Table of Content

29 June 2010, Volume 32 Issue 2

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Brief Report

SPATIAL ANALYSIS OF COSEISMIC DISPLACEMENT FIELD OF THE WENCHUAN M_S 8.0 EARTHQUAKE DERIVED USING D-INSAR

QU Chun-yan, SHAN Xin-jian, SONG Xiao-gang, ZHANG Gui-fang, ZHANG Guo-hong, GUO Li-min, LIU Yun-hua, HAN Yu-fei

2010, 32(2):  175-190.  DOI: 10.3969/j.issn.0253-4967.2010.02.001

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We used the radar data from the satellite ALOS/PALSAR of Japan and the differential interferometric synthetic aperture radar(D-InSAR)technology to derive the coseismic displacement field produced by the M_S 8.0 Wenchuan,Sichucan Province,China earthquake on 12 May 2008.Based on processing SAR data of 7 tracks and 112 scenes by the two-pass method,we obtained the interferometric map of 450km×450km covering the causative fault and determined the distribution range of incoherent zones.Proper phase unwrapping was performed to these tracks of continuous and discontinuous phases,yielding digital image of the interferometric displacement field,which is analyzed by displacement contours and the profile across the fault.The result shows that the Wenchuan M_S 8.0 earthquake has produced a vast area of surface deformation along the Yingxiu-Beichuan Fault,primarily concentrated in a near-field range of 100km wide on the both sides of the causative fault.In this field,the 250km-long and 15～35km wide incoherent zone nearby the fault has suffered the largest deformation with surface ruptures,of which the amount is too large to measure by InSAR.The secondary deformed areas are 70km wide on each side of the incoherent zone,where envelope-like fringes are clear,continuous and converging towards the fault,indicative of increasing gradient and amplitude of displacements which exhibit sunk north wall and uplifted south wall in sight line.With respect to the north and south edges of the data track,the maximum subsidence in the north wall is 110～120cm appearing northeast of Wenhcuan and Maoxian,and a big range of descents of 55～60cm occurred nearby the epicenter south of Lixian.The largest uplift 120～135cm in the south wall is present at the epicenter west of Yingxiu,north to Dujiangyan and around Beichuan.The maximum relative displacement between the north and south walls is up to 240cm that appears nearby the epicenter west of Yingxiu and north to Dujiangyan.In the far-field 70km away from the incoherent zones on the both sides of the causative fault,there are sparse fringes indicative of displacements less than 10cm.The profile across the fault indicates a highly variable gradient of deformation with profound heterogeneity near the fault and in its hanging wall,and a relatively uniform deformation in the foot wall.These differences of deformation can be attributed to complicated thrust faulting.Our analysis suggests that the fault rupture of the Wenchuan earthquake is a relative thrust between the two walls of the fault.

DETERMINATION OF PALEOEARTHQUAKE EVENTS USING OSL AND AM¹⁴C DATING TECHNIQUES AT THE LEIGU TRENCH ALONG THE LONGMEN SHAN YINGXIU-BEICHUAN FAULT ZONE

LIU Jin-feng, CHEN Jie, YIN Jin-hui, CHEN Li-chun, LU Yan-chou, YANG Hui-li

2010, 32(2):  191-199.  DOI: 10.3969/j.issn.0253-4967.2010.02.002

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Paleoseismic studies conducted along the surface rupture of the Wenchuan earthquake(M_S 8.0)of 12th May 2008 provide important information regarding earthquake reoccurrence intervals and slip rates of the Longmen Shan Fault zone.The Leigu trench was excavated along the middle segment of the surface rupture of the Yingxiu-Beichuan Fault zone.Based on the syntectonic sedimentary structure,two earthquake events,including the Wenchuan event,were identified at this site.This study utilizes Optically Stimulated Luminescence(OSL)dating of samples collected from the Leigu trench section,using SMAR(Sensitivity-corrected Multiple Aliquot Regenerative-dose)protocols.AMS¹⁴C dating was also carried out on charcoal from the same sediments.OSL ages are generally consistent with the calibrated AMS¹⁴C ages(cal a BP)for the same units.The penultimate earthquake event similar to Wenchuan earthquake scale in this region occurred(2.1?0.2)ka to(1.1?0.2)ka.

