Wulanwulahu-Yushu Fault is an important active fault at the boundary area between Bayankala block and Qiangtang block.The slip rate of this fault in the late Quaternary is of fundamental importance for analyzing the seismic hazards and kinematic characteristics of neighboring active blocks.Due to the limited field conditions,the study of the late Quaternary fault activity on the eastern segment of this fault is rare.On the basis of interpretations of remote sensing images and field investigation,the eastern segment of this fault is active since late Quaternary and the offset geomorphic features are prominent,displaying left-lateral and high-angle thrust faulting.Using the methods of detailed geomorphic mapping,geological survey,differential GPS survey,optically stimulated luminescence(OSL)dating and radiocarbon(¹⁴C)dating,this paper makes an analysis of the late Quaternary geomorphic features and slip rate of the eastern segment of this fault.Four typical sites on the fault,including Xiabatang,Zhada,Shangbatang and southwest corner of Batang Basin,were selected for the field investigation.At Xiabatang,the vertical slip rate of the fault is 0.23～0.28mm/a since around 22～27ka BP,0.40～0.51mm/a since 4～5ka BP,and left lateral slip rate is 6.0mm/a since 27ka BP.At Zhada,the vertical slip rate is 0.23mm/a since around 24ka BP.At Shangbatang,the vertical slip rate is 0.45～0.63mm/a since around 6～9ka BP.At southwest corner of Batang Basin,the left lateral slip rate is 4.0mm/a since 150ka BP.The average vertical slip rate of the eastern segment of Wulanwulahu-Yushu Fault is 0.23～0.28mm/a since late Pleistocene,the average vertical slip rate is 0.40～0.63mm/a since early-mid Holocene,and the left lateral slip rate is about 4～6mm/a since late Quaternary.

The Longling-Ruili Fault is an important northeast trending active fault in southwestern China which controls the development of the sedimentary series and magmatic action of the two sides of the fault,and also controls Longling Basin,Mangshi Basin and Zhefang Basin along the fault.Due to limited Quaternary sediments samples and harsh natural conditions,the study of late Quaternary fault activity on the northern segment of the Longling-Ruili Fault is lacking and presently the time of the newest faulting and the Quaternary slip rate are not clear.Based on interpretation of remote sensing images,quantitative geomorphologic deformation measurement and dating of young terrace deposits and alluvial fans,the paper obtains some new results as follows:The northern segment of Longling-Ruili Fault is a Holocene dextral strike-slip fault with a normal slip component.The terrace T1 composed mainly of alluvial deposits formed during 4ka BP was offset by the northern segment of the Longling-Ruili Fault,with left-lateral and vertical displacements of 8～12m and 2m,respectively.The late Pleistocene alluvial fan was dislocated with a left-lateral and vertical displacement of 70m and 18m,respectively.The strike-slip rate of the Longling-Ruili Fault is 2.2～2.5mm/a and vertical slip rate is 0.6mm/a since late Pleistocene epoch.The strike-slip rate of the Longling-Ruili Fault is 1.8～3.0mm/a and vertical slip rate is 0.5mm/a during the Holocene epoch.The proportion of horizontal to vertical displacement is about 4:1,that means the vertical rate on the northern segment of Longling-Ruili Fault is about 25%of horizontal slip rate.The left-lateral slip rate in late Holocene is well consistent with GPS measurement.The strike slip rate is in good consistency in different time scales since late Pleistocene epoch,indicating that the activity of the Longling-Ruili Fault is stable.

Field investigation on the surface ruptures of the M_S8.0 Wenchuan earthquake along the segment between Beichuan and Qingchuan shows that there is one surface rupture zone developed on this segment,which extends generally along the Beichuan-Qingchuan Fault zone.Observation at Huangjiaba Chenjiaba,Guixi,Pingtong,Nanba,and Shikan suggests that the surface ruptures on this segment spread continuously along the trend of the fault,with single structure and a length of 60～90km.The surface rupture has not reached Guanzhuang of Qingchuan county.The observable rupture zone is about 62km,between Beichuan and Shikan,trending 20°～55° in general,dipping NW with an angle of 70°,showing mainly thrusting with dextral strike-slipping.The most distinct feature of the surface ruptures of this earthquake is the vertical surface bending,which indicates the thrusting of the deep fault.Its horizontal motion on this segment displays as dextral strike slipping,without sinistral slipping component.The value of the vertical coseismic displacements decreases gradually from 3m at Huangjiaba to about 1.5m at Nanba and Shikan;The amount of the dextral displacements does not change evidently,generally between 1.5m and 2.0m.Features of the surface rupture show that the causative tectonics of this M_S8.0 Wenchuan earthquake is the Yingxiu-Beichuan-Qingchuan Fault,whose movement is characterized mainly by thrusting,with a dextral slipping component,and the thrusting direction is from west to east.

