Table of Content

30 June 2014, Volume 36 Issue 2

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KEY TECHNIQUES AND SEVERAL CASES ANALYSIS IN PALEOSEISMIC STUDIES IN MAINLAND CHINA(3)：RUPTURE CHARACTERISTICS, ENVIRONMENT IMPACT AND PALEOSEISMIC INDICATORS ON NORMAL FAULTS

RAN Yong-kang, LI Yan-bao, DU Peng, CHEN Li-chun, WANG Hu

2014, 36(2):  287-301.  DOI: 10.3969/j.issn.0253-4967.2014.02.001

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Normal faults, developed within extensional environment, are widely found in North China. Given the varieties in surface ruptures of different earthquakes and their depositional environment, some issues are needed to be paid much attention to in exposing the actual and complete history of paleoseismic events occurring along normal faults. In this paper, based on the existing knowledge about surface rupture characteristics of large earthquakes and indicators of normal fault, combining the cases study in China and the factors of geological, geomorphologic and climatic environment, some key techniques and methods in paleoseismic study on normal faults in mainland China are recommended as follows: (1)Choosing appropriate trenching sites according to local conditions. In the area where the faulted surface deposits are mainly alluvial-fluvial materials of piedmont or river and lake sediments, the trenching sites should try to meet following conditions: the geomorphy can reveal multiple fault events with not too large single displacement, the erosion(or denudation)of external force and the accumulation processes maintain relative balance, the sediments are medium-fine grained, and the samples for dating are easy to be collected. The sites where the faulted sediments are mainly composed of loess or secondary loess or sandy loam should be avoided to excavate trenches for paleoseismic study, however, if it cannot be avoided, the areas with weaker erosion and accumulation near small gullies are the choices to be considered, because these areas may have different deposits from upstream of the gullies, and some supplemental information such as tectonic landform are needed to substantiate the paleoseismic analysis.(2)Recording and analyzing the trench profiles in detail in the field. For the deposits(e.g. loess)with no stratification, the key observation point is the slight change in the color, grain and orientation, which may indicate the stratigraphic boundary. Indicating the scarp-derived deposits units such as colluvial wedge is the key to analyzing paleoseismic events, and the indicated elements conclude the messy configuration and nodules in the collapse facies, and the soil developed in the upper of the erosion facies. When the scarp-derived deposits are difficult to distinguish from normal strata, we should, by "brushing", "jabbing" or "microscopic analysis", try to analyze the color, grain, non-loess materials(e.g. small gravel, plant roots, etc.)and the enrichment degree of calcareous materials(e.g. calcium-mod, calcium-nodule, calcium-dot, calcium-filament, etc.), to identify the stratigraphic boundary.(3)Synthetically analyzing and checking the paleoseismic results combining other information. The appearances of the scarp-derived deposits revealed by trench are often obscure, so supplemental information from geomorphology and multiple trenches are necessary. Some techniques and methods, such as progressive constraining method of paleoseismic events, fault displacement constraining method, correlating method between multiple trenches, inversion and reconstruction of fault events, etc., are helpful for judging whether the paleoseismic results are actual and complete.

ELECTRIC STRUCTURE OF CRUST BENEATH THE SOUTH- WESTERN SEGMENT OF THE LONGMENSHAN FAULT ZONE

WANG Li-feng, ZHAO Guo-ze, ZHAN Yan, CHEN Xiao-bin, XIAO Qi-bin, ZHAO Ling-qiang, WANG Ji-jun, QIAO Liang, HAN Bing

2014, 36(2):  302-311.  DOI: 10.3969/j.issn.0253-4967.2014.02.002

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On 20 April 2013, the Lushan M_S7.0 earthquake happened in the southwestern segment of the Longmen Shan Fault zone, which is a shock only five years after the devastating 2008 Wenchuan M_S8.0(M_W7.9) earthquake that ruptured the northeastern section of the same fault zone. At least seven years before the Wenchuan event, the most parts of this tectonic zone looked seismically quiet, without M>4.0 quakes, apparently consistent with the low slip rates along the fault evidenced in geological research and GPS measurements. The occurrence of these two major shocks has stimulated researchers to explore what is the cause, particularly the deep conditions, that made the Longmen Shan Fault zone wreak subsequent havoc within five years.To probe the crustal structure beneath Longmen Shan, a magnetotelluric (MT) profile LS6 was deployed for data collection, which runs through the southwestern fault trace and facilitates comparison with another MT profile LS04 that crosses through the Lushan seismic area to the northeast. Using the advanced methods, MT data processing and 2D inversion have been carried out on both the two profiles. The comparison shows that the profile LS6 seems to reveal a more complex electric structure of crust with respect to that from the profile LS04, though both have similarities to some extent. The formation of such a deep structure is associated with the southeastward motion of the Songpan-Ganzi block in the northwest, which is hampered by the Longmen Shan Fault zone and by the rigid Sichuan Basin farther to the southeast. It is also influenced by the motion of the Chuandian block as well as active faults, such as the Xianshuihe Fault in the southwest. The occurrence of the 2013 Lushan earthquake implies a subsequent release of the elastic strain accumulated within the Longmen Shan Fault zone, which is an independent event to the 2008 Wenchuan earthquake, though both have some relationship between each other.

