The neotectonic movement in the middle reaches of the Jinsha River is active and the earthquakes occur frequently. Lacustrine sediments are commonly distributed on both sides of the river with stable sedimentary environment, good horizontal continuity and relatively developed stratification, which are good carriers for recording paleo-seismic events. In this study, a large number of soft sedimentary deformation structures are found in the riverside lacustrine sediments in the Taoyuan Town area in the middle reaches of Jinsha River, with strong deformation and large scale. We focus on the comprehensive analysis of four soft-sedimentary deformation profiles. In which the profiled strata are mainly medium-fine sand and clay. And the soft sedimentary deformation structures mainly include sand liquefaction, rootless faults, clay lumps and folds.
Causes analysis: In the profiles of soft sedimentary deformation structures, there are medium and fine sand layers whose thickness is from thick to super thick. Sedimentary bedding has not been observed in the sand layer; and a large number of clay debris or lumps are involved in the sand layer, which are often filled between the adjacent clay lumps; and there are quicksand channels in the sand layer. All the features indicate that the sand layer in the study profiles has been liquefied. In the study profile, we found that the soft sedimentary deformation structure has the following characteristics: The faults found in the study profile extend downward and terminate in the lower liquefied sand layer and a large number of clay lumps. There are clay lumps in the place where the clay fold structure develops, and a large number of liquefied sand bodies are filled between the fold structures. The deformation structures in the profiles are not contrastive in terms of extension, chaotic deformation characteristics and obvious stress direction. Based on the characteristics of sand liquefaction and clay deformation in the above profile, it is inferred that the deformation structure in the profile is mainly due to sand liquefaction. The liquefaction strength of sand layer determines the deformation degree of clay layer.
Trigger factors analysis: There are many factors that can trigger the liquefaction deformation of the unconsolidated sediment, such as flood, freeze-thaw, collapse and earthquake, which can cause the liquefaction deformation of the sediment under certain conditions. In this paper, the possible trigger factors are analyzed based on the combination of the structural characteristics of soft sedimentary deformation, sedimentary environment and geological background of the area. First the stratigraphic characteristics also reflect the hydrostatic sedimentary environment at that time. The soft sedimentary deformation on such a large scale could not be mainly caused by the disturbance of lake waves. The research profiles are located at a sheltered bay with weak hydrodynamics, and no alluvial strata have been found in the upper part of the soft sedimentary deformation stratum. Moreover, the soft sedimentary deformation structure caused by flooding is often a small-scale curly layered structure, which has a large difference with the deformation structure and scale in the study profiles. This suggests that alluvial and diluvial events are not the main triggering factors of the deformation. Although the landslide is likely to occur near the study area, no trace of bedrock landslide is found near the study profiles. Therefore, the invasion of bedrock landslide into the sedimentary layer cannot be the triggering factor. Moreover, the occurrence of lacustrine sedimentary layer is nearly horizontal, which is a relatively stable sedimentary state, and it is impossible to form such a large-scale slump structure due to its own gravity effect. And we don't find any sliding surface in the profiles. Therefore, the collapse is ruled out. According to the geological background and geological survey of the study area, this area does not have the conditions triggered by volcanism, glaciation and freeze-thaw. Because of the active neotectonic movement and frequent earthquakes in the study area, and seismic actions are the main trigger factors for liquefaction. So it is considered that seismic action may be the main trigger factor for the strong liquefaction deformation in the study area. According to the previous studies, the relationship between the soft sedimentary deformation structure, the liquefaction thickness and the seismic strength is discussed, the magnitude of this ancient seismic event probably reached 7 or higher.
There are sand layers in the section of “soft sedimentary deformation structure” caused by earthquake, the lower stratum is sand layer and the upper stratum is clay layer. The thickness and deformation strength of the lower sand layer determine the strength of the deformation structure of the overlying clay layer. The upper and lower surface of the sand layer are undulating, and there are clay lumps in the sand layer. The deformation structure of clay layer is complex and there is no obvious deformation rule.

