshuihe Fault zone, whereso it is an ideal is a perfect region for quantitative study of active faults and test of seismic rupture model test ----the independent rupture model or cascade rupture model. This fault zone attracts much attention of geologists all around the world. Further research about the Xianshuihe fault zone is a valid way to know more about the kinematics and dynamics mechanisms of the eastern margin of the Tibetan plateau, and also is a good window of understanding recurrence of major earthquakes on the large strike slip fault zone at block boundaries.Lots of achievements have obtained about the Xianshuihe fault zone, especially in geology. Many studies are concentrated on the geometry, dynamitic feathers on the fault zone, besides that, several scholars have studied the segmentation of the Xianshuihe fault zone, but the segment boundary is hardly been focused on. while, the research degree of the segment boundary is very important for knowing better about the fault activity and its relationship with strong earthquakes. So we choose the segment boundaries of the Xianshuihe Fault zone as the main study object for this thesis, and expect to know more about them.There have been numerous research of active fault in this region, which suppliy a good foundation as well as a higher requirement for this work., The thesis begins with the characteristics of the segment boundaries of the Xianshuihe fault zone ,building their evolution models, then discussing the relationship between the boundary feathers and the seismic rupture of fault segments. Finally, it discusses the fault acvitvity feathers of the Xianshuihe fault zone and the recurrence rules of strong earthquakes. the main aspects of this thesis rae, some progresses are as follows:1)I have analyzed the feathers of the segment boundaries of the Xianshuihe Fault zone from three aspects including fault activities, tectonic geomorphic feathers and stratum sedimentation feathers,and furthermore, discussed the relationship between the evolution process of the segmentation boundary and the fault activities. 2)I have determined the segmentation rupture behave features by analyzing of the evolution stages of segment boundaries.3) Seismic risk of the Xianshuihe fault zone has been evaluated briefly.By comprehensive application of geology, geomorphology, stratigraphy and so on, this work studied the activity law of fault segment boundaries, and reconstructed evolution process. Combined with the data of historical earthquake and paleoearthquake rupture, it analyzed earthquake rupture segmentation behavior for the rupture zone of the Xianshuihe, and then studied the stability of segment boundaries and provided the basis for analyzing the hazard of regional earthquake. Meanwhile, the above study result could be used as the scientific basis of understanding for the earthquake rupturing processes of large scale strike-slip fault zones and their special migration law, and modeling the kinematics model for Chuandian active block. Base on gathering and analyzing available research results, I investigated and analyzed the geological and geomorphic characteristics of the rupture zone. Combined with geology, surveying, chronology and so on, by means of remote sensing, GIS, geology, geodesy and so on, I studied the activity features, the geomorphologic features and the sedimentary pattern of the segment boundary in the Xianshuihe fault zone, and recovered and modeled the evolution history for the fault segment boundary, and then discussed the relationship between the segment boundary evolution and the segmental rupture behavior of the fault zone. Overall, some conclusions can be made below:1、Rupture segmentation of the Xianshuihe fault zoneCombined with the data of the geometrical distribution pattern of the fault zone, historical seismic ruptures seismic activity, deep structure, fault sipping rates, paleoearthquakes, recurrence law of major shocks, and the elapsing time of the last seismic event, the Xianshuihe fault zone is divided into 8 segments ,i.e Luhuo segment, Daofu segment, Qianning segment, Yalahe segment, Selaha segment, Zheduotang segment, Xuemenkan segment and Moxi segment.2、Segment boundary features and the relationship with segments rupture behavior.1)The fault activity evidence indicates that, the Luhuo segment of Xianshuihe fault zone has a trend of south-east extending, and the Daofu segment has the trend to migrate toward south-east; In the Xialatuo basin, there is a moving trend from northwest to southeast at the overlapped area of the two active boundary faults.According to well developed basin terrace characteristics and stratigraphic sedimentary feature, it can be seen that the new sedimentary center had moved from northwest to southeast. The motion of the major boundary faulting strength center controlled the basin evolution history and the moving of new sedimentary center, and the basin is still developing with a high ratio of length to width. The Xialatuo basin could control M7.5 earthquakes at the Luhuo segment and Daofu segment, but the greater earthquakes could accelerate the southeasetward of the Luhuo segment, and make the two segment link andgenerate cascade faults.2)The latest activity of the Daofu segment and Qianning segment of the Xianshuihe fault zone has a tendency of migration from north to south.The fault activity in the Xlialatuo basin migrated from outside to inner of the basin, and the activity of the boundary fault is affected by the normal faults in the basin include Zilong fault. Feathers of the terrace development and stratigraphic sedimentary reflect that the deposit center migrates from the east part of the basin to the center, which is controlled by the boundary faults activity on two sides of the basin. Earthquakes of ~M 7.5 on the Daofu segment and Qianning segment are restricted by the Daofu basin, but it seems highly possible that the two segments rupture together, with earthquakes of M 7.5-8,and the two segments rupture in a cascade manner.3)The evolution of the Qianning Basin is controlled by the Qianning segment of the Xianshuihe fault zone and Zhonggu fault,and the basin is now in the intial stage of development. The deposit center is in the southeast of the basin. The apart distance of this pull apart step-over and the adjacent segment south of it is more than 10km ,a thus , it can be regarded as a persistent segment boundary.? 4) The apart distance between the segments of the Xianshuihe fault zone around the Zheduoshan pop-up range are from 9 to 13km, and the segment boundary here can be considered as a persistent boundary of the Xianshuihe fault zone. There is a bend of 10-15°between the Yalahe segment and Xuemenkan segment of the Xianshuihe fault zone. Differences of the historical seismic rupture feathers, kinematic feathers (strike slip or reverse with strike slip) and slip rates between the two segments together with the local compressive uplift of the Gongga Mont. show that the bend is a permanent segment boundary, and it is almost impossible for the two segments to rupture together in a cascade mode .5)The segment boundary of the Yajiageng pull apart step-over is a nonpersistent boundary, which can’t stopt propagations of strong earthquakes.6)The Xialatuo basin and Daofu basin in the northwest of the Xianshuihe fault zone can be considered as a segment boundary, without persistence and stability,which may propagate, and the maximum magnitude would be M8.0 if the three segments all rupture through a at the same event. The Huiyuansi basin and Zheduoshan pop-up are persistent boundaries,resisting seismic ruptures propagating from each other, and so the Zheduotang segment,Selaha segment, and Yalahe segment on the middle of Xianshuihe Fault zone would rupture independently. The Yajiageng pull-apart step over on the southeast segment of the Xianshuihe fault zone has been overlapped before, and so it is an nonpersistent segment boundary.The Xuemenkan segment and the Moxi segment can be ruptured in a cascade rupture way.3 The potential seismic source zone and the upper limit magnitude analysisAccording to the principle and rule of the potential seismic source zoning, the Xianshuihe fault zone can be divided into eight basic potential seismic source zones as Luhuo segment, Daofu segment, Qianning segment, Yalahe segment, Selaha segment, Zheduotang segment, Xuemenkan segment and Moxi segment. In this case, their upper limit magnitudes are Ms7.7,Ms 7.7,Ms 7.5,Ms 7.7、Ms 7.3,Ms 7.3,Ms 7.5 and Ms 7.6,respectively. By considering the cascade rupture possibility, five main potential seismic source zones can be determined, and the upper magnitudes for which are Ms7.9、Ms 7.7,Ms 7.3,Ms 7.3,Ms7.8, respectively.