In-situ stress measurement was carried out near the eastern segment of Wanyaogou Fault,western segment of Yamalike Fault,and middle-eastern segment of Xishan Fault in the urban area of Urumqi.The characteristics of tectonic stress magnitude,direction and distribution were obtained.The borehole HFZK3 is located on the south side of the eastern segment of Wanyaogou Fault,the depth is 62.13m;The borehole HFZK5 is located on the south side of the western segment of Yamalike Fault,and the depth is 30m;The boreholes HFZK9 and HFZK12 are located on the middle and eastern segment of Xishan Fault,and the depth are 34m and 54.5m,separately.According to stress measurement data,the direction of maximum horizontal principal stress is NE-NEE,different with the direction of regional tectonic stress of SN-NNE.This indicates that,effected by fault activity,the stress state near fault is obvious different with the regional tectonic stress field.The activity of fault zone is discussed using stress measurement data.The horizontal stress increases with depth,and its relation with vertical stress is σ_H>σ_h>σ_v.Such stress state is propitious to reverse faulting.The result is consistent with geological study.According to Coulomb friction criteria and taking m=0.6～1.0,the stress state near fault reaches or exceeds the critical value of generating frictional sliding along reverse fault,indicating that the current tectonic activity is reverse faulting.

Based on the result of quantitative research on active faults,analysis of regional seismicity,and especially the detailed investigation and systematic summary on paleoearthquake in Urumqi region,the recurrence mode and probability models of Xishan Faults that reflect the seismic,geological and kinemics characteristics are established.The strong earthquake recurrence mode considers the Poisson mode and quasi-period mode and reflects the randomicity and determination with chanciness of seismicity on the fault.The probability model involves two levels of content:analyzing the paleoearthquake data and historical strong earthquake catalog on or near the faults to obtain the mean recurrence interval of surface-rupturing earthquakes(M_S≥6.7),and analyzing the modern instrumental earthquake catalog(1970～present,and the size of these earthquakes is in the range of 2.0～5.0) of potential seismic source zone where the active faults locate.The calculation of seismic hazard probability of active faults utilizes the expert-opinion method to assign different weights to the models such as Poisson model and BPT model,and then incorporate the weighted probability result to construct combined model,which is the core of the method.This data-processing method has two advantages:taking complexity of strong earthquake recurrence mode into consideration and reducing the possibility of overestimating the seismic hazard of active faults.The result shows that the recurrence probability of surface rupturing earthquakes is 4.0% in the coming 100 years,and 3.6% in the coming 50 years.This result is consistent with that of the method based on stress environment analysis.The maximum magnitude of Xishan Faults(belt)is M_S 7.5 estimated from paleoearthquake,dimension and dip-slip rate of fault,which is consistent with the upper limit magnitude of potential seismic source zone where the faults locate.This study integrates the newest research results on regional seismology and geology,better describes the complexity of seismic process along fault to a certain extent,and assesses comprehensively seismic hazards of active faults in Urumqi.This multidisciplinary method can be applied to the region like Urumqi where complete seismogeological data are unavailable.

Based on the deformation observation data of 11 cross-fault survey points in the capital circle region and taking the two walls of fault as indeformable rigid block,we analyze the quantitative relation between the relative sliding of fault plane and relative displacement of the sites on the two walls on the ground surface,and then taking the Tangshan earthquake and its aftershocks as time-bound,we divide the fault slip complexor into two time-intervals,fit the regional stress state of the two different time

Yanhuai Basin contains Yanfan Basin and Huaizhuo Basin,which are connected one another.According to field seismo-geological investigation and fault slip observation,the active faults in northeast direction in Yanfan Basin are mainly dextral strike-slip,with some cases of normal fault.But in Huaizhuo Basin,the situation is opposite,the active faults in northeast direction are mainly normal faults,with some cases of strike slip.However,in the whole Yanhuai Basin,the active faults in northwest direction are mo

Based on analysis of distribution of 2 660 earthquake focal mechanism solutions from “Database of Crustal Stress of China and Adjacent Area” (updated to December, 2003), the regional characteristics of focal mechanism solutions are studied. In the Northeast and North China stress regions, the orientations of maximum horizontal stress mostly are in the near EW direction and NEE-SWW direction. The strike-slip focal mechanism is predominant in Northeast and North China stress regions. In the South China stress region, the predominant orientation of maximum horizontal stress is in NE-SW direction. The focal mechanisms mainly include thrust faulting and strike-slip faulting. In the Xinjiang stress region, the predominant orientation of maximum horizontal stress is in near N-S direction. There is a significant number of thrust faulting events and strike-slip faulting events in Xinjiang stress region. In the south Tibetan stress region, the predominant orientation of maximum horizontal stress is in near N-S direction, and the focal mechanism solutions are of strike-slip and normal faulting. In the stress region of north Tibet plateau and in the northeastern edge, the predominant orientation of maximum horizontal stress is not concentrated, and the focal mechanisms mostly are of strike-slip faulting, with some normal faulting and thrust faulting.