Table of Content

18 December 2008, Volume 30 Issue 4

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Introspection on the earthquake

SOME THOUGHTS ON THE M_S 8.0 WENCHUAN, SICHUAN EARTHQUAKE

DENG Qi-dong

2008, 30(4):  811-827. 

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The M_S 8.0 Wenchuan earthquake caused tremendous disaster to the country and people,and meanwhile it also gives us many lessons worth thinking about.The severe earthquake disaster tells us the necessity of evaluating and surveying the seismic and geological environment in urban and township construction so as to avoid the unfavorable terrain conditions and keep away from the instable sites prone to landslide,soil liquefaction and mud-rock flow and etc,complying stringently the standards for disaster fortification regulated by the state and the relevant technical code for seismic design of buildings,including the houses in rural area,and intensifying the study on active tectonics to avoid the active faults and possible seismogenic tectonics.Since the historical earthquake record is too short and the recurrence interval of earthquakes is long within the continent,we should be cautious when using the principle of recurrence of historical earthquake in seismic hazard assessment,but promote the study on paleoearthquakes on active tectonics,so as to fully understand the history of earthquake recurrence.Big earthquake may also occur on low-slip-rate active fault,but with longer recurrence interval.So special attention be paid to the elapsed time of the latest event on it.The source rupture of the M_S 8.0 Wenchuan earthquake is characterized by a multi-point rupturing process,resulting from the expansion of deep fault from the detachment zone to the front-end ramp,and shown as double-ramp faulting on the surface.In the source rupture propagating process,the nature of the rupture changed.The southwest section is characterized with thrust faulting,and the northeast section turns to be dominated by strike-slipping.The initial thrust-type rupture occurring in the middle of the Longmenshan tectonic belt ran through its northeast section and became a new fault.The 1997 Mani,the 2001 west pass of Kunlun Mountains and the 2008 Wenchuan earthquakes with MS 7～8 are all the products of the movement of the Bayahar block.After the Wenchuan earthquake,a series of earthquakes with magnitude around 6 occurred in the Qinghai-Tibetan Fault block region,which characterizes the new episode of seismic activity of the Qinghai-Tibet Plateau block region.

UNDERGROUND FLUID ANOMALY AND MACRO ANOMALY OF M_S 8.0 WENCHUAN EARTHQUAKE AND OPINIONS ABOUT EARTHQUAKE PREDICTION

CHE Yong-tai, LIU Cheng-long, YU Jin-zi, GUAN Zhi-jun, LI Jun

2008, 30(4):  828-838. 

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Based on the analysis of fluid anomaly data and investigation of macro precursor of the M_S 8.0 Wenchuan earthquake,and having taken careful consideration of the difficulty and ability of predicting earthquake,we found that the failed prediction of the M_S 8.0 Wenchuan earthquake doesn't mean that earthquakes can't be predicted.There were,though not much in quantity,a certain amount of underground fluid anomalies and remarkable macro anomalies occurring before the earthquake.Though it is difficult to predict the earthquake basing on them,it is possible to be aware of the impending earthquake.To improve the ability of earthquake prediction,the paper proposes to innovate the present work and administrative systems,in which,earthquake monitoring,prediction and research are separate each other,and professional and local forces are separate each other,to change the present work state of computer-replacing-human brain and the work mode of the too early use of formality and standardization,and to attach greater importance to investigation and confirmation of precursory anomalies and to monitoring and studying macro anomalies.

Earthquake disaster and disaster reduction

AN ANALYSIS ON M_S 8.0 WENCHUAN EARTHQUAKE FAULT AND SEISMIC DISASTER

ZHAO Bo-ming, XU Xi-wei

2008, 30(4):  839-854. 

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Complex spatial distribution of seismic motion near faults has always been a concern to scientists,and it still remains as an uncertain problem due to insufficiency of events and information.The paper presents the main cases of seismic disaster by field investigations of M_S 8.0 Wenchuan earthquake,analyzes and discusses the relationship among earthquake fault,ground motion and earthquake disaster near the fault fractured zone,based on previous research of source rupture processes and source inversion of Wenchuan earthquake.Intensive deformation and ground surface rupture along the earthquake fault have caused obvious damage to buildings,so it is necessary to introduce the safety distance away from active fault and other measures.Possible reasons for buildings near surface rupture zone having withstood the strong earthquake are as follows:other than their performance of seismic resistance,firstly,most of them locate at hard sites or on bedrock in the surface rupture zone,and secondly,the effective stress drop and low rupture velocity may exist in the shallow asperities,resulting in a relatively lower ground motion at the period about 1 sec.

