For analyzing the role that reservoir impounding plays in triggering earthquake, the process of diffusion of pore pressure and its mechanism of action should be understood firstly. The temporal distribution of seismicity, which occurred before the M_S8.0 Wenchuan earthquake, following the impoundment of Zipingpu reservoir is studied in this paper. Then the mechanisms of the occurrence and development of reservoir triggered seismicity are discussed. A comparative analysis of the temporal distribution of seismicity and the submerged area by reservoir impounding is carried out firstly. Then the influence of various factors on modeling is analyzed in detail. After calculating, the pore pressure change by the Zipingpu reservoir impoundment is obtained. The following observations are made:(1)Conspicuous swarms of earthquakes, of which the sources are located on the same fault of the M_S8.0 Wenchuan earthquake, occurred orderly with the impoundment of Zipingpu reservoir.(2)Because of the influences of the terrain and the medium, the range of effect of pore pressure change by the impoundmemt is limited and anisotropic. Hydraulic diffusivities(D)of 0.7 and 0.35m²/s along the fault strike and the fault dip are reached respectively by a semi-quantitative assessment. Of course, the qualitative pressure constraints on the surface are also applied for the modeling.(3)The calculation results show that the temporal distribution of seismicity near the Zipingpu reservoir is related with the pore pressure change. After the pore pressure reached the threshold of triggering earthquake, whether the pressure head change is high or not, the change rate of pressure head change plays a key role in the decrease or increase of seismicity. It means that the triggered seismicity by pore pressure is a dynamic triggering process.

In this paper, using the double difference tomography method, the P wave and S wave velocity structures of the earth's crust beneath the Three Gorges Reservoir are inversed based on the high-resolution seismic data of seismological stations recorded from March 2009 to December 2010. According to the research results, the P wave and S wave crust velocity zones in the Three Gorges Reservoir area show a high V_P value area and a V_S value area with value low in the lower part and high in the upper part, distributing respectively at both sides of Shennongxi River to western Xietan in the north of Badong and near the outlet of the Xiangxi River at the northern section on Xiannvshan Fault. In the region from the two sides of Shennong River in the north of Badong to the western Xietan, microseisms are distributed in three zones in near east-west direction, with steep and north-dipping sections, spreading along the high-to-low velocity transition zone of the P and S wave. On the northern section of Xiannvshan Fault, small earthquakes are distributed along the NNW-trending Xiannvshan Fault, and the geological section reveals a steep and linear distribution along the transitional zone between the high V_P value area and the V_S value that is low in the upper and high in the lower part. Joint inversion results show a good consistency of the planes of the microseisms with the distribution of active faults.

Based on the differences of seismic activity,focal mechanism,geological structure,and hydrologic condition between reservoir induced seismicity(RIS)to natural earthquake,the basic features of reservoir induced earthquake from natural earthquake are studied statistically by the RIS database of 150 cases of reservoir induced earthquakes world-wide,and the results are shown as follows: ① Using statistical principle,we find that RIS occur in the areas near the reservoir banks in the impounding period,most of reservoir induced earthquake are distributed within a distance of 10km to the bank,with depth less than 10km,and a few events extend outwards as far as over 10km along fault zones,karst caves,and(thermal)springs; ②The frequency and magnitude of reservoir induced seismicity change with the reservoir water level. Most maximum magnitude earthquakes take place in the period before the first highest water level is reached,or in the following 2-3 impounding stages after the first highest water level,after then,a few would take place; ③The sequence of RIS is complete,usually as "foreshock-main shock-aftershock" or "foreshock-swarm-aftershock",most of them are microseisms of M_L1-3, so the b value is larger than natural earthquakes; ④ RIS differ from natural shocks in higher epicenter intensity,higher frequency of ground motion and higher peak ground acceleration,but faster attenuation with distance; ⑤The stress drop of RIS is smaller than that of natural earthquake of the same size,and the source size is larger; ⑥ The RIS occur in areas under lithological and geologic conditions of good permeability. Tensional fracture,tensional fault zone,unconsolidated fault plane,karst,and(thermal)springs are all good water penetration channels.

