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ABSTRACT "5.12" Wenchuan Earthquake (Ms=8.0) triggered tens of thousands of geo-hazards throughout an area of about 100,000 km2. The distribution of geo-hazards is definitely affected by the such factors as geomorphology, topography, lithology and human engineering activities, but the distribution of earthquake-triggered geo-hazards is mainly controlled by the co-seismic fault, for they present zonal distribution along the co-seismic fault. Through the studies of three geo-hazard concentration areas including Doujiangyan to Wenchuan Road, Beichuan to Anxian County area, and Magong to Hongguang area, it is found that Wenchuan Earthquake-triggered geo-hazards have the following co-seismic effects:Wenchuan Earthquake is a thrust fault earthquake. The distribution of geo-hazards show marked "hanging wall and footwall effects", manifested by that the distribution concentration in the hanging wall of seismic fault is higher than that in the footwall and the former coverage is wider than the latter as well as that the scale of geo-hazards is larger than that in the footwall; the strong development zones of geo-hazards are within the range of 7 km in the hanging wall of co-seismic fault. The ranges of 7 to 11km in the hanging wall and o to 5km in the footwall can be delimited as the middle development zones. A vast majority of large-scale landslides are distributed within the range of 5km from the fault, and large-scale landslides are unlikely to occur within the range of over 10km; the transitional and staggered positions of fault are the concentration areas of geo-hazards and large-scale geo-hazards are likely to occur in these positions; the preferred direction of landslide is NW-SE, basically vertical to the spreading direction of Yingxiu-Beichuan Fault. This may be because that Longmenshan Fault Zone has always been affected by the tectonic stress field of NW-SE extrusion area since Cenozoic Time; the earthquake-triggered geo-hazards mainly occur in the zones of seismic intensity IX and higher. The concentration of geo-hazards in the seismic intensity zones XI is the same as that in seismic intensity zones X; the concentration in the seismic intensity zones IX equals to 1/3 of the abovementioned one; the concentration in the seismic intensity zones VIII is only 1/10 of the abovementioned one. 1 INTRODUCTION At 14:28 May 12, 2008(Beijing time), a great earthquake measured at 8.0 Ms according to the China Seismological Bureau struck Yingxiu Town (31.0°N, 103.4°E) of Wenchuanxian County, 70 km northwest of Chengdu, the capital of Sichuan, causing the frontal and central faults (Jingyou-Guanxian Fault and Yingxiu- Beichuan Fault) of the Longmenshan Fault Zone between the Western part of the Sichuan Basin and the eastern margin of the Qinghai-Tibet Plateau to rupture, resulting in the formation of an earthquake rupture zone of around 300km long(Huang, R.Q., et al, 2008, b; He,H.L.,et al,2008). The Wenchuan earthquake had a high magnitude and large energy release causing a strong destructive force and large area of influence. The epicenter of the strong earthquake was located in the medium to high mountain areas just West of Sichuan Basin, where the geological environment is quite weak.