RAPID INVERSION OF FAULT RUPTURE CHARACTERISTICS OF WENCHUAN EARTHQUAKE WITH PGA RECORDS

KANG Lan-chi, JIN Xing, CHEN Hui-fang, LI Jun, WEI Yong-xiang

2014, 36(2):  312-321.  DOI: 10.3969/j.issn.0253-4967.2014.02.003

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A magnitude 8.0 earthquake occurred in Wenchuan, Sichuan Province of China at 14:28 on May 12, 2008, with the epicenter at north latitude 31 degrees, east longitude 103.4 degrees, and the focal depth of 14km. Result of focal mechanism was given by a number of research institutes and academics, from a few hours to several days, or even months after the earthquake. Such the situation determines that some of the methods cannot be used to earthquake emergency and rescue services, or applicable to rapid reporting of seismic intensity. A lot of strong motion acceleration records are accumulated in Wenchuan earthquake. This paper puts forward a new method which uses adequate Wenchuan earthquake acceleration records and can be applied in the future to the intensity assessment in fast report to quickly estimate the fault strike, rupture mode and rupture length.Assuming the seismic source is a line source model, the fault is discretized into sub-sources with 10 km each, the length of the initial fault rupture is determined using statistical relations;taking the initial rupture point as a fixed point, and for a fault of arbitrarily rupture direction, by moving sub-source at one side to the source at the other side, all possible rupture modes of fault along certain rupture direction can be obtained;Using the initial rupture point as the center and rotating the fault of any rupture modes, all possible rupture directions are acquired, then, combining the two together gets all the possibilities of rupture directions and modes of fault. The real earthquake must be one of the circumstances. Fault distance between each of the possible fault distributions and stations is calculated, and statistical regression method is used to analyze the peak ground acceleration attenuation relationships, then, variance analysis is conducted on the fitting results. The rupture direction and mode corresponding to the minimum variance of the attenuation law can most reasonably explain the spatial distribution of fault in the ground motion field, which, in another word, are the real rupture direction and mode of fault in the actual earthquake.The time it takes to fix intensity for quick report is less than 20 seconds by two-parameter method and within 2 minutes by three-parameter method, which can meet the time requirement for quick report. The calculation result is accurate, so this method is feasible.

A STUDY OF THE CRUST-MANTLE VELOCITY STRUCTURE BENEATH THE YUSHU EARTHQUAKE ZONE AND ITS ADJACENT AREAS

ZHANG Jian-shi, WANG Fu-yun, LIU Bao-feng, WANG Shuai-jun, ZHAO Jin-ren, ZHANG Cheng-ke, LI Yi-qing, LIU Qiao-xia, LIU Lan

2014, 36(2):  322-332.  DOI: 10.3969/j.issn.0253-4967.2014.02.004

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In this paper the crustal velocity structure is obtained along Yushu earthquake zone, using wide angle reflection and refraction data. The results reveal strong vertical and lateral heterogeneities in the crust, as well as the basic characteristics of the crustal velocity structure and the tectonics along the seismic sounding profile. Results show that crustal velocity structure is featured with significant regional heterogeneity both in the longitudinal and lateral directions. The crust is of layered structure, and the crystalline basement interface undulates greatly beneath the study area, which is about 8 km in thickness beneath Yushu and gradually thins northwards to 2.5 km beneath the Stake 400.0 km at Wenquan. There are good correlations of the depressions and uplifts on basement interface with different tectonic units. The crust gradually thins towards both south and north direction to a thickness of 62 km from 72 km beneath Nangqian and Yushu. There exist big undulations in 2D velocity contours and the interfaces in the crust between Stakes 200.0～400.0km, and there is presence of an arc-shaped depression in the Moho beneath the Yushu area.

VERIFICATION RESEARCH ON THE COSEISMIC DISPLACEMENT OF THE APRIL 20, 2013, M7.0 LUSHAN EARTHQUAKE AND PRIMARY EXPLORATION OF SURFACE DEFORMATION MODE

WEI Wen-xin, JIANG Zai-sen, ZOU Zhen-yu, YANG Yong-lin, ZHANG Long, WU Yan-qiang

2014, 36(2):  333-343.  DOI: 10.3969/j.issn.0253-4967.2014.02.005

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Considering the inconsistence of the focal mechanisms of the M7.0 Lushan, Sichuan earthquake of 2013 with the GPS coseismic displacement field, high-precision gradienter was used to measure the inclination of piers at 6 temporary GPS continuous observation stations, such as LS05, LS06 and so on. Repeat measurement was carried out on the elevation difference of the benchmark on the pier surface of SCTQ, and the surface deformation of this area was investigated. (1)The results of tilt measurement show that there was no obvious tilt for the piers at 5 stations, such as LS06, compared with the period when they were built. The pier at LS05 tilted relative to the time when it was built, with the amount of tilt estimated to be 0.4°～1.2°, and a dominant tilt direction in NW. No matter when the inclination of this pier occurred, the southward movement in the coseismic displacement was dominating. (2)The leveling results of SCTQ station pier surface show that there was no tilt at this site and it was a stable observation point. (3)Field surveys show obvious features of compression-shortening deformation of the crust. Around the site LS05, there were evidences of accelerated SW-directed motion and shear deformation. In summary, the characteristics of left-lateral strain release displayed at the GPS observation sites LS05, SCTQ and LS06 are reliable.