PRELIMINARY RESEARCH ON THE CHARACTERISTICS OF THE M_S 8.0 WENCHUAN EARTHQUAKE HAZARD

LI Zhi-qiang, YUAN Yi-fan, LI Xiao-li, ZHANG Qin, DAI Bo-yang, YE You-qing, GE Pei-feng, ZENG Jie

2008, 30(4):  855-876. 

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The M_S 8.0 Wenchuan earthquake on 12^th May 2008 caused heavy casualties and economic loss.According to the field investigations,the characteristics can be included as follows:The meizoseismal region with an intensity of XI,showing obviously zonal distribution,suffers serious destruction from the earthquake,and the destruction perpendicular to the rupture decreases sharply.At the same time,the intensity X and IX degree regions perpendicular to the rupture are narrow and therefore their coverage area is small.The intensity on both sides of the rupture attenuates rapidly,but the Ⅶ degree region and the VI degree region are wide,the latter region covering about 240,000 square kilometers.In the VI degree region,the damage area perpendicular to the rupture in the southern part is much larger than that of the northern part.Besides,many new understandings about the destruction types and destructive modes of all kinds of buildings,landform and terrain are gained in this paper.

GENERAL FEATURES AND THEIR DISASTER-REDUCTION EDUCATION IMPLICATIONS OF THE EARTHQUAKE DISASTER COGNITION AND RESPONSES OF THE SOCIAL PUBLIC IN M_S 8.0 WENCHUAN EARTHQUAKE-HIT AREA:A CASE STUDY FROM DEYANG PREFECTURE-LEVEL CITY, SICHUAN PROVINCE

SU Gui-wu, MA Zong-jin, WANG Ruo-jia, WANG Yue, DAI Bo-yang, ZHANG Shu-wei, NING Qian-wen, ZHANG Shao-song

2008, 30(4):  877-894. 

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By taking Deyang prefecture-level city,Sichuan Province,one of the worst affected area of the 12th May,2008,M_S 8.0 Wenchuan earthquake as the case study area,some general features of the earthquake disaster cognition and responses of the social public in this large earthquake-hit area were analyzed preliminarily through questionnaire survey and field interview methods.The main conclusions are as follows:1)The comprehensive level and capability of cognizing on earthquake disasters and responding to the actual earthquake event of the social public in the studied area is not ideal.2)From the adolescent to the elderly,their levels and capabilities in different cognition and response aspects all increase gradually first and then decrease step by step.The population with highest levels and capabilities are those whose ages are from 19 to 25 years old,while the poorest are those juvenile and aged people.In addition,the earthquake disaster cognition levels of part of those middle-aged people are also obviously not as good as expected.3)People's education degrees have significant positive correlations with people's cognition and response levels or capacities.4)The levels and capabilities of the female are inferior to,even significantly inferier to that of the male.5)People's earthquake disaster knowledge and people's earthquake disaster-coping skills both influence deeply how these people respond to an actual earthquake event.Especially,the impact from the latter is much stronger.That is,the people who have some higher cognition degrees usually have more active and reasonable response attitudes and behaviors,and vice versa.6)The official mass media,including radio,television and newspapers,especially those after M_S 8.0 Wenchuan earthquake and the aimed propaganda of local government and other relevant institutions constitute together the two main channels for the local people to obtain earthquake disaster knowledge and to learn earthquake disaster-coping skills.While,the school formal education did not get any advantages as expected.Another point worthy of being emphasized is that,among these knowledge and skills possessed by the people of the studied area in the time of two months after the M_S 8.0 Wenchuan earthquake,a considerable large quantity was obtained right after this large earthquake.These people's earthquake disaster cognition degree before this large earthquake is obviously rather difficult to let us to be optimistic.Based on these conclusions,some suggestions on strengthening the earthquake disaster-reduction education for the studied area were put forward.

COSEISMIC CHANGES OF WATER LEVEL AND WATER TEMPERATURE CAUSED BY M_S 8.0 WENCHUAN EARTHQUAKE

YANG Zhu-zhuan, DENG Zhi-hui, LIU Chun-guo, WANG Gui-qing, ZU Jin-hua, TAO Jing-ling, SONG Jian

2008, 30(4):  895-905. 

