Since the occurrence of the Wenchuan Earthquake on May 12, 2008, several papers on its seismotectonics analysis have been published (Lei et al. 2009; Yue, 2010; Chen et al. 2009; Tang et al. 2009; Zhu et al. 2009). Although their opinions were not uniform, most authors believe that the source of the earthquake was the Central Longmen Mountain Range fracture with a length of about 230km. However, this hypothesis cannot explain the following phenomena: 1) the focus, of which the depth was provided by the State Seismological Bureau, cannot be projected to the central fracture but is located on the front range fracture; 2) the epicenter is not the center of the isoseismal contour; 3) the surface rupture along the front range fracture and its thrusting displacement are similar to the surface rupture and displacement along the central fracture, and 4) the area in which the aftershocks (Ms≥4.0)occurred have a shape in the form of a ‘√’. The ambiguity results from the complicated tectonic environment of northwestern Sichuan. People generally pay more attention to the large boundaries of the triangular block in northwestern Sichuan (SCNWTB in Fig 1), but neglect the effect of the Minshan block (MSB) in the geostress conditions. The present authors think the “bottlle neck” geostress concentration in the Minshan block is the main controlling factor for the occurrence of the Wenchuan earthquake. The Minshan block is a sub-block of the northwest Block of Sichuan, it is bordered by Longmen Mountain Range fractures ((2) in Fig 1)in the south, the Huya fracture (5) in the east, the Maqin-Lueyang fracture in the north and the Mounigou Valley fracture (6) in the west. The block has been recognized earlier as an important tectonic element in the northwestern region of Sichuan (Tang et al. 2009; Jiang et al. 2004; Zhao et al. 1994a; Tang et al. 1991; Qian et al. 1999; Zhou et al. 2000). It not only includes the Minshan uplifted block in a narrow sense, but also the middle segment of the Longmen Mountain Range structural belt (Fig.1). Although the positions of the eastern and western block boundaries are still controversial, their existence and their recent activation are widely accepted. Analysis of seismological setting and the deformation data shows that the Minshan block has been activated by the north Mounigou Valley fracture ((6) in Fig 1) in the west, the Huya fracture (5) in the east, the back range fracture in the south and the MaqinLueyang fracture (1) in the north. The whole middle segment of the Longmen Mountain Range structures has been strongly pushed along the southern boundary of the Minshan block since the Mesozoic. The Paleo-

The Yigong rock slide – debris avalanche (YRA), which occurred on 9 April 2000, received worldwide attention as one of the largest nonseismic landslides in recent years, with a volume of 0.3 × 109 m3. Sixty-two days after this landslide event, a catastrophic flood happened because of landslide dam failure. One of the special features of this debris avalanche is liquefaction, which plays an important role in the entrainment and long run-out distance and high-speed movement of the debris avalanche. Numerous sand boils were found in the deposition zone, providing strong evidence for liquefaction. The YRA provides the first actual evidence for a theoretical model where the mechanisms of excess pore pressure and liquefaction induced by undrained loading, and entrainment and dissipation control the run out and deposition of the debris avalanche. The damage mode to trees and the presence of debris cones or molards with a rounded top is proven to be the result of strong air waves and eddies. These features all imply that the YRA is a solid–liquid–air mixed-debris avalanche.

High geo-stress and its engineering problems have severely affected the development of civil infrastructures in western China. The problems include high rock slope instabilities, rock burst, gas explosion and large-scale soft rock deformation in deep tunnels. This paper investigates the distribution of the high geo-stresses and the models of the stress concentration areas in the eastern margin of Qinghai-Tibet plateau so that a solid foundation can be formed to address the problems. The investigation is based on a comprehensive analysis of the previous research data of the eastern margin and uses remote sensing techniques, geophysics, geochemistry, and large scale geological surveying methods. The investigation has found that some special tectonic zones have high geo-stresses. The high geo-stresses are located at (1) the convergent boundary areas between two fault blocks with large strength differences, (2) the tectonic necks in front of active fault blocks, and (3) the intersection and/or termination areas of faults within the fault blocks. An example for (1) is the north Qilian high geo-stress area. Another example for (2) is the Minshan high geo-stress area in the northwest Sichuan. Furthermore, the investigation has summarized six basic models to characterize the high geo-stress concentration areas. The first one is the convergent stress concentration model at the boundary of two fault blocks. The other five stress concentration modes are oblique fissures or intersecting areas, areas without lower velocity layer in the crust, areas of compression induced tensile cracking, tectonic wedge areas, and tectonic neck areas, respectively.

The paper described a large rock avalanche and flow landslide in detail, which happened in Yigong, Tibet, China on April 9, 2000. According to the basic characteristics of the landslide, the whole event can be devided into three sections: rock avalanche, flow landslide, and deposition. The paper presented a detailed description of each section. Attention should be paid to some unique characteristics of the deposition section such as depositional cones, liquefaction holes, and remarkable wind damaged trees. Furthermore, in terms of some distinctive characteristics, the complex landslide can be subdivided into several subsections. The paper also provided a brief discussion on the mechanism of the large complex landslide to raise the research interests of the scholars both at home and abroad.