DISTRIBUTION OF COSEISMIC LANDSLIDES IN LUSHAN EARTHQUAKE AND DISCUSSION ON RELATED PROBLEMS

ZHOU Qing, JIANG Ya-feng, WU Guo, CHEN Guo-guang

2014, 36(2):  344-357.  DOI: 10.3969/j.issn.0253-4967.2014.02.006

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Geological disaster distribution induced by the 2013 M_S7.0 Lushan earthquake shows clear hanging wall effect and direction effect under reverse fault motion. No surface rupture was found in the post-earthquake emergency field surveys. We developed a detailed geo-spatial database of 2230 rock falls and landslides based on post-earthquake field surveys and examination of high-resolution aerial photographs across the disaster area. Landslides triggered by earthquake, dense aftershocks and isoseismal map provide clues to study the unrevealed causative fault.Statistics show that rock falls and landslides have a dominant slide direction of southeast(135°～144°), which is perpendicular to the strike of the causative fault. This result is consistent with the focal mechanisms. From distribution and density of seismic disasters, the macroscopic epicenter was relocated on the northern edge of Baosheng Town, ～3.6 km from the instrumental location. The probable surface rupture may appear on the edge of dense region of coseismic landslides and aftershocks, paralleling to the Shuangshi-Dachuan Fault. Because of difference in weatherability of rocks, linear cliffs or high-angle topography are formed between different strata or igneous rocks, and these regions are susceptible to rock falls and landslides under the action of strong earthquake.

STUDY ON THE FAULT MECHANICS INFLUENCES ON THE LANDSLIDES DISTRIBUTION：A CASE STUDY FROM THE WENCHUAN EARTHQUAKE

CHEN Xiao-li, HUI Hong-jun, ZHAO Yong-hong

2014, 36(2):  358-367.  DOI: 10.3969/j.issn.0253-4967.2014.02.007

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More and more earthquake-triggered landslides show the hanging wall effects on the spatial distribution, however, the effects of seismic fault mechanics on landslides development are not clear at present time. Based on a simplified fault model, the knowledge of seismic waves reflecting between the ground surface and the fault plane is applied to explain the reasons why landslides are asymmetrically distributed on the hanging wall side during earthquakes caused by thrust faults. Moreover, this paper discusses the relationship between fault movement directions and the initial slope aspects based on the data of large landslides which occurred during the Wenchuan earthquake. This study shows that fault dip angle can affect landslides distribution, namely with the increasing of fault dip angle, the width of the region subjected to the landslides on the hanging wall side decreases. Also, the slopes whose aspects are in the similar directions as the initial movement direction of seismic fault are prone to failure. The knowledge obtained in this study will help to improve the accuracy in regional earthquake-triggered landslide susceptibility assessment and promote the research on the large landslides mechanism, major influencing factors as well as the possible landslide scales.

CHARACTERISTICS OF PARTICLE SIZE DISTRIBUTION OF THE WENCHUAN EARTHQUAKE FAULT ZONE：INSIGHTS FOR CATACLASTIC MECHANISMS

CHEN Jian-ye, YANG Xiao-song

2014, 36(2):  368-379.  DOI: 10.3969/j.issn.0253-4967.2014.02.008

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Particle size distribution(PSD)of fault rocks has been widely used to investigate the fracturing mechanism, frictional property and energetic partitioning of earthquakes. Sieve-weighting and laser diffraction analysis are two methods of quantifying 3-D PSD of fault rocks. However, each of them can only cover grain size in roughly three orders of magnitude, so they cannot reflect the full range characteristics of typical fault rocks, from nanometer scale to millimeter scale. This study performs measurements combining these two methods, on natural fault rocks collected from two surface exposures of the Wenchuan earthquake fault zone, with particle size measured ranging from 0.2μm to 16mm. Our measurements apparently show the following results: 1)There exists a critical particular size d_c(0.95～1.90μm), and particles with larger and smaller size than d_c follow different power law relations between particle number(N_d)and particle size(d), indicating that the PSDs of the fault rocks studied are non-self-similar. 2)The fractal dimensions(D-value)obtained from the particles with d>d_c are well relevant to the fault rock types. This means that the D-values obtained by sieve-weighting method and by laser diffraction analysis are consistent with one another. The D-values of fault rocks analyzed show an increase trend towards the slip surface, from 2.6 for fractured breccia, to 3.0 for cataclastic breccia, and to nearly 3.5 for fresh fault gouges. By contrast, particles with d<d_c have low D-values of 1.7～2.1. We infer that variation in fractal dimension for different particle size intervals reflects the change in cataclasis mechanisms. The critical particle size d_c, probably corresponds to the grind limit of the gouge material. 3)From the PSD results, the surface fracture energy calculated is 0.63MJ/m² for the Wenchuan earthquake fault gouge.

