Table of Content

30 September 2012, Volume 34 Issue 3

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CONTENTS

KEY TECHNIQUES AND SEVERAL CASES ANALYSIS IN PALEOSEISMIC STUDIES IN MAINLAND CHINA(2): SURFACE DEFORMATION CHARACTERISTICS OF WENCHUAN EARTHQUAKE AND PALEOSEISMIC INDICATORS ON FOLD-REVERSE FAULT

RAN Yong-kang, CHEN Li-chun, CHEN Wen-shan, WANG Hu, LI An

2012, (3):  385-400. 

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The M_S8.0 Wenchuan earthquake is a rare earthquake of fold-reverse fault type in mainland China. The rupture zone of Wenchuan earthquake is an indispensable case which can be used to study the surface co-seismic deformation of reverse fault and discuss the paleoseismic records. Based on the geologic and geomorphic features along the rupture zone of Wenchuan earthquake,we choose the Pingtong,Dengjia and Liulong three sites which were deformed only in the Wenchuan earthquake to analyze the characteristics of co-seismic deformation. And we combine with paleoseismic studies in the Yingxiu and the Guixi areas to discuss some key techniques in the paleoseismic study of the fold-reverse fault type. The conclusion shows surface deformation types include fault dislocation,bending dislocation and fold deformation. The cut-cover relationship of colluvial wedge,fault and strata is the feasible evidence of the fault dislocation type. But the indicator for bending dislocation and fold deformation types emphasizes the unconformity on the hanging wall,growth strata on the footwall and sudden change of maker strata position between the hanging wall and footwall. The multiple relationship of fault scarp height is related to paleoseismic times in some degree,but it is not wise to decide the paleoseismic times simply by a direct division of the height of fault scarp by the co-seismic displacements. In addition,there are two important paleoseismic indicators for identifying paleoseismic event on low-angle thrust,that is,the sudden change of displacements in maker strata and the cut-cover relationship of thin-long colluvial wedge,fault and strata. A good paleoseismic study needs to consider many factors and use more evidences with consideration of local conditions to support or supplement the analysis.

THE ACTIVITY AGE OF TARWAN FAULT AND GENESIS OF THE TOPOGRAPHIC SCARP

CHEN Tao, LIU Yu-gang, MIN Wei, ZHOU Ben-gang

2012, (3):  401-414. 

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Many NW-trending faults have been developed on the north of the eastern segment of Altyn Tagh Fault. The Tarwan Fault,about 10km long and striking NW on the whole,is the western segment of the largest Tarwan-Dengdengshan-Chijiaciwo Fault among these faults. The fault appears as a straight linear scarp in the satellite image and a geomorphic scarp of dozens of centimeters to 5 meters high,topographically. The scarp dips NE and is composed mainly of beds of early Pleistocene conglomerate and Holocene aeolian sandy soil. As revealed by a measured topographic profile,the scarp composed of Holocene aeolian sandy soil is about 5m high,and that of early Pleistoscene conglomerate is about 1m high. Field investigation and trenches excavated on the vertical scarp have revealed the Tarwan Fault is a thrust fault,striking NW and dipping SW.The Geogene mudstone is thrust over the early Pleistocene conglomerate,with a throw of 0.5m. The Holocene aeolian sand and late Pleistocene gravel layers overlying the fault are not dislocated. The hanging wall of the fault is Geogene mudstone with rich groundwater and well-developed vegetation. Due to the protection and control of sand movement with vegetation,aeolian sand was accumulated constantly and preserved,and as a result,the aeolian sand layer became higher gradually. The foot wall of the fault consists of a Gobi gravel layer of a few centimeters thick on the surface and hard cemented conglomerate of early Pleistocene under it,with groundwater and vegetation being undeveloped. Therefore,Holocene aeolian sand is only developed on the hanging wall of the fault,and there is no Holocene stratum developed in the footwall. The height of the scarp formed on the early Pleistocene conglomerate is far lower than that on the Holocene aeolian sand. These findings indicate that the topographic scarp composed of Holocene aeolian sand was produced by external dynamic process rather than faulting,and that the Tarwan Fault is an early-middle Pleistocene thrust fault.

DETERMINATION OF FAULT PARAMETERS AND SLIDING BEHAVIOR OF THE 1879 SOUTHERN WUDU M8.0 EARTHQUAKE

LIU Bai-yun, YUAN Dao-yang, ZHANG Bo, CHEN Wen-kai, NIU Yan-ping

2012, (3):  415-424. 