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The paper studies the coseismic changes of water level and water temperature caused by the M_S 8.0 Wenchuan earthquake.The differences of water level and water temperature variations caused by the M_S 8.5 Sumatra earthquake on Sep.12,2007 and the M_S 8.0 Wenchuan earthquake on May 12,2008 are analyzed.The result shows that the well water level change caused by Wenchuan earthquake is dominated by rising.Spatial distribution of wells with water level rising or descending exhibits regional difference.The proportion of wells is higher with water level and water temperature changes in the same directions than in the reverse direction.Water temperature mainly dropped when water level fluctuated.Compared to remote earthquake,near earthquake caused bigger changes in water level and water temperature in wells.Coseismic changes of all the well water level and most of water temperature kept the same direction regardless of distance,magnitude,focal mechanism of earthquakes or epicentral directions,though water temperature direction changes occurred only at some peculiar wells.The water temperature direction changes were caused by changes of artesian condition and water level response from fluctuations to steps.The direction of water level changes might be controlled mainly by both local geological structures and hydro-geological conditions of the well.However,the direction of water temperature relates with mixing of water in the well,the location of the water temperature probe and other factors.The mechanisms of water temperature coseismic change are more complicated.

Earthquake source physics and model experiment

EXPLORATION OF SUSPECTED SURFACE RUPTURES OF THE M_S 8.0 WENCHUAN EARTHQUAKE AT FRONTAL AREAS OF LONGMENSHAN USING SHALLOW SEISMIC REFLECTION

LIU Bao-jin, YANG Xiao-ping, FENG Shao-ying, KOU Kun-peng

2008, 30(4):  906-916. 

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The M_S 8.0 great Wenchuan earthquake of May 12th,2008 was generated by abrupt faulting of the Yingxiu-Beichuan Fault along the Longmenshan Fault zone.The earthquake produced not only the surface ruptures along the Yingxiu-Beichuan Fault and Guanxian-Jiangyou Fault,but also surface rupture,highway's pavement arching,sand-boils and waterspouts in various degrees in the areas such as Shifang and Mianzhu of the Chengdu plain.To know the shallow geological structures under the surface rupture zone,a high-resolution shallow seismic reflection profile of 6350m long in near-EW direction was completed.This profile is located at the Shigu town,Shifang city,where a suspected earthquake surface rupture zone was discovered.In this study,a trace interval of 3m,shot interval of 18m,and a 300-channel 25-fold observation system was used.In order to give consideration of both near-surface reflections and dipping interface imaging,we adopted the split-spread geometry and asymmetrical zero-offset receiving technique.For suppressing random-noise and raising signal to noise ratio of seismic data,30 times vertical stacking of vibrator signals was made for each common-shot gather after correlation of individual records.By using the above work method and spread geometry,we obtained high-resolution images of structures in the depth range of 15～800m after data processing.The result shows that there are the buried thrust fault thrusting to the plain and the back-thrust fault under surface rupture zone,and that the activity of the buried thrust fault may be the main causes for folding and deformation in near-surface strata and for the coseismic surface rupturing.

DISCUSSION ON RELOCATION AND SEISMO-TECTONICS OF THE M_S 8.0 WENCHUAN EARTHQUAKE SEQUENCES

L? Jian, SU Jin-rong, JIN Yu-ke, LONG Feng, YANG Ya-qiong, ZHANG Zhi-wei, TANG Lan-rong, LI Chao

2008, 30(4):  917-925. 

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The M_S 8.0 Wenchuan earthquake and its 2216 aftershocks were relocated using the double difference algorithm.The horizontal and vertical errors of the 2061 relocated hypocenters are approximately 1～2km and 2～3km,respectively.The epicenter of main shock is approximately 31.00°N,103.38°E,the focal depth is about 13km and the seismogenic structure is the central fault of Longmenshan Fault zone.The total length of spatial distribution of aftershocks along the strike of the fault is about 330km and the predominance distribution of focal depth is 3～20km,which shows obviously the characteristic of segmented activity.The seismicity of the southern part mainly concentrates on the central fault of Longmenshan Fault zone,and some earthquakes occurred on the range-front and range-back faults;the dip of the three faults seems to become gentler gradually from west to east,forming imbricate ruptures.The central fault and Pingwu-Qingchuan Fault of the Longmenshan Fault zone are involved in the seismogenic process,and the seismic rupture is both of thrust napping and right-lateral strike-slipping.

CLASSIFICATION OF FOCAL MECHANISM SOLUTIONS AND CHARACTERISTICS OF LATEST CRUSTAL DEFORMATION OF SICHUAN-YUNNAN REGION AND ITS ADJACENCY

MA Wen-tao, XU Xi-wei, CAO Zhong-quan, YU Gui-hua, LI Hai-ou, XU Chang-peng

2008, 30(4):  926-934. 

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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.

THE COULOMB FAILURE STRESS CHANGE ASSOCIATED WITH THE M_S 8.0 WENCHUAN EARTHQUAKE AND THE RISK PREDICTION OF ITS SURROUNDING FAULTS

ZHANG Guo-hong, SHAN Xin-jian, LI Wei-dong

2008, 30(4):  935-944. 

