Table of Content

18 September 2008, Volume 30 Issue 3

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Brief Report

THE M_S 8.0 WENCHUAN EARTHQUAKE:PRELIMINARY RESULTS FROM THE WESTERN SICHUAN MOBILE SEISMIC ARRAY OBSERVATIONS

LIU Qi-yuan, CHEN Jiu-hui, LI Shun-cheng, LI Yu, GUO Biao, WANG Jun, QI Shao-hua

2008, 30(3):  584-596. 

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In the western Sichuan(100°～105°E,26°～32°N),a mobile array consisting of 297 broadband seismic stations has been deployed by the State Key Laboratory of Earthquake Dynamics,Institute of Geology,China Earthquake Administration since October of 2006.Until June of 2008,a total of 690 teleseismic events(m_b>5.0,30°≤Δ≤90°)have been recorded.The May 12 Wenchuan earthquake(M_S8.0)provides an opportunity to test the western Sichuan array.The preliminary data analysis of the May 12 Wenchuan earthquake and its larger aftershocks has been carried out in this study.Our results show: 1)The event parameters of the May 12 Wenchuan earthquake sequence need to be modified and their location error reaches to 8～24km.A more reasonable estimation of the location of the main shock is possibly at the depth of 19km.2)the wavefield analysis of the Lixian earthquake(M_S5.9)of May 16,2008 manifests that the surface waves of this event are not fully developed,and thus its source depth should not be very shallow.The peak values of the ground-motion velocity on the vertical and horizontal component have an abnormal increase by 4 times and more of the normal attenuation,which is related closely to the faults within the range of 200～250km,when the topography and site effects are not considered.3)The preliminary analysis of the crustal and upper mantle structure beneath the Sichuan basin and the Songpan-Ganzi block manifests that the crust beneath the Sichuan basin thickens along the western direction and its lower crust displays the hard structure.The crustal thickness in the northeast of Chengdu City reaches 46km.The crustal structure beneath the Songpan-Ganzi block has complex lateral variations.The crustal thickness in the Wenchuan earthquake source region reaches 52km.In the depth range of 14～20km,its crust has a complex high-velocity structure with the averaged velocity larger than 4.0km/s.The Wenchuan earthquake should be located within the area with high-velocity medium.In the lower middle crust,a low-velocity layer exists with the S-wave velocity of～3.6km/s,which could provide a relaxed boundary condition for the upper crust movement-deformation.This observation is consistent with the abnormal attenuation of ground motion with the epicenter distance obtained from the wavefield measurements.

THE M_S8.0 WENCHUAN EARTHQUAKE SURFACE RUPTURES AND ITS SEISMOGENIC STRUCTURE

XU Xi-wei, WEN Xue-ze, YE Jian-qing, MA Bao-qi, CHEN Jie, ZHOU Rong-jun, HE Hong-lin, TIAN Qin-jian, HE Yu-lin, WANG Zhi-cai, SUN Zhao-min, FENG Xi-jie, YU Gui-hua, CHEN Li-chun, CHEN Gui-hua, YU Shen-e, RAN Yong-kang, LI Xi-guang, LI Chen-xia, AN Yan-fen

2008, 30(3):  597-629. 