THE DISTRIBUTION PATTERN OF WENCHUAN EARTHQUAKE SURFACE RUPTURE ZONE AND ITS SIGNIFICANCE OF SEISMIC DAMAGE AROUND HANWANG TOWN,MIANZHU COUNTY,SICHUAN PROVINCE

SUN Chang-bin, XIE Xin-sheng, JIANG Wa-li

2010, 32(2):  200-212.  DOI: 10.3969/j.issn.0253-4967.2010.02.003

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The epicenter intensity of the M_S 8.0 Wenchuan earthquake of 12^th May 2008 is Ⅺ.Hanwang town,Mianzhu county,Sichuan Province,located on the edge of seismic intensity Ⅹ zone,suffered the seismic intensity one degree higher than the surroundings,in the respects of both the houses damaged or the number of victims and missing persons,as close to the destruction in meizoseismal region.In this paper,the distribution pattern of the earthquake surface rupture zone around Hanwang town is discussed,focusing on the causes for the aggravation of seismic damage.The paper introduces two left-step en echelon surface rupture zones,which are 1.5km apart,located at the north and south of Hanwang town.The one on the north of Hanwang town has a vertical displacement of 1.4m and horizontal dextral displacement 0.44m,respectively.The other on the south of Hanwang town has a vertical displacement of 3.0～0.2m,descending from west to east.Although the two surface rupture zones did not pass through the center of the town,since Hanwang town is located right in the sinistral stepover clamped by the two surface rupture zones,the buildings in the stepover were subject not only to the vibration from the seismic source but also the compression inside the stepover,thus aggravating the damage of the buildings in Hanwang town.This also shows the diversity of building damage by the surface rupture zone.The seismic damage of Wenchuan earthquake around Hanwang town shows that the stepover of the surface rupture zone is also the special structural part subject to aggravated damage in earthquake.

CRUSTAL STRUCTURE UNDER BANGONG-NUJIANG SUTURE AND TERRAIN ASSEMBLY MODEL OF LHASA AND QIANGTANG BLOCKS IN THE WESTERN QINGHAI-TIBET PLATEAU

LI Hai-ou, XU Xi-wei, JIANG Mei, MA Wen-tao

2010, 32(2):  213-221.  DOI: 10.3969/j.issn.0253-4967.2010.02.004

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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.

STUDY OF THE LATE QUATERNARY SLIP RATE ALONG THE NORTHERN SEGMENT ON THE SOUTH BRANCH OF LONGLING-RUILI FAULT

HUANG Xue-meng, DU Yi, SHU Sai-bing, XIE Fu-ren

2010, 32(2):  222-232.  DOI: 10.3969/j.issn.0253-4967.2010.02.005

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The Longling-Ruili Fault is an important northeast trending active fault in southwestern China which controls the development of the sedimentary series and magmatic action of the two sides of the fault,and also controls Longling Basin,Mangshi Basin and Zhefang Basin along the fault.Due to limited Quaternary sediments samples and harsh natural conditions,the study of late Quaternary fault activity on the northern segment of the Longling-Ruili Fault is lacking and presently the time of the newest faulting and the Quaternary slip rate are not clear.Based on interpretation of remote sensing images,quantitative geomorphologic deformation measurement and dating of young terrace deposits and alluvial fans,the paper obtains some new results as follows:The northern segment of Longling-Ruili Fault is a Holocene dextral strike-slip fault with a normal slip component.The terrace T1 composed mainly of alluvial deposits formed during 4ka BP was offset by the northern segment of the Longling-Ruili Fault,with left-lateral and vertical displacements of 8～12m and 2m,respectively.The late Pleistocene alluvial fan was dislocated with a left-lateral and vertical displacement of 70m and 18m,respectively.The strike-slip rate of the Longling-Ruili Fault is 2.2～2.5mm/a and vertical slip rate is 0.6mm/a since late Pleistocene epoch.The strike-slip rate of the Longling-Ruili Fault is 1.8～3.0mm/a and vertical slip rate is 0.5mm/a during the Holocene epoch.The proportion of horizontal to vertical displacement is about 4:1,that means the vertical rate on the northern segment of Longling-Ruili Fault is about 25%of horizontal slip rate.The left-lateral slip rate in late Holocene is well consistent with GPS measurement.The strike slip rate is in good consistency in different time scales since late Pleistocene epoch,indicating that the activity of the Longling-Ruili Fault is stable.