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Using the focal mechanisms and slip distribution model of the M_S 8.0 Wenchuan earthquake,we inverted the co-seismic Coulomb failure stress change.The inverted Coulomb stress change based on Chen J's slip distribution model shows that about 83%of its aftershocks lie in regions where the Coulomb stress increased by 0.01MPa,given the receiver fault' strike/dip/rake as 204?/56?/98? respectively.In contrast,the distribution of aftershocks is sparse in areas where the inverted Coulomb stress change is decreased by 0.01MPa.From this we can tell that aftershocks are abundant where the Coulomb stress change rose by more than 0.01MPa,and aftershocks are sparse where the Coulomb stress dropped by a similar amount.This study and the forerunner,Ma et al(2005),demonstrate that even when the source geometry and slip distribution are complex,Coulomb stress change is correlated with the distribution of aftershocks.And its main characteristic is that for most of the region in the northern wall of the fault the Coulomb stress dropped by 0.01MPa and for most of the region in the southern wall of the fault,it increased by 0.01MPa.The tendency of change of the Coulomb stress field is progressed to the direction of N-E and S-W.At last,we calculated the Coulomb stress change on most of the existing faults based on Deng's active fault data.The results show that the Coulomb stress change on several strike-slip faults is decreased by about 0.01MPa,including Maerkang Fault,southeast Chengdu thrust fault,Huya Fault,Min Jiang Fault,west Qinling north-edge fault and Qinling north-edge fault.This means these faults may be less dangerous in future earthquakes.There are several strike-slip faults on which the Coulomb stress change are increased by about 0.01MPa,including eastern Kunlun Fault,northern part of Chengdu thrust fault,Wenxian Fault,and western part of west Qinling north-edge fault.These results are consistent with that of Tom Parsons et al.What brings contradiction is that when comes to Xianshuihe Fault,our results show an increase by about 0.01MPa in the middle part of Xianshuihe Fault,at longitude 101.2皌o 102癊 and latitude 30皌o 31.04癗.The Coulomb stress change in most part of Xianshuihe Fault is decreased,which means the Xianshuihe Fault system has a potential seismic risk lower that it had before the M_S 8.0 Wenchuan earthquake.

EXPERIMENTAL STUDY ON RUPTURE PROPAGATION OF BURIED REVERSE FAULT AND ITS SEISMOGEOLOGICAL SIGNIFICANCE

DAI Shu-hong, MA Sheng-li, PAN Yi-shan, HE Yong-nian

2008, 30(4):  945-956. 

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In this paper,the rupture propagation along reverse fault in bedrock towards the overlying sediment layer has been studied using modeling tests and the evolution of the vertical displacement field on the surface of sediment layer has been measured using fringe projection technique,in which the effects of fault displacement,thickness of sediment layer and fault dip have been analyzed.The research results indicate that fault in bedrocks propagates gradually towards the overlying sediment layer and a gradient zone of vertical displacement(i.e.a deformation zone)surrounding the projection line of the upper end of fault in bedrock is formed on the surface of sediment layer as the fault slips,given a constant thickness of sediment layer and a constant fault dip.The larger the fault displacement is,the larger the width of the deformation zone and the deformation intensity are.However,the width of deformation zone keeps constant and only the deformation becomes more concentrated as fault displacement increases after the fault propagates to the surface of sediment layer.Under the same fault displacement and the same fault dip,the thicker the sediment layer is,the wider the deformation zone controlled by the fault in bedrock is,i.e.the larger the influencing extent of the fault is.When the thickness of sediment layer exceeds a critical thickness,an extensional rupture may occur on the surface of the sediment layer caused by uplifting of the sediment layer.Under the same thickness of sediment layer and the same fault displacement,the width of the deformation zone on the surface of sediment layer decreases but the deformation intensity increases as fault dip increases.The experiment results imply that the critical fault displacement,the critical thickness of sediment layer and the fault dip play important roles in determining the surface deformation and rupture of earthquake generated by buried reverse fault.The results obtained in this paper are valuable for further understanding the characteristics of surface deformation caused by Wenchuan earthquake.

Earthquake surface rupture zone

THE DEFORMATION OF THE XIONGPO ANTICLINE AND THE ACTIVITY OF PUJIANG-XINJIN FAULT

ZHENG Wen-jun, WANG Wei-tao, LI Chuan-you, WEI Zhan-yu, GAO Xiang

2008, 30(4):  957-967. 