COMPARISON AND ANALYSIS OF WELL WATER-LEVEL VARIATIONS CAUSED BY LUSAN M_S 7.0 EARTHQUAKE IN 2013 AND WENCHUAN M_S 8.0 EARTHQUAKE IN 2008

YANG Zhu-zhuan, DENG Zhi-hui, YANG Xian-he, TAO Jing-ling

2014, 36(2):  380-391.  DOI: 10.3969/j.issn.0253-4967.2014.02.009

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With the Wenchuan and Lushan epicenters as the center, 8 wells were selected to make a comparative analysis of coseismic variations of well-water level caused by the two earthquakes. Emphasis was laid on the three wells, namely, the Huajiang well in Rongchang, the Liuyin well in Beibei, and the Luguhu well in Sichuan Province, where reverse coseismic variations of water-level were observed during the two earthquakes. We further collected the coseismic variations of water-level data of the three wells caused by two remote earthquakes, the Japan M_S9.0 earthquake in March 11, 2011 and the Sumatra northern sea area M_S8.6 earthquake in April 11, 2012. We also computed the coseismic volumetric strain caused by Wenchuan and Lushan earthquakes respectively in order to interpret the mechanisms. During the Wenchuan earthquake, the coseismic variations of water-level of the Huajiang well in Rongchang, the Liuyin well in Beibei, and the Luguhu well coincided with the coseismic volumetric strain at the wells, that is, the wells located in the compressed area of coseismic volumetric strain showed ascending in the coseismic variations of water-level, while the wells located in the dilatational area of the coseismic volumetric strain showed descending in the coseismic variations of water-level. During the Lushan earthquake, the coseismic volumetric strain and the coseismic water-level variations of the three wells were inconsistent, which are similar to the coseismic variations caused by the Japan and Indonesia earthquakes.

3-D CRUSTAL P-WAVE VELOCITY STRUCTURE IN WESTERN YUNNAN AREA AND ITS TECTONIC IMPLICATIONS

YANG Ting, WU Jian-ping, FANG Li-hua, WANG Chang-zai

2014, 36(2):  392-404.  DOI: 10.3969/j.issn.0253-4967.2014.02.010

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Using the absolute relocation method, we relocate 1593 earthquakes from the seismic waveform data recorded by the dense temporary seismic array from August 2011 to August 2012, which is deployed in the southern segment of the north-south seismic belt. Then seismic traveltime tomography method is applied to obtain the 3-D crustal P-wave velocity structure in the western Yunnan area. The inversion results indicate that high-velocity anomaly extends to the middle crust from the surface in the Panzhihua area, with the high velocity on its west merging into a large-scale high-velocity block. We speculate that the high-velocity block plays a certain impediment to southward escaping of the Tibetan plateau materials, which caused the rapid uplift of the northern sub-block of the Sichuan-Yunnan active block. In the south of the Jinshajiang-Red River Fault, low-velocity anomaly exsists in the lower crust of the Tengchong block and Baoshan block. Since it is located in the subduction boundary between the Indian plate and Eurasian plate, we consider that the low-velocity anomaly may be caused by the high temperature derived from the upper mantle due to the Indian plate's eastward subduction. Jinshajiang-Red River Fault is the southern boundary of the Sichuan-Yunnan block. The high-velocity crustal structure and the weak seismic activities in the middle segment of the Red River Fault(Midu to Red River areas)imply that this region is locked currently. However, as the boundary between the blocks, it should be one of the key monitoring areas in the future.

CHARACTERISTICS OF FOCAL MECHANISMS AND STRESS FIELD OF THE CHUAN-DIAN RHOMBIC BLOCK AND ITS ADJACENT REGIONS

LUO Jun, ZHAO Cui-ping, ZHOU Lian-qing

2014, 36(2):  405-421.  DOI: 10.3969/j.issn.0253-4967.2014.02.011

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Because of the special geographical location and complex tectonic background of the Chuan-Dian rhombic block and its adjacent areas, researches on the tectonic stress and fault mechanics of the region have been a concern for scientists, recently. Focal mechanism solutions and focal depths of 75 M≥3.5 moderate earthquakes occurring in the Chuan-Dian rhombic block and its adjacent regions from Aug. 1st, 2007 to Apr.21th, 2013 are obtained by using CAP method. Combining the results with the historical focal mechanism solutions from Harvard CMT, the characteristics of the spatial distribution of the focal mechanisms and focal depths are investigated, and their tectonodynamic background is discussed. Main conclusions are drawn as follows. 1)The spatial distributions of focal mechanisms differ among different faults and sub-blocks inside the block and those outside the block, which indicate the heterogeneity of the regional stress filed. 2)Spatial distribution of focal mechanisms is consistent with the tectonic background, suggesting that the recent faulting and earthquake mechanisms of the Chuan-Dian rhombic block and its adjacent regions are controlled mainly by the faulting of respective fault zones and the interaction between sub-blocks. 3)The special features of focal mechanisms, P axis and T axis distribution in the Lijiang-Xiaojinhe Fault zone further confirm the absorption and shielding effect of the fault on the plateau's escaping masses. 4)The predominant focal depth in the Chuan-Dian rhombic block and its adjacent region is 5 to 15km, suggesting that brittle seismic layer beneath the Chuan-Dian rhombic block is in the upper crust.