In this paper,a statistical analysis is made using the gray clustering method on the earthquake inducing factors,such as dam height,storage capacity,regional stress state,fault activity,lithology medium conditions and seismic activity background,based on a database of reservoir-induced earthquakes,which includes 150 reservoir-induced earthquake cases and 532 large reservoirs data. Corresponding earthquake occurrence ratios are obtained and used as predictors for reservoir-induced earthquakes. According to the six quake-inducing factors,assessment is made on the maximum magnitude of reservoir-induced earthquake for the Three Gorges Reservoir dam area,the bayou of Xiangxi stream and the cross-river section of the Xiannushan Fault zone,the west Xietan,Badong,and on both banks of Shenlong stream. The results show that the upper-limit of magnitude at dam area is less than magnitude 3,that at the bayou of Xiangxi stream and the cross-river section of Xiannushan Fault zone is between magnitude 4～5,that at west of Xietan is less than magnitude 3,and that at the northern bank of badong and on both banks of the Shenlong stream is between magnitude 3～4.The results have been validated by the actual reservoir-induced seismicity after the impoundment of the Three Gorges Reservoir, with the M_L 2 earthquake at Xiaoxikou 4 km far away from the dam of the Three Gorges Reservoir on February 6, 2009, the M_S 4.1 earthquake at the bayou of Xiangxi stream and the cross-river section of the Xiannushan Fault zone on November 22,2008, the M_S 2.6 earthquake at the west Xietan on August 22, 2009, and the M_L 3.5 earthquake at the bank of Shenlong stream on December 14,2009.

Based on the interpretation of satellite image,field investigation and geomorphic survey and sample dating of surface,the strath terrace and fill terrace at the outlet of Moleqiehe River on the western segment of Altyn Tagh Fault zone(ATF)are used to study the tectonic uplift rate,uplift model and aggradation rate,and cooperated with data of the regional climate,the response to climate evolution of development of terrace are discussed.The previous studies of the terraces related with the Altyn Tagh Fault zone are mainly focused on the horizontal offset in order to obtain sinistral-slip rate,but few studies involve the uplift using terraces.As a structural zone with strike-thrust characteristic,the ATF is a boundary structure of the northern fringe of Qinghai-Tibet Plateau,and its thrusting and uplifting movement is of significance for controlling the uplift of the northern fringe of the Plateau.Therefore, the study of uplift of the ATF will be helpful for understanding the uplift model and mechanism and promoting the kinematic study of the northern fringe of the Plateau.The formation of strath terrace is closely related with tectogenetic movement,the landform age of the terrace represents the starting time of uplift.Based on the height and strath and landform age,the uplift rate can be calculated.The fill terrace is formed by climate forcing,the surface age represents the end time of one aggradation event.If the starting time of aggradation is obtained,the aggration rate can be calculated.There are four stream terraces at the outlet of Moleqiehe River(T₄,T₃,T₂,and T₁). T₄ and T₃ are strath terraces,T₂ is fill terrace,and T₁ are fill-cut terraces.The landform ages of T₃,T₂,and T₁ are 18.98±1.42ka BP,13.08±1.01ka BP,and 5.72±0.43ka BP,respectively.The existence of T₃ reveals the uplift rate of 6.66±0.50mm/a since 18.98±1.42ka BP.The existence of T₃ and T₂ reveals the time of fast uplift movement and aggradation events between 18.98±1.42ka BP to 13.08±1.01ka BP,the uplift rate is bigger than 20mm/a and the aggradation rate bigger than 10mm/a.The model of tectonic uplift shows tilted uplift from south toward north across the ATF,and this model is one of the types of the Qinghai-Tibet Plateau extending toward north.The aggradations,that constructed the T₂,are the result of the coactions of fast uplift and deglaciation climate between 15ka BP to 12ka BP.

