The Ebomiao Fault is a newly discovered active fault near the block boundary between the Tibetan plateau and the Alashan Block. This fault locates in the southern margin of the Beishan Mountain, which is generally considered to be a tectonically inactive zone, and active fault and earthquake are never expected to emerge, so the discovery of this active fault challenges the traditional thoughts. As a result, studying the new activity of this fault would shed new light on the neotectonic evolution of the Beishan Mountain and tectonic interaction effects between the Tibetan plateau and the Alashan Block. Based on some mature and traditional research methods of active tectonics such as satellite image interpretation, trenches excavation, differential GPS measurement, Unmanned Aircraft Vehicle Photogrammetry(UAVP), and Optical Stimulated Luminescence(OSL)dating, we quantitatively study the new activity features of the Ebomiao Fault.
    Through this study, we complete the fault geometry of the Ebomiao Fault and extend the fault eastward by 25km on the basis of the 20km-fault trace identified previously, the total length of the fault is extened to 45km, which is capable of generating magnitude 7 earthquake calculated from the empirical relationships between earthquake magnitude and fault length. The Ebomiao Fault is manifested as several segments of linear scarps on the land surface, the scarps are characterized by poor continuity because of seasonal flood erosion. Linear scarps are either north- or south-facing scarps that emerge intermittently. Fourteen differential GPS profiles show that the height of the north-facing scarps ranges from (0.22±0.02)m to (1.32±0.1)m, and seven differential GPS profiles show the height of south-facing scarps ranging from (0.33±0.1)m to (0.64±0.1)m. To clarify the causes of the linear scarps with opposite-facing directions, we dug seven trenches across these scarps, the trench profiles show that the south-dipping reverse faults dominate the north-facing scarps, the dipping angles range from 23° to 86°. However, the south-facing scarps are controlled by south-dipping normal faults with dipping angles spanning from 60° to 81°.
    The Ebomiao Fault is dominated by left-lateral strike-slip activity, with a small amount of vertical-slip component. From the submeter-resolution digital elevation models(DEM)constructed by UAVP, the measured left-lateral displacement of 19 gullies in the western segment of the Ebomiao Fault are(3.8±0.5)~(105±25)m, while the height of the north-facing scarps on this segment are(0.22±0.02)~(1.32±0.10)m(L₃-L₇), the left-lateral displacement is much larger than the scarp height. In this segment, there are three gullies preserving typical left-lateral offsets, one gully among them preserves two levels of alluvial terraces, the terrace riser between the upper terrace and the lower terrace is clear and shows horizontal offset. Based on high-resolution DEM interpretation and displacement restoration by LaDiCaoz software, the left-lateral displacement of the terrace riser is measured to be(16.7±0.5)m. The formation time of the terrace riser is approximated by the OSL age of the upper terrace, which is (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface, and(11.4±0.6)ka at (0.89±0.03)m beneath the surface, the OSL age (11.2±1.5)ka BP at (0.68±0.03)m beneath the surface is more close to the formation time of the upper terrace because of a nearer distance to sediment contact between alluvial fan and eolian sand silt. Taking the (16.7±0.5)m left-lateral displacement of the terrace riser and the upper terrace age (11.2±1.5)ka, we calculate a left-lateral strike-slip rate of(1.52±0.25)mm/a for the Ebomiao Fault. The main source for the slip rate error is that the terrace risers on both walls of the fault are not definitely corresponded. The north wall of the fault is covered by eolian sand, we can only presume the location of terrace riser by geomorphic analysis. In addition, the samples used to calculate slip rate before were collected from the aeolian sand deposits on the north side of the fault, they are not sediments of the fan terraces, so they could not accurately define the formation age of the upper terrace. This study dates the upper terrace directly on the south wall of the fault.
    Since the late Cenozoic, the new activity of the Ebomiao Fault may have responded to the shear component of the relative movement between the Tibetan plateau and the Alashan Block under the macroscopic geological background of the northeastern-expanding of the Tibetan plateau. The north-facing fault scarps are dominated by south-dipping low-angle reverse faults, the emergence of this kind of faults(faults overthrusting from the Jinta Basin to the Beishan Mountain)suggests the far-field effect of block convergence between Tibetan plateau and Alashan Block, which results in the relative compression and crustal shortening. As for whether the Ebomiao Fault and Qilianshan thrust system are connected in the deep, more work is needed.