STUDY ON THE ASSOCIATION BETWEEN THE SURFACE LATENT HEAT FLUX(SLHF)IN TENGCHONG REGION WITH THE EARTHQUAKES IN SICHUAN-YUNNAN REGION

CHEN Mei-hua, DENG Zhi-hui, LIANG Qi-zhen, DING Liu-wei, TAO Jing-ling

2014, 36(2):  422-433.  DOI: 10.3969/j.issn.0253-4967.2014.02.012

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Domestic and foreign scholars found that SLHF anomaly occurred before some strong earthquakes in coastal area or in inland region with abundant water. The author noted that SLHF anomaly in some special tectonic position is not only associated with the strong earthquakes nearby but also with the strong earthquakes far away in the same tectonic belt. The dynamic variation rule of SLHF anomaly in the special tectonic position prior to earthquakes might reveal some trend of regional seismic activity and contribute to the research of mechanism of SLHF anomaly prior to earthquakes. Tengchong area in Yunnan Province is such a special tectonic position located in the south of the eastern Himalayan syntaxis and is one of the youngest volcanic regions on the Chinese mainland with frequent magmatic activity and strong hydrothermal activity.The earthquakes (M_S≥6.4) since 2000 in Sichuan-Yunnan region(including the Yao'an M_S6.5 earthquake in 2000, the Ning'er M_S6.4 earthquake in 2007, the Wenchuan M_S 8.0 earthquake in 2008, the M_S7.0 Myanmar earthquake in 2011 and the Lushan M_S7.0 earthquake in 2013)are selected as cases to study the variation characteristics of the SLHF in Tengchong region before and after the above earthquakes.It is found that significant SLHF anomalies occurred in Tengchong area not only before earthquakes happening in the vicinity but also earthquakes far away in the surroundings. The SLHF anomalies occurred mostly within one and half months before the earthquakes, except Wenchuan M_S 8.0 earthquake. The SLHF anomalies occurred two months before Wenchuan earthquake, the earlier occurrence of the anomalies might be due to the relatively long distance between Wenchuan earthquake and Tengchong area. It is also found that the amplitude of anomaly is related to earthquake magnitude, the larger the magnitude of earthquake, the larger the amplitude of the anomalies. For example, the SLHF anomalies in Tengchong region prior to Burma M_S 7 earthquake and Wenchuan M_S 8.0 earthquake are very strong, far more than the reference maximum values, but the SLHF anomalies prior to Yao'an M_S 6.5 earthquake and Ning'er M_S 6.4 earthquake are relatively small.The sensitive response of SLHF in Tengchong region to the surrounding earthquakes on the one hand is related to the development of active faults and the present-day intensive crustal deformation;on the other hand, there are strong volcanic activities in Tengchong area, with springs widely developed and water heat exchanging quickly. So as a result, the SLHF of this region is sensitive to surrounding tectonic movement.

LATE-QUATERNARY DEFORMATION AND GEOMORPHIC FEATURES ALONG THE LONGLING-RUILI FAULT

HE Gen-wen, FU Bi-hong, LIU Ming, YANG Shun-hu, SHI Pi-long, LIU Feng

2014, 36(2):  434-448.  DOI: 10.3969/j.issn.0253-4967.2014.02.013

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Longling-Ruili Fault zone(LRF)is located at the boundary between China and Burma, striking in northeast direction for about 135 km and controling the formation and evolution of Longling, Luxi, Zhefang and Ruili Basins. This study utilizes the high-resolution remote sensing data, such as ALOS and ASTER and SRTM digital elevation model data in combination with field work and RTK-GPS(Real Time Kinematic-Global Positioning System)to present the tectono-geomorphic features and geologic structures along the LRF. Analysis from the remote sensing data reveals that the LRF always develops along the south and north edges of the basins, and forms the NE-striking linear mountains and valleys between the basins, with a general strike in N55°E direction. In the field, many geomorphic features, including saddle-shaped landform, fault facets and shutter ridges, are developed along the LRF;While at Xintangfang and Ganlanpo, a several meters-wide shear zone crops out with fresh fault gouge. All the descriptions above demonstrate that the LRF is sinistral strike-slip fault and has been active since late Quaternary.To gain more insight into the activity of the LRF, a trench was excavated at Zhujiazhai which reveals the fresh fault plane at the base rocks and late Quaternary alluvial deposits above, but no evidence found indicating the fault offset those young deposits. Radiocarbon dating of charcoal samples from the bottom of alluvial fans indicates that there is no destructive earthquake happening along the north segment of LRF since 1150 B.P. Finally, analysis for the 1976 Longling earthquake and distribution of historical earthquake imply that the 1976 destructive earthquake may be triggered mostly by the NNW-striking newly formed fault zone rather than the NE Longling-Ruili Fault zone.

