Table of Content

30 June 2013, Volume 35 Issue 2

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Research Paper

DEFORMATIONAL CHARACTERISTICS OF THE NIUSHOUSHAN-LUOSHAN FAULT ZONE AND ITS TECTONIC IMPLICATIONS

WANG Wei-tao, ZHANG Pei-zhen, LEI Qi-yun

2013, 35(2):  195-207.  DOI: 10.3969/j.issn.0253-4967.2013.02.001

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As the outermost fault zone in the northeastern margin of the Tibetan plateau,the deep structures,distribution,movement feature and deformational mechanism of the Niushoushan-Luoshan Fault zone are crucial to understand the formation and evolution of the arcuate fault zones in the northeast corner of the Tibetan plateau. In this paper,we analyze four seismic reflection sections across the Niushoushan-Luoshan Fault zone and map in detail the area within the fault zone. These data indicate that the Niushoushan-Luoshan Fault zone is a discrete fault zone. The fault zone can be subdivided into three parts: the south part,i.e.the Luoshan Fault,is characterized by positive flower structure,shown as remarkable right lateral strike-slip; in the middle segment,that is,the Niushoushan Fault,no active fault exists on the east flank of the Niushoushan,and this region is dominated by intensive folding; the north part,the Sanguankou Fault,is a left-lateral strike-slip fault. The discontinuity and segmentation feature of the Niushoushan-Luoshan Fault zone suggest different deformational styles in different locations of the fault zone associated with the process of northeastward propagation of the Tibetan plateau.

MAJOR ACTIVE FAULTS IN LINGQIU BASIN AND THE SEISMOGENIC STRUCTURE OF THE EARTHQUAKE IN 1626

MA Xing-quan, LI Yan-bao, RAN Yong-kang, CHEN Li-chun

2013, 35(2):  208-221.  DOI: 10.3969/j.issn.0253-4967.2013.02.002

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Lingqiu Basin is located in the northeast of the Shanxi graben system,where a M_S 7.0 earthquake occurred in 1626.The achievement of active fault research in this basin could contribute not only to the study of the seismogenic structure of the earthquake in 1626,but also to the research of the types of large earthquakes in Shanxi graben system. Much work has been conducted here,laying the foundation for the active fault study in this area. However,the spatial distribution and activity of several major faults,and the seismogenic structure of the earthquake in 1626 are still in discussion. This paper analyzes the geomorphologic characteristics in the whole basin via interpreting SPOT5 images,SRTM3 and fieldwork,and acquires some new knowledge of the major faults in combination with trenching. The activity of the main segment of the piedmont fault of Taibaiwei Mountains is limited to the late Pleistocene; The NEE-striking Shuijian-Luoshuihe Fault has obvious geomorphic features to the west of Lingqiu County,and the geomorphic feature of the fault is not remarkable to the east of the county. Its latest event left a 1m-high fault scarp on the surface. The NW-striking Huashanhe Fault behaves as a hinge fault. In the northern basin,the fault dips west,producing a height difference of about 10m in terrace T1 of the Huashanhe River. In the southern basin,the fault dips east. Profiles and geomorphic features show the south segment of the fault is an active strike-slip fault with a high angle. Thus,we consider the earthquake in 1626 resulted from the conjugated action of the NEE-striking Shuijian-Luoshuihe Fault and the NW-striking Huashanhe Fault.

TOPOGRAPHIC DIFFERENCE AND MORPHOLOGIC EVOLUTION BETWEEN THE TWO SIDES OF THE NORTHERN MARGIN OF WEST QINLING-LAJISHAN FAULT