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Field investigations show that the M_S8.0 Wenchuan earthquake of 12^th May 2008 ruptured two NW-dipping imbricate reverse faults along the Longmenshan Fault zone at the eastern margin of the Tibetan Plateau.This earthquake generated a 240km long surface rupture along the Beichuan-Yingxiu Fault characterized by right-lateral oblique faulting and a 90km long surface rupture along the Guanxian-Jiangyou Fault characterized by dip-slip reverse faulting.Maximum vertical and horizontal dispacements of 6.2m and 4.9m,respectively,were observed along the Beichuan-Yingxiu Fault,whereas a maximum vertical displacement of 3.5m occurred along the Guanxian-jiangyou Fault.This co-seismic surface rupture pattern,involving multiple structures,is among the most complicated of recent great earthquakes.Its surface rupture length is the longest among the co-seismic surface rupture zones for reverse faulting events ever reported.Aftershocks recorded by local network clearly outline the hanging wall of the Beichuan-Yingxiu Fault and indicate that the fault dips about 47? to the west.Industry seismic lines,in addition to surface ruptures and aftershocks,allow us to build a 3D model for the rupture geometry that shows crustal shortening is the dominant process along the Longmen Shan to accommodate long-term deformation.Oblique thrusting accomplished by the earthquake indicates that the east-southeastward extrusion of Tibet Plateau accommodates,in part,the continuing penetration of the Indian plate into the Eurasian plate,and this extrusion is transformed at the eastern margin of the Tibetan Plateau into crustal thickening and shortening along the Longmenshan Fault zone that is responsible for the growth of high topography in the region.

PRIMARY ANALYSES OF IN-SITU RECURRENCE OF LARGE EARTHQUAKE ALONG SEISMOGENIC FAULT OF THE M_S8.0 WENCHUAN EARTHQUAKE

RAN Yong-kang, CHEN Li-chun, CHEN Gui-hua, YIN Jin-hui, CHEN Jie, GONG Hui-ling, SHI Xiang, LI Chen-xia

2008, 30(3):  630-643. 

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In the historical rocords,there have been no comparable earthquakes with the May 12 Wenchuan earthquake in Chengdu and Longmenshan mountain region.Then,whether the ancient earthquake traces with comparable magnitude in the geological records can be found has become an important scientific issue.The authors and other members of Wenchuan earthquake geological investigation team did fieldwork in the earthquake region for more than one month.Four trenches and one geologic section were excavated along the middle segment of surface faulting of both the central fault and the mountain front fault.And geomorphologic surfaces of deformation were measured.In this paper,we discuss the fact that there is prehistory large earthquake along the seismogenic fault of Wenchuan earthquake from analyzing the accumulated deformation of old and young fault scarps or geomorphologic surfaces,trenching,and comparing the activities of related faults and so on.The result shows that whether at Xiaoyudong,Leigu located on the central fault or Bailu,Hanwang on the mountain front fault,or at other places along the surface faulting of the the Wenchuan earthquake,the height of the fault scarp on the second terrace has a multiple relation with that on the first terrace after the Wenchuan earthquake.The 4 trenches reveal that the dislocation of the marker strata of the second terrace on both sides of fault is twice of that of the Wenchuan earthquake,which shows that there was an earthquake event with the same scale of surface deformation of this M_S8.0 Wenchuan earthquake between the formation time of the second terrace in Lomhmenshan area and the May 12 Wenhucan earthquake.

Research Paper

GEOLOGICAL AND GEOMORPHIC EVIDENCE FOR THE QUATERNARY ACTIVITY ON THE NORTHEAST SEGMENT OF LONGMENSHAN FAULT ZONE

YANG Xiao-ping, FENG Xi-jie, GE Tian-yong, SONG Fang-min, SHI Ya-qin, LIU Yu-fa

2008, 30(3):  644-657. 