GRAIN-SIZE CHARACTERISTICS OF PYROCLASTS IN CHANGBAISHAN MOUNTAIN AREA

ZHAO Bo, XU Jian-dong, YU Hong-mei

2010, 32(2):  233-243.  DOI: 10.3969/j.issn.0253-4967.2010.02.006

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The pyroclastic deposits from tephra fall,pyroclastic-flow,base-surge,and lahar are well developed in Changbaishan Mountain area where volcanic activity is active in the Holocene.These different pyroclastic deposits are difficult to distinguish from each other in the field because of their similar physical and geological appearance.Through field investigation of pyroclastic deposit sections in the study area,we systematically collected different pyroclastic deposit samples at different sites,and then calculated the grain size parameters and plotted out the probability cumulative curves of all samples in the laboratory.Our results indicate that the pyroclastic samples from different origins can be distinguished by characteristic grain-size diagrams,e.g.median diameter(Md_Ф)versus sorting(σ_Ф),F1 versus F2 diagram.In terms of texture,most pyroclastic deposits show similar characters.However,with the increase of distance from the volcano crater,grain size decreases correspondingly.Finally,Laser Particle Size analyzer is utilized at first time to analyze the subpopulation of fine ash(≤62.5μm)of our samples.On the basis of the fine ash subpopulation histogram distribution and weight cumulative curves,we found that the fine ashes with grain-size from 0.02μm to 62.5μm in pyroclastic-flow deposits and pyroclastic-surge deposits are significantly different due to their different formation origin and different transportation processes.

DISCOVERY OF NATURAL DEFORMATION VESTIGE AT GAIXIA RUINS,GUZHEN COUNTY,ANHUI PROVINCE AND ITS IMPLICATIONS

YAO Da-quan, SHUO Zhi, LIU Jia-can, TANG Jie-ping, WANG Zhi, SHEN Xiao-qi, CHEN An-guo, ZHENG Hai-gang

2010, 32(2):  244-251.  DOI: 10.3969/j.issn.0253-4967.2010.02.007

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Recognition of natural deformation during excavating ancient ruins and tomb is an important work of tracing or enriching the history and the historic records about prehistoric deformation.Recently,the earthquake department in cooperation with the cultural relic archaeology department conducted the special excavation research at the archaeological site of Gaixia ruins,Guzhen,Anhui,and discovered fault and cracks,which are demonstrated by preliminary study to be the vestiges of two coeval strata deformation events.The high speed deformation characteristic possibly represents two strong earthquake events,and the time is approximately in the Late Dawenkou culture stage.The crack was formed earlier than the fault.The discovery is the first in the eastern China.This work has filled the gap of research on recognition of natural dislocation event in Late Dawenkou culture stage and may improve the recognition of seismological relics in cultural stratum of human being in eastern China,especially in the transition belt between the north and south of China in the Changjiang-Huaihe area.

THE EQUIVALENT STATIC WATER LEVEL OF FLOW VALUE AND COMPREHENSIVE DYNAMIC WATER LEVEL AND EXPLORATION OF ITS CALCULATION MEANS

WANG Hua, ZHANG Zhao-dong, WANG Wei, LI Yue-qiang

2010, 32(2):  252-259.  DOI: 10.3969/j.issn.0253-4967.2010.02.008

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In our country,the water level and discharge flow rate are observed simultaneously when observing the dynamic water level in the seismic ground fluid observation network.Thus,two types of dynamic variations are obtained generally,and they are handled and analyzed separately.Two contrary results are obtained sometimes in the information of aquifer stress state change.Facing this irrational status,the relationship of water level to flux in dynamic water level observation well was explored based on the fluid dynamics theory.The equivalent static water level was put forward,synthesizing the two observation terms into one unified physical quantity.The calculation means to synthesize the water level and flux into one quantity was proposed and tested using the observation data of wells Dongshui-3 and Liaogu-1.