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The southern Wudu M8.0 great earthquake occurred in 1879,many places in Wudu and adjacent areas had suffered destruction in various degrees. So far,the research results on the causative structure of this earthquake are less and inconsistent. Because it occurred in lofty mountains and the traffic is inconvenient,it is hard to make detailed field study on the earthquake site. Based on the assumption that clustered small earthquakes often occur in the vicinity of fault plane of large earthquake,and referring to the morphology of the major axis of the meizoseismal area obtained by the predecessors,we selected a strip-shaped zone from the relocated earthquake catalog which occurred near the earthquake rupture zone in the period from 1985 to 2009 to calculate fault plane parameters of the earthquake,such as strike and dip,with the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms. On this condition,the rake angles of the fault plane are further inferred from regional tectonic stress parameters. We discussed the causal mechanism of the earthquake and finally identified the length and location of the seismogenic fault. In addition,clustered small earthquakes occurred frequently in the Xionghuangshan area west of the mezoseismal area,but we didn't find clear fault planes in field investigation,so,they should not have relation with the 1879 M8 southern Wudu earthquake.

THE IMPACT OF 2011 TOHOKU-OKI EARTHQUAKE IN JAPAN ON CRUSTAL DEFORMATION OF NORTHEASTERN REGION IN CHINA

CHEN Wei-tao, GAN Wei-jun, XIAO Gen-ru, LIANG Shi-ming, SHENG Chuan-Zhen

2012, (3):  425-439. 

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This paper studies the impact of the M_W9.0 Tohoku-Oki earthquake,March 11,2011,Japan on the crustal deformation,active fault movement and earthquake risk in northeastern China by using continuous GPS data. The result shows that the direction and amount of principal strain is different between coseismic and interseismic strain field. The Japan earthquake mainly affects the level of strain accumulation in northeastern China. The coseismic strain accumulation is equivalent to about 12.7-years long-term background strain accumulation. In other regions,the impact of coseismic strain on the background strain rate isn't obvious. Based on the coseismic strain and background strain rate,we select 4 major active faults for the analysis by using the relative movement time sequence of endpoints of GPS baselines which cross the Tanlu Fault,Zhangjiakou-Penglai Fault and Taihangshan piedmont fault,respectively. We find that the earthquake produced obvious left-lateral slip and extensional slip on the north segment of Tanlu Fault,and there is a tendency of slight continuous left-lateral movement and extension after the earthquake. But on the middle segment of Tanlu Fault,there is no obvious evidence of coseismic movement,but a slight increase of the right-lateral slip rate after the earthquake. As for Zhangjiakou-Penglai Fault located in North China,the fault was characterized by obvious right-lateral movement and extrusion in about one year after the earthquake, then resumed quickly to the original left-lateral movement and extension. This means that the fault's accumulated stress was released temporarily by the earthquake,then the regional stress resumed to its original level. The movement of Taihangshan piedmont fault isn't affected by this earthquake,therefore we infer that the earthquake affected areas did not spread to the Shanxi Rift System. In one word,the crustal strain accumulation and stress level of northeastern region in China is released by Tohoku-Oki earthquake,so we believe that the earthquake risk is reduced to a certain extent.

MAARS IN XILINHOT-ABAGA VOLCANIC CLUSTER

WANG Xi-jiao, BAI Zhi-da, TAN Lin, WU Zhi-li, WANG Yan

2012, (3):  440-448. 

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Xilinhot-Abaga volcanic cluster is located in Xilin Gol League of Inner Mongolia Autonomous Region,and it is in the centre of Daxinganling-Datong volcanic eruption belt. There are more than 300 Quaternary basaltic volcanoes of different types. Maars are found for the first time there,with Haotewula in the southeast of Abagaqi,Chelewula and Esigewula in the northwest of Abagaqi being the most representative. They all have large scale,and the diameter of cone is generally 3～4km,the largest is up to 6.5km. They have relatively intact structure,similar two-ringed mountain landscape and obvious phreasic eruption which contains strong phreatomagmatic eruption at the earlier stage,later weak magmatic eruption and at last basaltic lava overflowing phase. This eruption sequence reflects the interaction between magma and groundwater,the change in intrusive speed and the flow-out rate of magma. The sand and gravel of Middle Pleistocene depositing in the valley are covered by basal surge deposits. The TL age of sandstone-shale inclusion which was found in the spattering deposits close to crater is(0.112±0.0096)Ma,which indicates that the eruption times of the maars are early Late Pleistocene.