GAO Ming-xing, XU Xi-wei, LIU Shao-feng

2013, 35(2):  222-233.  DOI: 10.3969/j.issn.0253-4967.2013.02.003

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The northern margin of West Qinling-Lajishan Fault is reckoned as an important geomorphologic and geological boundary. The elevation and local relief differences between the two sides of the boundary are distinct. The aim of this study is to understand the topographic differences and morphologic evolution mechanisms between the eastern and the western side of the northern margin of West Qinling-Lajishan Fault. The digital elevation model combined with geologic data has been used to: 1)Extract local relief and swath profiles to quantitatively reveal the topographic characters of the eastern and the western side of the boundary; 2)Make statistics on the elevation modes of the Cenozoic sandstones on both sides of the fault. The results show that the older strata lie at higher elevations in the eastern region of the boundary; And for the west of the boundary,older strata(Paleogene and Neogene)perch on lower elevations,however,the Quaternary strata lie in the lowest elevation. The differential topographic evolution history of the two sides of the boundary indicates the complexity of Cenozoic strata development. Moreover,the lithological combination and sedimentary facies of the Cenozoic strata in the eastern and western side of the northern margin of West Qinling-Lajishan boundary are summarized,and the denudation and aggradation history of the two sides of the boundary are compared in this study. A longer constant denudation history in the eastern side than the western side has been suggested. The results also indicate the differential tectonic evolution history between the two sides of the boundary. Outward growth of the Tibetan plateau appears in the eastern side. The deformation and uplifting decrease as propagating northeastward. For the western side,the deformation pattern is more complex with different evolution histories of faults under stronger compression. The limited river erosion has enhanced the up-growth of this part of the plateau.

LATE-QUATERNARY FOLDING OF THE MINGYAOLE ANTICLINE SOUTHWESTERN TIP,PAMIR-TIANSHAN CONVERGENT ZONE

LI Tao, CHEN Jie, XIAO Wei-peng

2013, 35(2):  234-246.  DOI: 10.3969/j.issn.0253-4967.2013.02.004

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Folding growth in three dimensions involves shortening in transversal direction,uplift in vertical direction and lateral propagation in longitudinal direction. The impact of these three components changes along the fold's strike: the middle part is dominated by shortening and uplift,and deformation neighboring the fold tip involves not only shortening and uplift,but also strong lateral propagation. Previous studies are focused on the middle part,and the fold tip,a relatively special part,however,is poorly investigated. Thereby,how does the fold tip grow,what is the deformation difference between fold tip and the middle part,and how do terraces deform in response to folding growth？Our study to the southwestern tip of the Mingyaole anticline,located at the Pamir-Tianshan convergent zone,indicates terrace surfaces are strongly back-tilted,and display increasing dips with age,implying a limb rotation mechanism. According to the OSL ages of the T_2b,T_3b and T_4a,as well as a magnetostratigraphy age of underlying bedrock,rotation angle increments of the dip domain 46° display a parabola tendency with the age of<～0.35Ma,(93.9±18.7)ka,(82.6±16.5)ka and(19.4±2.9)ka,and the average rotation rate is>(0.13±0.01)°/ka,(0.08±0.02)°/ka,(0.05±0.01)°/ka and(0.04±0.01)°/ka,which display an obviously decreasing tendency too. However,the shortening rate absorbed by this dip domain keeps constant.The fluvial terraces display not only tilted and uplifted in response to the shortening and uplift of the fold,but deformed in response to lateral propagation. Toward west,density,width and depth of gullies on the terraces decrease,and elevation to the riverbed of the terrace surface,height of the terrace riser as well as rotation angles of terrace surfaces display a decreasing tendency too,both of which are consistent with the fold's western-ward propagation. Based on the magnetostratigraphy age of～1.6Ma at the Kapake valley section,the average western-ward lengthening rate is about 16～16.8mm/a.

CO-SEISMIC RUPTURE AND DISPLACEMENT IN THE XIAOYUDONG AREA PRODUCED BY THE 2008 WENCHUAN EARTHQUAKE,CHINA,AND ITS MECHANISM

TAN Xi-bin, YUAN Ren-mao, XU Xi-wei, CHEN Gui-hua, CHANG Chung-pai

2013, 35(2):  247-260.  DOI: 10.3969/j.issn.0253-4967.2013.02.005

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The M_W 7.9 Wenchuan,China,earthquake is a large oblique reverse slip shock,whose main fault is dominated by reverse slip with right-lateral strike-slip component. It generated one of the longest and most complicated surface ruptures,and to many of the phenomena,we haven't had an appropriate interpretation or a common understanding,e.g.on the 7km-long NW-trending Xiaoyudong Fault and the coinstantaneous fracture on the two parallel thrust faults which are 11km apart on the north of Xiaoyudong area. Field investigation in the Xiaoyudong area shows clear co-seismic rupture and displacement,and on these bases,we analyzed the mechanism of the surface rupture in the Xiaoyudong area. Our study indicates that the change of attitude of Beichuan-Yingxiu Fault(BYF),that is,the ca.3.5km step-over in the west of the Xiaoyudong area,is the primary cause of the above complex phenomena. Specific mechanisms are as follows: 1)The dextral strike-slip of the BYF results in compressional uplift in the left-restraining step-over,creating a frontal reverse fault,known as the Xiaoyudong Fault. ; 2)The Pengguan Fault,which is parallel to and 11km apart from the BYF,is activated in the north of the step-over by a combination of the increased dip angle in the north of the step-over due to the ca.3.5km left step of the BYF and the lateral push of the hanging wall to the footwall of the BYF caused by the dextral strike-slip of the BYF.These results are helpful in deepening our understanding of the dynamic processes that produced surface ruptures during the Wenchuan earthquake. We also suggest that more attention shall be paid to the impact of the dextral strike-slip component,the change of primary fault's attitude and the difference of the rocks of the fault's two walls on the process and distribution of surface rupture.