Hexi Corridor is located at the northeastern margin of the Tibetan plateau. Series of late Quaternary active faults are developed in this area. Numerous strong earthquakes occurred in history and nowadays. Jinta Nanshan fault is one of the boundary faults between the Qinghai-Tibet block and the Alxa block. The fault starts from the northwest of Wutongdun in the west, passes through Changshan, Yuanyangchi reservoir, Dakouzi, and ends in the east of Hongdun.
Because the Jinta Nanshan fault is a new active fault in this region, it is important to ascertain its paleoearthquakes since late Pleistocene for the earthquake risk study. Previous studies were carried out on the western part, such as field geomorphic investigation and trench excavation, which shows strong activity in Holocene on the western segment of Jinta Nanshan fault. On the basis of the above research, in this paper, we carried out satellite image interpretation, detailed investigation of faulted landforms and differential GPS survey for the whole fault. Focusing on the middle-eastern part, we studied paleoearthquakes through trench exploration on the Holocene alluvial fan and optical luminescence dating.
The main results are as follows:Early Pleistocene to late Pleistocene alluvial strata are widely developed along the fault and Holocene sediment is only about tens of centimeters thick. The Jinta Nanshan fault shows long-lasting activity since late Quaternary and reveals tens of centimeters of the lowest scarp which illustrates new strong activity on the middle-east segment of this fault. Since late Pleistocene, 4 paleoearthquakes happened respectively before(15.16±1.29) ka, before(9.9±0.5) ka, about 6ka and after(3.5±0.4) ka, revealed by 4 trenches, of which 2 are laid on relatively thicker Holocene alluvial fan. Two events occurred since middle Holocene, and both ruptured the whole fault.

A series of NWW striking faults are obliquely intersected by the NEE striking Altyn Tagh fault zone in the western Qilian Mountains. These faults were mostly active in late Quaternary and play an important role in accommodating regional lateral extrusion by both reverse and sinistral slip. Detailed studies on late Quaternary activity, tectonic transformation, paleoseismology, and strain partitioning not only significantly affect our recognition on seismogenic mechanism and zones of potential large earthquakes, but also provide useful information for exploring tectonic deformation mechanism in the northern Tibetan plateau. The Danghenanshan Fault, Yemahe-Daxueshan fault, and Altyn Tagh Fault form a triplet junction point at southwest of Subei county. The Yemahe-Daxueshan fault is one important branch fault in the western Qilian Mountains that accommodated eastward decreasing slip of the Altyn Tagh Fault, which was active in late Holocene, with a length up to 170km. Based on geometry and late Quaternary activity, the Yemahe-Daxueshan fault was subdivided into 3 segments, i.e. the Subei fault, Yemahe fault and Daxueshan Fault. The Yemahe Fault has the most prominent appearance among them, and is dominated by left-lateral slip with a little normal component. The heights of fresh scarps on this fault are only several tens of centimeters. We dug 2 trenches at the Zhazhihu site, and cleaned and reinterpreted one trench of previous studies. Then we interpreted trench profiles and paleoseismic events, and collected ¹⁴C and Optical Stimulated Luminescence samples to constrain event ages. Finally, we determined 3 events on the Yemahe fault with ages(6 830±30) a BP-(6 280±40) a BP, (5 220±30) a BP, (2 010±30) a BP, respectively. The elapsed time of most recent earthquake is(2 010±30) years before present, which is very close to the recurrence interval, so the possibility of major earthquakes on the Yemahe fault is relatively large.

Three-dimensional scanning with LiDAR has been widely used in geological surveys. The LiDAR with high accuracy is promoting geoscience quantification. And it will be much more convenient, efficient and useful when combining it with the Unmanned Aerial Vehicle (UAV). This study focuses on UAV-based Laser Scanning (UAVLS)geological field mapping, taking two examples to present advantages of the UAVLS in contrast with other mapping methods. For its usage in active fault mapping, we scanned the Nanpo village site on the Zhangxian segment of the West Qinling north-edge fault. It effectively removed the effects of buildings and vegetation, and uncovered the fault trace. We measured vertical offset of 1.3m on the terrace T₁ at the Zhang river. Moreover, we also scanned landslide features at the geological hazard observatory of Lanzhou University in the loess area. The scanning data can help understand how micro-topography affects activation of loess landslides. The UAVLS is time saving in the field, only spending about half an hour to scan each site. The amount of average points per meter is about 600, which can offer topography data with resolution of centimeter. The results of this study show that the UAVLS is expected to become a common, efficient and economic mapping tool.

