Table of Content

20 September 2015, Volume 37 Issue 3

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THE RUPTURE MODEL AND TIME SERIES OF PALEOEARTHQUAKES OF THE HEJING THRUST FAULT-FOLD BELT IN THE NORTH MARGIN OF THE YANQI BASIN

LI An, YANG Xiao-ping, Yiliyaer

2015, 37(3):  661-674.  DOI: 10.3969/j.issn.0253-4967.2015.03.001

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Paleoearthquake study is a basic research that can be favorable for the understanding of deformation pattern, intensity and time scale of the fault structure. The Yanqi Basin is an intermountain basin located in the eastern part of southern Tianshan Mountains. The present-day tectonic stress field of the basin is dominated by compression with strike-slip component. Both the north and the south marginal faults are Holocene active faults. The Kaiduhe Fault on the southern margin is a strike-slip fault. The Hejing Fault on the northern margin is a neogenic thrust-fold belt dominated by thrust faulting and extending towards the basin. The Hejing thrust fault has the potential to generate M7 earthquake in the future. Therefore, it is important to study the rupture model and time series of paleoearthquakes of the Hejing thrust fault-fold belt. According to the surveys, the major thrust fault at the south limb of the Haermodun anticline thrusts inwards the basin, with a dip angle of 30°, producing three paleoseismic scarps in T₁ terrace and washland. We determine the time series of six paleoseismic events by the cut-cover relationship of marker stratums in five trenches, date the age of stratums and colluvial wedges by ¹⁴C and OSL dating method, and obtain the recurrence intervals of the paleoseismic events using progressive constraining method. The rupture models of the Hejing fault are summarized. The fault F₁ ruptured in every paleo seismic event, but fault F₂ only ruptured in event E, F₃ only ruptured in event D and E. In other words, Event D ruptured the three faults simultaneously, Event E ruptured two faults, and the other events only ruptured fault F₁. There exist both certainty and uncertainty in the rupturing of Hejing thrust fault in the paleo-earthquakes.

LATE PLEISTOCENE SHORTENING RATE ON THE NORTHERN MARGIN OF YANQI BASIN, SOUTHEASTERN TIAN SHAN, NW CHINA

HUANG Wei-liang, YANG Xiao-ping, LI An, ZHANG Ling, LI Sheng-qiang, YANG Hai-bo

2015, 37(3):  675-696.  DOI: 10.3969/j.issn.0253-4967.2015.03.002

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How strain is distributed and partitioned on individual faults and folds on the margins of intermontane basins remains poorly understood. The Haermodun(Ha) anticline, located along the northern margin of the Yanqi Basin on the southeastern flank of the Tian Shan, preserves flights of passively deformed alluvial terraces. These terraces cross the active anticline and can be used to constrain local crustal shortening and uplift rates. Geologic and geomorphic mapping, in conjunction with high-resolution dGPS topographic surveys, reveal that the terrace surfaces are perpendicular to the fold's strike, and display increased rotation with age, implying that the anticline has grown by progressive limb rotation. We combine ¹⁰Be terrestrial cosmogenic nuclide(TCN) depth profile dating and optically stimulated luminescence(OSL) dating to develop a new chronology for the terraces along the Huangshui He since 550ka. Our in situ ¹⁰Be dating of fluvial gravels capping strath terraces suggests a relationship between the formation and abandonment of the terraces and glacial climate cycles since the middle-late Pleistocene. These data indicate that the formation of the four terraces occurred at ~550, ~430, ~350, and~60ka. We suggest that episodes of aggradation were facilitated by high sediment supply during glacial periods, followed by subsequent incision that led to abandonment of these terraces during deglaciation. Combining uplift and shortening distance with ages, we found the vertical uplift gradually decreased from 0.43 to 0.11mm/a, whereas the shortening rate was constant at ~0.3mm/a since the anticline began to grow. The shortening rates of the Ha anticline from geomorphology agree with current GPS measurements, and highlight the importance of determining slip rates for individual faults in order to resolve patterns of strain distribution across intermontane belts.

CONTRIBUTIONS OF FOLD-ACCOMMODATION FAULTS IN FOLD SCARPS TO THE CALCULATION OF SHORTENING INCREMENT: A CASE STUDY FROM QIULITAGE ANTICLINE (SOUTHERN TIAN SHAN, CHINA)