Seismic Intensity Estimation Through Questionnaire Survey In the 2008 Wenchuan Earthquake, Sichuan, China
Yan, Changgen (Kanazawa University , Highway College,Chang'an University,Xi'a) | Miyajima, Masakatsu (Kanazawa University) | Kitaura, Masaru (Kanazawa University) | Murata, Akira (Kanazawa University) | Toshikazu, _ (Kanazawa University)
ABSTRACT Seismic intensity around the epicentral area of the 2008 Wenchuan, Sichuan Province of China earthquake is estimated using a questionnaire survey conducted one month after the earthquake. The survey results show that the seismic intensity of Chengdu city is about VII, Beichuan County and Pengzhou Longmenshan Town about X, and Dujiangyan City about IX in MMI scale, above all are basically in agreement with the survey results of Chinese Earthquake Administration (CEA). The analysis results show that there are two subjective factors affecting the survey results of seismic intensity: one is the sexuality and the other is age. The seismic intensity value by survey of men is often lower than the women's. When the surveyed people is elder than 60 years or younger than 19 years, the seismic intensity value by surveyed those people usually is higher than the others'. In general, the questionnaire survey is a good method to estimate the seismic intensity near to the populated area, but it is unsuitable in an unpopulated area. 1. INTRODUCTION A severe earthquake (30.986°N, 103.364°E, Depth=14km), which measured 8.0 Ms, 8.3 Mw, according to Chinese Earthquake Administration (CEA), occurred at 14:28:01.42 CST (06:28:01.42 UTC) on 12 May 2008, in Wenchuan County, southwestern of China. It is also known as the Wenchuan earthquake, after the earthquake's epicenter in Wenchuan County, Sichuan Province. The length of ruptured fault, Longmenshan northeast extension fault, is more than 300 km. The earthquake was so strongly that it can be felt in most part of China, 69,197 people were killed and 18,341people were missing in this earthquake. In addition to such devastating human loss, countless buildings were collapsed or heavy damaged, most of the lifeline suffered heavy damage and heavy geological disaster caused around the ruptured fault, the local economy was suffered fatal destroyed, and the direct economic loss of this earthquake is 845.1 billion RMB (It is about 122.5 billion dollars). It is the deadliest and strongest earthquake to hit China since the 1976 Tangshan earthquake, which killed at least 240,000 people. 1.1 The situation of damage buildings Many engineered as well as non-engineered structures were severely damaged and even collapsed during the earthquake. In Beichuan County and Penzhou Longmenshan Town, Un-reinforced masonry (URM) structures and wooden structures were commonly collapsed because of poor construction of their bearing walls, and most of buildings were suffered heavy damage (Plate 1. to Plate 3.). In Dujianyan City and Chengdu City the buildings were suffered different extent damage. For example, in Chengdu City, a few buildings appeared cracks, whereas, in Dujiangyan City, most buildings appeared cracks, some heavy damaged or even collapsed (Plate 2.), and the damaged degree depending on the distance from the ruptured fault. Most of the destroyed villages were subjected to very intense earthquake motion without attenuation because they were very near the ruptured fault. The site effects amplified the earthquake motion in some areas, causing excessive damage. In general, village near the epicenter and ruptured fault suffered greater damage.
ABSTRACT One ofthe lessons learned from the catastrophic Wenchuan earthquake is that extremely strong event has occurred first time within peoples' memory in the region with millennia-long seismic record. It clearly shows that extreme events that can occur in favorable seismotectonic conditions should be considered for seismic hazard and slope stability assessment irrespective of presence or absence of similar phenomena in the historical and instrumentally recorded occurred along the northern and southern boundaries of this mountain system, while its central part have no record of such events in the past. However, this record hardly extends for 200 years. At the same time Central (Inner) Tien Shan is extremely rich of surface ruptures, rockslides and other features that can be interpreted as geological traces of strong past earthquakes. It hydraulic projects are proposed and implemented here. Possibility of a very strong earthquake occurrence should be considered both for dams' design and for the reservoir slopes stability assessment. 1 INTRODUCTION The catastrophic 2008 M8.0 Wenchuan earthquake gave us several tragic lessons, one of which, may be the most important, is that this extremely strong event has occurred first time within people' memory in the region with the millennia-long seismic record (Catalogue … 1989, Wang 2004). None of this numerous strong earthquakes that have occurred in this seismotectonic zone exceeded M7.0–7.5. Another lesson is that location and kinematics of surface ruptures, which mark the causative fault, play a critical role in the spatial distribution of the earthquake- triggered landslides. Most of slope failures occurred within the hanging wall of the 180-km long surface rupture with up to 6-m lateral displacement (Chigira et al. 2008). Largest river-damming rockslides also concentrate just along main active faults (Yin et al. 2008). The same phenomena were observed at the 1999 M7.3 Chi-Chi earthquake in Taiwan (Lin et al. 2004), at the 1911 M8.2 Kemin earthquake in Kyrgyzstan (Bogdanovich et al. 1914, Delvaux et al. 2001) and in many other cases. These lessons mast be applied to other seismically active regions, where the historical record of hazardous natural phenomena is much shorter than expected recurrence interval of strongest earthquake and where the possibility of large-scale slope failures is high enough due to rugged terrain and geological conditions. One of such regions is the Tien Shan, its inner (Kyrgyzian) part in particular. Spatial distribution of strong earthquakes, both historical and instrumentally recorded along the northern and southern boundaries of this mountain system, while its central part, which is neotectonically as active as boundary zones, has almost no records of such events in the past (Figure 1). It should be poited out, however, that the Tien Shan historical record of earthquakes hardly extends up to 200 years, since this region was inhabited by nomads lacking written history. At the same time the entire Tien Shan, including its central part, is extremely rich of surface ruptures, rockslides and other features that can be interpreted as geological traces of strong past earthquake (Figure 2).