Using the observations of the Zipingpu reservoir seismic network(including seven short-period seismic stations,with an average station distance of 10km)of the period from July 2004 to 2009,and the data from January-December 2009 recorded by the intensive seismic observation network(six short-period seismic stations with the station spacing of 3km),930 foreshocks near Zipingpu reservoir,the main shock,and 5789aftershocks,as well as the initial rupture of the M_S 8.0 Wenchuan earthquake were relocated with double difference location algorithm.Combined with the geological field investigations,the analysis of seismic activity in Zipingpu reservoir area,the distribution of coseismic surface rupture,the depth of initial rupture location of Wenchuan M_S 8.0 earthquake,and the aftershock distribution,we find that the occurrence of M_S 8.0 Wenchuan earthquake has close relation with Zipingpu reservoir: 1) The seismic swarms at the Shuimo,and Dujiangyan and Shenxigou area are 10 kilometers away from the reservoir,the seismic strain release caused by Zipingpu reservoir increased by 200%,and the release was closely related to water level changes and accelerated before the Wenchuan earthquake; 2) There are two northeast directed coseismic surface rupture zones with 1m displacement along Zhongtanpu Fault; 3) the aftershocks of Wenchuan earthquakes are mainly distributed on northwestern wall of the Zhongtanpu Fault; 4) The relocation revealed that the Wenchuan earthquake occurred at 27minutes,59.5 seconds,the depth is between 6～9km; and 5) by comparison,the focal depths of the fore-shocks occurring on 2008-04-05 are about 7.8km,and the initial time of waveforms of this swarm is same as that of the M_S 8.0 Wenchuan earthquake.

Earthquakes induced by reservoirs always cause more serious damage to reservoir dam due to its epicenter location and destroying effect.Reservoir induced earthquakes have different characteristics from the natural ones,their hypocenters are always shallow,less than ten kilometers,and the magnitude is small too.Till now the biggest earthquake induced by reservoir is the MS6.1 earthquake in Xinfengjiang reservoir in 1962.The paper analyzes the effects of different factors related to reservoir induced earthquake based on GIS technology and lots of reservoir induced earthquakes data from the world.The primary results show that reservoir's local geological and hydrogeological settings have closer relations with induced earthquake than regional geological and seismic activity setting does.The reservoirs with dams higher than 50m or reservoir volume greater than 5?1010 m3 are prone to induced earthquake.Moreover,the reservoir region,where limestone,carbonatite,karst landform,or joints and high permeable structure are developed,will have a higher probability to induce earthquake.

815 aftershocks of Wenchuan earthquake recorded by 12 seismic stations in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the epicenters of aftershocks concentrate relatively in three regions.Focal depth distribution characters of different concentration regions take on obvious difference.This means earthquakes in different concentration regions may have different causes.Compared with relocations of earthquakes taking place before the Wenchuan earthquake done by other researchers,locations of the seismic concentration regions in the reservoir area changed noticeably after the Wenchuan earthquake.These variations are related with the stress adjustment of local part in the reservoir area and may also be related with the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.

136 earthquakes,taking place in the west of Xietan area,recorded by portable stations deployed in the Three Georges reservoir area were relocated using the double difference algorithm.The relocations show that the root-mean-square shifts of the relocations in the directions of E-W,N-S and U-D are 0.38km,0.33km and 0.98km,respectively.The earthquakes in clasolite area with the focal depths about 4～5km take on linear distributions from shallow to deep part.These earthquakes were deduced as reservoir induced earthquake of fault fracture type.In the contrast,the earthquakes in limestone pavement with the focal depths about 2～3km take on slightly divergent distribution and have the characters of reservoir induced earthquake of the karts collapse type.