ZHANG Ling, YANG Xiao-ping, HUANG Wei-liang, LI Sheng-qiang

2015, 37(3):  697-708.  DOI: 10.3969/j.issn.0253-4967.2015.03.003

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Fold-accommodation faults, secondary faults subordinated to the principal fold, are of much significance to accommodate strain variation in different parts of the rock during the evolution of folding. They are generally found in groups. And each of them has limited displacement and does not connect with the main detachment. After the geological survey in the East Qiulitage anticline zone, we find that the secondary faults accompanying fold scarps in this area are out-of-syncline thrusts and also give an instance of secondary faults occurring later than the folding. The fact that the secondary faults in fold scarps force the hanging wall to move upward relative to the footwall not only makes the terrace tilting and increases the slope of fold scarps, but also affects the authenticity in calculating regional shortening increment. The theoretical results show that if we do not consider the increased fold scarps height influenced by the secondary faults, the shortening increment is 51.42m. Otherwise, the value will be 45.23m and the difference between them is 6.19m. Because the deviation is 13.7% of the total shortening increment, the contributions of fold-accommodation faults to the calculation should not be ignored. The fold scarps in the northern and southern flanks of the East Qiultiage anticline depend on same bedrock type and formation mechanism. But three levels of fold scarps were found in the cross section of less than 300 meters in horizontal distance. This fact indicates that the active kink band of northern part is more closed because of higher compressive stress and faster lifting, which produce a large number of secondary faults in the fold scarps only in the northern flank. Therefore, the study of secondary faults is of significance in understanding of regional tectonic evolution and interaction between folds and faults. But there still exist many problems: 1)Limited by the observing scope, discontinuous distribution of secondary faults and variations of displacement along fault, we may underestimate the influence of secondary faults and the theoretical result should be the minimum. 2)What is the quantitative relationship among the increased height of fold scarps, the transfer slip and the dip of secondary faults?3)If secondary faults only grow in active kink band, how will they affect fold scarp?More examples of fold-accommodation faults are needed for further research.

GEOMORPHIC INDICES INDICATED RECENT DIFFERENTIAL TECTONIC UPLIFT OF THE LINTAN-DANGCHANG FAULT AND THE MINXIAN-ZHANGXIAN EARTHQUAKE

GAO Ming-xing, CHEN Gui-hua, XU Xi-wei

2015, 37(3):  709-718.  DOI: 10.3969/j.issn.0253-4967.2015.03.004

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The July, 22, 2013 M_S6.6 Minxian-Zhangxian earthquake occurred in the Lintan-Dangchang Fault belt which is located in the northeast Tibet Plateau. Recent tectonic activities show that the Lintan-Dangchang Fault is a thrust fault with strike-slip component. However, the spatial pattern and mechanism of the recent tectonic activity along this fault have not been analysed in detail before the occurrence of the Minxian-Zhangxian earthquake. For better understanding the differential recent tectonic uplift of this fault, drainage basins and river networks were extracted based on ASTER GDEM data in this study. Hypsometric integral and the stream length gradient were calculated. The geomorphic indices analysis results suggest the differential tectonic uplift along the Lintan-Dangchang Fault. The areas experiencing strong tectonic uplift were identified which locate at the west of the Lintan County and around Minxian County. Besides, high geomorphic indices values are associated with epicenters of the historical earthquakes, which demonstrates the importance of the quantitative geomorphic analysis. Our results suggest that the overall tectonic uplift along the Lintan-Dangchang Fault is not very strong. However, certain segments which are experiencing relative high tectonic uplift have the potential of generating moderate or even large earthquakes. Under the regional NE-SW compressional stress caused by the northeastward expansion of the Bayankala block, the tectonic uplift of the fault may relate to the local stress concentration.

A DISCUSSION ON THE SOURCE AND SEISMOGENIC STRUCTURE OF M_S4.5 AND M_S4.7 ZIGUI EARTHQUAKES IN 2014

WU Hai-bo, YAO Yun-sheng, SHEN Xue-lin, ZHAO Ling-yun

2015, 37(3):  719-730.  DOI: 10.3969/j.issn.0253-4967.2015.03.005

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On 27 and 30 Mar. 2013, an M_S4.5 and M_S4.7 earthquake occurred at Zigui County of Hubei Province, which were two larger earthquakes in this region since the impoundment of the There Gorges Reservoir in 2003. Characteristics of the focal and seismogenic tectonic of the two earthquakes are analysed and discussed in this paper. Moment tensor solutions of two earthquakes are inverted by the Kiwi method which uses the wideband waveform of 13 stations from Hubei and Chongqing seismic network and a six-layer crustal velocity structure model. The inverted results show that there is a good fit between the observed waveform spectrum and theoretical waveform spectrum, and the non-fitting errors of two earthquakes are less than 0. 57. These indicate that the inversion results are reliable. Rupture modes of the earthquakes are both strike-slip with a small amount of thrust component, but the former earthquake is of sinistral strike-slip and the latter is the dextral one, and the fact that there is less DC component and more ISO component in their moment tensor solutions is likely the manifestation of the effect of the reservoir water on physical properties of crustal strata. More than 500 earthquakes from Mar. 27 to Apr. 27 in this sequence are relocated by Double-Difference Method using the waveform recording of 15 sub-stations from the Three Gorges Seismic Network, and the results show that aftershocks are distributed along NNW and NE, but mainly along NE direction. Two depth profiles along NNW and NE direction show that focal depths of the sequence are from 4.5 to 10km. Two significant planes in the deep strata are formed along NE direction by these aftershocks, which are in accord with the occurrence of the NE-striking nodal plane of the focal mechanisms. The field intensity survey points out that the isoseismal line of the intensity Ⅴ meizoseismal area is an ellipse, the major axis is along NNW direction and the minor axis is along NE direction. Combining the results of field intensity survey with the tectonic setting of the epicenter area, we deduce that the rupture plane along NE direction at the north of Xiannüshan Fault is the seismogenic fault plane of the two earthquakes, the distribution of aftershocks along NE and NNW, the shape of the seismogenic fault plane and the characteristic of the depth profiles all indicate that these earthquakes are controlled and influenced by Xiannüshan Fault and Jiuwanxi Fault.