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The Longmenshan Fault zone is an important thrust belt on the eastern margin of the Qinghai-Tibet Plateau,consisting of the back-range,the central and the front-range faults,which differ from each other in size and activity.The rupture zone of the Wenchuan earthquake of 12 May 2008 occurred over a length of～270km along the Yingxiu-Beichuan Fault(a segment of the Central Fault)and a length of～70km along the Guanxian-Anxian Fault(a segment of the Front-Range Fault).The northern end of the fracture zone is at the Nanba region in Central Fault.In this work,we make a detailed field investigation on the northeast segment of the Longmenshan Fault zone.Qingchuan Fault is the northeast segment of the Longmenshan Back-range Fault,and the Chaba-Lin'ansi Fault is the northeast segment of the Longmenshan Central Fault.Along the above two faults,we make geological and geomorphologic mapping of Tuguanpu,Da'an and Hujiaba regions,where the Qingchuan Fault runs through the Tuguanpu and Da'an area,and Chaba-Lin'ansi Fault runs through the Hujiaba area.Based on the field investigation,there are five terraces in the northeast Longmenshan area along the major rivers.The height above the river of T1 terrace is about 3～5m,and the formation time is Holocene.The heights of T2 and T3 terraces are 10m and 30～35m above the river,and the deposition time of alluvium and diluvium is Late Pleistocence.The remnant of T4 terrace's sediment covers on some hills,with the height above the river of about 60～70m.In the remnant,granite cobble and sandstone cobbles have been air slaked,these gravels have the shapes only.T5 terrace's height is about 90m,the sediment on it has been eroded.Qingchuan Fault and Chaba-Lin'ansi Fault were strongly active faults in the times before T3 and after T4 formed.Some fault grooves were formed on T4 or T5 terrace,they have 30～180m in width,and 8～20m in depth.The vertical displacement of T4 terrace's gravels is 10～15m.Fault groove didn't form on T3 terrace,or the terrace height on a fault wall is consistent with other fault wall.At some places,T3 terrace's gravels overlie the fault zone.

Brief Report

RUPTURE OF THE M_S8.0 WENCHUAN EARTHQUAKE ALONG BAISHAHE RIVER

HE Hong-lin, SUN Zhao-min, WEI Zhan-yu, DONG Shao-peng, GAO Xiang, WANG Shi-yuan, WANG Ji-qiang

2008, 30(3):  658-673. 

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At 14:28,May 12,2008,a great earthquake of M_S8.0 occurred in Wenchuan County,Sichuan Province.It produced two ruptures along the Longmenshan Thrust Belt,with the major one of ca.240km along the central fault zone and minor one of ca.70km along the mountain front fault zone,respectively.Baishahe rupture section,located at the southern end of the rupture of Wenchuan earthquake,extends 14km along Baishahe River,11km north of Dujiangyan City.This rupture section presents complicated geometric features,and is composed of several segments.Almost each segment has different strike from others(from 0 to 90°),but the section has an average strike of 50°.Co-seismic slip along the rupture section also shows complexity and multiformity.Up-thrust of the northwest wall is the main vertical co-seismic slip,and the maximum height of fault flexure scarp is 6.5m,but local backward flexure scarps exist there;the right-slip is the main co-seismic strike slip,and the maximum right-slip offset is 4.8m,but local left-slip offset also appears.A rupture plane of 40°/NW∠76° crops out at the northern end of the rupture section.Two groups of striations present on the plane,75°SW and 80°SW.This indicates not only that reverse faulting is major,and strike-slip faulting is minor,but also two rupture events in the earthquake.Based on the analysis of the rupture geometric features and co-seismic slip distribution,we have found the following knowledge: 1)The gradual change of the rupture plane from low angle at source depth to high angle at surface compels the horizontal shortening to transform into uplifting,causing directly the ratio of strike-slip to horizontal shortening approaching to 1,and forming backward fault flexure scarp behind the main rupture zone;2)The direction and amount of strike slip are determined by three factors of strike of rupture,azimuth of reference and gravity;3)The thickness of the overlying unconsolidated deposits affects the scope and style of rupture at the surface.

SEVERAL ISSUES IN THE QUANTITATIVE ANALYSIS OF CO-SEISMIC SURFACE RUPTURE PRODUCED BY REVERSE FAULTS:A CASE STUDY FROM THE 2008 WENCHUAN EARTHQUAKE

CHEN Gui-hua, LI Feng, ZHENG Rong-zhang, XU Xi-wei, YU Gui-hua, WEN Xue-ze, AN Yan-fen, LI Chen-xia

2008, 30(3):  674-682. 