The southeast section of Zhongdian-Daju Fault is located in the northern part of Haba and Yulong Snow Mountain, belonging to the southwestern boundary of the secondary block in northwestern Sichuan, an important boundary fault striking 310°~320° on the whole. The nature of the fault, the age of its activity and the slip rate are of great significance for the analysis of the secondary block movement in the northwestern Sichuan and the intersection relationship with the eastern piedmont fault of Yulong Mountains.
Based on the 1 ︰ 5 million-scale active fault geological mapping, this paper studies in detail the stratigraphic landform, scarp landform, surface rupture, typical fault profile and river terrace along the fault. Based on the research results, we divide the southeastern section of Zhongdian-Daju Fault into two sub-segments, the Majiacun-Daju sub-segment and the Daju-Dadong sub-segment, according to the geometric structure, fault landforms and fault activity.
(1)Fault scarp:In the Majiacun-Daju sub-segment, the fault parallelly controls the two sides of the Haba fault depression. It cuts the late Pleistocene moraine deposits, forming a fault scarp of about 4.5km long and(14±2)m high. The continuity of the scarp is very good, and it is also very obvious in the remote sensing image. In the Daju-Dadong sub-segment, a scarp with a height of about 2m is formed, and an optical luminescence dating sample is collected from the upper part of the gravel layer on the second-order terrace to obtain an age of(22±2.2)ka.
(2)Horizontal dislocation:In the Majiacun-Daju sub-segment, through the analysis of the development of outwash fans in the area and the measurement and induction of the gully dislocations, it is considered that there are at least three stages of outwash fans developed in the area and there may be four phases of faulting. That is, the earliest-stage outwash fan and gully are horizontally dislocated about 1km; the second-stage outwash fan and gully are horizontally dislocated about 47m, and the vertical dislocation is about(14±2)m; the gully in the third stage outwash fan is horizontally dislocated twice, the first dislocation formed a beheaded gully with a dislocation of 22m, and the second formed a beheaded gully with a dislocation of 8.5m. It is further proved that the fault has strong activity since the Holocene in the Majiacun to Daju area. In the Daju-Dadong sub-segment, there are no obvious horizontal dislocations in the alluvial deposits since the Holocene. Only 3~4 gullies are found to be offset right-laterally in the ridges east of Wenhe Village, with the maximum dislocation of 210m, which may be the older phase dislocation.
(3)Surface rupture:In the northwest direction of Dabazi Village on the T3 terrace in the basin between Majiacun and Daju, an earthquake surface rupture zone is found, extending in the NW direction. The rupture zone left clear traces on the about 1m-thick, hard T₃ terrace surface formed by calcification of sand gravels, and the overburden either upwarps and bulges, or ruptures, generates ground fissures, or forms small pull-apart "depressions" locally. However, the rupture zone is not large in size, about 350m long, 60m wide at the widest point, and 0.3~1.5m high. It is partially en-echelon or obliquely arranged, dominated by compressive ruptures. Through observation, the possibility of artificial transformation is ruled out for these upwarping bulges, ruptures or ground fissures. The fault section is found in the southeast direction of the rupture zone. The slickensides at the section show that the fault is dominated by right-lateral strike-slip with a small amount of thrust. In the eastern sub-segment, only intermittently distributed surface ruptures are found in the northern part of the village, and the scale is small.
In summary, through the field geological survey, it is found that the Majiacun-Daju sub-segment is a Holocene active segment. Though the Daju-Dadong sub-segment also offset the late Pleistocene to Holocene strata, it is considered that its Holocene activity is weak in terms of either the dislocation amount or the slip rate of this segment.
By analyzing the geological and geomorphological evidences, such as fault scarps, horizontal dislocation and surface ruptures along the fault, it is considered that the Majiacun-Daju sub-segment is a right-lateral strike-slip fault with a normal faulting component, and its vertical slip rate since the late Pleistocene is(0.4~0.8)mm/a, the horizontal slip rate is 1.5~2.4mm/a. The Daju-Dadong sub-segment is dominated by right-lateral strike-slip with a normal faulting component, and its vertical slip rate since the late Late Cenozoic is 0.1mm/a.