TOPOGRAPHIC VARIATION ALONG THE MIDDLE-EAST SEGMENT OF HAIYUAN FAULT ZONE AND ITS IMPLICATIONS

CHEN Tao, ZHANG Hui-ping, WANG Wei-tao

2014, 36(2):  449-463.  DOI: 10.3969/j.issn.0253-4967.2014.02.014

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Tectonic geomorphology introduces the quantitative topographic factors to describe and characterize the landform in real world, which is accepted as one efficient approach in neotectonics study now. More and more qualitative and quantitative researches have been implemented to delve into the response of surface topography to tectonic activity, or discuss the further subsequent geomorphology evolution. The various topographic characteristics along the middle-east segment of the Haiyuan Fault, which is located on the northeastern margin of Tibet plateau, indicate the different geotectonic backgrounds and evolution processes. Five quantitative topographic factors (i.e. elevation, slope, local relief, residual relief and channel steepness)derived from 90-m-grid SRTM DEMs all demonstrate higher values on the western section, lower values on the eastern section and middle section as well. However, all factors slightly increase to local peak values at the southeastern tailing end. Combining with average annual precipitation and geologic mapping, we discuss the basic mechanism about how geotectonic, climate and bedrock type would impose and build up various landforms. As demonstrated by our analysis our analysis, the precipitation is thought to aggravate the surface erosion and accelerate the landform evolution, and there is no significant correlation between the distribution of topographic factors and the bedrock type. Statistic result indicates the relative strong extrusion uplift on the western section. The middle part is a transitional zone and affected by Yellow River incision and widespread fluvial terraces. The influence of compressive folding at the southeastern tail of large strike-slip fault is also revealed by topographic variations.

ACTIVITY AND SLIP RATE OF THE NORTHERN SECTION OF YELLOW RIVER FAULT REVEALED BY DRILLING

LEI Qi-yun, CHAI Chi-zhang, ZHENG Wen-jun, DU Peng, XIE Xiao-feng, WANG Yin, CUI Jin, MENG Guang-kui

2014, 36(2):  464-477.  DOI: 10.3969/j.issn.0253-4967.2014.02.015

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Yellow River Fault is the longest, deepest fault in the Yinchuan Basin, also is the eastern boundary of the basin. Because its north section is buried, its activity and slip rate remains unknown, which made a negative impact on understanding the evolution and seismic hazard of the Yinchuan Basin. In this study, a composite drilling section with a row of drillholes were laid out along the northern section of the Yellow River Fault based on the results of shallow seismic exploration near the Taole Town, where oil seismic exploration data are available. Fault activity and slip rate are obtained by measuring the age of samples of holes. The results show that the northern section of the Yellow River Fault is a late Pleistocene or Holocene Fault, its accumulative displacement is 0.96m since (28.16±0.12)ka BP, with an average slip rate of 0.04mm/a, which is significantly lower than the southern section. The activity intensity of the northern section of the Yellow River Fault is significantly lower than the southern section since Late Quaternary. In the Yinchuan Basin, the Helanshan eastern piedmont fault is the most active fault since late Quaternary, next is the Yellow River Fault, then, the Yinchuan buried fault and Luhuatai buried fault. Although the Yellow River Fault is the deepest and the longest fault, its maximum potential earthquake is magnitude 7, this seismogenic capability is weaker than the relatively shallower Helanshan eastern piedmont fault, on which occurred the Pingluo M8 earthquake in 1739 AD. Yinchuan Basin is the result of long-term activities of the four major faults, which shaped the special structure of the different parts of Yinchuan Basin. The Yellow River Fault controlled the evolution of the south part of Yinchuan Basin. The two-layer crustal stretching model can help us understand the structural deformation between the upper crust and the lower crust beneath Yinchuan Basin. Deformation of the upper crust is controlled by several brittle normal faults, while the deformation of the lower crust is controlled by two ductile shear zones. The shear sliding on Conrad discontinuity coordinates the extensional deformation of different mechanical properties between the upper and the lower crust. Yellow River Fault might have cut deeply into the Moho in Mesozoic, the tectonic activity in Yinchuan Basin began to migrate and was partitioned into several faults since the beginning of the Cenozoic, mainly in the Helanshan eastern piedmont fault. This may be the reason why the Yellow River Fault has lower seismogenic capability than the shallower Helanshan eastern piedmont fault.