The Basic Characteristics And Development Trends of Wenchuan Earthquake Geohazards
Li, Zhiqing (Hohai University) | Hu, Ruilin (Institute of Geology and Geophysics) | Xu, Wenjie L. (The China Institute of Water Resources and Hydropower Research) | Wang, Lichao (China Institute of Geo-Environment Monitoring)
ABSTRACT A strong earthquake of Ms 8.0 happened on 12 May 2008 in Wenchuan, Sichuan Province, China. The secondary geologic hazards were very serious. Wenchuan geohazards investigation team organized by Chinese academy of sciences entered into Wenchuan to carry out the post-earthquake geohazards investigation in October 2008. Based on the investigation, the paper discusses the geological background of Wenchuan earthquake, the basic characteristics of secondary geohazard and its development trend. Some suggestions are put forward for the postearthquake reconstruction in disaster area. 1 INTRODUCTION A strong earthquake of Ms 8.0 happened on 12 May 2008 in Wenchuan, Sichuan Province, China. The secondary geohazards were so serious that it damaged not only the huge amounts of buildings, farmlands, traffic and casualties, but also hindered Chinese urgent rescue work. It is noteworthy that collapse hazard was one of the most widely distributed geological disasters. It distributed widely and had the different scale. Whenever the latent collapse incidents may happen because of the fragmentation of mountains triggered by the earthquake. It is very necessary and urgent to strengthen the scientific research on the forming mechanism and dynamic process of collapse triggered by the earthquake. Our China is a multi-earthquake country. There were many strong earthquakes occurring in history. The earthquake which is one of the severest natural disaster threats is in seismic active period. Also our China is a mountainous country, in which hilly and mountainous areas account for two-thirds of the whole land areas. There is massive earthquake fault zone development in most areas and the most active seismic zones are in the alpine gorges, such as Longmenshan frature. So it is very outstanding of the secondary geological hazards triggered by the earthquake. Wenchuan geohazards investigation team organized by Chinese academy of sciences entered into Wenchuan to carry out the post-earthquake geohazards investigation in October 2008. 2 GEOLOGICAL BACKGROUND AND BASIC CHARACTERISTICS OF WENCHUAN EARTHQUAKE The basic generation reason of Wenchuan strong earthquake is that the strong broaching force transferred to Longmen mountain fault zone in the west of Sichuan basin from Tibetan plateau because of the sustained broaching effect between Indian plate and Eurasia plate. The sustained accumulation of stress and energy producing in this area was finally more than the affordability of central main fracture in Longmenshan fault zone. So that the long-term strain energy accumulation formed the huge earthquake (Runqiu Huang, 2008). The structural deformation of continental lithosphere is a kind of kinetic formulation of bidirectional extension deformation from crustal derm to deep extension and from crust-mantle transition zone to upper crust (Xuelin Cai, 2008). The earthquake hazards brought out great damage mainly because that seismogenic zone is located in mountain area in the west of Sichuan basin, in which there are extremely vulnerable geological environment, large height difference terrain and relative dense population. The hazards were triggered on the one hand by the earthquake, on the other hand by geohazards (Plate in full paper) It is found that the intense activity of secondary geohazards further intensified the risk.