The formation of the NW-trending surface rupture zone found in the Daju Basin is very young, where there are only two major earthquakes, namely, the M_S6.4 1966 Zhongdian earthquake and the 1996 Lijiang M_S7.0 earthquake, and both earthquakes produced NW-oriented surface rupture zones. Therefore, it cannot be ruled out that the rupture zone is a product of the 1966 Zhongdian M_S6.4 earthquake or the 1996 Lijiang M_S7.0 earthquake.

We estimate the slip rates of three faults in the central segment of Longmenshan Fault zone, namely, the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault, based on the measurement and dating of deformation of Late Quaternary terraces along the Minjiang River and its tributaries. The three-level Late Quaternary terraces, T₁, T₂ and T₃, are well developed along the Dujiangyan-Wenchuan reach of the Minjiang River. The cross profiles show that T₃ and T₁ have undergone a complete development process. The thermoluminescence ages of the upper deposits on T₁, T₂ and T₃ are 9～13ka BP, 19～30(ka BP) and 51～58ka BP, respectively. Therefore, we can constrain the ages of T₁, T₂ and T₃ surfaces to 10ka BP, 20ka BP and 50ka BP, respectively. The synchronous three-level terraces also exist in the valleys of the Baishahe River and other tributaries of the Minjiang River. These river terraces are laterally and vertically dislocated by the Longmenshan Fault zone. According to the dating and vertical displacements of river terraces, the Late Quaternary reverse slip rates of the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault are estimated to be 0.5mm/a, 0.6～0.3mm/a, 0.2mm/a, respectively. According to the lateral displacements of river terraces, the Late Quaternary dextral strike-slip rate of the Maowen-Wenchuan Fault and the Beichuan-Yingxiu Fault averages 1mm/a each. The late Quaternary faulting of the Longmenshan Fault zone is characterized with intermittency. Three faulting stages can be identified. The first one is between 50ka BP to 20ka BP, the second one between 20ka BP to 10ka BP, and the latest starts at 10ka BP. The existence of thick alluvial deposits in the present channel indicates that the uplift of Longmenshan Mountains is more complex than previously estimated.

Segmentation of thrust fault zone is a basic problem for earthquake hazard evaluation. The Yingjing-Mabian-Yanjin thrust fault zone is an important seismic belt trending northwest in the southeast of Tibetan plateau. The longitudinal faults in the thrust zone are mainly of thrust slipping. The late Quaternary motion modes and displacement rates are quite different from north to south. Investigation on valleys across fault shows that the transverse faults are mainly of dextral strike-slipping with a bit dip displacement. Based on their connections with longitudinal faults, three types of transverse faults are generalized, namely, the separate fault, the transform fault and the tear fault, and their functions in the segmentation of the thrust fault zone are compared. As the result, the Yingjing-Mabian-Yanjin thrust fault zone is divided into three segments, and earthquakes of the three segments are compared. The trisection of Yingjing-Mabian-Yanjin thrust fault zone identified by transverse faults reflects, on one hand, the differences in slip rate, earthquake intensity and pace from each segment, and the coherence of earthquake rupturing pace on the other hand. It demonstrates that the transverse faults control the segmentation of thrust fault zone to a certain degree, and each type of the transverse faults plays a different role.

Basic features of the ground fissure zones which have occurred in the Xi'an city area since late 1970's are described by using field observations and survey data. The focus is put on expounding the spatial-temporal relationship between the dynamic deep-well water level changes, inhomogeneous ground subsidence and ground fissure activities. It is suggested that the large-scale ground subsidence induced by overextraction of confined ground water is the primary cause of the ground fissure zones in the Xi'an city while the local geological conditions play a controlling role to the development of these fissure zones.