USING DEFORMED FLUVIAL TERRACES TO CONSTRAIN GROWTH MECHANISM AND RATES OF THRUST RELATED FOLD：METHODS AND RECOGNITION

LI Tao, CHEN Jie

2014, 36(2):  478-488.  DOI: 10.3969/j.issn.0253-4967.2014.02.016

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The foreland basin of contractional orogen commonly occurs in thrust related fold. Quantitative constraint on fold's growing process is important to understand the spatial and temporal evolution of orogens, and for seismic hazard assessment and hydrocarbon resource prospect evaluation. Fluvial terrace, as a kind of widely distributed, easy dating and passively deformed geomorphic marker, is more and more applied to defining the growth mechanism and rates of active folds. Combining deformed fluvial terrace with pre-growth and growth strata, we can acquire the total shortening and the initiation age, and retrieve the whole evolving history of a fold.Based on geometry and kinematical characteristics, thrust related fold can be classified into fault bend fold, fault propagation fold and detachment fold. As occurred on the tip of a fault, the fault propagation fold and detachment fold are generally named fault tip fold. According to different stress situation, folding deformation includes no shear, simple shear and pure shear, and displays as two end-member models: kink band migration and limb rotation. In different models, the fluvial terrace shows different deformation characteristics, which can be used to distinguish the models.We summarized and discussed how to use deformed terraces to constrain the shortening rates and uplift rates of the sinusoidal and chevron flexural-slip fold, classical fault bend fold, as well as pure shear fault tip fold, and briefly introduced the conception of fold scarp. However, the natural fold is usually formed by combination of different mechanisms, which is much more complicated than the models mentioned. Therefore, the future study should be concentrated on modeling the fold produced by different mechanisms, rather than only one mechanism. In addition, we mentioned three different types of fold scarps: fold scarp occurring in classical fault bend fold and fold scarp formed by kink band migration and limb rotation in detachment fold.

THE DEEP GEOLOGICAL STRUCTURE CHARACTERISTICS OF PARTS OF LIAOJI PALEORIFT

LIU Xiao-yang, XUE Lin-fu, LIU Zheng-hong, Jiang Yan, LI Tong-lin

2014, 36(2):  489-500.  DOI: 10.3969/j.issn.0253-4967.2014.02.017

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Measured on the basis of four high-precision gravity and magnetic profiles and the MT profiles, combined with regional geological data and field geological research, the deep-seated geological issues of the Liaoji Paleoproterozoic rift, such as the boundaries, the internal geological structure and the substrate composition, are discussed. The newly acquired non-seismic geophysical profile data provide important information for revealing the deep geological structure of the Liaoji rift. It is found along the MT inversion profile that a clear south-dipping structure interface exists in the area of Lianshanguan-Saima, which is an extensional fault surface that controls the northern boundary of Liaoji rift. In the past, the line of Benxi city-Benxi county, where the northern boundary of Liaohe Group is distributed, was considered to be the ancient geographical boundary of Liaoji rift. It is confirmed that the difference between lithofacies of the north and south Liaohe Group is caused mainly by phase transition. MT inversion profiles reveal that the interior of the Liaoji rift is characterized by double-fold structure, with the basement depth reaching over 10km. Regional geological studies and joint gravity-magnetic-electric inversion results show that two types of basement are developed beneath Liaoji rift: (1)Archean supracrustal rocks-granite, and(2)Archean granite-Proterozoic granite.

RADIATION CHARACTERISTICS OF ARTIFICIAL CORNER REFLECTORS AND PARAMETERS SELECTION FOR ITS PIXEL LEVEL PRECISION POSITIONING

ZHANG Gui-fang, GUO Li-min, SHAN Xin-jian, QU Chen-yan

2014, 36(2):  501-512.  DOI: 10.3969/j.issn.0253-4967.2014.02.018

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Accurate positioning of corner reflector (CR) is the precondition to monitor the small deformation. CR can be looked as a better point target and it is expected that the energy remains more or less the same when processing different looks with fractional azimuth and range bandwidth. The intensity, the mean to standard deviation ratio (MSR) of energy and the spectral correlation are used to measure the spectral diversity. The paper begins with the performance of CRs, then describes in detail the intensity image, spectral correlation image and MSR image of CRs in two SAR data: Envisat data and TerraSAR data. Based on these work, we draw the below conclusions: 1)corner reflector is obvious on the intensity image, and its strong reflectance to radar wave tends to be met also by some adjacent pixels. In Envisat data CRs appear as bright targets, and CRs in TerraSAR data show as clear crosses;2)In MSR image CRs signals are too weak to be easily detected;3)One advantage of the correlation measure is that it is "well focused", so in the case of CRs high values, they are typically found only in one pixel, and the correlation value of this pixel is obviously higher than adjacent pixels. Generating the list of coordinates of CRs with the three criteria parameters: high intensity value, high MSR value and high spectral correlation value, respectively, and then analyzing comparatively the CRs' pixels location from different criteria parameters and the corresponding phase information, we can get the results as follows: 1)the intensity is susceptible to disturbance from the background environment, and the CRs pixels located by this parameter are offset when their environment is complex;2)the difference is so small between the CRs' MSR value and the background that it can't detect the CRs;3)compared with the CRs' phase located with correlation parameter and other parameters, the phase of former is more stable, indicating that correlation parameter can locate the CRs accurately.