Jinta Nanshan Fault is an important fault in northeast front of Qing-Zang Plateau, and it is crucial for determining the eastern end of Altyn Tagh Fault. However, there is still debate on its significant strike-slip movement.
In this paper, we study the Late Quaternary activity of Jinta Nanshan Fault and its geological and geomorphic expressions by interpreting aerial photographs and high-resolution remote sensing images, surveying and mapping of geological and geomorphic appearances, digging and clarifying fault profiles and mapping deformation characteristics of micro-topographies, then we analyze whether strike-slip activity exists on Jinta Nanshan Fault.
We get a more complete fault geometry than previous studies from most recent remote sensing images. Active fault traces of Jinta Nanshan mainly include 2 nearly parallel, striking 100°~90° fault scarps, and can be divided into 3 segments. West segment and middle segment form a left stepover with 2~2.5km width, and another stepover with 1.2km width separates the middle and east segment.
We summarize geomorphic and geologic evidence relating to strike slip activity of Jinta Nanshan Fault. Geomorphic expressions are as follows:First, fault scarps with alternating facing directions; second, sinistral offset of stream channels and micro-topographies; third, pull-apart basins and compressive-ridges at discontinuous part of Jinta Nanshan Fault. Geologic expressions are as follows:First, fault plane characteristics, including extremely high fault plane angle, unstable dip directions and coexistence of normal fault and reverse fault; second, flower structures.
Strike-slip rate was estimated by using geomorphic surface age of Zheng et al.(2013)and left-lateral offset with differential GPS measurements of the same geomorphic surface at field site in Fig. 4e. We calculated a strike-slip rate of (0.19±0.05)mm/a, which is slightly larger than or almost the same with vertical slip rate of (0.11±0.03)mm/a from Zheng et al.(2013).
When we confirm the strike-slip activity of Jinta Nanshan, we discuss its potential dynamic sources:First, eastern extension of Altyn Tagh Fault and second, strain partitioning of northeastward extension of Qilian Shan thrust belt. The first one is explainable when it came to geometric pattern of several E-W striking fault and eastward decreasing strike slip rate, but the former cannot explain why the Heishan Fault, which locates between the the Altyn Tagh Fault and Jinta Nanshan Fault, is a pure high angle reverse fault. The latter seems more explainable, because oblique vectors may indeed partition onto a fault and manifest strike-slip activity.

The Hexi Corridor-Qilian Fault systems,the Altyn Tagh Fault and the Haiyuan Fault jointly control the north boundary and deformation of the Qinghai-Tibet plateau. The Changma Fault,as one of the Hexi Corridor-Qilian Fault systems,is a highly active strike-slip fault,and connects the Altyn Tagh Fault and the Haiyuan Fault. Based on the active characteristics and geometrical distribution,this fault is divided into four segments. We obtain the left-lateral strike-slip rates of three segments,which are 1.33±0.39mm/yr at the west segment,3.11±0.31mm/yr at the middle-west segment,and 3.68±0.41mm/ya at the middle-east segment,respectively,and the shortening rate at the west segment(0.70±0.20mm/yr).The result shows that the sinistral slip rate of the fault is significantly increased from west segment to east segment. The activity of Changma Fault accommodates～30%reduction of Altyn Tagh Fault slip rate. The studies in this paper confirm that the sinistral slip and shortening of Changma Fault and other secondary faults in accompany with deformation inside the basin absorb most of displacement of east segment of Altyn Tagh Fault,and this structural change mode supports the hypotheses that the northeastern margin of Qinghai-Tibet plateau has a continuous crust thickening mode with lateral displacement.