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Xiongpo anticline locates in Chengdu Basin,to the southeast of Longmenshan tectonic zone.It is an important deformation area where the Longmenshan thrust-nappe structure intrudes into the Chengdu Basin.The Pujiang-Xinjin Fault is an associated fault to Xiongpo anticline.The deformation mode between the fault and anticline fold is in concordance obviously.The geologic section across the anticline indicates that the south segment of Xiongpo anticline is an asymmetric fold and the northeast segment is symmetric,wide and gentle relatively.The fold includes Mesozoic and pre-Mesozoic strata.The topographical investigation reveals that the faulting is always associated with folding.At the northeast part of fold,the fold axial direction is parallel to the fault strike.Near the fault,the strata dips are remarkably different and the height difference of topography is large.The investigation of the Pujiang-Xinjin Fault did not reveal any obvious fault profiles and new activity characteristics.The Pujiang-Xinjin Fault has no influence on the gullies and T1 terraces widely developed in this area,but it controls the pluvial terrace which corresponds to T4 terrace of Nanhe river(the first-order branch of Minjiang River).The OSL age of the pluvial terrace is older than 130ka.All above indicates that the activity age of the Pujiang-Xinjin Fault is at the early-Quaternary.By the late-Quaternary,the faulting weakened or was nearly inactive.So in the area,the major tectonic character is faulting associated with fold deformation,which is also the major deformation mode of Xiongpo anticline.

PRELIMINARY INVESTIGATION ON DEFORMATION CHARACTERISTICS AND ACTIVITY OF LONGQUANSHAN FAULT BELT IN SICHUAN

WANG Wei-tao, JIA Dong, LI Chuan-you, ZHENG Wen-jun, WEI Zhan-yu

2008, 30(4):  968-979. 

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Based on the field work and seismic reflection profiling data,the paper investigates the deformation characteristics of the Longquanshan Fault zone.The main thrust fault of the Longquanshan Fault belt lies in the west of the Longquanshan anticline and has different properties from northeast to southwest.In the north segment and south segment of the Longquanshan Fault,the plane of fault dips to northwest and is uncontinuous,but in the middle segment,the plane of fault dips to southeast and is continuous.Therefore,the middle part of the fault is the main segment of the Longquanshan Fault.Structural geometries of the middle segment of the fault suggest classical fault-propagation folding and the fault ruptured along different axial directions.Historical earthquakes and geomorphological response to activity of the Longquanshan Fault indicate that the fault was active from the early Pleistocene to late Pleistocene,and its activity is weak since the late Pleistocene,and gradually decreases from south to north.

TECTONIC DEFORMATION OF LACUSTRINE SEDIMENTS IN QIANGYANG ON THE MINJIANG FAULT ZONE AND ANCIENT EARTHQUAKE

AN Wei-ping, ZHAO Jin-quan, YAN Xiao-bing, LI Zi-hong, SU Zong-zheng

2008, 30(4):  980-988. 

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During the post-disaster recovery in Maoxian country,we found that lacustrine sediments are exposed intermittently ～30km north of the present Diexi barrier lake(Xiaohaizi)along the Minjiang River and its tributaries.According to the study on lacustrine sediments around Qiangyang village,we obtained the evidence of activities of the Minjiang Fault in the Holocene.It is concluded that the tectonic deformation of paleo-lacustrine sediments in Qiangyang possibly reflects multi-time ancient seismic activity of Minjiang Fault zone.It is more accepted that the 1st seismic activity resulted in the formation of dammed paleolake in Qiangyang,and accumulation of the 1st set of lacustrine facies deposits;the 2nd seismic activity led to deformation of the lacustrine facies stratum;the 3rd seismic activity caused the deformation of the 1st and 2nd set of lacustrine facies strata;and the 4th seismic activity ruptured the youngest deposit overlying the lacustrine facies stratum.Total Station Instrument measurements indicate that the vertical displacement of the last ancient seismic activity is about 2.6～3.6m.

SURFACE RUPTURES IN BEDROCK OF THE M_S 8.0 WENCHUAN EARTHQUAKE

LI Xi-guang, YU Gui-hua, XU Xi-wei

2008, 30(4):  989-995. 

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During the M_S 8.0 Wenchuan earthquake,most of surface deformation zones show as flexural scarps or fault scarps,and surface ruptures in bedrock are rare.However,earthquake surface ruptures are entirely exposed in bedrock and discovered by authors near Xiaojiaqiao town of Anxian county.Based on careful analysis of the deformation characteristics and the inner structure of this rupture zone,and in combination with features of the regional earthquake surface ruptures,the authors hold that the earthquake surface ruptures are developed and extend mainly along the preexisting Yingxiu-Beichuan Fault during this earthquake,the faulting is right-reverse-slip movement at a low angle,and the maximum vertical co-seismic dislocation is 5.4 meters,which coincide with the measurements of the flexural scarps or fault scarps.