ABSTRACT Instability analysis about saturated rocks is carried out to understand the initiation and evolution of rock landslides and rock-falling. The spatial development of a single shear band in saturated rocks for anti-plane shear deformation is mainly investigated here. It is shown that if the angle between the proportional loading path and the direction of the localization ∆θ = π/2, then the deformation is stable. This corresponds to the case of neutral loading. The case ∆θ > π/2 corresponds to elastic unloading. ∆θ < π/2 corresponds to loading and the instability develops the fastest when ∆θ is equal to zero. Positive solutions for α require that the rate of pore pressure softening exceed the rate of strain hardening. INTRODUCTION Shear band is often observed in saturated rocks and is the main reason that causes the failure of foundations, the emergence of landslide etc (Kebeasy, 2008). Such as during the earthquake occurred in Wenchuan, Sichuan province, China at 14:28 on May 12, 2008 (Beijing time), many rock landslides are excited. For rock landslides, the initiation and evolution of cracks in rocks which may cause the lose of the strength is the key problem. The development of shear band in rock determines the evolution of cracks. Thus to study the initiation and evolution of shear band in rocks is very important for understanding the initiation of rock landslides. Shear bands is closely dependent on the material characteristics and the local strain (Rice, 1975). For an example, when the pore pressure softening overcomes the strain hardening (Sulem, 2006; Lu 2000), localization shear may form. The issue of the inception of localized shearing in rocks subjected to undrained deformation has been the object of both theoretical as well as experimental research. Theoretical contributions are related mainly to stability and bifurcation analysis of diffused and localized failure models (Rice, 1975). Typically, the stability problem is formulated by considering small perturbations in field variables (e.g. displacement and pore pressure). Classical continuum approach leads, in this case, to the ordinary diffusion equation for the perturbation in pore pressure (Rice, 1975). The results indicated that the uniform response is often followed by the onset of a diffused, nonhomogeneous deformation model, after which distinct shear bands form. However, this problem is often discussed under inertia-free undrained conditions (Rice, 1975; Pietruszczak et al 1993). Anti-shear is a commonly load bear by Rocks in the condition of earthquake, wind load etc.. Nevertheless, study on the failure under this type of load is few. In viewpoint of above, we will investigate the spatial development of a single shear band for anti-plane shear deformation in this paper so as to obtain a comprehensive and precise picture of the instability phenomenon by using of the method of Douglas &Chen [1985]. CONTROLLING EQUATIONS Consider anti-plane shear deformation, for which the motion is governed by(Equation in full paper) PERTURBATION ANALYSIS (Equation in full paper) Substitution of the equation above into the controlling equations (3) and (12) gives the first order.
Investigations On the Active Tectonics And Surface Ruptures of the Wenchuan Earthquake
Yong, Li (University of Technology) | Runqiu, Huang (University of Technology) | Densmore, Alexander L. (Department of Geography, Durham University) | Sang, Chan Lung (Department of Earth Sciences, The University of Hong Kong)
ABSTRACT Wenchuan earthquake is a thrust with strike-slip type, and surface ruptures are located in Beichuan fault, Pengguan fault zone, Xiaoyudong tear fault, and Leigu tear fault. The scratches of the surface rupture reveal the thrust movement occurred early and the strike-slip movement occurred lately during the earthquake. A dynamical model to illustrate possible links between surface processes and upward extrusion of lower crustal flow channel at the eastern margin of the Tibetan plateau have been studied, and the results is the material in lower crust in the Longmen Shan moving as nearly-vertical extrusion and uplift, resulting in the surface rate of tectonic movement differing according to depth rate as well as the occurrence of large shallowWenchuan earthquake. 1.INTRODUCTION The Ms 8.0 Wenchuan Earthquake in May 12th, 2008 is one of the most disastrous earthquakes since the foundation of P.R. China, which destroyed not only the epicenter of Sichuan province but also several closed provinces. It was felt in most region of China, as well as nations outside China. This tragic event provides the opportunity to advance the subject of seismic sciences. Based on our accumulated activitiy on active tectonics in Longmen Shan area, we have undertaken several new field surveys, including international collaborative efforts after the earthquake. This paper compiles 70 sets of data from accumulated past and new surveys, detailing surface rupture and seismic disasters since our work began in the 1990s(Li et al,2000,2001,2006;Zhou et al,2006,2007;Densmore et al,2007; Li et al,2008). 