On July 22,2013,the Minxian-Zhanxian M_S 6.6 earthquake occurred at the central-northern part of the South-North Seismic Belt. In the area,complicated structural geometries are controlled by major strike-slip fault zones,i.e.the Eastern Kunlun Fault and the Northern Frontal Fault of West Qinling. The distribution of related seismic disasters,namely,the ellipse with its major axis trending NWW,is in good accord with the strike of the Lintan-Tanchang Fault. Severe damages in the meizoseismal area of the Minxian-Zhangxian M_S 6.6 earthquake are located within the fault zone. So it is considered that the earthquake related damages are closely related to the complicated geometry of the Lintan-Tanchang Fault,and it also indicates that the earthquake is the outcome of joint action of its secondary faults. Based on field investigations,and by integrating the results of previous studies on active tectonics,structural deformation and geophysical data,it can be inferred that the southward extension of the Northern Frontal Fault of West Qinling and the northeastward extrusion of the Eastern Kunlun Fault in the process of northeastward growth of Tibetan plateau are the main source of tectonic stress. Basic tectonic model is provided for strong earthquake generation on the Lintan-Tanchang Fault.

The northern Yumushan Fault located on the northern flank of Qilian fold system is an active fault in Holocene.The fault is about 60km long,trending NWW.It is a trust fault with left-lateral strike-slip component.The activity of the fault produced a series of scarps along the mountain front.The fault zone is divided into three segments,and the middle part is the most active.In this paper,palaeo-earthquake events on the fault are studied.With the study of trench profiles,two palaeo-earthquake events are determined.Event I occurred at 4.066±0.086ka BP,and event II is between 6.852±0.102ka BP and 6.107±0.082ka BP.The last palaeo-earthquake event on this fault occurred in 4.066±0.086ka BP. So,the northern Yumushan Fault is not the seismogenic fault of the M 7 1/2 Biaoshi earthquake of 180 AD.The elapse time from the latest event has been 4000yr,so the possibility of generating destructive earthquake in future should be recognized sufficiently.

This paper makes a comprehensive analysis of the recent progress of the seismic active fault prospecting in Lanzhou city. Based on the satellite and aerial photos interpretation,geological and geomorphic investigation,geochemistry prospecting,shallow seismic investigation,resistivity imaging,drilling,especially large-scale trenching along the 7 active fault zones in Lanzhou city,we have achieved very important progress and gained new knowledge about the recent activity of main active faults and deformation features in Lanzhou Basin. The main conclusions are summarized bellow: (1) The Jinchengguan Fault is a thrust fault,constituting the northern boundary of the Lanzhou Tertiary Basin. It is revealed by geophysical prospecting and drilling that the newest strata offset by the Jinchengguan Fault are the early-Pleistocene sandstone and conglomerate,and that the overlying second and third terraces of the Yellow River remain intact. So,it's an early and middle Pleistocene active fault.(2) The Liujiabu Fault and Shengouqiao Fault constitute the northern and western boundaries of the Qilihe Subsidence,respectively. Revealed by geophysical prospecting,drilling and large trenching,they are not faults but lithologic boundaries of different rocks between Pliocene and early Pleistocene.(3) The Leitanhe Fault is the eastern boundary of Qilihe Subsidence,a boundary fault separating the Tertiary Lanzhou Basin into the east and west basins. According to the geophysical prospecting and drilling,the Leitanhe Fault is a thrust fault and its newest activity age is early and middle Pleistocene. It is not active since late Quaternary and does not cut the third terrace of the Yellow River.(4) The Siergou Fault is the southwestern boundary of Lanzhou Basin,a thrust fault too. It's an early and middle Pleistocene active fault and does not offset the forth terrace of Yellow River. While the Xijincun Fault is much nearer to the south margin of Lanzhou Basin and forms the southern boundary of the Tertiary Lanzhou Basin. It's an early Pleistocene fault.(5) The northern margin of Maxianshan Mountains fault is a major seismic fault on the southern margin of Lanzhou Basin,and its movement is characterized by segmentation. The east segment,the Neiguanying sub-fault,is a late Pleistocene fault. The middle segment,the Maxianshan and Qidaoliang faults,are active during late Pleistocene and early Holocene. The west segment,the Wusushan sub fault,is active during late Pleistocene and Holocene,and it's also the seismic fault of the M7 Lanzhou earthquake.On the whole,we correct the previous recognitions about the activity times of 4 faults,i.e. the Jinchengguan Fault,Leitanhe Fault,Siergou Fault and Xijicun Fault. They are all early and middle Pleistocene instead of late Pleistocene active faults. Especially,we find that the Liujiabu Fault and Shengouqiao Fault directly across Lanzhou city are not late Pleistocene or Holocene active faults but lithologic boundaries between Pliocene mudstone and early Pleistocene conglomerate. The results are very important for the urban planning and engineering construction,and will produce obvious economical and social benefits.