In accordance with the requirements of "Eleventh Five-Year" national scientific and technological support projects,an intensive seismic observation network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009(21 short-period seismographs,5 broadband digital seismographs).From March to December,2009,2995 M_L-0.8～2.9 earthquakes were observed during the trial impounding of the Three Gorges Reservoir(water level rose from 145m to 172.8m).Using the double difference earthquake location algorithm,2837 earthquakes were relocated precisely.The results show: The pattern of small local earthquake swarms in Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution,the mass-like clusters of events were generally within the distance of 5km from waterfront,and the linear distribution of the earthquakes could be extended to a distance of 16 kilometers away from waterfront.In Hubei section of Three Gorges Reservoir,earthquakes were mainly concentrated in the northern end of Xiannüfeng and Jiuwanxi Fault in the vicinity of the Xiangxihe river,and along the banks of the Yangtze river at the east of Xietan village and at Shenlongxi area on the northern bank in Badong region;the focal depths were less than 10km,and 4km in average.Earthquake frequency of the reservoir region had a positive correlation with reservoir water level fluctuation,indicating that the seismicity belonged to the reservoir induced earthquakes.Along the Shenlong river in the reservoir area,earthquakes showed three linear distributions in the northern Badong County,which was related to karst distribution.There are underground rivers along carbonate strata.When reservoir was impounded,water permeated into the underground rivers,thus inducing earthquakes.Earthquakes in the areas on the cross-river segment of Xiannüfeng Fault,the Jiuwanxi Fault and at the areas west of Xietan,Shazhen and Xizhen,may be related to the softening of discontinuities,such as the Nukou Fault,the Xiannüfeng Fault or the bedding joints,etc.,which would lead to failure of rock masses,thus,inducing earthquakes.However,convincing conclusions about the triggering mechanism still need further study.In addition,in the areas south of Wenhua and Yanglin of Zigui County and at Rangkou town east of Badong county,there are mining-induced earthquakes at the mines nearby.And on the shores of the reservoir,there are some collapse earthquakes.

Teleseismic receiver functions(RFs)migration method has been used to image the spatial variation of the converters in the crust and upper mantle along the north Hi-Climb broadband seismic array in the western Qinghai-Tibet Plateau.Results of RFs migration image show the crustal structure under Bangong-Nujiang Suture(BNS)takes on opposite underthrust form,i.e.,the Lhasa block exhibits northward overthrust and underthrust of upper crust and lower crust,respectively,the Qiangtang block southward overthrust and underthrust of upper crust and lower crust,respectively.The above underthrust form means Lasha and Qiangtang blocks may have complex assembly process.Combined with petrology achievements from previous researchers,terrain assembly model has been constructed based on the opposite underthrust form under BNS.Terrain assembly model illustrates that oceanized crust of the New-Tethys north ocean basin has been subducted southward beneath the Lasha block since the closure of the ocean basin and the continental collision of Lhasa and Qiangtang blocks.Since the continental collision of the Indian and Eurasian Plates at 60～50Ma ago,the lower crust of Lasha block however has been subducted northward and may have made the southward subduction plate delaminated,and then Indian Plate subducted beneath the Qiangtang block.

In the paper,according to the related factors and characteristics of identified reservoir induced earthquakes,the parameters of reservoir induced earthquake database and library structure have been determined.Based on the database functions of ARCGIG geographic information system software,the reservoir induced earthquake database contains 131 reservoir induced earthquake cases in the would, 110 Class-Ⅰlarge reservoirs(larger than one thousand million cubic meters),200 ClassⅡlarge reservoirs (larger than one hundred million cubic meters and less than one thousand million cubic meters) and 70 reservoirs with dams higher than 100 meters in China.It has the functions of quick query,statistics and icons.After the Wenchuan earthquake,aftershocks endangered many reservoir's dams and people who were taking part in earthquake rescue and relief operations.Two days after the M_s8.0 Wenchuan earthquake,the distribution and basic information of large reservoirs in Sichuan Province and nearby regions was submitted.The government made timely earthquake emergency response decisions to ensure a safe relief.The reservoir induced earthquake database is not only a method provided to the government for quick decision-making,but also can be used nation-wide.

As of today,our understanding of the reservoir induced earthquakes is very limited and its study is still at an exploratory stage.The large amount of natural occurrences of reservoir induced earthquakes provide us an essential and invaluable database for in-depth study on this issue.It will certainly expedite the study if one can centralize and manage these data scientifically and efficiently.We introduce in this paper a reservoir induced earthquake database management system,which is based on Visual Studio 2008 development platform and C++ programming language.As it relies on Object Oriented Design(OOD) and Object Oriented Analysis(OOA),the system not only has a user-friendly interface and is easy to operate,but also has good maintainability,scalability and upgradability.The system supports a wide range of data formats for input and output.It is convenient for users to search, query,retrieve and update data.This reservoir induced earthquake database management system is undoubtedly a great research tool for studying the reservoir induced earthquakes.