DEEP STRUCTURES OF YISHU FAULT ZONE DERIVED FROM GRAVITY DATA

WANG Xin, ZHANG Jing-fa, FU Ping-jie, GAO Min

2015, 37(3):  731-747.  DOI: 10.3969/j.issn.0253-4967.2015.03.006

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Yishu Fault zone is the Shandong segment of Tan-Lu Fault zone, which is characterized by remarkable neotectonic activities and is one of the strong earthquake activity belts in North China. Wavelet multi-scale analysis method is applied to separate gravity fields effectively to study the features of crust structures and spatial distribution of faults with collected Bouguer gravity data of this area. Moho depths are inversed by using the variable density model. The following conclusions are concluded: (1)The gravity fields show that the Yishu Fault zone forms a large-scale NNE-striking gravity gradient zone, which separates the western Shandong block and eastern Shandong block as a major geophysical boundary in this area. (2)The local gravity fields show that the structure of mid and upper crust is complex. The gravity anomaly pattern of 1 horst trapped between 2 grabens appears in the Yishu Fault zone and 5 main faults distributed in the east and west grabens form a linear gradient zone. Many NW-striking active faults in Western Shandong block intersect with Yishu Fault zone in the deep part. The majority of these faults intersect to the west graben of Yishu Fault zone. Only Mengshan Fault and Cangni Fault traverse the Yishu Fault zone. The structure of lower crust is relative simple, fold structures are evident, and there is typical characteristics of large-scale high and low density anomalies alternating in the lower crust.(3)In the Moho depths image, the east part is high and the west is low. The Yishu Fault zone forms the Moho abrupt change zone, creating the separating pattern. Uplift of Moho occurs along the east Weifang-Juxian-Linyi regions, providing deep conditions for strong earthquake preparation.(4)Earthquake epicenters are mainly located in conversion zones between the high and the low-density anomaly, especially in the transitional area from the low-density to high-density anomaly. The occurrence of earthquake is closely related to activity of fault. The Yishu Fault zone sees the strongest seismic activity in this area, and the seismicity in east graben is higher than that in west graben.

SIMULTANEOUS INVERSION OF EARTHQUAKE HYPOCENTERS AND VELOCITY STRUCTURE IN ZIPINGPU RESERVOIR AREA

WANG Liang, ZHOU Long-quan, HUANG Jin-shui, JIAO Ming-ruo, LIANG Yi-jing, YANG Mu-ping

2015, 37(3):  748-764.  DOI: 10.3969/j.issn.0253-4967.2015.03.007

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In this study, using the earthquake observation reports recorded by 7 local earthquake stations in Zipingpu reservoir area and 10 regional seismic stations during the period from August 2004 to May 2008, and with the method of simultaneous inversion of earthquake hypocenters and velocity structure, we invert the velocity structure and conduct the relocation of small earthquakes in the Zipingpu area using the software Simulps14.
Nodes method is used in model parameterization and ART-PB is used in forward calculation. Damped least square method is used in inversion. Regarding the evaluation of solutions, the RDE and DWS are both given in this study.
By calculating, we determined the precise locations of almost all earthquakes in Zipingpu area and the distributions of P wave velocity and V_P/V_S ratio on layers 0km, 3km, 6km and 10km. The relocations of earthquakes show that the seismicity mainly concentrates in the three areas of Hongkou, Yutang town and Shuimo.Tomographic imaging result of P wave velocity structure and wave velocity ratio elaborated the effect of reservoir seepage on Wenchuan M_S8.0 earthquake. We find the effect of reservoir seepage in the southwest of Zipingpu reservoir, which is closest to the epicenter of Wenchuan earthquake, is not more than 8km in depth, and there is no obvious low velocity anomaly at the depth of the Wenchuan main shock, nor obvious high value anomaly of V_P/V_S ratio. This means penetration of water didn't reach the depth of Wenchuan main shock, that is, water was not the direct factor causing the Wenchuan earthquake.