This paper defines the boundaries of north Qinghai-Xizang plateau active tectonic block and offers quantitative information of active faults within the north Qinghai-Xizang plateau active tectonic block in the past dozen years. The information mainly include: the serial number,name,attitude of active fault,main geological geomorphic signs,activity age,fault segmentation,fault slip rate,paleo-earthquake and its date,and the major features of earthquake rupture zones and so on. These data indicate that great earthquakes with M≥8 concentrate on the boundary active faults of the north Qinghai-Xizang plateau active tectonic block,and the slip rate of the faults all reaches about 5～12mm/yr. Earthquakes of about M7 occurred on the smaller-scale active faults inside the north Qinghai-Xizang plateau active tectonic block. Generally,the slip rates of these active faults are 1～3mm/yr.The north Qinghai-Xizang plateau active tectonic block can be divided into several sub-blocks,and these active sub-blocks are dominated with deformation,and no rotation occurred. Our result supports the hypothesis of continuous deformation of Qinghai-Xizang plateau active tectonic block.

According to the results of 1/10,000 stripped geologic mapping of the northwest segment of the Maxianshan north marginal fault and historical accounts of past events,we discuss in this paper the range of the magistoseismic area,seismogenic fault,pattern and combination feature of the surface ruptures of the 1125 A.D.Lanzhou M7 earthquake.The results show that the magistoseismic area of this earthquake is located in Lanzhou City and its southwest,and the epicenter can be located at the Xianshuigou area.The seismogenic fault of this earthquake is the Xianshuigou-Maquangou sub-segment on the northwest segment of the Maxianshan north marginal fault.This earthquake has produced a surface rupture zone of about 7km long and 300～1000m wide,extending along the seismogenic fault.The surface ruptures consist of earthquake fractures,fault scarps,seismic fissures,seismic landslides,and seismic pits.The surface rupture zone can be sub-divided into 2 sub-segments:the Maidiwan-Xianshuigou sub-segment in the southwest and the Damajiatan-Maquangou sub-segment in the northwest.Among them,the Maidiwan-Xianshuigou sub-segment consists of two parallel surface ruptures,while the Damajiatan-Maquangou sub-segment comprises a single surface rupture.Basing on large scale mapping,it is determined that the left-lateral displacement produced by this event is 2.4～2.5m,and the vertical offset is 0.45～0.92m.Regionally,the Maxianshan north marginal fault is located at the junction of the northern margin of the Qinghai-Xizang plateau and the northern segment of the North-south tectonic belt,which have been strongly active since neotectonic period.A rhombic block confined by major faults of different strikes is developed in this region,and we call it the Gansu-Ningxia rhombic block.The 1125 A.D.Lanzhou M7 earthquake just occurred on the western edge of the rhombic block,i.e.the Wuwei-Zhuanlanghe-Maxianshan Fault zone.The strong uplift and northeastward pushing of the active Qinghai-Xizang block may cause the stress relief on the boundary faults of the Gansu-Ningxia rhombic block,and hence the occurrence of several strong earthquakes.

Tianshui area locates at the northeastern margin of Qinghai-Tibetan plateau and the middle segment of the north margin of western Qinling tectonic zone.In history,several strong destructive earthquakes happened in the area.Because of the very long history and that the historical records are not quite clear,it is very difficult to study them in nowadays,and most of them became historical knotty problems.The Tianshui earthquake in 734 AD is one of the large earthquakes.On March 23,734 AD,a large earthquake happened in Qinzhou of Tang Dynasty,now the vicinity of Tianshui City,causing serious seismic disasters as "the earth ruptured and closed again,nearly all the houses damaged,about 4000 people dead,hills changed into valleys,and towns covered by landslip,and so on".According to the detail textual research of the historical earthquake records,the meizoseismal area of the 734 AD Tianshui earthquake is in the Qinzhou and Maiji area,now,the Qincheng,Beidao and Maijishan area of Tianshui City.The epicenter intensity is about Ⅹ,the magnitude is about M 7 1/2.The direction of long axis of isoseismal line is NW,consistent with the strike of the eastern segment of Wushan-Gangu Fault of the northern margin of western Qinling active fault zone.The meizoseismal area is just in the step-over zone between the left-lateral strike-slip faults,i.e.the Tianshui-Baoji Fault and the Gangu-Wushan Fault,an area prone to strong earthquake.Comprehensive analysis indicates that the causative structure of the 734 M 7 1/2 Tianshui earthquake is the east segment of the Gangu-Wushan Fault of the northern margin of western Qinling active fault zone.