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The Wenchuan earthquake(M_S=8.0)of May 12,2008 in Sichuan Province,China produced surface ruptures along the seismogenic reverse faults.It is very important to analyze the co-seismic surface ruptures quantitatively for the comprehension of tectonic behavior of the great earthquake.We discuss several issues about the quantitative analysis of co-seismic surface ruptures produced by reverse faults with some examples from Wenchuan earthquake.With the geometric relations between the parameters of deformation and survey data from planar and linear geomorphologic benchmark,solution methods for those parameters are provided.The vertical slip,horizontal dip-slip and dip angles on survey profiles were measured to calculate the actual deformation of the fault.The relationship between the apparent deformation and the real value was provided quantitatively for oblique profiles and linear benchmarks.We also made some discussions on the application of the survey data to the analysis of regional structural geometry and kinematics.

CHARACTERISTICS OF THE SURFACE RUPTURE ZONE OF THE M_S8.0 WENCHUAN EARTHQUAKE, CHINA ALONG THE SEGMENT NORTH TO BEICHUAN

LI Chuan-you, YE Jian-qing, XIE Fu-ren, ZHENG Wen-jun, HAN Yong-bing, LIU Yu-fa, WANG Wei-tao, WEI Zhan-yu, ZHAO Dong, MA Bao-qi, REN Jun-jie

2008, 30(3):  683-696. 

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Field investigation on the surface ruptures of the M_S8.0 Wenchuan earthquake along the segment between Beichuan and Qingchuan shows that there is one surface rupture zone developed on this segment,which extends generally along the Beichuan-Qingchuan Fault zone.Observation at Huangjiaba Chenjiaba,Guixi,Pingtong,Nanba,and Shikan suggests that the surface ruptures on this segment spread continuously along the trend of the fault,with single structure and a length of 60～90km.The surface rupture has not reached Guanzhuang of Qingchuan county.The observable rupture zone is about 62km,between Beichuan and Shikan,trending 20°～55° in general,dipping NW with an angle of 70°,showing mainly thrusting with dextral strike-slipping.The most distinct feature of the surface ruptures of this earthquake is the vertical surface bending,which indicates the thrusting of the deep fault.Its horizontal motion on this segment displays as dextral strike slipping,without sinistral slipping component.The value of the vertical coseismic displacements decreases gradually from 3m at Huangjiaba to about 1.5m at Nanba and Shikan;The amount of the dextral displacements does not change evidently,generally between 1.5m and 2.0m.Features of the surface rupture show that the causative tectonics of this M_S8.0 Wenchuan earthquake is the Yingxiu-Beichuan-Qingchuan Fault,whose movement is characterized mainly by thrusting,with a dextral slipping component,and the thrusting direction is from west to east.

TRENCH LOGS OF EARTHQUAKE SCARP OF THE M_S8.0 WENCHUAN EARTHQUAKE IN THE SEGMENT NORTH OF BEICHUAN

ZHENG Wen-jun, LI Chuan-you, WANG Wei-tao, YIN Jin-hui, WEI Zhan-yu

2008, 30(3):  697-709. 