WAVELET MAXIMA METHOD IN IDENTIFYING SINGULARITIES IN ELECTROMAGNETIC SIGNAL

HAN Bing, TANG Ji, ZHAO Guo-ze, BI Ya-xin, WANG Li-feng, CHENG Yuan-zhi

2015, 37(3):  765-779.  DOI: 10.3969/j.issn.0253-4967.2015.03.008

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Wavelet maxima method as a kind of data mining method has been applied to earthquake science research, which gives us a direct way to identify the singularities of different time and frequencies in the long time observations. This paper introduces how to identify the electromagnetic anomalies using the wavelet maxima, i.e., the wavelet coefficients are calculated by using continuous wavelet transform and then calculate the maximum value of wavelet coefficients in each scale and identify the singularities associated with the earthquake. The identified singularities are further examined by Lipschitz-exponent α. The proposed method has been employed using the 35 days' data of the electromagnetic field recorded in Baosheng station in Sichuan after the Lushan M_S7.0 earthquake, and three electromagnetic anomalies are collected, then, the relationships between the electromagnetic anomalies and the earthquakes are discussed. This method cannot give a certain relationship between the electromagnetic anomaly and earthquake, but it proves the method's effectiveness in extracting the electromagnetic anomaly in continuous observation data.

DISCUSSION ON SEISMOGENIC STRUCTURES AND SEISMIC ACTIVITY PROCESS BEFORE THE YUTIAN M_S7.3 EARTHQUAKE ON FEBRUARY 12, 2014, XINJIANG

SONG Chun-yan, MA Jin, RAN Hui-min, HUANG Fu-qiong

2015, 37(3):  780-791.  DOI: 10.3969/j.issn.0253-4967.2015.03.009

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The Yutian M_S7.3 earthquake occurred on February 12, 2014 in Xinjiang Uygur Autonomous Region, China and the epicenter is located in the western part of Altyn Tagh Fault. This is the second M≥7 earthquake following the March 21, 2008 Yutian M_S7.3 earthquake in the south of the Tarim Basin. Aftershocks of this Yutian M_S7.3 earthquake are distributed mainly along the NE direction, and that in the southwest part of the aftershock area presents a near-NS distribution. Most of the aftershocks including foreshocks (accounting for 85% of the total aftershock sequence) are densely distributed in the southwest of the aftershock zone, the vast majority of strong aftershocks (all of the M_S≥5 and 81% magnitude 4 earthquakes)are distributed in this area. Aftershocks of the first day are mainly distributed in this area and in a near-NS distribution. The aftershocks extended from west to east. The authors noted that there occurred several M5~M6 earthquakes in 1982, 2011 and 2012 along the near-NS direction of this Yutian earthquake and this Yutian earthquake filled up the empty section. Based on the regional tectonic environment, earthquake focal mechanism solutions and aftershocks distribution, etc., we analyzed the process of this earthquake and found that the earthquake occurred at the branch fault of Altyn Tagh Fault zone on the south margin of Xiaoerkule Basin. Affected by the tectonic stress of Altyn Tagh Fault zone, Xiaoerkule Basin suffers the local near east-west extension, earthquake rupture occurred first along the near-NS direction, the unlocking of this tectonic position promotes the left-lateral movement of the Altyn Tagh Fault, producing NE-oriented rupture, and stress transferring to the east. This paper also discusses the seismogenic structures of historical earthquakes above M_S7 occurring on the Altyn Tagh Fault zone and their impact on the fault zone.

SPATIAL-TEMPORAL CHARACTERISTICS OF THE FOCAL MECHANISM CONSISTENCY PARAMETER IN TIANSHAN (WITHIN CHINESE TERRITORY)SEISMIC ZONE

LI Jin, ZHOU Long-quan, LONG Hai-ying, NIE Xiao-hong, GUO Yin

2015, 37(3):  792-803.  DOI: 10.3969/j.issn.0253-4967.2015.03.010

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Based on 306 focal mechanism solutions of M_S≥3.5 earthquakes from 2003 to 2014 in Tianshan seismic zone, we divide the Tianshan seismic zone into grids of certain size and use multiple focal mechanism solutions around each grid node to do the stress tensor inversion, and then calculate the temporal and spatial distribution of the focal mechanism consistency parameter for each grid node. On this basis the stress state of the Tianshan seismic zone is analyzed and the relationship between temporal and spatial distribution of focal mechanism consistency parameter and strong earthquakes is discussed. The result shows that the principal compressive stress of Tianshan is quite similar to the former research result; the P axis of the whole Tianshan is mainly in NS direction and in NNE and NNW in localized areas. There is a corresponding relationship between the temporal-spatial distribution of the focal mechanism consistency parameter and the strong earthquakes; strong earthquakes in the middle-east area of Tianshan Mountains often occur in the low value zones of focal mechanism consistency parameter or near the edge of the zones. Since the second half of 2011, the focal mechanism consistency parameter in the middle-east area of Tianshan Mountains presents a process from disorder to consistency, which corresponds to the group activity of strong earthquakes in Xinjiang in this period; the focal mechanism consistency parameter in the southern section of Tianshan also decreased before 2008 Wuqia M_S6.9 earthquake. In addition, we can see from the b-value image of Tianshan that earthquakes with magnitude larger than 5 occur mostly in the relatively low b value area.