INVESTIGATIONS ON FEATURES AND GENESIS OF HIGHER HIMALAYAN CRYSTALLINES

YANG Yong-xin, YANG Xiao-song

2013, 35(2):  261-277.  DOI: 10.3969/j.issn.0253-4967.2013.02.006

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The widely distributed migmatites in Higher Himalayan Crystallines(HHC)are an important clue to investigate the anatexis,crustal rheology,origins of Higher Himalayan leucogranites(HHL)and evolution of orogenic belts. We studied in detail the petrographic,petrologic and geochemical features of three basic constituent units of the migmatites,i.e.leucosome,mesosome and melanosome,collected from Nyingchi and Nyalam regions in Tibet. In order to study the chemical features of each migmatite unit,the major,trace elements and rare-earth elements of leucosomes,mesosomes and melanosomes separated from migmatites have been analyzed. Our studies indicate that leucosome is the product of crystallization of migrated melt,and mesosome is regarded as source rocks of migmatites or as a reactant of unsegregated melt and restite at source region. The melanosome represents the aggregation of melanocratic minerals crystallized from melt or the products of retro-metamorphic reaction between segregated melt and mesosome. Leucosomes and melanosomes are usually characterized by igneous textures,coarse grain size and isotropic fabric,which are important identifications to distinguish them from mesosomes. Positive Eu anomaly of leucosomes may be attributed to partially melting of plagioclase significantly and /or early crystallization of plagioclases from primary melt in nearby source region. In contrast,leaving feldspars behind source region and crystallization differentiation of plagioclase during melt aggregation contributed to negative Eu anomaly displayed by leucograintes. It seems that the existence of minerals formed in dehydration melting cannot be regarded as judgment standard about partial melting because the diagnostic minerals of dehydration melting may disappear during retrograde reactions.

CRUSTAL STRUCTURE AND ITS FEATURES IN THE SOUTHWEST MARGIN OF BOHAI BAY AND ADJACENT AREAS

WANG Shuai-jun, WANG Fu-yun, ZHANG Jian-shi, ZHENG Yan-peng, ZHANG Cheng-ke, ZHAO Jin-ren, LIU Bao-feng, Huang Can

2013, 35(2):  278-289.  DOI: 10.3969/j.issn.0253-4967.2013.02.007

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DSS data of Bohai Bay profile was processed in August 2011 and the result obtained in this paper and the results of other profiles,which cross this profile,were interpreted comprehensively in this paper. The DSS data were calculated and interpreted synthetically using 1-D and 2-D processing techniques in order to find out the basic features of 2-D velocity structures,spatial distribution of faults,geological structure of shallow and deep crust in the southwest margin of Bohai Bay and adjacent areas. The result shows that obvious layered structure appears along the profile,and the crustal velocity structures in different regions have obvious heterogeneity in the lateral and vertical directions. The crystalline basement near the Bohai Bay is gradually thinning southwestwards,and beneath the 220km Stake,the depth of G interface is 7.4km. The thickness of the middle layer varies greatly,with the change range up to 4.0km. The crustal depth varies relatively moderately,with a change range of about 2.0km. The Moho deepens gradually from coastal area to the inland along the southwest direction.