The Maxianshan north marginal fault belongs tectonically to the Kunlun-Qilian-Qinling Caledo～nian-Variscan orogenic belt.The northwest segment of the fault locates within the Mesozoic Lanzhou basin,consisting of Xianshuigou-Maquangou,Xinchenggou and Qingshizui sub-segments.The Xianshuigou-Maquangou sub-segment is 7km in length,and comprises two sub-parallel faults,having a general strike of 290°～300°,dipping NE or SW at an angle of 60° or more.The faults dissect mainly the Cretaceous system,and locally act as the boundary of the Cretaceous system with the Ordovician and Jurassic systems.Upwards,the faults cut through the late Pleistocene loess or the gravel bed of gully terrace,appearing as fault scarp or fault escarpment.This sub-segment was the active segment of the whole fault during late Pleistocene to Holocene periods.The faulting of this sub-segment was dominated by left-lateral strike-slipping.The left-lateral displacement along this sub-segment since late Holocene is 5～8m,and the displacement rate is 0.5～1.72mm/yr.The Xinchenggou sub-segment is about 1.6km long,striking 325°and dipping southwest at the angle of greater than 60°.This sub-segment can be assigned to reverse fault,dissecting the Cretaceous system,and is covered with the gravel bed of the third level terrace of the Yellow River and the late Pleistocene loess.This sub-segment,therefore,has no longer been active since late Pleistocene.The Qingshizui sub-segment is about 2.5km long,striking 280°～310°and dipping northeast at angles of 58°～80°,and can be assigned to normal fault.The fault dissects mainly the Cretaceous system,and locally becomes the boundary between the Cretaceous and Ordovician systems.The fault is also covered with the gravel bed of the third level terrace of the Yellow River and the late Pleistocene loess.This may indicate that this sub-segment has ceased its activity since late Pleistocene.Macroscopically,the middle and eastern segments of the Maxianshan north marginal fault,together with the Zhuanglanghe Fault have made up a right-stepped en echelon zone.The faulting process of the former during late Pleistocene-Holocene was dominated by left-lateral strike-slipping,while that of the later by right-lateral strike-slipping,so a compressional step-over was formed between the two faults.Therefore,the Xianshuigou-Maquangou sub-segment can be assigned to shear fault within the compressional step-over,and hence the latest activity of this sub-segment is later than that of the middle and eastern segments of the Maxianshan north marginal fault.