SPATIAL AND TEMPORAL CHARACTERISTIC OF STRESS FIELD FOR WENCHUAN M_S8.0 AND LUSHAN M_S7.0 EARTHQUAKE SEQUENCE

ZHANG Zhi-wei, ZHOU Long-quan, LONG Feng, RUAN Xiang

2015, 37(3):  804-817.  DOI: 10.3969/j.issn.0253-4967.2015.03.011

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Based on focal mechanism solutions of Wenchuan M≥4.0 and Lushan M≥3.0 aftershocks, using inversion method of stress field to analyze the spatial distribution characteristic of compressive stress (S₁) and stress tensor variance of Wenchuan and Lushan aftershock zones, the relation between spatial and temporal distribution of stress tensor variance and strong aftershock activity are studied. The results show that (1) The orientations of compressive stress (S₁) are complex in Wenchuan and Lushan aftershock zones, there exists obvious regional difference spatially, the S₁ orientations all present disorder feature near the main shock rupture zone, and the stress tensor variances are obviously higher. (2)Along Wenchuan aftershock zone from southwest to northeast, the compressive stress orientation gradually changes from EW to NW-SE, finally, it presents near EW direction at north segment of aftershock zone, and the stress type is all thrust; the S₁ orientation presents NEE direction in Lixian branch of Wenchuan aftershock zone, the stress type shows strike-slip type, the compressive stress presents a near horizontal feature in whole aftershock zone. (3)The compressive stress orientation presents NW direction in Lushan aftershock zone, the dip angle is near horizontal, the stress type is thrust near the main shock and strike-slip and thrust in other areas. (4)The spatial and temporal distribution of stress tensor variance has certain indicative significance for occurrence of strong aftershock, most later-phase strong aftershocks of Wenchuan and Lushan earthquake occurred in the low-value area of stress tensor variance and its fringe areas, the origin time of Wenchaun strong aftershocks are also at the time frame of low value of stress tensor variance.

THE RESEARCH ON FAULT PARAMETER AND SLIDING BEHAVIOR OF THE 1927 GULANG M8.0 EARTHQUAKE

LIU Bai-yun, ZENG Wen-hao, YUAN Dao-yang, LI Qiu-hong, CHEN Wen-kai

2015, 37(3):  818-828.  DOI: 10.3969/j.issn.0253-4967.2015.03.012

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The Gulang M8.0 great earthquake occurred in 1927, many places in Gulang and adjacent areas had suffered destruction in various degrees. So far, divergences exist in the former studies on its seismogenic structure. It is known that clustered small earthquakes often occur in vicinities of fault plane of large earthquake. In this study, the precisely relocated earthquakes which occurred near the earthquake rupture zone between 1985 and 2012 are used, and two strip-shaped zones of clustered small earthquakes are chosen according to the previous studies of the causative structure of this earthquake. Based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms, we obtained fault plane parameters of the earthquake such as strike, dip, and location using data of densely concentrated small events. On this condition, rake angles of the fault plane are further inferred from regional tectonic stress parameter. Then we discussed briefly the seismogenic environment and causal mechanism of the earthquake, combined with the results of deep tectonics and surface investigations. The focal fault we inverted locates within the meizoseismal area (intensity Ⅺ)of the Gulang M8.0 in 1927, suggesting that the focal fault obtained by inversion is possibly the causative structure of this earthquake. Besides, we found a clustered zone of small earthquakes near the south-central part of the main fault, and a fault plane could be derived from them, which we think might be a tensional seismic fault developed on the main fault when the whole earthquake-hit region rotated counterclockwise during the big earthquake.