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On 12 May 2008,a magnitude 8.0 earthquake occurred beneath the steep eastern margin of Tibetan plateau in Sichuan,China.Rupture occurred over a length of～240km along the central Longmenshan Fault(Beichuan-Yinxiu Fault)and ～72km along the Longmenshan piedmont fault(Guanxian-Jiangyou Fault).In order to know clearly the activity of the middle segment of Longmenshan Fault,we surveyed the earthquake rupture and excavated 5 trenches in the north part of the central Longmenshan Fault.Four of the five trenches have revealed the deformation of the Wenchuan earthquake and the characteristics of strong earthquake activity on this segment.The 4 trenches are briefly described as follows.The Fenghuang village trench locates on T₂ terrace or T₃ terrace of Pingtong River.The trench logs show that there is another earthquake event except the Wenchuan earthquake.As a tectonic deformation character,the thrust fault is exposed on the surface,the underground soils were thrust over the cultivated surface soil,forming thrust nappe and extrusion wedge.The bedrock near the fault has been compressed and fractured seriously,which is represented by overlap of some old scarps with new ones on the ground surface.The trench on T₁ terrace at Pingtong Panxuanlu records the zigzag deflections of marker bed.According to the recovery of marker bed,it is possible that there was an earlier earthquake event whose magnitude is equivalent to the present Wenchuan earthquake,because the footwall is higher than the hanging wall after the marker bed flattened,and the Wenchuan earthquake scarp on flood plain and T1 terrace is much lower than the scarp at the trench site.The scarp with a length of～20m and height of～2.7m is revealed by the trench excavated near the Da′ai School,but we can't see obvious signs of fault and faulting.Perhaps the fault displacement is represented by slightly folding of each deposit layer,which is one of the surface deformation models of thrust faulting.On T₁ terrace at Miaoziwan village,Nanba town,the trench displays that the vertical displacement of arc-deflection on marker bed is equal to the earthquake scarp height,indicating that this phenomenon is caused by the present earthquake event alone.Now,there are no results of dating samples,so we obtained the topographical age by comparing the adjacent surfaces.According to the dating results of terrace,the forming time of T₁ terrace is about 3000～5000a and T₂ terrace about 12000～20000a.It is revealed that the recurrence interval of strong earthquake on the northern segment of central Longmenshan Fault is more than 3000a.

INVESTIGATION OF LATE QUATERNARY ACTIVITY ALONG THE NORTHERN RANGE-FRONT FAULT, LONGMENSHAN

CHEN Li-chun, CHEN Jie, LIU Jin-feng, LI Feng, YANG Xiao-ping, RAN Yong-kang

2008, 30(3):  710-722. 

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With the occurrence of the 12May 2008 Wenchuan,Sichuan,China earthquake,two high-relief surface ruptures were formed on the Yingxiu-Shikan segment of the range-front fault and the Bailu-Hanwang segment of central fault along the Longmenshan Fault zone,and around the faults almost all buildings were razed.Furthermore,surface ruptures on the central fault broke through the accustomed activity segmentation boundary and extended to the northern segment by about 60km.Aftershocks also have a trend of migrating to the northern segment from the middle segment of Longmenshan Fault zone.It would be interesting to know as what kinds of effects this giant earthquake has on the earth's surface along the northern segment of Longmenshan Fault zone,and whether there have been earthquakes occurring since late Quaternary? In order to accumulate more reliable and detailed data and have a primary understanding to solve these questions and for post-quake reconstruction,we made a detailed geological and geomorphologic survey of the co-seismic earth's surface transformation and the late Quaternary Fault activity along the northern segment of the range-front fault on the basis of researches by predecessors.And then we excavated trenches at two relatively heavily-damaged and suspicious sites with legible linear shadows on image to investigate whether there exists active fault or fold.Our observation suggests that the northern segment is quite distinct in geology and geomorphology,and the coseismic earth's surface transformation is different from that in the middle segment of front-range fault and there is no clear trace to indicate that it is a late Quaternary active fault.So the late Quaternary active segment of range-front fault may terminate nearby the south of the Yong'an town.Our observation also suggests that the so-called active fault scarp as argued by some researchers near the Yong'an village is in fact an eroded river bank.

QUANTITATIVE ANALYSIS OF THE CO-SEISMIC SURFACE RUPTURE OF THE 2008 WENCHUAN EARTHQUAKE, SICHUAN, CHINA ALONG THE BEICHUAN-YINGXIU FAULT

CHEN Gui-hua, XU Xi-wei, ZHENG Rong-zhang, YU Gui-hua, LI Feng, LI Chen-xia, WEN Xue-ze, HE Yu-lin, YE You-qing, CHEN Xian-cheng, WANG Zhi-cai

2008, 30(3):  723-738. 