CRUSTAL THICKNESS AND POISSON RATIO OF LIAONING PROVINCE FROM RECEIVER FUNCTION STUDY

JIA Li-hua, CHONG Jia-jun, LIU Yuan-yuan, NI Si-dao, DAI Jin, WANG Shuai

2010, 32(2):  260-268.  DOI: 10.3969/j.issn.0253-4967.2010.02.009

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Songhuajiang-Liaohe Basin is surrounded by several folded mountains,with some major deep faults running across them.Among these faults,the Tanlu Fault is one of biggest active faults in the east China continent.This kind of geological environment makes Liaoning and its adjacent areas be capable of highly active tectonic movement and high seismicity.As crustal thickness and Poisson ratio are closely related to seismogenic structure,studying on these parameters will be very important for us to understand the seismogenic process of Liaoning and its neighboring regions.In this work,teleseismic P wave records from 15 permanent broadband seismic stations in Liaoning province are collected and processed by inverse convolution method in spectral domain to get the P wave receiver functions.The H-Kappa stacking method is further used to obtain the crustal thickness and Poisson ratio under each seismic station.The result shows that Poisson ratio in this region is between 0.25and 0.29,and crustal thickness ranges from 31km to 36km.Comparing with the Harbin Basin,the crustal thickness of the west and east fold belts in this region is about 2～4km thicker.The distribution pattern of crust thickness shows that the crust is getting thicker from east to west and from north to south,with an average crustal thickness about 31km in Songliao Basin.This pattern shows that the Songhuajiang-Liaohe Basin is a typical rift basin,its seismogenic process may mainly be controlled by the subsidence of the basin and the horizontal extension from the east and the west boundaries.

ANALYSIS TO THE CHARACTERISTICS OF COGNITION AND RESPONSE OF THE PUBLIC TO WENCHUAN M_S 8.0 EARTHQUAKE DISASTER:A CASE STUDY FROM MIANXIAN COUNTY,SHAANXI PROVINCE

YU Yao-chuang, ZHOU Qi, WANG Chang-yan

2010, 32(2):  269-281.  DOI: 10.3969/j.issn.0253-4967.2010.02.010

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In order to find out the cognition and response level of the public to Wenchuan M_S8.0 earthquake and supply a scientific basis for government to establish and implement the disaster-reduction strategy,the paper formulated a questionnaire on the public's cognition and response to earthquake disaster according to the structure system of cognition and response to earthquake disaster.The main index systems in the questionnaire include cognition to the earthquake disaster knowledge,techniques and abilities of defending and reducing the earthquake disaster,the self and mutual help and information dissemination after earthquake.Based on weight and evaluation,the paper constructed the evaluation model for each part of index system and overall evaluation model on cognition and response to earthquake disaster and calculated comprehensive score of the public's cognition and response to earthquake disaster based on it.The results show that in earthquake disaster area of Mianxian county the synthetical score of cognition and response to the earthquake disaster in the public is 49.74,the score of self and mutual help is 53.8,the score of information dissemination after earthquake is 50.9,the score of cognition to the earthquake disaster knowledge is 49.79,and the score of technique and ability of defending and reducing the earthquake disaster is 47.2.The main conclusions are as follows.(1)The comprehensive level and ability of the public's cognition and response to earthquake disaster is very low,and the earthquake knowledge acquisition and technique and ability of defending and reducing earthquake disaster need to be improved greatly.(2)The public's technique and ability of defending and reducing earthquake disaster is the worst,the knowledge acquaintance level of earthquake disaster comes second,and the self and mutual help and information dissemination are a little better.(3)The public's understanding to earthquake knowledge and views still stays in superficies,and the understanding to the complex abstract earthquake knowledge,especially the accurate acquaintance and deep understanding of earthquake knowledge need further improved.(4)In view of the main channels for the public gaining information after the earthquake,to enhance the publicity by the government on the knowledge and skill of preventing earthquake disaster and publicize earthquake information are one of the effective ways to reduce earthquake disaster.

STUDY ON POTENTIAL STRONG EARTHQUAKE RISK IN MABIAN AREA,SOUTHERN SICHUAN

YI Gui-xi, WEN Xue-ze, ZHANG Zhi-wei, LONG Feng, RUAN Xiang, DU Fang

2010, 32(2):  282-293.  DOI: 10.3969/j.issn.0253-4967.2010.02.011

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Based on seismic data of the regional network of the last 34 years,we have analyzed current faulting behaviors of major fault zones in Mabian area,southern Sichuan,and preliminarily identified the risky fault-segments on which potential strong and large earthquakes may occur in future,with the method combining the spatial distribution of b-values with activity background of historical strong earthquakes and current seismicity.Our results mainly show:(1)The spatial distribution of b values displays significant heterogeneity in the study area,which reflects the spatial difference of cumulative stress level along various fault zones and segments in the area;(2)Three anomalously low b-value areas with different sizes exist on Mabian-Yanjin Fault zone,these anomalies can be identified as asperities under relatively high cumulated stress levels,in which,two asperities,located at north of Mabian county and Lidian town in western Muchuan county,and near Yanjin at the south end of the fault zone,respectively,may be the potential seismogenic sources of large earthquakes in Mabian area in the near future,and the third asperity with a small size located at southern Suijiang may be the potential strong-earthquake source;(3)An asperity at south-western segment of Longquanshan Fault may be the site of potential moderate to strong earthquakes;and(4)the asperity on the segment between Huangmu town in Hanyuan county and Longchi town in Emeishan city on Jinkouhe-Meigu Fault has potential for moderate to strong earthquakes.