FOCAL MECHANISMS AND SEISMOGENIC STRUCTURES OF THE M_S4.6 RUICHANG-YANGXIN EARTHQUAKE OF 2011

ZHANG Li-fen, YAO Yun-sheng, LIAO Wu-lin

2013, 35(2):  290-299.  DOI: 10.3969/j.issn.0253-4967.2013.02.008

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Ruichang-Yangxin earthquake is another moderate-size earthquake in the Yangxin-Jiujiang area since the M5.7 Jiujiang-Ruichang earthquake in 2005.In order to have a better understanding of the seismic activities in this area,we investigate the moment tensor solution and the seismogenic structure of Ruichang-Yangxin earthquake. Precise earthquake relocation shows that the main shock occurred on the southwestern part of the NE-trending fault and aftershocks are distributed along both NNE and NW directions. By comprehensive analysis of the earthquake distribution,isoseismal features,focal mechanism,and regional structure characteristics,it is inferred that the this earthquake is caused by the southern segment of the NNE-trending Tanlu Fault(TLF).In addition,it has close relationship with the conjugated NW-trending fault as well.

MODERATE-STRONG EARTHQUAKES AND THEIR TECTONIC CORRELATION IN THE LIAODONG PENINSULA

WAN Bo, JIA Li-hua, DAI Ying-lei, SUO Rui

2013, 35(2):  300-314.  DOI: 10.3969/j.issn.0253-4967.2013.02.009

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Moderate-strong earthquakes happened very frequently in the Liaodong peninsula. In addition to the Haicheng M7.3 earthquake in 1975,there are also 19 earthquakes of M≥5 recorded in this area. The earthquakes are distributed regularly,mainly restricted to seismic belts or to the grid nodes. The spatial distribution is accordant to the NE-NNE trending faults such as the Jinzhou Fault,the Yalujiang Fault,and to the NW-trending conjugated ones. Moderate-strong earthquakes happened mostly near the tectonic basins where two sets of faults intersect,or close to the NW-trending tectonic belt. The NE-NNE and NW-trending faults are a pair of conjugate sheer fracture planes characterized mainly by lateral strike-slip movement under the action of the contemporary NEE-directed near-horizontal principal stress. The NE-NNE faults belong to the inheritance fault,which control the regional geological tectonic pattern and its evolution,often large in size; while the NW ones mostly belong to newly-generated fault,and their size is usually related to the size of NE-NNE faults they conjugate with. The study shows that the moderate-strong earthquakes in the Liaodong peninsula are controlled mainly by the NE-NNE structures,and the seismogenic structures are basically the NW faults. Meanwhile,correlated with the size of the NW faults,the moderate-strong earthquake activities show a trend of decreasing gradually from north to south and from west to east,and the magnitude of them decreases roughly from M7～7.5 to M5.5～6 or so.

THE TEXTUAL RESEARCH ON THE WANGTIAN’E VOLCANO ERUPTED ON OCTOBER 6,1597

LI Yu-che

2013, 35(2):  315-321.  DOI: 10.3969/j.issn.0253-4967.2013.02.010

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The Wangtian'e volcano is situated in the middle part of the Changbai Korean Autonomous County in Jilin Province,China,whose summit elevation is 2051.4m and 35km to the Changbaishan Tianchi(Mt. Baekdu Sky Lake)volcano. There are the historical records concerning the earthquake and volcano eruption occurring on October 6,1597 in the Korea historical document 'the Annuals of the Choson Dynasty’.The paper investigates the historical materials based on the old map of Korea and the local chronicles of the Samsu County of Hamgyong Province in Choson Dynasty,and suggests that the 1597 volcano eruption occurred in a chain of mountains at the bottom of the Wangtian'e basaltic lava platform located between the Shisandaogou Village and the Shisidaogou Town in the Changbai Korean Autonomous County,Jilin Province,China. The geographic position of the 6 October 1597 Wangtian'e volcano eruption is about 30km to the summit of Mt. Wangtian'e and about 60km to the Mt. Chanbai(Mt. Baedu)Tianchi Lake.

OSL DATING OF FAULT ACTIVITY IN THE WEST OF ULAANBAATAR

SUN Gao-yuan, GONG Zhi-jun

2013, 35(2):  322-327.  DOI: 10.3969/j.issn.0253-4967.2013.02.011

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Several faults are developed around Ulaanbaatar city. In this study,optically stimulated luminescence(OSL)dating is applied to the Emeelt and Hustai Faults in the west of Ulaanbaatar,in order to evaluate the hazards around Ulaanbaatar. Totally,two profiles are studied: the profile ET for the Emeelt Fault and the profile KI for the Hustai Fault. The fault-related sediments in the two profiles are dated by OSL dating to infer the date of the past fault activities. The OSL dating results for samples from profile ET suggest that the age of the last fault movement is between 9.6±1.0ka and 11.3±1.3ka. The deformed strata and the Heilu soil in the fault scarp on the Hustai Fault suggest a maximum age of 24.2±4.9ka and a minimum age of 0.87±0.08ka for the faulting.