REGIONAL CHARACTERISTICS OF WAVE VELOCITY RATIO IN JILIN AREA AND THEIR TECTONIC IMPLICATIONS

ZHANG Hong-yan, ZHANG Guang-wei, WANG Xiao-shan, ZHANG Chen-xia, KANG Jian-hong

2015, 37(3):  829-839.  DOI: 10.3969/j.issn.0253-4967.2015.03.013

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Using the analog and digital P and S wave data of 254 events from January 2001 to June 2013, we obtained the wave velocity ratio with the multi-station method and got the mean value for every earthquake. This study analyzes the spatial variation of the wave velocity ratios of Jilin area. To ensure the accuracy and stability of the calculated results, we carefully winnow the collected data and the calculated results. Our results show that there are strong lateral heterogeneity and difference between the south and north of Jilin area. The wave velocity ratio in the north part of Yitong-Shulan Fault shows strong lateral heterogeneity, the orientation of anomaly area for high and low wave velocity is in accordance with the NE-trending Fuyu-Zhaodong Fault; The high wave velocity ratio areas in the south part of Yitong-Shulan Fault mainly concentrate in the volcanic region, the wave velocity ratio in the southwest is little higher than that in the north part of Changbai Mountains volcanoes, suggesting that the low velocity abnormal body maybe locates in the southwest under Changbai Mountains, The study suggests that the high value zone and the transitional zone of high to low value are earthquake-prone zone by analyzing the distribution characteristic of wave velocity ratio and the earthquake activity.

AN UNSTRUCTURED GEOMETRIC MULTIGRID GENERATION ALGORITHM FOR ARBITRARY GEOELECTRIC MODEL

QIAO Liang, CHEN Xiao-bin

2015, 37(3):  840-850.  DOI: 10.3969/j.issn.0253-4967.2015.03.014

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With the growing number of observational data, increasingly complex geoelectric model, and high-dimensional magnetotelluric method, the efficient and stable forward and inversion technologies become more and more required. Multigrid method enables the asymptotically optimal approximate numerical solution to elliptic partial differential equations including the MT 2D forward modeling problem. Currently, the coarsening and refining operations are based on regular grid which is easy to understand and implement. However, when the geometry of a region is complex or the region needs local refinements, regular grid becomes not well applicable. Unstructured triangular grid has better geometrical adaptability than structured grid. The Delaunay triangulation ensures that each triangle is generated to be Delaunay. This property is particularly suitable for numerical interpolation and FEM calculation. Besides, the Delaunay triangulation has a strong mathematical theory foundation which makes it very convenient for local refinement and sparseness. The key to solve the complicated geoelectric model using multigrid method is the automatic generation of unstructured multi-level grids. In this paper, we present an algorithm for generating unstructured grid for Multigrid method based on Delaunay triangulation. The algorithm will automatically generate the coarse and fine grid for complex input region, and all the triangle elements abide by the Delaunay criteria which ensure numerical accuracy and high convergence rate.

THE U SERIES DATING AND ITS SIGNIFICANCE FOR CALCAREOUS COATS OF EARTHQUAKE-GENERATED REGIONAL ROCKFALL ALONG YUSHU FAULT ZONE, IN EASTERN QINGHAI-XIZANG PLATEAU

TIAN Ting-ting, WU Zhong-hai, MA Zhi-bang, ZHOU Chun-jing, ZHANG Ke-qi, JIANG Yao, ZHANG Xu-jiao

2015, 37(3):  851-868.  DOI: 10.3969/j.issn.0253-4967.2015.03.015

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In 2010, the M_S7.1 earthquake in Yushu, Qinghai Provence caused an obvious phenomenon of earthquake-generated rockfall in many places along the seismogenic fault zone. In the course of surface investigation, it was found that in several typical development areas of latest earthquake-generated rockfall, there are entirely multi-period in-situ recurrent earthquake-generated rockfalls with most typical characteristics. It is so common with thin calcareous coats on the surface of earthquake-generated rock fall. On the basis of systematic sampling for the calcareous coats, which are on the surface of earthquake-generated rock fall in several typical distribution areas near the Yushu active fault belt, the author made an attempt on U-series dating analysis for calcareous coats of the earthquake-generated rock fall, by applying dilute acid leaching method. The ages are mainly distributed in the range of about 69 980a BP, 36 300a BP, 14 900~12 700a BP, 6 030a BP, 4 720a BP, 3 560~3 530a BP, 2 010~1 090a BP, about 760a BP and 230 a BP. This result tallies basically with the multi-period characteristic of earthquake-generated rock fall by surface observation, and primarily with the seismic events reflected from paleo-earthquake trench and paleo-earthquake landslide. It is preliminary confirmed that the development of rock fall near the Yushu Fault zone is of obvious multi-period, and the multi-period rock fall is in close genetic relationship with paleo-earthquake activities. In addition, combined with results of field investigation and the data of paleo-seismic research, it can be judged that the phenomenon of several extensive earthquake-generated rock falls is mainly a reflection of larger intensity of paleo-earthquake events since the late of Q³_p period, of which the paleo-earthquake events occurring in about 3 560~3 530a BP and 14 900~12 700a BP may be the biggest. The exploratory research on U series dating method for calcareous coats in Yushu earthquake-generated rock fall may provide a new way for the future study of paleo-earthquake activity in active tectonic region.