On December 14,2002,an earthquake of M_S5.9 occurred in Yumen,Gansu Province. Field investigation revealed that both the macroscopic and measured epicenters of this event,the coordinate of which are 39.8°N,97 3°E and 39.7°N,97 3°E,respectively,are located on the northern marginal fault of the Qilian Mountains. The epicentral intensity was determined to be Ⅶ. The epicentral area appears as an elongated ellipse with N65°W-trending long axis of 15km long and N25 E-trending short axis of 12km long. The earthquake took place in the Yumen area,which is located on the northern margin of the Qinghai Tibet Plateau and the western section of the Qilian Mountains. There are several Holocene active faults,of which the most famous ones are the Altun and the Changma faults. Historically,several large earthquakes occurred on these two faults,such as the Changma (M_S=7.6) earthquake of 1932 and the earthquake (M_S=5.5) occurred on the eastern section of the Altun fault in 1933. The Yumen earthquake is a moderate-strong one,which didn't produce distinct surface rupture zone. The event was the result of the most recent movement on the secondary fault of the northern marginal fault zone of the Qilian Mountains,namely the Hanxia-Dahuanggou Fault. The evidence is as follows: Field investigation shows that the macroscopic epicenter of this earthquake is located in the area involving the Hanxia Coal Mine of Yumen,Yuerhong Village of Subei County and Yaomoshan. The trend of the epicentral area is just consistent with the Hanxia-Dahuanggou Fault. Focal mechanism solution shows that the nodal plane A appears as thrust fault trending to 147°with a relatively small dip angle of about 26°,in accord with the general strike of the Hanxia-Dahuanggou Fault. The P axis is oriented in 277° direction having an elevation angle of 27°. This may indicate that this earthquake was the result of stress concentration due to the action of nearly SN-trending horizontal compressive stress. Aftershocks of this event were very frequent,and 113 aftershock of M_S≥0.5 had been recorded before Feb. 20,2003. Most aftershocks were distributed near the Hanxia Dahuanggou Fault and located to the south of the main shock,making a NE-trending zone. This may indicate that the faulting propagated from south to north along the fault,and that the event was the result of thrusting along the reverse fault. Several lines of evidence shows that a low angle major detachment plane exists at a weak zone 6～9 km beneath the surface of Yumen area. All folding and thrusting deformation occurred in the strata above the detachment plane (Fig.6). The northern marginal fault of the Qilian Mountains consists of several secondary reverse faults dipping southwest at an angle of 25°～70°. They all converge on the low angle detachment plane. The Hanxia Dahuanggou Fault is a part of the fault zone,and has a nappe structure. The focal depth of 31 recorded aftershocks coincides well with the depth of the detachment plane. Therefore,it is unlikely that a greater earthquake will occur on this fault in the near future,and this recognition may provide credible evidence for earthquake trend assessment after the Yumen earthquake.

The Tianqiaogou Huangyangchuan Fault (called also Gulang Fault) lies to the east of Lenglongling,the highest peak on the eastern section of the Qilianshan Mountains. It is one of the important active faults in eastern Qilianshan. From west to east,the fault is about 86km long,initiating from Hongyaoxian,extending eastward along Tainqiaogou,Qianjin,Guanjiatai,and Huangyangchuan villages,and ending at Jiapigou. The western segment of the fault is NWW-trending,the middle is nearly E-W-trending and the eastern is NEE-trending. Among them,the middle segment is convex slightly to the south. The fault is basically continuous,except that a small step-over exists near Guanjiatai village,dividing the fault into two sub-segments:the Tianqiaogou sub-segment in the west and the Huangyangchuan sub-segment in the east. Field investigation revealed that offset landforms are well developed along the fault,such as offset gully,fault scarp,sag pond,offset terrace and alluvial fans. All the offset landforms indicate that the fault was dominated by left lateral strike-slip with a component of normal faulting. Tracing along the fault and trenching across 6 typical offset landforms revealed the evidence of Holocene activities and several palaeo earthquakes along the fault. By the use of progressive constraining method,7 Holocene palaeo-earthquakes and one historic earthquake are identified. The occurrence time of these events are determined to be 10,743±342a BP (Event Ⅰ),9038±39a BP (Event Ⅱ),6910±438a BP (Event Ⅲ),4847±185a BP (Event Ⅳ),3562±190a BP (Event Ⅴ),2476±194a BP (Event Ⅵ),1505±253a BP (Event Ⅶ) and 1927AD (Event Ⅷ,possibly the Gulang earthquake). The distribution and recurrence behavior of strong earthquakes along the fault show a distinct linearity and sub-periodical recurrence,for which the linear regression equation of event-time sequence is T_i=1516i-11734. Moreover,during field investigation we found new evidence indicating that surface rupture occurred along the Tianqiaogou-Huangyangchuan fault during the Gulang M_S8.0 in 1927.