Special Review

RHEOLOGICAL COMPLEXITY OF CONTINENTAL LOWER CRUST BASED ON CREEP TESTS OF MAFIC ROCKS

ZHOU Yong-sheng

2013, 35(2):  328-346.  DOI: 10.3969/j.issn.0253-4967.2013.02.012

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It is crucial to research on creep tests of granulites in understanding rheology of lower crust,continental dynamics and seismogenic environment. However,based on analysis of creep data of mafic rocks,it was found that most of experimental studies of mafic rocks focused on creep of single-phase and two-phase aggregates,and creep data of mafic granulite is very limited. There are a lot of factors affecting rheology of mafic granulite. Besides the external causes of experimental conditions(temperature,pressure,strain rate),the internal causes,such as mineral components,grain size of samples,trace water,partial melt,mineral reactions,are very important influencing factors,which induced the results of creep tests on mafic rocks to be more complex. Therefore,there are a lot of uncertainties in discussing the rheological structures of lower crust according to creep data of single-phase and two-phase aggregates,which could not fulfill the requirement for studying lower crustal rheology. The calculation of the rheological strength of granulite constituted by multi-phase minerals based on end-member flow law parameters and empirical equation is only a kind of simple approximate,which could not replace experimental studies on rheology of granulite. The development trend of the experimental study on mafic rocks in the future will be to perform creep tests of multi-phase natural mafic granulite and obtain flow laws of samples which can be used to study the rheology of continental lower crust quantitatively. The scientific questions and technological problems during experimental studies of creep tests of mafic rocks are multi-phase mineral rheology,partial molten under high temperature and interactions of mineral reaction and rheology,and all of those need more detailed experimental studies under high temperature and pressure.

APPLICATION OF SATELLITE REMOTE SENSING IN VOLCANIC ASH CLOUD MONITORING

YIN Jing-yuan, SHEN Di, LI Cheng-fan

2013, 35(2):  347-362.  DOI: 10.3969/j.issn.0253-4967.2013.02.013

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A large volcanic eruption can produce large amounts of volcanic ash,water vapor and heat,and form the volcanic ash cloud. The volcanic ash cloud is mainly composed of volcanic ash debris in diameter less than 2mm and gases including SO₂, H₂S,CO₂,the mixture of the two can form acidic aerosols which can stay in the atmosphere for a long time. It not only destructs the balance of earth's surface solar radiation and causes the depletion of the ozone layer,the greenhouse effect,air pollution,acid rain,anomalies of air temperature and precipitation,and other major global climate and environmental changes,but also damages and corrodes the structure of an aircraft,reduces the visibility and jams the radio communication system. The most serious problem is that the volcanic ash debris particles are capable of cooling and adhering to the aircraft engine blades after high-temperature melting,resulting in the flameout of aircraft engine. Under the background of globalization and the boom of air-transport industry,the volcanic ash cloud is a serious threat to aviation safety. Remote sensing technology can quickly and accurately obtain the information of the surface's and the atmosphere's changes,therefore it is playing an important role in monitoring volcanic activity. In recent years,with the advancement of sensor technology,the thermal infrared remote sensing technology has become an important means of monitoring the volcanic ash cloud. Currently,there have been a variety of remote sensors for volcanic ash cloud monitoring. Meanwhile,based on that,a series of volcanic ash cloud monitoring algorithms have also been developed for different remote sensors. However,most of the volcanic ash cloud monitoring algorithms have limitations of a low accuracy and a narrow scope. This paper tries to conduct a more comprehensive overview of the different types of remote sensors and the different algorithms for volcanic ash cloud monitoring. First,the damage of volcanic ash cloud to the natural environment and aviation safety is expounded; secondly,with the volcanic ash cloud remote sensing monitoring platform,this study systematically introduces the remote sensing sensor types of volcanic ash cloud remote sensing monitoring and the development situation of monitoring methods,and focuses on the moderate resolution imaging spectradiometer(MODIS)sensor which has better volcanic ash cloud monitoring effect; then,the research foundation of volcanic ash cloud remote sensing monitoring in China is reviewed; and finally the prospect and thoughts of their further development are discussed.