OCCURRENCE CONDITIONS FOR GREAT EARTHQUAKES

LIANG Hai-hua

2010, 32(2):  294-302.  DOI: 10.3969/j.issn.0253-4967.2010.02.012

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Two necessary conditions for occurrence of great earthquakes,especially for M_S≥8.0 ones are:The high stress which must exceeds the elastic limit of rock;and that the rock should maintain elastic behavior in a longer period than the time for strain accumulation in rock(usually several thousand years).To check the first condition,the actions of the three plates surrounding China are analyzed.The subduction zone between the Pacific plate and Asian plate is far away from the Chinese continent,and the horizontal component of the subduction force is relatively small,so there is no significant effect on China.The collision zone between the Philippine Sea plate and the Asian plate makes Taiwan and Fujian Province of China become high stress area.As the collision boundary is short(only 200km long),the stress declines rapidly along the northwest direction.The high stress area in northwest China and north China is due to the Indian plate's strong collision to Asian plate.The collision boundary between Indian plate and Asian plate is composed of three arc parts with different directions and magnitudes of force.Based on this data,we try to recognize the three seismic belts.To predict the possible locations of great earthquakes,three geologic units(rigid block,sub-rigid block and fold belt)are divided and their rheology is anayzed.Based on the calculated data,the elasticity of rigid block can maintain as long as 1million years,while there is only 1000 years for fold belt.So,the location of M_S 8.0 earthquake is related closely to rigid blocks.

CONTRAST OF SEVERAL EXPERIMENTS OF QUARTZ ISOLATION AND PURIFICATION FOR COSMOGNEIC DATING

ZHENG Rong-zhang, CHEN Gui-hua, XU Xi-wei, LI Jian-ping

2010, 32(2):  303-311.  DOI: 10.3969/j.issn.0253-4967.2010.02.013

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Cosmogenic dating(in situ cosmogenic nuclides)has been widely used in geologic and geographic domain along with the application of accelerator mass spectrometry(AMS)and highly sensitive conventional noble-gas mass spectrometry in the early 1980s,since Raymond Davis and Oliver Schaffer proposed that cosmogenic nuclides produced within minerals at the surface of the earth could be applied to geological problems.At present,this dating method is used to study glacier,desert and relief evolution,volcano development and active structure movement etc.There are a lot of materials which can be used in cosmogenic dating method.Because of its tight crystal structure,quartz can minimize possible contamination by meteoric ¹⁰Be,and also for its low ²⁷Al content,it becomes an ideal material for cosmogenic dating method.During the process of the isolation of ¹⁰Be and ²⁶Al from quartz sample,quartz isolation and purification are one of the key steps.HCl/H₂O₂ and HF/HNO₃ leaching is a safe method used to isolate and purify the quartz samples widely.However,there is defect about the method.It takes more time for sample preparation and curtails the life of instrument.Based on the above method,three experiments were designed to compare with the primary one.The results of experiment reveal that the purification efficiency can be enhanced by increasing the leaching solution concentration and the amount of sample,and ultrasonic cleaner can be replaced by hotplate /magnetic stirrer.

DISCUSSION ON THE ISSUE OF PROPER DEPTH IN CALCULATING EQUIVALENT SHEAR WAVE VELOCITY FOR SITE CLASSIFICATION

HUANG Ya-hong, L? Yue-jun, LAN Jing-yan, SHI Chun-hua, SHI Bing-xin

2010, 32(2):  312-319.  DOI: 10.3969/j.issn.0253-4967.2010.02.014

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Based on analysis of the shear wave velocity data of 823 boles from the northern,eastern,southern,northeastern and northwestern areas of China,we studied the difference of the equivalent shear wave velocities calculated with the soil-and-rock depths of 20m and 30m,respectively.We also discussed the features and differences among the Chinese,American and European seismic regulations in site classification using the parameter of equivalent shear wave velocities of boles.Our results indicated that:① The equivalent shear wave velocity will increase about 10 to 60m/s when we increase the calculation depth of soil-and-rock from 20m to 30m.The average increase is about 24m/s.② Comparing with the American and European seismic regulations,the current Chinese seismic regulation(GB 50011-2001)requires the information of the equivalent shear wave velocities calculated with the soil-and-rock depths of 20m,as well as the depth of the overburden in the site when classifying the site.However,it is usually difficult to obtain the usable overburden depth in many engineering sites to serve the site classification.Therefore,it is necessary for us to use for reference of American and European seismic regulations to increase the depth of soil-and-rock when calculating the equivalent shear wave velocity so as to get a more reliable parameter to serve for the site classification.