In this paper,the theoretical foundation of classification of focal mechanism solutions is drawn by the theory of shear dislocation,where the mechanisms of earthquake can be divided into the strike-slip fault,reverse fault and normal fault.The 124 focal mechanisms from Harvard University have been classified by the triangle diagram method in the area of Sichuan and Yunnan Provinces in China.The focal mechanisms of strike-slip,reverse and normal faulting have been discussed.These are the three deformation types in this area.The results show a SE-directed block movement of the Sichuan-Yunnan region.Along the Xianshuihe Fault,the Anninhe Fault,the Zemuhe Fault and the Xiaojiang Fault,large strike-slip movement and deformation have occurred.On the Sagaing Fault and the Longmenshan Fault,the reverse faulting mechanism dominates.The quick movement of the Red River Fault towards southeast driven by the Xiaojiang Fault forms a large tensional stress area between the Jinshajiang and the Xiaojinhe Fault.This constitutes the latest pattern of recent crustal deformation of Sichuan-Yunnan region and its adjacency.

In the paper,the seism ic damage to reservoirs in Sichuan Province has been analyzed by field in-vestigation and data collection of the damages of the M_S8.0 W enchuan earthquake.The results show that the damage of the M_S8.0 W enchuan earthquake is 1 or 3 degrees higher than the seism ic intensi-ties provided by the reservoir seism ic safety assessments in Sichuan Province.This reveals certain questions behind the reservoir seism ic safety assessment in this region.Because of the short earth-quake record history,there has been no pred icting of earthquake larger than the largestmagnitude of historical earthquake,and thus,the risk of the Lomenshan active faultwas greatly underestimated,re-sulting that fissures,seepage and partial collapse occurred widely on the dams ofmany reservoirs.The paleoearthquake method is an importantmeans of d istinguishing active fault in reservoir seism ic risk assessment and improving the accuracy and reliability of the assessment results.

After Wenchuan Earthquake(M8.0),surface fractures in Beichuan and Yingxiu were investigated.It was found that the earthquake deformation zone strikes in NE-SW.Earthquake fracture is characterized by thrust faulting with small amount of strike-slip movement.The compressional shortening is 3～4m in Beichuan and the left-lateral strike-slip displacement is 0.4～0.5m in Yingxiu.

Some destructions in meizoseismal region of Wenchuan earthquake with magnitude 8,Sichuan,China are shown,and their seismic intensity is determined according to "The Chinese Seismic Intensity Scale".The type of earthquake-generating fault and some features of seismic destruction are discussed briefly.

Shallow seismic surveying is a common method applied to the research of blind fault activity at the Quaternary stratum region,especially in urban area.The bifurcating,uniting,bending,intermitting and ending off of reflection waves on stacked time section are important signs which are often used to judge the existence of blind fault.At the northern area of Songhuajiang River in Harbin City,we finished two seismic reflection profiles.On the shallow seismic reflection profiles of Lugangtun and Jubaotun,the T₀ wave group corresponding to the interface between Cretaceous sand-mud stone and lower Pleistocene gravel is clear.T_0-1 reflecting wave group corresponding to the contact surface between gravel and clay,or fine sand and gravel,is also clear.Based on the stacking seismic profiles and borehole data,we conclude that the Ashihe fault offset the bottom stratum of lower Pleistocene,and Binzhou fault cut off the bottom stratum of upper Pleistocene.By building high quality composite drilling geological profiles,dating the samples from bores,and contrasting stratum from one bore to the other,we consider that the Ashihe fault does not offset the Quaternary formation,the fault may only exist in Cretaceous sand-mud stone.The Binzhou fault is only an early Pleistocene fault;it doesn't offset the late Pleistocene formation.Finally,we think that the bifurcating,bending,intermitting and ending off of seismic reflection wave group on Luegangtun and Jubaotun seismic profiles are not the result of fault activity,but the phase change of Quaternary formation.