A PRELIMINARY RESEARCH ON THE LATEST TECTONIC DEFORMATION STYLE AND THE ACTIVITY AGE OF THE DAYI FAULT IN THE PIEDMONT OF THE LONGMEN MOUNTAINS

DONG Shao-peng, HAN Zhu-jun, YIN Jin-hui, LI Feng, AN Yan-fen

2008, 30(4):  996-1003. 

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Primary evidence of the latest tectonic deformation style and the activity age of the Dayi Fault in the Sichuan Basin are obtained by field geological mapping,and surveying with the help of 3D scanner and total station.In the region of Wenshangou-Longfengchang,northeast of Dayi county on the northwestern wall of the fault,the time of Cretaceous and Neogene strata's gentle folding was measured to be between late Neogene and early Pleistocene.And obviously its forming time is later than the Longmen Shan Fault which controls the northwest boundary of Sichuan Basin's Mesozoic strata and shows a thrust-nappe structure.With the help of oil prospecting data,fault plane inclines to northwest,dip angle changes from 10°～20° in the deep to 70° at near-surface,and depth of the upper faulted point is about 200～300m.Geological and geomorphologic evidence shows that Holocene is its latest active time.The latest tectonic deformation on the Dayi Fault is represented by Holocene fold.Topographically,the deformation appears as continuous piedmont hillocks with traceable length of 2.5km,distributed en echelon in the piedmont of the Longmen Mountains and with the plane shape of single hillock being ellipse.Accordingly the Dayi Fault is inferred as a Holocene active blind fault on the basis of hillocks' section shape.

THE CAUSATION OF MAXIMUN HORIZONTAL DISPLACEMENT OF WENCHUAN EARTHQUAKE ALONG THE BAISHA RIVER

GAO Xiang, HE Hong-lin, WEI Zhan-yu, DONG Shao-peng

2008, 30(4):  1004-1011. 

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Maximum co-seismic displacement is an important parameter for research of active tectonics,estimation of fault's activity characteristic and analysis of earthquake hazard.It relates to the quantitative research of determining the upper limit of maximum magnitude of earthquake in potential seismic source area and recurrence interval of large earthquake.Co-seismic displacement distribution of earthquake surface rupture is complex,and the influencing factors are various.The M_S 8.0 Wenchuan earthquake is a typical thrust event,whose co-seismic displacement is the vertical displacement,with dextral strike-slip component,which is identical to the results from focal mechanism solutions and scratches on the fault plane.The maximum co-seismic dextral horizontal displacement is 4.5m,and near the site of the maximum displacement,the co-seismic vertical displacement is 5m.The distance between the site of maximum vertical displacement of 5m(observation point 1)and the horizontal displacement of 4.5m(observation point 2)is only 50m;The earthquake made the ground move to SE.The strike between observation sites always changes,arcing towards NW.Generally speaking,it is impossible for a thrust event changing to a dextral event in 50m.The stratum in the hanging wall is shortened under the effect of earthquake.Because of the change of the strike of fault,the thrust direction is perpendicular to the strike in some part of the fault,and in other part they are oblique.When thrust direction is oblique to the strike,thrust motion can be decomposed in three directions:extrusion perpendicular to the fault(SE),uplift perpendicular to the surface and shear parallel to the fault.We suppose that the fault dip is γ in observation point 1,α in point 2,the angle between fault's movement and fault strike in observation point 1 is β,and the angle difference between strike in observation point 1 and point 2 is θ.According to space geometry,we can obtain formulas among these parameters,and calculate them.Geometric and kinematical analyses of the ruptures show that the maximum co-seismic horizontal displacement may result from thrust motion decomposed in different directions and the change of fault dip.

RESEARCH OF CUMULATIVE VERTICAL SLIP OF THE SEISMOGENIC FAULTS OF THE M_S 8.0 WENCHUAN EARTHQUAKE

WANG Lin, TIAN Qin-jian, MA Bao-qi, ZHANG Shi-min, HAO Kai, YU Jian-qiang

2008, 30(4):  1012-1022. 