ACCURACY VERIFICATION OF SEISMIC PHASE ARRIVAL TIMES OF THE MAY 12 WENCHUAN EARTHQUAKE RECORDED BY ZIPINGPU RESERVOIR SEISMIC NETWORK

HAN Jin, XIE Rong-hua

2012, (3):  449-455. 

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The Zipingpu Reservoir Seismic Network is the nearest network to the epicenter of the M_S8.0 Wenchuan earthquake and it recorded the rare data of this earthquake event. In order to discuss the accuracy of these data,we compared the seismometers used in this network in terms of the types,hardware components and performance index with the US RefTek seismometer and analyzed the processing results of the earthquake records for the verification. Results show that the seismometer of the network has a maximum error of <1.8ms. There is a difference of 0.02s in the phase picking result of the M_S 8.0 Wenchuan earthquake from the recordings of this network with that from the US RefTek recordings. And compared with the earthquake occurrence time determined from the recordings of the Sichuan seismic network,the maximum difference is 0.48s,and the minimum -0.03s. The time-history curve plotted using P-wave arrivals and epicentral distances of 21 stations(including the network stations)has a good linearity. This indicates that the error generated from the seismometers used in Zipingpu network is far less than the error from phase picking,and the impact of it is also smaller than that caused by the heterogeneity of medium in seismic wave propagation. The time accuracy of the seismometer used in this network can satisfy completely the needs of modern earthquake analysis.

CONTENTS

THE INFLUENCE OF THE EARTH'S CURVATURE ON THE LONG-PERIOD MAGNETOTELLURIC SOUNDING METHOD

QIN Qing-yan, LUO Wei, ZHANG Wei

2012, (3):  456-466. 

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Based on the electromagnetic theory,the forward formula is obtained for the long-period magnetotelluric(LMT)method in a layered spherical earth. The computer program for computing the LMT response is worked out and the forward modeling is completed for some theoretical models. According to the results of forward modeling,we analyzed how the earth's curvature influences the electromagnetic response. The results show that the apparent resistivity will decline and the phase will increase with an increased probing depth. However,the impact of the Earth's curvature can be ignored in the case of one-dimensional base regarding the measurement accuracy of the LMT instrument at present; But with the development of instrument,it is necessary to take the earth's curvature into account in the future.

APPLICATION OF ELECTRICAL RESISTIVITY TOMOGRAPHY TO THE DETECTION OF PUDU RIVER FAULT IN YUXI BASIN

ZHU Tao, ZHOU Jian-guo, SHEN Kun, JIN Zhi-lin

2012, (3):  467-476. 

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Yuxi basin is one of the potential earthquake risk areas with high seismic frequency and intensity in Yunnan Province. In order to set up the "standard" structure model to be applied directly in earthquake prevention and disaster reduction,we used electrical resistivity tomography(ERT)to detect the electrical structure of mountain-basin transition zone in Yuxi basin and Pudu River Fault which is buried in and goes through the western margin under the support of the Special Project for the Fundamental R&D of Institute of Geophysics,China Earthquake Administration. In data acquisition,6 ERT measuring lines,as an important supplement to the seismic reflection survey,were designed to control the fault,and Wenner α arrays were used,with the electrode spacing of 6m,maximum depth factor of 30～75 and the electrode number of each array being 112～288.Our results indicate that Pudu River Fault in Yuxi basin consists of two NNE-striking branches,the east branch and the west branch. From south to north,the width of the fault zone becomes wider from Wenquan town to Heicun Village at first and then becomes narrower through Lianchi and Meiyuancun Village to Yukun Iron & Steel Co. Ltd. Its hanging wall is a low resistivity fracture zone,while its footwall appears as a homogeneous high-resistivity zone. The east branch (F_1-2) has an apparent dip direction of SE/S near and to the north of Meiyuancun Village and of NW near and to the south of Lianchi Primary School,with the apparent dip angle of 56°～73°,while the west branch(F_1-1) has an apparent dip direction of SE/E/S and the apparent dip angle of 38°～66°.The seismic results showed that both of the east and west branches of Pudu River Fault have the apparent dip direction of SE/E and apparent dip angle of 75°～80° in Yuxi basin(He et al., 2012),which are slightly different from our results.

P-WAVE REFLECTION TIME-DISTANCE CHARACTERISTIC OF SHOT GATHER FROM 3D SEISMIC CURVED-INTERFACE

SONG Li-hu, HAO Chong-tao, YAO Chen, CAI Ming-gang

2012, (3):  477-487. 