Academic discussion

PETROGRAPHIC AND MICRO-TEXTURAL FEATURES OF THE YELLOW PUMICE AT TIANWEN PEAK OF CHANGBAISHAN TIANCHI VOLCANO

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

2013, 35(2):  363-370.  DOI: 10.3969/j.issn.0253-4967.2013.02.014

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A series of deposits with tens of meters in thickness and in black,grey and yellow colors,from explosive eruptions in the Holocene,are developed at Tianwen peak,the north summit of Tianchi caldera of Changbaishan volcano. Among the deposits,a relatively large scale unit in yellow color,so-called "yellow pumice" by the previous studies,is characterized by coarse tephra and well-developed pores with large-sized trachyte blocks. In this study,we analyzed the petrographic and micro-textural features of the yellow pumice,as well as the grey pumice which is believed to be produced by the millennium eruption in 946 AD.Our results indicate that: 1)the yellow pumice can be classified into alkaline rhyolite as like the grey pumice,but is slightly more basaltic than the grey pumice,suggesting that they all are probably from the identical magma source; 2)remarkably similar as the grey pumice in texture,the yellow deposits are pumiceous with abundant pores in various sizes and few phenocrysts around which moniliform pores are well developed with typical flowing features; 3)parameters of feldspar of the yellow pumice from SEM and X-ray diffraction by predecessors are distinguishable from that of the other pumice,indicating that the yellow pumice is presumably produced by the isolated explosive eruption event. In all,our findings provide strongly petrographic evidences to support the conclusion that the yellow pumice deposits at Tianwen peak of the volcano summit are the product of an explosive eruption prior to the millennium eruption of Changbaishan volcano.

Application of new techniques

CONFIGURATION AND APPLICATION OF TEM OUTSIDE-LOOP SOUNDING

HE Jian-guo, LEI Yang, XI Zhao, LI Ji-an

2013, 35(2):  371-379.  DOI: 10.3969/j.issn.0253-4967.2013.02.015

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In this paper we introduce a new kind of TEM sounding configuration: Outside-loop sounding configuration,which measures a sounding spot 20～25 meters outside the loop instead of directly measuring with central-loop sounding. The characteristic of the outside-loop sounding configuration is that during high-precision sounding survey,this method can massively avoid the influence from the self-inductance of the transmitter loop compared to central-loop sounding,thus, can raise the resolution and the exploration depth of the survey. By comparative tests with central-loop sounding configuration and application to a sounding survey carried out in Turpan-Hami Basin,Xinjiang,we find that the method is effective and shall be put in use in future.

EXPERIMENTAL STUDIES OF PRECISE CALIBRATION OF GPS ANTENNA PHASE CENTER VARIATION

CHEN Tao, HU Zhi-gang, LI Tao

2013, 35(2):  380-387.  DOI: 10.3969/j.issn.0253-4967.2013.02.016

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In the high-precision GPS positioning applications,the antenna phase center calibration significantly impacts the survey accuracy. This article introduces the experiment study of precise calibration of GPS antenna phase center variations based on automatic survey robot GPS which is funded by Crust Movement Observation Network of China project. In this paper,the main derivations of the principle and implementation procedure are described step by step. Comparing with the known calibration parameters,the horizontal accuracy is estimated about 2mm and the vertical accuracy is estimated about 3mm. This study is of practical significance to improve the accuracy of GPS positioning and to popularize the application of calibration of antenna phase center variation based on survey robot.

CONTENTS

THE SURFACE RUPTURE SIGNS OF THE LUSHAN "4.20"|M_S 7.0 EARTHQUAKE AT LONGMEN TOWNSHIP, LUSHAN COUNTY AND ITS DISCUSSION

HAN Zhu-jun, REN Zhi-kun, WANG Hu, WANG Ming-ming

2013, 35(2):  388-397.  DOI: 10.3969/j.issn.0253-4967.2013.02.017

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The co-seismic surface rupture signs of the "4.20" Lushan M_S7.0 earthquake are found at Longmen Township,Lushan County. The sites of rupture signs have a linear distribution with a 2～3km length and N40°～50°N strike. The maximum shortening of the rupture is about 8cm,uplifting is about 1～2cm. Strike-slip component is not observed,but the dynamic process of the earthquake is characterized by compression from northwest to southeast. The observed co-seismic surface ruptures can be oblique shear-fissure,or thrusting crack,however most of them are extensive fissures,which can be explained by the local extensive stress-field on the top of the thrust bending. Although these ruptures have different geometric shapes or variant mechanic features,they similarly reflect the northwest-southeast compression and the surface lift-bending on the top of a thrusting seismogenic structure. Comparing with Dachuan-Shuangshi Fault(frontal fault)and Dayi-Mingshan Fault(piedmont fault),Lushan-Longmen presumed blind fault is more likely the seismogenic fault,which is also consistent with the results of the Lushan earthquake sequence relocations and the seismic intensity contours. As the seismogenic fault of the Lushan earthquake has surpassed the frontal fault of Longmen Shan,it may be a new-generated tectonics,which implies that it is important to re-evaluate the seismic risk at the piedmont area of the Longmen Shan. However,the conclusions are still very primary and geophysical survey is needed to demonstrate the existence of the Lushan-Longmen presumed blind fault.