2. GGOLOGICAL SETTING OF LONGMEN SHAN SEISMIC BELT The Qinghai-Tibetan Plateau is the highest and the youngest uplift region in the world with more than 4000 m altitude. The Indo-Asian collapse is the most important tectonic event, inducing the uplift, deformation and thickening of the Qinghai-Tibetan Plateau. Concerning the relationship between this event and Cenozoic tectonic geology, two famous theories have been presented: one is the crust thickening mode(England,et.al,1990) and another one is lateral extrusion mode(Avouac, et.al,1993). The former emphasizes the shortening of the south-north direction as well as crust thickening; the later emphasizes the east-lateral extrusion mode along main strike-slip faults. The core object is the relation between uplifting process (vertical motion) and deformation process (horizontal motion) in the Qinghai-Tibetan Plateau, and the relation between the motions and Indo-Asian collapse. The Longmen Shan thrust belt is located at the west margin of Sichuan basin and the joint between Songpan-Ganzi orogeny and Yangtze Craton. It is both the east margin of Qinghai-Tibetan Plateau and the west margin of Longmen Shan foreland basin (fig. 1). It is enclosed by Guangyuan in the north, Tianquan in the south, Dabashan orogeny in the northeast and Xianshuihe fault in the southwest, forming a northeast-southwest strike belt with 500 km long and 30 km wide. Consisting of a series of parallel imbricated thrust, it develops, from the west to the east, the Maoxian-Wenchuan, Beichuan and Pengguan faults. Along the fault planes, there are multi-classic added thrust nappe structure belts: Wenchuan-Maowen ductility fold structural belt, the Beichuan cuiductilit thrust nappe structural belt.
- Phanerozoic > Cenozoic (0.50)
- Phanerozoic > Mesozoic (0.46)
ABSTRACT The Ms 8.0 earthquake that hit Wenchuan on May 12, 2008 caused a number of barrier lakes threatening lives and properties in the downstream. The early reported 34 lakes in the quake-hit area are scattered within a distance of 15 km to the central fault of the Longmenshan Fault Zone. These lakes are distributed along the faults. Emergency measures have been taken to the lakes with high risks. This paper reports the background, environment, damming structure, countermeasures, and its effects with particular interest to the Tangjiashan and Xiaogangjian barrier lakes. Finally, problems for future study are commented 1 INTRODUCTION Barrier lakes are usually formed by landslides, moraines, debris flows, and volcanic eruptions or flows when they run into and dam up rivers. Barrier lakes induced by earthquake are one of many secondary disasters of the earthquake. Historically, quake-induced lakes are common in mountain areas. These dams are formed by quick accumulations and usually unconsolidated, loose in structure, and likely to collapse due to seepage and erosion, resulting in catastrophic flood or debris endangering the downstream (Seismological Bureau of Sichuan Province, 1983). Landslides, rockfalls, debris flows, and collapses induced by the Wenchuan earthquake on May 12, 2008, have formed many barrier lakes and 34 of high risk ones have been treated by emergent countermeasures, from the early report. This paper provides an outline of the barrier lakes, their distribution, and the countermeasures. 2 Distributions and main features 2.1 Distributions of the barrier lakes The Wenchuan earthquake has stricken the mountain areas with complexity of geology and geotectonics, making them prone to landslides and collapses. Many slopes are unstable and likely to fail under the vertical and horizontal forces of the quake, large landslides occur at some locations. The landslides and collapses poured into the streambed that dammed the flow and finally formed lakes (Cui Peng, 2008). By May 19, seven days after the earthquake, 34 barrier lakes were reported (Figure 1, Table 1, 2). 2.2 Distributions of the barrier lakes along the major faults The epicenter is located in the central faults of the Longmenshan fault zone, which consists of a group of faults in NE30°-40° of Yingxiu-Beichuan fault, Remote sensing data indicate that the major fracture stretches along the Yingxiu-Beichuan faults, from the Shuimo town of Wenchuan in the south through the Chaping village of Qingchuan in the north. The barrier lakes are distributed along the major fracture, all within 15 km to the major faults (Figure 2). Among them, 24 lakes are within 5 km; and the number of lakes along the front and back faults are 24 and 10 respectively. (Table in full paper) 2.3 Distributions of the barrier lakes in terms of rivers In rivers, the barrier lakes form strings of beads. For example, in the Tongkou (Jianjiang) River within 30 km lie 9 lakes; and in Mianyuan Rive there are 4 within 10 km(distribution density is 0.4 lakes/km); in Shiting River, 7 within 8 km(distribution density is 0.88 lakes/km).