PRELIMINARY STUDY OF RE-OSL ON WATER-DEPOSITED SEDIMENTS

JI Hong-lei, ZHAO Hua, WANG Cheng-min, MAO Hong-liang, LU Yan-chou

2010, 32(2):  320-326.  DOI: 10.3969/j.issn.0253-4967.2010.02.015

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According to measuring the natural TT-OSL signals of the fine-grained quartz from both fluvial and lacustrine sediments,the dose recovery experiment and the comparison of the multiple aliquots fine-grained quartz SMAR and Re-OSL dating results,the possibility of Re-OSL dating with fine-grained quartz for water-deposited sediments has been studied.The results show that,the Re-OSL signals accumulated in the fine-grained quartz from fluvial sediments are lack of regularity with the depth,and the Re-OSL signals of the fine-grained quartz can not yield the dose values;While the Re-OSL signals of the lacustrine sediments increase gradually with the depth.The Re-OSL dose measured and laboratory additional dose are consistent within 2%error range,which indicates that the Re-OSL dating with the fine-grained quartz for lacustrine sediments is feasible to some range of time.The Re-OSL dating method has much potential for dating the lacustrine samples more than 100ka years.

THE INFLUENCING FACTOR OF ELASTIC ANISOTROPY IN MIDDLE TO LOWER CONTINENTAL CRUST

ZHANG Guo-ling, YANG Xiao-song, CHEN Jian-ye, YAN Xiao-bing

2010, 32(2):  327-337.  DOI: 10.3969/j.issn.0253-4967.2010.02.016

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Elastic anisotropy is an important geophysical property of crustal rocks.Due to the compositional and textural complexity of crustal rocks,the potential factors influencing its elastic anisotropy are various.Microcracks preferred orientation plays an important role on elastic anisotropy in superficial crust.While at deeper crust,lattice preferred orientation(LPO)and shape preferred orientation(SPO)take over and control the elastic anisotropy.Statistic of previous experimental work shows that the content of mica and amphibole is somehow related to the anisotropy of intermediate-acid and basic rocks,respectively.Otherwise,the oriented melt can largely enhance rock's anisotropy,which is a preferable explanation to the observed high anisotropy within the middle to lower crust under Tibetan plateau.

CHARACTERISTICS OF SURFACE RUPTURE OF THE M_S 7.1 YUSHU,QINGHAI PROVINCE EARTHQUAKE AT TWO REPRESENTATIVE PLACES

SUN Xin-zhe, XU Xi-wei, CHEN Li-chun, TAN Xi-bin, SU Gui-wu, WANG Ji, LI Zhi-min, ZHANG Xiao-qing

2010, 32(2):  338-344.  DOI: 10.3969/j.issn.0253-4967.2010.02.017

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The M_S 7.1 Yushu,Qinghai earthquake on April 14th,2010 generated a surface rupture zone about 65km long.The west of Ganda Village,D1,and Guo Qing Yi Rong Song Duo,D2,are the two most representative places of surface rupture characteristics of the M_S 7.1 earthquake in Yushu,Qinghai Province.Survey results on co-seismic surface rupture characteristics at the two locations show that:(1)Surface rupture extends along the remains of late Quaternary activities on the pre-existing fault,and at the west of Ganda Village,the surface rupture is composed of tensile shear fcractures arranged en echelon,with mole tracks.Most part of the surface rupture is distributed along the paleoseismology trench,and through measuring an offset wall,we get the offset of 1.4m.(2)At Guo Qing Yi Rong Song Duo,the surface rupture zone consists of a series of right-step en echelon sub-cracks with a spacing about 30m each other,and the sub-crack is formed by the right step cracks,with a spacing of 3～5m.The mole track-tension crack and crack belt compose the single rupture,which are along the slope wash in front of the mountain.The surface rupture across the river valley appears as compressional ridge and sag pond in the valley.The offset of a fence is measured to be 1.4m.(3)The surface rupture shows left-lateral strike-slip characteristics,with no significant vertical component.The surface rupture mode shows a typical strike slip character.The surface rupture distributed along the pre-existing offset landforms reflects that the Ganzi-Yushu Fault,which is an active fault of Late Quaternary,is the seismogenic fault of this earthquake.The fault has the character of in situ recurrence of large earthquakes.