After the accumulation slope above the left bank intake of Xiluodu Hydro-electrical Power Station is excavated, it has been discovered by drill-monitoring that there exist obvious deformations at the deep-seated of the slope. Based on a lot of studies, we think that the deformation should be caused by excavation. Being characterized by stable-creep, the deformation is practically a kind of deformation-adjustment due to stress re-distribution during cutting. Although the safety factor with respect to strength is more than 1.0 (under natural conditions), it is not satisfied with safety standards of building slope engineering.

There are many weak fractured zones and net-cracks inside a right damabutment at a hydro-electrical power station.In order to evaluate their influences to deformation and stability of the right dam-abutment,the method of FLAC~(3D)(Vision 2.0) study is carried out on the basis of analysis on the geological conditions.It is shown that total deformation of the right dam-abutment is small, and the differential deformation between two dam-abutments is also small.The deformation of the right dam-abutment decreases rapidly as time-step under rock-fill dam load or dam and reservoir load.There is no obvious influence of the weak fractured zones on deformation and stability of a right dam-abutment.

Seti bridge,located in Pokhara city,Nepal,is one of the key bridges in main national highway network,connecting Kathmandu with Plkhara,and plays an important role in Pokhara city traffic. Its construction started in 1995 and was completed in September 1998. During floods in June 1995,three cracks appeared at left bank. Till now,the largest crack width is about 4 m,threatening the safety of the bridge. In March 2001,a group of seven Chinese scholars investigated the deformation and failure of bank rock slope. Investigation results show that the main deformation mechanisms of the Seti bridge river bank rocks are the combination actions of its peculiar conditions:special meteorological-hydrographic condition (intensive rainfall),valley topographical features (deep-cutting and narrow valley) and special geological structures (banks consisting of bonded gravel with “easy-scouring” rock stratum underneath,as proved by new investigation results,2002),and the erosion of the bottom rock strata to form a reentrant by strong water flow. The deformation and failure mechanisms of bank rock slope at Seti bridge site are very special,named as a fracturing-toppling-falling pattern. Under intense scouring actions,easy-scouring strata at the bottom of valley are hollowed out by rushing water. Tensile cracks will then develop in the relatively rigid rock mass overlaid,and induces toppling failure like cantilever due to gravity. The rock mass among the tensile cracks starts to fall. This pattern of deformation and failure can explain reasonably failure phenomena that appeared in deformation process of bank slope. At present,the inside crack at Kathmandu bank is 17 meters away from Seti Bridge,and the tensile cracks have the possibility to develope,and the crack are threatening the safety of Seti Bridge. Therefore,it is necessary to reinforce both of bank slopes with appropriate measures at bridge site.

Based on a detailed investigation on a consequent rock slope,it is found that a intermediate state of cataclastic-loosing before failure is there between the stages of both the strong bending-swell and the instability of rock body;for which a preliminary analysis is made on the forming condition,and then a geomechanical mode of "four stages" is put forward herein.

The east part of the inverted-triangle-shaped fault block in the northwest of Sichuan is situated in the northeast edge of the Qingzang Plateau. It extends across the northwest plateau of Sichuan and the valley of transitional area between that and Sichuan Basin. The NE-directional active rift zone of the Longmen Mountains is the southeastern boundary active rift zone of the east part of the inverted-triangle-shaped fault block in the northwest of Sichuan. The recent EW-directional active rift zone of the southern margin of the west of Qinling Mountains is its northern boundary active rift zone. The Minjiang River faults and Huya faults which consist of the recent SN-directional active rift zone of the Minshan Mountains are situated inside the east part of the northwest fault block of Sichuan. The three active rift zones consist of A-type active tectonic system controlling earthquakes. The authors discover the seismic activity law of this region. Generally speaking, inlensive seismic activity belts formed near 104°E . Besides, the frequency and maximum earthquake magnitude inside the fault block are higher than those in boundary faults and the seismic intensity in the north is far larger than that in the south. As far as the space is concerned, intensive earthquakes in 104°E can transfer from boundary faults to those inside and vice versa, when it comes to the time, the active period and dormant period alternate and the earthquake whose magnitude is larger than 7.0 tends to accelerate.