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It's very difficult and time-consuming to calculate the precise location of the reflection points in the reflection time-distance simulation of 3D complex tectonic model. In order to analyze the P-wave reflection time-distance characteristic systematically and accurately,the paper regards 3D curved interface as the envelope of 3D dipping interface with different tendencies and inclinations,and presents a new method for calculating P-wave reflection time-distance from 3D curved interface,using the principle of reflection from 3D dipping interface. Based on the dip layer CDP theory,the location of shot points and reflection travel times are obtained from the positions of receivers and surface CDPs,finally the 3D P-wave reflection records of shot gather are received,assuming that the distribution of interface reflection points is known. Numerical simulation results of shot gather show that this method has high computing efficiency and can be applied to analysis of the P-wave reflection time-distance characteristic with different positions of shots; especially it can reveal the relation between reflection points and time-distance curve. The reflection time-distance feature comparison of 3D curved interface and 3D dipping interface shows that it's very hard to distinguish time-distance characters between two types of interfaces only from the same survey line orientation,and this problem can be solved by comparing time-distance differences from different survey orientations.

RESEARCH ON THE CHARACTERISTICS OF ANOMALIES IN ELF ELECTROMAGNETIC OBSERVATIONS IN YUNNAN PROVINCE

SUN Wei-huai, TANG Ji, ZHANG Ping, YANG Xue-hui

2012, (3):  488-499. 

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Located in an earthquake-prone region,the geological structures in Yunnan Province are complex. Taking into account that Tonghai County is located at the intersection of Xiaojiang Fault and Honghe Fault,an F-4M ELF electromagnetic instrument was installed at Tonghai seismic station,which has produced continuous reliable data. The author collected the data and information of the year 2009 & 2010,and did analysis on the variation characteristics of both geomagnetic fields and electrical resistivity. The result shows that the 1Hz and 39Hz electromagnetic power spectra are 0.2 to 1.4 orders of magnitude higher than the normal values immediately before many earthquakes. The anomalies are represented by the abrupt changes of the electric and magnetic field power spectra in earthquake and aftershock sequences,and the amplitude of change is related to the size of earthquake magnitude and the epicentral distance. The electrical resistivity also changes obviously. So,further research on the anomalous characteristics of ELF electromagnetic data will be meaningful to the use of this instrument in earthquake prediction in the future.

THE METHOD OF LATENT DEVELOPMENT LEVEL OF EARTHQUAKE-PREGNANT STRUCTURES AND THE ANALYSIS OF RELEVANT SEISMOGEOLOGICAL INFORMATION

HUANG Xiu-ming

2012, (3):  500-515. 

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The approach of latent development level of earthquake-pregnant structures in the crustal movement(LDLEPS)is a seismogeologic dynamic indicator for evaluating seismic risk quantitatively,developed based on the net deformation ratio using GIS techniques. The following concerns are addressed in this paper,including the working principle and applying conditions for LDLEPS,the drawing of LDLEPS maps,the relationship between the distribution characteristic of LDLEPS and earthquake in the North China region,and the seismogeologic analysis of the LDLEPS anomaly areas in North China. An example is presented of the use in forecasting earthquakes with LDLEPS method,and more discussions are made on the origin of formation of LDLEPS.Contrasting the spatial distribution of LDLEPS with earthquakes of 1966-1998 in the North China region in Fig 1,we found that they match and correspond completely to each other in terms of lineament,location,and size. This demonstrates that the LDLEPS method can be used in quantitative evaluation of seismic risk. To verify this conclusion,we drawn the LDLEPS maps of Sichuan and Qinghai regions and used them to forecast experimentally the earthquake trend in these regions in 2006,and the afterward outcomes have proved that the forecast is basically correct,as shown in Fig. 2 and 3.As a consequence,the 2008 M8 Wenchuan earthquake in Sichuan Province and the 2010 M7.1 Yushu earthquake in Qinghai Province just happened in the expected abnormal LDLEPS regions and times,respectively. Formation mechanism of earthquake-pregnant structures is discussed in detail in the paper. According to the geophysical exploration and seismogeologic survey data,a bilayer-structured earthquake-generation model is proposed,derived from the crustal thermal processes and tectonic reworking resulting from the late-stage evolution of sub-mantle plumes and mantle-branch structures in North China,i.e.the semi-plastic rheologic layer or lower layer in the middle crust and the rigid layer or upper layer in the upper crust,and that the dynamic of the lower layer induces the formation and development of the seismogenic structures in the upper layer,causing tectonic deformation and generating earthquakes. Based on LDLEPS maps,we divide the North China region into 6 seismogenic structure anomaly zones,as shown in Fig. 1.Of them,the anomalous seismogenic structure zone B,located in the northwestern Beijing,is rich in geothermal resources,where hot fluids upwell into the fault planes of the upper layer from the lower rheologic layer,reactivating the faults and generating the moderate and lower-magnitude earthquakes,with a causative mechanism similar to that of reservoir-induced earthquake. In the anomaly zone A of seismogenic structures(Bohai Sea area and its margins),the powerful driving force from the lower rheologic layer controls directly the generation and development of the upper seismogenic structures,producing moderate and lower-size earthquakes. Researches on palaeoseismic traces from trench profiles and seismites in marine sediments reveal that there are many strong seismic events with magnitude ≥6 and palaeotsunami events in this region in the prehistoric times. The nowadays high level of LDLEPS and strong earthquake events are the continuation and repetition of their past activities. It is also noted that there are obvious differences in structural landforms and earthquake trends between the South and North China blocks owing to difference of their tectonic background. Having the advantage of novelty,uniqueness,visibility,operability,effectiveness and computerization,the LDLEPS method can be widely applied to areas of earth science research,earthquake prediction and engineering.