The Western Jiuquan (Jiuxi) Basin is located in the westernmost part of the Hexi Corridor. The basin is bounded by the Qilian Mountain fault on the south, by Alytn Taugh fault on the north, and by Jiayuguan fault on the east, respectively. The Hexi Corridor is one of the seismically active regions in western China. According to historical records, a large number of strong earthquakes had occurred in this area. Recently, we have discovered three Holocene active faults through the interpretation of aerial photos and field investigation in the Jiuxi basin. These three faults are called Xinminpu, Yinwashan and Yumen faults, respectively. The Xinminpu fault is a Holocene thrust fault, which is 17km in length, striking 315°and dipping southwest, located in the northern part of the basin. A fault scarp of 14m height was developed on the hanging wall of the fault, and it is superposed by the newly formed fault scarp with free surface. The rate of vertical motion along the fault is determined to be 0.24mm/yr. The Yinwashan fault is a Holocene thrust fault located on the alluvial fan at the eastern piedmont of the Yinwashan Mountain, striking 315°with a length of 17km and dipping southwest. The rate of vertical motion along the fault is determined to be 0.18mm/yr. Two Holocene paleoearthquake events have been identified through trenching on the fault. The first event occurred 10.64±0.83ka B.P., while the second event occurred between 4.09±0.31ka B.P. and 8.22±0.63ka B.P. The Yumen fault is also a Holocene thrust fault, which is nearly EW-striking and south dipping, located on the alluvial fan at the northern piedmont of the Qilianshan Mountain. A fault scarp of less than 2m height was developed along the fault. The fault scarp was perhaps produced by a historical earthquake. The rate of vertical motion along the fault is determined to be 0.25mm/yr. Two Holocene paleoearthquakes were revealed by trenching on the fault. The first earthquake occurred at 3.05±0.24 ～3.20±0.25ka B.P. The second occurred after 3.05±0.24ka B.P. As mentioned above, all the three Holocene faults belong to thrust fault, and thus no obvious horizontal displacement can be observed along the fault. This may indicate that this area is dominated by compressional deformation. According to historical records, the Jiuquan earthquake of 756 A.D. is the latest historical earthquake occurred in this area. It is postulated that the Xinminpu fault or Yumen fault would be ruptured during this earthquake, but currently we are unable to determine which fault was ruptured by this earthquake on the basis of available dating data. The ages of paleoearthquakes and the characters of surface ruptures along the three faults suggested that the three faults were activated independently or sometimes in cluster.

On 14 November 2001, an extraordinarily large earthquake occurred on the Hoh Sai Hu segment of the Eastern Kunlun Fault, northern Tibetan Plateau. This event, named as Hoh Sai Hu (Kunlanshan) Earthquake, is the largest earthquake occurred in China continent for the past 50 years. The moment magnitude of this earthquake reaches 7.7 to 7.9 (USGS National Earthquake Information Center, 2001; Harvard CMT Catalog, 2001) and the surface wave magnitude reaches 8.1 (China Digital Seismic Network, 2001). Field investigation indicates that the surface rupture zone produced by this earthquake is striking N80°±10°W with a length of 350 km, which initiates from 91°E in the west nearby the east of Buka Daban Feng, a snow-capped summit with an altitude of 6 800m, extends eastwards along the fault traces of the Hoh Sai Hu segment, and terminates at the 94.8°E in the east. The surface ruptures of this earthquake consist of shear fractures, transtensional fractures, tension gashes and mole tracks arranged in en echelon. The shear fractures are N80°～90°W trending and dominated by left-lateral slip. Transtenssional fractures are several to tens meters long, the strike of which varies from N62°E to N65°E or from N70°E to N75°E, and are dominated by left-lateral slip with a component of tensile opening, the width of which decreases with depth. The shear and/or transtensional fractures are arranged in left-stepping or right-stepping to form releasing or restraining steps, on which tension gashes or mole tracks are developed. Tension gashes strike N45°～50°E and are developed at a releasing step to connect with the boundary shear or transtensional fractures which constrain the step in most cases. The tension gashes may also be arranged in en echelon pattern along the surface rupture zone, and especially at the termination of the surface ruptures. The mole tracks of 1.5 to 3 m height are trending 295°to 330°, which are well developed at the right-steps of the shear and/or transtensional fractures of different scales along the surface rupture zone. This surface rupture pattern appears to be purely strike-slip characterized by several meters of left-lateral offset. The maximum left-lateral offset we observed reaches 6 m at a site (93°05.384’E, 35°47.623’N), where a shallow channel bed was left-laterally offset by a single pure shear fracture. The macroscopic epicenter of the Hoh Sai Hu (Kunlunshan) earthquake is then inferred to be located at the piedmont area to the northeast of Hoh Sai Hu Lake, about 80 to 90km west of Kunlunshan Pass, in terms of the features of surface ruptures. It is postulated that this earthquake may trigger the occurrence of future large earthquake on the Dongdatan-Xidatan segment to the east of the Hoh Sai Hu segment of the Eastern Kunlunshan Fault,reflecting the eastward motion or flowing of the Tibetan Plateau along the fault.