Owing to strong and permanent Cenozoic re-orogenic processing, a lot of EW-striking active thrusts and folds have been developed in Tian Shan, resulting in crustal shortening in NS direction. There also exist NW-striking transform-like strike-slip faults that cut the Tian Shan and accommodate uneven crustal shortening larger in the west and smaller in the east. The seismogenic structures in and around the Tian Shan mainly include EW-striking thrust ramps or blind thrusts and NW-striking transform-like strike-slip faults. The 2003 AD Bachu-Jiashi earthquake is located at south of the Kalpintag nappe. A NE-trending deep seismic reflection profile about 50km long across the epicenter has been conducted after the earthquake. From this reflection profile four blind faults are identified. Together with earthquake relocation, these identified blind faults are used in the paper to interpret the seismogenic structures of the 1997 AD Jiashi strong earthquake swarm and 2003 AD Bachu-Jiashi earthquake. The 1997 AD Jiashi strong earthquakes were generated mainly by a NW-striking buried transform-like strike-slip fault, while the 2003 AD Bachu-jiashi earthquake by blind thrusts in front of the Kalpintag nappe.

Geological, geomorphic and geophysical methods, including drilling and chronological dating, are applied to determine whether the suggestively buried Taoyuan fault exists. Stratum units at two sides of the fault are continuous. The “normal fault”, found at east side of Yuanjiang River, Yaohe town, is actually a fissure caused by un-loading. Weak cementation of the Eogene breccia is also favorable to formation of the extensive fissure. One meter of offset along the fissure is not the result of tectonic movement, but comes from up-plate falling under the gravitational action. Quaternary thicknesses at the two sides of Yuanjiang River show no abrupt change, but a normal sedimentation. According to the geomorphic feature and the fault location, suggested by previous work, three lines of shallow seismic survey are arranged at the two sides of Yuanjiang River. The sections of shallow seismic survey show that boundaries at different layers are clear and can be pursued continuously. There are no evidences to demonstrate that the suggestively buried Taoyuan fault may exist. Two drilling bores are arranged at Xunyangping Quaternary-covered area and they are located at two sides, 20m away from the suggestively buried fault. The drilling results show that the Quaternary layers are crossed almost at the same depths and the lower basal rocks are revealed, which are red clay-sand rocks of the Eogene. Thermoluminescence dating samples are collected respectively at depths of 4.5m, 9.8m and 16.8m. The experiment results are (2.2±1.8)×10⁴a, (2.9±2.5)×10⁴a and (13.1±11.0)×10⁴a, which shows that the Quaternary strata, formed at the end of mid-Pleistocene, are continuous at two sides of the suggestively buried fault. There is no neo-activity. In conclusion, there is no evidence to support the existence of the suggested Taoyuan fault. The Taiyangshan fault zone, on which the 1631 M6³/₄ earthquake occurred, is bounded at the north of Changde-Yeyang-Changsha fault zone. It provides a reasonable geological background for division of seismic belts and potential seismic sources and is also important for better understanding of structural conditions of the 1631, M6³/₄ Changde, Hunan, earthquake.

This paper introduces the results of shallow seismic exploration on five traverse lines across the Huangzhuang Gaoliying buried fault in the vicinity of Lishuiqiao,Beijing area. The section of the Huangzhuang Gaoliying fault in the vicinity of Lishuiqiao can be distinguished distinctly on the spot map,but the other sections of the fault along river valley are undistinguishable. The shallow seismic exploration and geological information reveal that the velocity model of the shallow part around the Lishuiqiao area is characterized by 4-layer structure. The layers 1 and 2 are located at a depth range of 0～150m beneath the surface,and they can be assigned to Tertiary or Quaternary deposits,having a velocity of 800m/s to 2000m/s. The layers 3 and 4 are identified at a depth range of 130m to 300m,which are bedrock consisting of mudstone and sandstone,having a velocity of larger than 2000m/s or 2500m/s. At shallow depth,the Huangzhuang-Gaoliying Fault in Lishuiqiao area is composed of two sub-parallel faults about 1300m apart from each other. The two faults are N23°E-striking,dipping southeast at an angle of 22° for the western fault and 87° for the eastern one. At a depth of 634m the two faults converge into one fault,appearing as a branching fault. The buried depth of the highest point of the hanging wall of the fault is 101m,while that of the footwall is 109 m. The throw of the fault is about 8m. It is concluded,therefore,that the fault is a normal fault with strike-slip component,dissecting the T₂ and T₃ stratigraphic interfaces.