This paper studies the activity and characteristics of main active faults in the Lijiang region, discusses the evolution of structural stress field and features of modern stress field, and probes into the mechanism of the Lijiang earthquake on February 3, 1996. By using the discrete element method, the stress field is simulated, which deepens the understanding of the stress field of the Lijiang earthquake. The Lijiang region is situated in a triangle tectonic block bordered by the NS-trending Chubo-Baihanchang fault, the NE-trending Lijiang- Xiaojinhe fault and the NW-trending Zhongdian-Yongsheng fault. Geological and seismological studies indicate that in the region the maximum principal stress is in the NNW-SSE direction. In such a stress field, the NE-trending Lijiang-Xiaojinhe fault is dislocated reversely with the left-lateral movement, and the NW-trending Zhongdian-Yongsheng fault is dislocated right-laterally. It is such a dislocation pattern of faults that causes the Judian-Jiuzihai block between the faults to move westward, thus producing local tensile stress of the east-west direction, leading to normal faulting of the fault at the east of Yulongxueshan Mountain, generating the M 7. 0 earthquake on February 3 of 1996. The mechanism of an after-shock on February 5 with a magnitude of 6. 0 south of the epicenter further indicates that the earthquake was induced by the strong concentration of tensile stress at the intersection of the fault at the east slope of Yulongxueshan Mountain with the Zhongdian-Yongsheng fault.

工程水文地质问题产生于人类工程活动与地质环境中岩土体和地下水之间的相互制约和相互作用过程中,由于问题本身的复杂性,目前对这类问题的研究尚处于相对分散的单一研究状态,缺乏系统的从地质机制分析到地质模型抽象,进而由模拟再现到发展趋势预测以及治理措施论证的全过程研究.本文在系统分析、总结水电工程中水-岩和水-工程建筑的作用类型及特征基础上,认为可能出现的工程水文地质问题主要是:水库诱发地震、岩溶渗漏、库岸及高边坡的稳定、坝基、坝肩抗滑稳定、枢纽区深层承压水问题等.

By systematic numerical modeling by the distinct element method with UDEC code, the effect of physical and mechanical properties of rocks including internal friction angle, Young's modulus, Poisson's ratio, and cohesion on stress field in the vicinity of fractures are analyzed. It is shown that the variation of all of these properties may cause the change of stress fields in the vicinity of a fault. Meanwhile, the effect of physical and mechanical properties of rocks co-exists with that caused by the change of mechanical properties of fractures and boundary conditions. In the modeling, both the single fracture model and compound fracture model are studied respectively. It is found that the Young's modulus of rocks has a strong influence on the variation of rock stresses in the case of compound fracture model.

Based on finite element analysis, the authors studied the regional stability of the Hutiaoxia (region) systematically. The result shows that the activities of the fractures change from place to place under the current regional stress-field. The activity of NW strike Zhongdian-Yongsheng fracture is most active and the safety degree is lowest, that of NE strike Lijiang-Xiaojinhe fracture is modest, and that of nearly SN fracture such as Chubo-Baihanchang fracture is weak and its safety degree is highest.

It has been discovered by exploration that besides the normal stress-release cracks there exist a lot of mud-intercalated fracture zones in the right bank slope. In order to ascertain the genesis and developing tendency of these deformations and their possible affection on the hydro-electrical construction in future, a lot of studies have been carried out. The results of these researches are discussed in this paper.

西南某坝区右岸由海西期花岗岩组成.除断裂构造发育外,右岸岩体蚀变类型多,分布广,风化和卸荷强烈,深度大.上述现象组合在一起,使右岸岩体极为复杂.实地调查表明,这一复杂岩体的形成主要是由于右岸丫口后山穹隆新生代以来曾经经历过较强烈隆升导致放射状和环状张裂隙发育的结果.

讨论官地水电站坝区存在的复杂化学成分的高水头承压水形成的地质控制因素.该区特定的地貌位置、岩体结构、裂隙的组合关系和地应力环境共同决定了本区承压水的形成、分布特征.

According to the result of joint statistics, the damage tensor of each rockmass structure zone is calculated by the method of damage mechanics. The model evaluating the quality of engineering rockmass with the maximal principal damage value is set up and used in engineering practice.

应用离散元方法,分别研究了断裂刚度、内摩擦角、内聚力和几何形态等对其附近地应力场的影响.结果表明:这些因素对断裂附近的地应力场均有不同程度的影响,能造成主应力方位和量值发生变化;这种变化主要限于断裂附近一定距离内,远离断裂,逐渐趋于与区域应力场一致;断层附近应力方位变化的幅度和发生变化的范围,是因断层的力学性质及几何形态等的不同而不同.认为断层的内摩擦角(ψ)对断层附近应力方位变化的范围和幅度影响最大.

在印度、欧亚两大板块相互碰撞推挤的特定地球动力学环境条件下,自新生代以来中国西南地区构造应力场的发展演变总体上经历了五个阶段,地壳岩体的应力-形变作用呈现出三个不同的发展阶段.

岷江上游地处我国著名的南北向地震带的中段,因其特定的地质环境导致区内表生地质灾害极为严重.通过对岷江上游(汶川以上)河段的崩塌、滑坡、泥石流等表生地质灾害的调查研究,其分布沿岷江两岸具有明显的分段特征与河谷地貌分段基本一致,它们形成发展与特定地形地貌、易崩滑或软弱地层、特殊的构造部位、降雨等密切相关.