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A 240km-long co-seismic surface rupture was produced along the Beichuan-Yingxiu Fault during the 2008 Wenchuan earthquake.We made a detailed survey at representative sites along the surface rupture and analyzed the data based on the geometry between the benchmark and deformation.The co-seismic vertical slip,horizontal dip-slip shortening,strike slip and moving direction of the hanging-wall were calculated based on the survey data of these sites.Results show that the spatial distribution of the co-seismic deformation of the fault varies a lot along the fault.The maximal horizontal slip,as we got till now,is located in the Shenxigou site of Hongkou with a value of 4.98m,and the maximal strike slip is also located in the same site with a value of 4.5m.The maximal vertical displacement is located to the northeast of Shenxigou with a value of 5.7～6.7m.The average horizontal slip for the NE trending fault is 1～2m,and the average vertical slip is 3m.But horizontal and vertical slip for the NW-trending branch from Xiaoyudong to Caoba is only 0.5～1.5m.The data from Leigu town show that the gravity deformation resulting from the fault-related landslide was perhaps superimposed on the tectonic one.The dip angles of the fault at the surveyed sites calculated from the horizontal shortening and vertical displacement indicate that the Beichuan-Yingxiu Fault is a steep dipping reverse fault with some strike-slip.From the comparison between field results and geophysical inversion,we believe that the spatial distribution of co-seismic fault-slip is related to the barriers and rupture process along the fault plane.

COSEISMIC SIGNAL ASSOCIATED WITH AFTERSHOCK OF THE M_S8.0 WENCHUAN EARTHQUAKE

TANG Ji, ZHAN Yan, WANG Li-feng, XU Jian-lang, ZHAO Guo-ze, CHEN Xiao-bin, DONG Ze-yi, XIAO Qi-bin, WANG Ji-jun, CAI Jun-tao, XU Guang-jing

2008, 30(3):  739-745. 

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The measurement of electromagnetic field for monitoring aftershock series has been done by continuous observation after the M_S8.0 Wenchuan earthquake on May 12,2008 in Hanwang Observatory,Longnan,Gansu Province and vicinity for 22 days since May 22,2008.Two V5-2000 instruments for MT measurement made by Phoenix Company were set up 2.5km away from each other near Hanwang Observatory where a geophone for strong earthquake recording was set up.The coseismic signals of aftershocks exist in all components of the magnetic and electric fields of the two electromagnetic sites.Comparing with the seismic data at Hanwang station,the signals arrive simultaneously with the seismic waves and do not at the origin time of the earthquake.The main frequency of both seismic waves and electromagnetic signals is almost the same.The signal produced at the origin time of the earthquake seems apparent in the EM data,but the signal amplitude is much smaller than those of seismic wave arrival.

FEATURES OF THE MAY 12 M8.0 WENCHUAN EARTHQUAKE SEQUENCE AND DISCUSSION ON RELEVANT PROBLEMS

JIANG Hai-kun, LI Ming-xiao, WU Qiong, SONG Jin

2008, 30(3):  746-758. 

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Aftershock activity ofMay 12 M8.0 W enchuan Earthquake sequence shows an obvious segmen-ted feature,most of the large aftershocks are d istributed in the south and north parts of aftershock zone.Thrusting is dom inant in the south partwith a small strike slip component.The aftershock activ-ity decays gradually and presents the sequence feature ofmainshock-aftershock pattern.The north part is the end ing area of the mainshock fracture,where the strike slipping is dom inant and shows an obvi-ous swarm feature,it therefore became the major area for large aftershocks.Modulation of earth tide on aftershock activity is remarkable,most of large aftershocks occur during the period of flood and neap tide.The time period around 16:00 is the dom inant occurring time for large aftershocks.Thep-val-ue,a parameter ofmod ified Omori formula,increases gradually with time,and reaches about 1 in the final.Based on the previous study,the sequence patterns,magnitude ofmaximum aftershock,as well as the duration of aftershock activity have been d iscussed.The primary results also show that the mag-nitude d ifference between mainshock and the maximum aftershock is proportional to the rupture size of the mainshock for huge earthquakes of about M8.0.This means that,when the magnitudes of earth-quakes are nearly the same,large rupture size corresponds to the sufficient energy release.