In this paper,the relation of time,space and intensity characteristics,as well as focal mechanisms of earthquakes to fault activity in Beijing area has been analyzed.A tectonic deformation model for the upper crust of this region is developed in this paper in the light of the quantitative analysis of fault activity.The results show that the earthquakes in this region are distributed mainly along the NWW direction,forming a concentration belt of earthquakes.In addition,several earthquakes of above magnitude M_S 4.5 occurred along the NE-or NEE-trending faults.These two sets of fault form a conjugate fault system,on which earthquakes occur alternately in different active periods,and the average release rate of creep energy corresponds to the occurrence of one M_S 4.2 earthquake per year.The crust of Beijing region can be divided in to the Yanshan,North Shanxi,Taihangshan and Central Hebei blocks,which have an irregular boundary along the NWW direction.Under the action of the NEE-directed principal compressive stress,these NWW-arranged blocks were easily activated,resulting in concentration belts of earthquake of a certain scale along block boundaries.Moreover,earthquakes of above magnitude M_S 4.5 occurred along the conjugated NE-or NEE-trending faults.

This paper demonstrates the results of shallow seismic exploration of the Liuwu-Dafosi Fault in the vicinity of Lhasa City. On spot map, the segment of the fault in mountainous area can be identified distinctly, but the segment in river valley is unidentified. Shallow seismic exploration reveals that the fault dissects Tertiary or Quaternary strata. Geological interpretation of shallow seismic reflection profile reveals a velocity structure of five layers. Layer 1 to layer 4 have a velocity ranging from 0.8km/s to 2.0km/s or 2.8km/s, and can be assigned to Tertiary or Quaternary strata. Layer 5 has a velocity ranging from 3.0km/s to 4.0km/s, which is supposed to be granite bedrock according to its scattered distribution in CDP stacked section and geological evidence. The Liuwu-Dafosi Fault can be identified at 320m distance of the shallow seismic reflection profile located to the southwest of the Lhasa City. The fault is dipping northeast at an angle of 80?, the highest point of which is at 100m depth. It is determined that the fault is an active reverse fault.

A large number of observed data about the widths of earthquake surface ruptures produced on diverse types of active faults, as well as the widths of intense deformation zones in trench logs across active faults are presented in this paper. Combining these data with the close relation between the damage zone of surface construction and the spatial position of active fault, the authors propose that a minimum distance(safety distance)of 30m away from active fault is essentially required for constructions to prevent the effect of faulting. The more accurate safety distance required for various types of active faults can be tested and modified through the analysis of the deformation features of strata in trench logs across the fault. The required safety distance in some specific sites, such as the step-overs of active faults, as well as the area defined by sub-parallel secondary faults on both sides, should be the sum of the width of the area and 15m from area boundaries. It is suggested that this "safety distance" should be taken as a legal regulation for constructions and buildings. In addition, further attention should be paid to the identification of active fault and precise determination of the geometric structures of the surface fault strand, so that earthquake hazards can be positively and effectively reduced.

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.

Gas explosions in karst caves might be a cause for reservoir induced seismicity.The occurrence condition and mechanism of gas explosion are discussed and the possible magnitude of induced earthquake is estimated in this paper.Phenomena of gas explosion have been found at the reservoirs in karst area in China. But the induced earthquakes at Wujiangdu reservoir which have been considered as typical gas explosion are not caused by the gas explosion.