CRYPTOTEPHRA DISCOVERED IN GUSHANTUN PEAT OF NE CHINA AND ITS SIGNIFICANCE

ZHAO Hong-li, LIU Jia-qi

2012, (3):  516-530. 

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Gushantun,200km to Tianchi volcano,is located in the Longgang volcanic area of northeastern China. Cryptotephra were discovered in the sediment at 40～43cm depth,and the particular shape,such as bubbles,flat and sharp rim,suggests that the tephra shards are the result of direct sinking to the bottom rather than the one of secondary transportation. Electron microprobe analysis showed that the volcanic ash was derived from rhyolitic lavas which are different from the characteristics of the Longgang area volcanism in Quaternary.
The age of the bed at depth 40～43cm in Gushantun is 260～420cal a BP calibrated by using AMS ¹⁴C.This is very close to the Tianchi volcano eruptions which happened in 1702,1668 and 1597AD.Moreover,these three eruptions are all rhyolitic eruptions which have the similar characteristics with tephra shards detected in Gushantun. All of the information shows the Gushantun shards could come from Tianchi 1702,1668 or 1597AD eruptions. There are few records about these eruptions,however,they happened after the millennium eruption of Tianchi volcano(1199-1200AD)and have geochemical characteristics very close to that of the millennium eruption. In the future work,if the tephra shards of these three eruptions are discovered in the sediments,we can do more careful work as tephra subsampling and extracting in the underlying layers to look for the possible Tianchi tephra. This will be very helpful for searching tephra shards of the Tianchi millennium eruption in the research area. If more Tianchi tephra shards are discovered in Longgang volcanic area,the firm tephrochronology and tephrostratigraphy frame can be established to provide the time basis for stratigraphic comparison and trace the evolution of volcano eruption. Moreover,the use of multi dating methods could provide precise and higher resolution time scale for recording the climatope,which is very significant for studying the past global changes.

SPATIOTEMPORAL PATTERN OF b-VALUE BEFORE MAJOR EARTHQUAKES IN THE SICHUAN-YUNNAN REGION

WANG Hui, CAO Jian-ling, JING Yan, LI Zhen

2012, (3):  531-543. 

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Because b-value can be used as an indicator to measure the level of crustal stress,the seismicity pattern recognition based on b-value is considered as one of the most intuitive methods for earthquake forecast research and experiments. The intense seismicity in the Sichuan-Yunnan region makes the area to be an ideal field for earthquake forecast experiment. We investigated the spatial and temporal variations of b-values before nineteen earthquakes with M≥6.5 in the Sichuan-Yunnan region from 1981 to 2008 using regional small earthquake data. The results show that spatial b-value variations portending M≥6.5 earthquakes in the Sichuan-Yunnan region are insignificant. Less than 50%of the major earthquakes occurred in the regions with low b-values relative to the entire study area. In terms of temporal evolutions,the b-values in seismic source regions showed a systematic drop before the occurrence of earthquake. They showed an obvious decreasing trend during the period one year prior to strong earthquakes in 58%of the seismic source regions. Combined with other geological and seismological data,the short-term seismicity data can provide certain enlightenment for the estimation of potential seismic hazard in specific regions.