RELOCATION OF THE M_S8.0 WENCHUAN EARTHQUAKE SEQUENCE IN PART:PRELIMINARY SEISMOTECTONIC ANALYSIS

ZHU Ai-lan, XU Xi-wei, DIAO Gui-ling, SU Jin-rong, FENG Xiang-dong, SUN Qing, WANG Ya-li

2008, 30(3):  759-767. 

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2741 aftershocks of the May 12,2008 M_S8.0 Wenchuan earthquake from May 12 to June 26 were relocated using the double difference algorithm to investigate tentatively the seismotectonic deformation and rupture process of the earthquake.Most relocated aftershocks occurred in the upper crust at depths of 0～20km,and fewer in the lower crust from depth of 25 to 40km,which may correspond to faulting in the lower crust.An aseismic layer appears beneath the Longmenshan thrust nappe zone between 20 and 25km depth after relocation,which may be correlated with the detachment for thrusting of the nappe zone.Spatial distribution of the relocated aftershocks and focal mechanism solutions show that the earthquake ruptured from south to north unilaterally with segmentation characteristics.The seismic belt formed by the aftershocks may be divided into two segments by the Gaochuan stepover in Anxian.The width of the seismic belt becomes narrower from south to north in map view,implying that the dip of the seismic fault may become steeper northward.The focal mechanism solutions suggest that rupturing in the south segment where the initial rupture point locates may be dominated by thrust slip with less component of strike slip,while it may have both thrust slip and dextral strike slip in the north segment.From Beichuan to the northeast end of the rupture zone,the seismic belt cannot be correlated with any fault in the north segment of the Longmenshan thrust nappe zone,even it cuts athwart the Qingchuan Fault.We can conclude that no controlling fault in the north segment of the Longmenshan thrust nappe zone was involved in earthquake generating and rupturing.The hypocenters in this part delineate a near vertical rupturing plane perpendicular to the strike of the seismic belt,and the focal mechanism solutions show great dextral strike slip component.

SOME INSIGHTS INTO THE MACRO-EPICENTER AND MEIZOSEISMAL REGION OF WENCHUAN EARTHQUAKE

LI Zhi-qiang, YUAN Yi-fan, LI Xiao-li, HE Ping

2008, 30(3):  768-777. 

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The M_S8.0 Wenchuan earthquake is one of the most devastating earthquakes in recent 30 years in the world.Based on a number of on-the-spot investigations,preliminary research on the macro-epicenter and meizoseismal region has been done.The analysis reveals that the meizoseismal region,whose intensity is Ⅺ and shows obviously zonal distribution,is controlled by the rupture process and there exists a close relationship between the meizoseismal region and the rupture zone.What's more,the macro-epicenter of Wenchuan Earchquake is quite different from those of previous earthquakes,not just one point any more but a group of points.This group of points begins from the village of Caijiagang of Xuankou town of Wenchuan county,passes through Yingxiu town,Yinxing town of Wenchuan county,the north of Hongkou town of Dujiangyan city,the north of Xiaoyudong town and Longmenshan town of Pengzhou city,the north of Hongbai town of Shifang city,and ends at Qingping town of Mianzhu city.Skipping over Anxian county,it continues from Leigu town of Beichuan county to Xuanping town,Qushan town,Chenjiaba town,Pingtong town of Pingwu county,and ends at the north of Nanba town.The shape of the macro-epicenter area of the Wenchuan earthquake can be considered as a narrow belt or line,broken in the middle.This point of view will influence our overall understanding of quake-hit areas to a great extent.Besides,the realization that the macro-epicenter of a severe earthquake probably presents like a line is of great significance when we cope with severe earthquakes later,especially when determining key relief areas,planning the relief work,evaluating the earthquake situation,identifying key areas of scientific investigation and so on.

INVESTIGATION ON WIDTHS OF SURFACE RUPTURE ZONES OF THE M8.0 WENCHUAN EARTHQUAKE, SICHUAN PROVINCE, CHINA

ZHOU Qing, XU Xi-wei, YU Gui-hua, CHEN Xian-cheng, HE Hong-lin, YIN Gong-ming

2008, 30(3):  778-788. 