Monographic papers on the April 20, 2013 M_S 7.0 Lushan earthquake

ANALYSIS ON THE CHARACTERISTICS OF THE M_S7.0 LUSHAN, SICHUAN PROVINCE,EARTHQUAKE HAZARD ON APRIL 20,2013

LI Zhi-qiang, HOU Jian-sheng, LI Yang, SU Gui-wu, JIANG Li-xin, SUN Bai-tao, YUAN Yi-fan, LIN Jun-qi

2013, 35(2):  398-410.  DOI: 10.3969/j.issn.0253-4967.2013.02.018

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At two past eight a.m.on April 20,2013,an earthquake measured 7.0 in Richter scale,whose focal depth is 13km deep,hit Lushan County(30.3 degrees north,103.0 degrees east)of Yaan City of Sichuan Province and caused 196 deaths. After the quake,China Earthquake Administration declared a first-grade emergency response. Though Lushan County has a larger population than Yushu County,the casualties caused by Lushan earthquake are far fewer than that caused by Yushu earthquake of the same size. The quake-affected area had also been hit by Wenchuan earthquake. After Wenchuan earthquake,most of the damaged houses in Lushan County were reinforced or directly pulled down and thus newly-built buildings make up a high proportion. The seismic performance of the newly-built rural houses is largely improved due to the various anti-seismic measures. As a result,few of them were totally collapsed in Lushan earthquake,which is the key reason of comparatively fewer casualties. However, the newly-built rural buildings failed to strictly meet the national anti-seismic requirements issued after Wenchuan earthquake, brought heavier damage than it should be. The goal of the first phase of earthquake disaster mitigation is to reduce casualties,and we made it in Lushan earthquake. But the rural buildings with fewer anti-seismic measures for earthquake mitigation were heavily damaged in Lushan earthquake and we still have a lot to do for the goal of property loss reduction.

INELASTIC TRIGGERING OF THE 2013 M_W 6.6 LUSHAN EARTHQUAKE BY THE 2008 M_W 7.9 WENCHUAN EARTHQUAKE

LEI Xing-lin, MA Sheng-li, SU Jin-rong, WANG Xiao-long

2013, 35(2):  411-422.  DOI: 10.3969/j.issn.0253-4967.2013.02.019

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Stress changes due to the co-seismic slip on the source fault of the 2008 M_W7.9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake,seismicity shows progressively increasing in a wide region. However,the south segment of the Longmenshan Fault did not show any significant change in seismicity,where positive Coulomb failure stress change(ΔCFS)was estimated under the elastic half-space model. Under such a background,the 2013 M_W6.6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0.2～0.4bar,a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M6.2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area,which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake,which ruptures the entire gap,may occur in the near future if there are no other unknown factors.