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The May 12,2008 M_S 8.0 Wenchuan earthquake in Sichuan Province,China produced a 240km long co-seismic surface rupture zone along the Longmenshan Central Fault.Our investigation focuses mainly on the three regions along the co-seismic surface ruptures,i.e.the Nanba Town and the Fenghuang village on the northern segment of the rupture zone,and the Yingxiu town on the southern segment.We studied the river terraces in these regions offset by the active fault,measured the surface ruptures and the fault scarps on the multilevel terraces and obtained the height of the fault scarps on each terrace,namely,the cumulative vertical slip of the active fault recorded by terrace,through data calculation and analysis.If we use the vertical slip of this earthquake as the average vertical slip of the paleoearthquakes,then the ratio of the cumulative vertical slip of each terrace to this average vertical slip is the cumulative times of paleoearthquakes recorded by each terrace.The research results show that T₁ of every study area has undergone only one paleoearthquake event ever since its formation,T₂ about 5 paleoearthquake events,T₃ about 9～11 paleoearthquakes and T₄ about 20 paleoearthquakes.Based on the research result of this paper,and combined with the previously dating ages of the terraces,we can obtain some reliable data about the recurrence intervals of the paleoearthquakes.

RUPTURE ANALYSIS OF THE WENCHUAN EARTHQUAKE AT TWO OBSERVATION STATIONS BY GEOMETRY AND KINEMATICS

WEI Zhan-yu, HE Hong-lin, DONG Shao-peng, CHEN Jie, GAO Xiang, SUN Hai-qing, JIA Shi-xu, WANG Shi-yuan

2008, 30(4):  1023-1032. 

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Surface ruptures of the M_S 8.0 Wenchuan earthquake are distributed mainly on the Longmenshan central fault and front-range fault,extending 235km and 72km,respectively.The ruptures exhibit complicated characters in geometry and kinematics.On the riverbed of Baisha river,a backthrust scarp was formed in the southwest of the major fault that comprises four right-step sub-faults,and precise topographical measurement shows a kinematic character of fault block and surface tilting in the rupture process;In Shiyan village,the master fault and the secondary fault formed an imbricate structure,and show as flexures and earthquake swells on ground surface.We analyzed the ruptures at Tongmakan village and Shiyan village,both locating on the central fault,by geometry and kinematics,and the results indicate that the earthquake rupturing along the central fault is mainly characterized by thrusting associated with right-lateral strike-slipping.This result is consistent with the focal-mechanism solutions which are promulgated by U.S.Geological Survey,Harvard University and China Earthquake Networks Center.In addition,the profiles of Tongmakan and Shiyan display different tilting directions,and the reason is that the former locates at the trailing edge of the fault,while the latter in the leading edge of fault.

THE DISCUSSION ON THE CALCULATION METHOD OF SURFACE SHORTENING AMOUNT DUE TO REVERSE-FAULTING SURFACE RUPTURE:WITH THE EXAMPLE OF SURFACE DEFORMATION OF THE M_S 8.0 WENCHUAN EARTHQUAKE

WANG Hu, RAN Yong-kang, CHEN Li-chun, SHI Xiang

2008, 30(4):  1033-1045. 

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A reasonable and authentic estimate of surface rupture shortening amount of reverse fault is essential to a thorough understanding of the co-seismic surface deformation parameters,while we have no effective or feasible methods to deal with it,presently.The paper attempts to analyze and calculate horizontal shortening amounts of reverse fault with trench excavation basing on the investigations of the surface ruptures of the M_S 8.0 Wenchuan earthquake.On this condition,we conclude three genesis models of fault scarp,namely,faulting,flexuring,and superposition of faulting and flexuring.The paper proposes several calculation methods about horizontal shortening amounts of reverse fault based on these fault scarp models,and also gives detailed mathematic proof and constraint factors when considering complicated phenomena.Moreover,combined with these models,we discuss how to correctly apply and interpret the reverse surface rupture information revealed by trench.Finally,we demonstrate the calculation process and results of the reverse fault's horizontal shortening amounts in the trench close to the Central School of Bailu and the trench at Quanxin village of Hanwang,which are(2.83?0.3)m and(0.61?0.11)m,respectively.

Academic discussion

INTEGRATED ANALYSIS OF STRESS AND REGIONAL SEISMICITY BY SURFACE LOADING—A CASE STUDY OF ZIPINGPU RESERVOIR

LEI Xing-lin, MA Sheng-li, WEN Xue-ze, SU Jin-rong, DU Fang

2008, 30(4):  1046-1064. 

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To investigate the mechanism of seismicity due to impoundment of a reservoir,we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion.As an example,possible role of Zipingpu Reservoir on nearby seismicity was studied in detail.The epicenter of the M_S 8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir.On one hand,several statistical properties including event rate(n),cumulative energy release(E),seismic b-value in the magnitude-frequency relation,and spatial correlation length(SCL)of earthquakes occurring in Zipingpu area from 2004.8 to 2008.5.11 were estimated in detail.On the other hand,we quantitatively examined change of Coulomb Stress(ΔCFS)due to the impoundment of the reservoir.Both weight loading and pore pressure diffusion resulted in significant ΔCFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults,which are considered as the source faults of the Wenchuan earthquake.Some clear correlations were verified between the local seismicity and stress change,thus we concluded that the impoundment of Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

Application of new techniques

THE APPLICATION OF DIFFERENT HIGH-PRECISION SURVEY METHODS IN THE INVESTIGATION OF THE M_S 8.0 WENCHUAN EARTHQUAKE SURFACE RUPTURES

LI Feng, XU Xi-wei, CHEN Gui-hua, DONG Shao-peng, WEI Zhan-yu

2008, 30(4):  1065-1075. 