INVERSION OF PRESENT-DAY FAULT SLIP RATE ALONG ALTYN TAGH FAULT CONSTRAINED BY GPS DATA

LI Yu-hang, WANG Qing-liang, CUI Du-xin, HAO Ming, WANG Wen-ping, QIN Shan-lan

2015, 37(3):  869-879.  DOI: 10.3969/j.issn.0253-4967.2015.03.016

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The Tibetan plateau is bounded by Altyn Tagh Fault in its northern edge, this well known for the characteristic of left-lateral strike-slip from late Quaternary, but its magnitude of left-lateral slip rate measured by geological way, either on a high level (20~30mm/a) or on a lower level (~9mm/a), is hotly debated, and that is central to reviewing the existent deformation mechanism of Tibet plateau. The present-day fault slip rate along Altyn Tagh Fault has revealed by Global Positioning System (GPS), however the limited GPS stations and its poor distribution might increase the uncertainties of the predicted fault slip rates, especially to this so mega fault with 1500km length approximately.
A dense GPS velocity field (from 2009 to 2013) has obtained along Altyn Tagh Fault and its vicinity, which provides us a good opportunity to study its slip rates along its different fault segments in detail. In this paper, we use the spherical linear elastic block theory constrained by new geodetic observations from GPS stations we have mentioned, to estimate fault slip rates along the Altyn Tagh Fault and other major faults in its vicinity. Our 3D geometric block model is based on the previous researches of active block. Then the optimal locking depths of Altyn Tough Fault are fixed by trail tactics, the result of optimal locking depths shows that it is from 10km to 15km in the southern part of Altyn Tagh Fault, in its middle and north segment the locking depths are deeper than its southern part, and in its north terminal the locking depth is 11km. Then the fault slip rates along Altyn Tough Fault are obtained as well as other fault slip rates of major faults in its vicinity. The left-lateral fault slip rates of different segments along Altyn Tough Fault are (7.8±0.2)mm/a (south of Qaidam Basin), (7.5±0.1)mm/a (south of Subei), (5.3~5.5)mm/a (from Subei to Changma) and(1.0±0.4)mm/a (north of Changma), which trend to decreasing from south to north along the fault strike, and the decreasing of the slip rate is mainly confined within the Qilian Range, and converted to the crustal contraction in this area.

DEBRIS-FLOW HAZARD EVALUATION BASED ON GEOMORPHIC INFORMATION ENTROPY AND LANDSLIDE MATERIALS IN THE LUSHAN EARTHQUAKE AFFECTED AREA

LIU Li-na, XU Chong, CHEN Jian

2015, 37(3):  880-892.  DOI: 10.3969/j.issn.0253-4967.2015.03.017

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Geomorphic information entropy is a quantitative indicator used to determine the evolutionary stage and express the erosion degree of watershed geomorphic surface, which is a reflection of topography factors. In order to do a better job for the upcoming rainy season's debris-flow hazard zone planning and provide a reference for disaster prevention, with GIS technology as a platform and the Lushan seismic landslides' volume data as source material of debris flow, and by combining geomorphic information entropy method, the paper carries out debris-flow hazard evaluation for 55 debris flows based on landslide material sources. The results show that: The range of the value of geomorphic information entropy is between 0.003 2~0. 938 1 in debris-flow valley basin of the study area, and valley geomorphic evolution is distributed from childhood to old age; the area of debris-flow hazard zone shows a decreasing trend basically from high to low hazard zone, with 80. 77% of the study region locating in the debris flow prone area, and the presence of debris-flow valley in its juvenile to mature stage increases the risk of debris flow. The response rate of seismic slope mass movement(RRSSMM)of the debris flow basin varies from 0~133. 24mm, and the area of material source sensitive area of low and very low degree accounts for 72. 93% of the valley basin area of the study area, which indicates that nearly 3/4 of debris-flow valley basins are insensitive to landslides material source. The result of debris-flow hazard evaluation based on landslides material source indicates that more than 2/5 of the debris flow valley basin is located in moderate or above hazard zone, where debris flow activity seems more active.