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The 12 May 2008 Wenchuan Earthquake created about 240km-long surface fault ruptures along the Central Fault and about 72km along the Mountain Front Fault,two of the three sub-parallel secondary faults of Longmenshan thrust faults striking NE-SW,according to field investigation of surface faulting.From north to south,most of the widths of intense surface rupture zones are less than 40m,and above 1/2 are between 10～30m.Many buildings along fault surface ruptures were destroyed,including those with frame structure or reinforced concrete structure,and we also find some houses or buildings have withstood the strong earthquake and its aftershocks for their excellent performance of earthquake-resistance.The distance between fault scarps and the buildings are from 10 to 30 meters.Based on the field investigation,on the widths of surface rupture zones of historical strong earthquakes,and considering "crustal shortening" for inverse faulting and other various uncertainties,it is suggested that safety distance away from active fault in rebuilding is 25m.Within this distance,only one or two-storeyed buildings with higher standard of earthquake-resistance can be constructed,and public buildings,like schools and hospitals should be prohibited to build.

Application of new techniques

InSAR DEFORMATION OBSERVATION AND PRELIMINARY ANALYSIS OF THE M_S 8 WENCHUAN EARTHQUAKE

SUN Jian-bao, LIANG Fang, SHEN Zheng-kang, XU Xi-wei

2008, 30(3):  789-795. 

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After the May 12,2008 M_S 8 Wenchuan earthquake,we extract the high-precision deformation data covering the whole quake-stricken area(～500km×350km)by using SAR Interferometry approach(InSAR),which provides important information for the earthquake mechanism research.By analyzing InSAR imaging geometry and the characteristics of ALOS PALSAR data,we get some qualitative knowledge of the Wenchuan earthquake.This work provides helpful information for the related research and a basis for the following quantitative inversion of the earthquake rupture.The InSAR application of this earthquake shows the scientific significance of satellite geodesy to earthquake research.

News in brief

VERIFYING THE RESERVOIR SEISMIC SAFETY ASSESSMENTS IN SICHUAN PROVINCE WITH THE M_S8.0 WENCHUAN EARTHQUAKE

MA Wen-tao, LI Hai-ou, YANG Zhu-en, CHEN Gui-hua, CHEN Xian-cheng, YANG Qing-yuan, DENG Zhi-hui, SUN Qian

2008, 30(3):  796-803. 

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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.

Application of new techniques

AN INTRODUCTION TO THE GEODATABASE OF SCIENTIFIC INVESTIGATION OF THE M_S8.0 WENCHUAN EARTHQUAKE

LI Chen-xia, AN Yan-fen, YU Gui-hua, XU Xi-wei, CHEN Gui-hua, WANG Shi-yuan, GONG Hui-ling, YANG Hu, GUO Ting-ting, ZHANG Lan-feng

2008, 30(3):  804-810. 

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The geodatabase of scientific investigation of Wenchuan earthquake stores many field investigation data,such as the data of field geological survey sites,the surface rupture belts,the Quaternary faults in the earthquake region,paleoseismological trenches,folds,etc.Moreover,it includes some collection data,such as strata data,the catalog of the Wenchuan mainshock and aftershocks,the relocation catalog of aftershocks of the Wenchuan earthquake,the catalog of historical strong earthquake.The field data are recorded,edited,analysed,mapped and output with ArcGis,a powerful function in Gis software.The geodatabase of scientific investigation of M_S8.0 Wenchuan earthquake was initially established,which includes the basic seismological information and realizes the integrated management of spatial location and attribute information.Data can be queried,analyzed and processed for the need of the correlation analysis of the data.Seismic tectonic map and surface rupture map can be drawn with the geodatabase,which can provide the basis for the determination of safety distance from the risky seismic rupture zones in post-disaster reconstruction and for the construction of the Wenchuan earthquake geographic information system.