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In the scientific investigation on the M_S 8.0 Wenchuan earthquake,different high-precision measuring instruments,such as electronic total station,GPS RTK,3D laser scanner,etc.were used widely to survey the landscape of co-seismic surface ruptures,and a wealth of information with the earthquake deformation were recorded timely,which ensured the quantitative analysis of related data.In this paper,we explored the characteristics of different means used in the micro-geomorphology survey and analyzed the advantages of different portfolios of the above-mentioned methods.Then,focusing on the examples,we discussed the advantages and limitations of three-dimensional laser scanner in recording and analyzing the information of seismic surface ruptures.Also we gave the prospects of the methods' application.

ANALYSIS ON THE CHARACTERISTICS OF InSAR COSEISMIC DEFORMATION OF THE M_S 8.0 WENCHUAN EARTHQUAKE

QU Chun-yan, SONG Xiao-gang, ZHANG Gui-fang, GUO Li-min, LIU Yun-hua, ZHANG Guo-hong, LI Wei-dong, SHAN Xin-jian

2008, 30(4):  1076-1084. 

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Using D-InSAR technology and by processing 7 track 56 scenes ALOS/PALSAR data,the surface deformation field of Wenchuan,Sichuan earthquake on May 12,2008 has been extracted.The deformation field covers a 500km?450km area and crosses Jinchuan-Shimian,Heishui-Leshan,Songpan-Pengshan,Nanping-Jianyang,and Kangxian-Chongqing regions,including the severely earthquake-hit areas,such as Lixian,Wenchuan,Maoxian,Beichuan,Qingchuan,and so on.The results show that the deformation field scope is large and the Sichuan basin has been deformed to different degrees.The incoherent belt near earthquake fault shows that the main earthquake surface rupture zone is on the Beichuan-Yingxiu Fault zone.The trackable surface rupture zone runs from the southwest of Yingxiu near the macroscopic epicenter to the north of Suhe in Qingchuan county,about 230km long.The southwest section of the seismogenic fault from Wenchuan to Maoxian shows an incoherent band width obviously larger than that of other incoherent parts,which is closely related to the surface rupture from Dujiangyan to Anxian on the Pengxian-Guanxian Fault(the Mountain Front Fault),and this surface rupture zone is about 70km long.Away from the seismic fault region,the northwest wall of the fault uplifted and the southeast wall subsided.However,both walls in the vicinity of the seismic fault uplifted locally,and along the fault the distribution is very uneven,showing strong segmentation,which indicates the fault is characterized by reverse thrust.The differences of epicentral positions and earthquake origin time given by Harvard,USGS,NEIC,CENC also show that the Wenchuan earthquake rupture process is a multi-point breakdown process.The largest relative deformation amounting to 260cm occurred in the epicentral region on the west of Yingxiu;if converted into vertical deformation,the relative vertical deformation of the two regions is up to 3.3m.In Ya'an and Emei mountain area,the settlement is about 35cm.In Chongqing and to its south,there is about 25cm small-scale uplift.

RETRIEVING THE LATENT HEAT FLUX OF WENCHUAN AREA OF SICHUAN PROVINCE USING A DUALTEMPERATURE-DIFFERENCE METHOD

YAN Yan, DENG Zhi-hui, CHEN Mei-hua, ZU Jin-hua, TAO Jing-ling

2008, 30(4):  1085-1094. 

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Research shows that anomalous increase of latent heat flux was discovered before several earthquakes.Basing on the Modis data,high-spectral data and ground meteorological data of the working area,we retrieved the sensible heat flux and calculated the latent heat flux from surface energy balance equation using a simplified two-layer model(dualtemperature-difference method).In this paper,we also discussed the characteristics of this model,and retrieved the latent heat flux of the area where the Longmenshan Fault goes through.Finally,we found that there were significantly high values of latent heat flux in the epicenter area of the M_S 8.0 Wenchuan earthquake and the surrounding area of Longmenshan Fault 48 hours before the earthquake,the latent heat flux on May 12 reached the maximum of 300～400W/m²,which might be the anomalous increase of latent heat flux before the M_S 8.0 Wenchuan earthquake.