ANALYSIS ON THERMAL INFRARED ANOMALOUS TEMPERATURE INCREASE OF MODIS SATELLITE BEFORE YUTIAN M_S7.3 EARTHQUAKE

WEN Xiang, CHEN Mei-hua, YAN Chun-heng, ZHOU Bin

2015, 37(3):  893-905.  DOI: 10.3969/j.issn.0253-4967.2015.03.018

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In this study, the continuous thermal infrared data of MODIS/Terra satellite remote sensing of the Yutian M_S7.3 earthquake area from January 2014 to February 2014 are collected, and by cloud-removing, the thermal infrared data between 4:00am-6:00am in Beijing time which is the best period for observation to conduct land surface temperature data retrieval, are selected. Time evolution process of land surface temperature anomalies before and after the earthquake is analyzed as well as the relationship between abnormal spatial distribution and active fault. Then, the impact of non-structural factors such as topography of earthquake area, seasonal climate, rain and snow on land surface temperature anomalies is discussed. The result shows that: a)There was phenomenon of thermal infrared abnormal temperature increase appearing near the epicenter area one month before the Yutian earthquake, and there was a certain correspondence between the abnormal temperature increase and earthquake occurrence time. The significant temperature increase happened in the first half of the month, while 5 to 6 days before the earthquake, the abnormal increase reached its peak, and the temperature dropped rapidly after the earthquake. b)Through the relative analysis of non-structural factors such as earthquake area's topography and landform, seasonal weather, rain and snow, the anti-seasonal structural "temperature increase" signals were discovered, and the rain and snow had a certain degree of influence on the abnormal temperature increase of the earthquake area. c)Due to the complex and sensitive fault structures of the valleys and basins at the southwest of the epicenter, the thermal infrared abnormal temperature increase usually starts from this area and gradually migrates to the epicenter along the faults. d)Abnormal temperature increase zone presents strip distribution which is in consistent with the NE-striking main fault zone. After full consideration of the influence of non-structural factors on the abnormal temperature increase, it was inferred that this thermal infrared temperature increase could be a short-imminent precursor before the earthquake.

THE NUMERICAL SIMULATION OF THE STRAIN ENERGY DENSITY CHANGING RATE AND STRONG EARTHQUAKE RECURRENCE INTERVAL OF THE SICHUAN-YUNNAN BLOCK

ZHU Ai-yu, ZHANG Dong-ning, JIANG Chang-sheng, LI Ming

2015, 37(3):  906-927.  DOI: 10.3969/j.issn.0253-4967.2015.03.019

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Based on the deep structure imaging of Sichuan-Yunnan region, a three-dimensional viscoelastic finite element model is established, and the simulation is constrained by tectonic stress field, Global Positioning System(GPS)observations and fault activities. In this paper, we focus on the discussion about the effect of gravitational potential energy of high altitude terrain, the slip rate of major active faults, the difference of elastic parameters and the creep feature of the rock medium on the distribution characteristics of strain energy density. We obtain the strong earthquake recurrence interval of the major active block which is reduced by strain energy density and the main active faults, and also we compare the simulated results and geological observations. The simulation results show that the gravitational potential energy accumulated by the high altitude terrain and the interaction between blocks are the main geodynamical factors, which control the active fault slip rate, the activity of the second class block, and the recurrence interval of strong earthquakes in the Yunnan-Sichuan area.

THE RESEARCH ON THE IMPACT OF DISPLACEMENT FIELD OF JAPAN 3·11 EARTHQUAKE ON THE SOUTH CENTRAL SECTION OF TANLU FAULT ZONE

YIN Jing-yuan, LIU Lan, WANG Hua-lin, LI Cheng-fan

2015, 37(3):  928-938.  DOI: 10.3969/j.issn.0253-4967.2015.03.020

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Based on the finite element models, in this paper, the impact of Japan 3·11 earthquake on the south-central section of Tanlu Fault zone is discussed in the view of displacement field. From the displacement magnitude acquired by the numerical simulation of Japan 3·11 earthquake, this earthquake has a certain impact on Tanlu Fault zone in mainland China area. The simulation results show that the coseismic horizontal displacement near the Tanlu Fault zone caused by Japan 3·11 earthquake has reached millimeter to centimeter level. On the east side of the fault zone, the maximum displacement value in Northeast China reaches to 30 millimeters, and the maximum displacement value in Yantai, Weihai and other places of North China also reaches to 12 millimeters. On the west side of the fault zone, the maximum displacement value in Northeast China reaches to 20 millimeters, and the maximum displacement value in North China is only 8 millimeters. The displacement value difference is very obvious in both sides of Tanlu Fault zone. In addition, the simulation results also show that the displacement values in the east-west direction is much bigger than the one in the south-north direction of the Tanlu Fault zone and nearby areas; especially the displacement direction is basically near to east-west direction in North China. Meanwhile, bigger NS-direction displacement appeared in Northeast China, and the maximum displacement value even reaches to 8 millimeters. Therefore, the impact of Japan 3·11 earthquake on this area is more complex. Considering the actual delay effect of crustal medium to stress transfer, the impact of Japan 3·11 earthquake on the Tanlu Fault zone will have a delay, so the follow-up effect will stay for a certain time. Although the displacement value in eastern North China(Tanlu Fault zone and its nearby area)is smaller than the one in Northeast China, the impact of Japan 3·11 earthquake still cannot be ignored. Considering that many magnitude 4 or 5 earthquakes have occurred in Kenli, Changdao, Rongcheng, Laizhou in Shandong Province and in the Rudong, Gaoyou in Jiangsu Province since 2011, it cannot be excluded the possibility that a larger earthquake will happen in this area in the future. Thus, due attention has to be paid to this.