STRONG GROUND MOTION DISTRIBUTION AND SIMULATION BASED ON FINITE FAULT MODEL OF LUSHAN 7.0 EARTHQUAKE ON APRIL 20,2013

ZHANG Dong-li, HUANG Bei, ZHANG Xian-bing, XU Xi-wei, ZHENG Wen-jun

2013, 35(2):  423-435.  DOI: 10.3969/j.issn.0253-4967.2013.02.020

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Field investigation and damage evaluation of the Lushan M7.0 earthquake have revealed that the seismogenic fault of this earthquake is a typical blind fault with thrust component and there is no distinct surface rupture or deformation zone. The earthquake caused severe damage and failure such as mountain landslide,bedrock collapse,sand liquefaction on near-fault region and tensional fractures. In order to estimate the influence of strong ground motion on damage distribution,based on the inversion of slip distribution and rupture process on the source fault of the Lushan 7.0 earthquake,strong ground motion simulation is carried out with finite-fault model and three-dimension crust model of Lushan area and its adjacent region. In the finite-fault model,the rupture source is characterized as a low-angle fault plane with inhomogeneous slip distribution. The maximum slip on fault plane is up to 150cm. For the three-dimension crust model,deep fault structure,steep terrain and basin have been taken into account and described by different physical parameters. In this paper,the numerical simulation results of strong ground motion about Lushan main earthquake reveal two following major characters. The first is that the distribution characteristics of peak acceleration values,peak velocities and peak displacements on the ground surface shows good consistency with the seismic damage investigation. On the hanging-wall of the causative fault,high intensity of strong ground motion mainly concentrates on Baosheng,Longmen and the northern area of Lushan,which are located within the IX meizoseismal area. Around the area of Longmen town,the maximum acceleration of UD component reaches up to 350gal and the maximum transient displacement is up to 110cm,which are consistent with recordings and investigations. The second conclusion from simulation results is that the strong ground distribution and propagation process are influenced by basin effect and steep terrain. Seismic wave propagated back and forth in intermountain basins,combining with the amplification of thin soil layers,which directly caused and increased the earthquake damage.

SPATIAL ANALYSIS OF LANDSLIDES TRIGGERED BY THE 2013 M_S7.0 LUSHAN EARTHQUAKE: A CASE STUDY OF A TYPICAL RECTANGLE AREA IN THE NORTHEAST OF TAIPING TOWN

XU Chong, XIAO Jian-zhang

2013, 35(2):  436-451.  DOI: 10.3969/j.issn.0253-4967.2013.02.021

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In this paper,a rectangle area of 20km?10km at the northeast of Taiping Town,which suffered strong shaking during the April 20,2013 M_S 7.0 Lushan earthquake,was selected as the study area for spatial analyses of landslides triggered by the earthquake. Landslide distribution map of the study area was prepared based on quick field investigations and visual interpretation of high-resolutions aerial photographs. It is showed that at least 688 landslides were triggered by the Lushan earthquake and the landslide number density(LND)of the study area is 3.44 landslides/km².Correlations of landslide number density with topographic,geologic and seismic parameters were analyzed based on the landslide inventory. The results show that the steeper the slopes,the greater the landslide number density values; the highest LND value appears at ranges from 1 600m to 1 800m in elevation. The landslides have preferred orientations,dominated by the east and SE directions. LND values of convex slopes are relatively higher. The limestone and dolomite of the Yangxin Group,Permian(Py)and granitic rocks of Proterozoic(Pt)experienced more concentrated landslides. In general,the higher the PGA value and seismic intensity zone,the greater the LND value. Correlation of landslide number density with distance from the Shuangshi-Dachuan Fault shows that there was not sudden change of LND value near the fault. The factor interaction statistics show that the slope angle and PGA affect the occurrence of earthquake-triggered landslides independently.

CHARACTERS OF SURFACE DISPLACEMENT DUE TO BLIND CURVED FAULT REVEALED BY NUMERICAL MODELS

ZHANG Zhu-qi, CHEN Tao, REN Zhi-kun, WANG Wei-tao

2013, 35(2):  452-460.  DOI: 10.3969/j.issn.0253-4967.2013.02.022

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A possible three-dimensionally highly-curved fault,suspected as the ruptured structure of the Lushan M7.0 earthquake,is revealed by relocated aftershocks. A recent study shows that obvious differences exist between curved fault and straight fault under the ground in regard to dislocation patterns and co-seismic stress responses on the planes ruptured during an earthquake. Infinite half-space dislocation models reveal that the characters of surface displacements due to a curved fault are similar to that from straight reverse fault as a whole. Nevertheless,the horizontal displacements due to slip on a curved fault show closer trend parallel to the direction of regional shortening and higher magnitude than that from a straight fault. Subsequently,the curved fault is suggested to be more capable of transferring horizontal movement of hanging-wall materials in large area. Relative to the case on a straight fault,horizontal displacement in foot-wall area of a curved fault decays more with distance from source fault. On the other hand,the curved fault generates obviously less co-seismic uplift while larger and more extensive surface drop somewhere than the reverse fault or left-lateral reverse fault of the same size but with straight fault planes does. For relatively small magnitude of main shock,it is not easy to determine whether the structure of rupture during Lushan earthquake is highly-curved fault or not due to the sparse observations on co-seismic deformation like GPS.Dense and high-resolution observations should be required to survey the features of focal structure in detail.