A STUDY OF WATER LEVEL ANOMALIES USING HYDROCHEMICAL AND PHYSICAL METHODS

ZHANG Lei, LIU Yao-wei, SUN Xiao-long, FANG Zhen

2014, 36(2):  513-522.  DOI: 10.3969/j.issn.0253-4967.2014.02.019

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Regional groundwater level change, such as rising or falling, may provide earthquake precursor information. The groundwater levels in earthquake observation wells have been rising in Jiangsu Province since 2008. By analyzing hydrochemical and physical characteristics of the Su-18 well water and surface water nearby, we studied the relationship between groundwater level anomaly and supply of surface water and atmospheric precipitation. Furthermore, we discussed the source of water supply to Su-18 well and the corresponding relationship between the rising water level and the M_S4.9 earthquake. The results show that, the groundwater of Su-18 well and reservoir water nearby are recharged by meteoric waters based on analysis of ion contents and hydrogen and oxygen isotopes. The method of assessment of precipitation recharge to groundwater by the chloride concentration found that the rise of water level is related to the increased rainfall over the same period. There are also hydraulic connections between the surrounding reservoir and Su-18 well. Impoundment activities of reservoir have impact on the groundwater recharge. In summary, the factor affecting the groundwater level anomaly at Su-18 well in 2011 is impoundment of reservoir which was affected by atmospheric precipitation. Reservoir water supplies well water through lateral permeation. This conclusion provides a basic reference for determining water level anomalies in earthquake observation wells in the future.

STUDY ON REGIONAL GRAVITY CHANGES BEFORE THE M_W 9.0 JAPAN EARTHQUAKE DETECTED BY GRACE

WANG Wu-xing, GU Guo-hua, CHEN Shi

2014, 36(2):  523-535.  DOI: 10.3969/j.issn.0253-4967.2014.02.020

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Regional gravity changes before the M_W 9.0 Japan earthquake are obtained from the observations of GRACE satellites. At the same time, time series of vertical displacements at GPS fiducial stations of TSKB, DAEJ, SUIY and HLAR are acquired in the ITRF 2000 reference frame. There are mainly two stages of the pre-earthquake regional gravity changes. The first is a process with slightly gravity change behaved as gravity rise and then decline during August 2002 to the end of 2007 or 2008, which can be fitted with quadratic polynomial. The second stage is showed as gravity increasing rapidly and then decreasing after the first process. Significant gravity decrease appeared in Heilongjiang and Ussuri and its surrounding region in 2008. The gravity increased significantly in vast region along the Pacific subduction zone in 2009 and 2010. And the areas of positive and negative gravity changes migrated towards the southwest. Comparing with 2009, the positive and negative gravity change areas are divided into west and east distribution along the plate boundary. Gravity decreased rapidly at locations of the eastern part from the beginning of 2010 until the occurring of the earthquake. While gravity changes at locations in the western part were still positive with high value, which began to decline after the earthquake. Far-field GPS vertical displacements showed an uplift beginning from the middle of 2006, and this trend turned in early 2009. Comparison with the gravity changes caused by variation of surface water contents shows that the regional gravity changes are mainly caused by changes of surface water contents. But gravity changes at locations P01 and P03 in the ocean side have the same characteristic. This suggests that regional gravity anomalies may be related to underground mantle migration and heat which caused regional climate changes. The relationship between these abnormal variations and seismogenesis needs to be studied in the future.

SURVEY AND ANALYSIS OF AWARENESS OF THREE GROUPS OF PEOPLE ON THE DIFFERENCE OF EARTHQUAKE EMERGENCY RESCUE INFLUENCING FACTORS：A CASE STUDY OF TANGSHAN

GAO Na, SU Gui-wu, DENG Yan, NIE Gao-zhong, WU Huan-jie, TAO Peng

2014, 36(2):  536-546.  DOI: 10.3969/j.issn.0253-4967.2014.02.021

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Based on a large number of previous researches and the international similar experiences, and in combination with the characteristics of China earthquake emergency management, a county-level earthquake emergency rescue capability indicators evaluation system is set up in this paper, which consists of 34 indicators and is thought to be an ideal and comprehensive indicators system. Then taking the indicators system as the study object, and based on AHP and questionnaires, we conduct face-to-face interviews and analysis on the awareness of the importance of these indicators among three different groups, namely, the local scientific and technical personnel engaged in earthquake research, the managements of the local earthquake administrations, and the personnel engaged in earthquake study at grass-roots level. The results show that: (1)Three different groups all think that different indicators have different effects on earthquake emergency rescue. Overall, the impact factors are listed as follows in the order of importance: seismic capacity of buildings, degree of settlements concentration, hazard source control, and evacuation site and so on.(2)For different respondents, they have different understanding about the weights to specific indicators or factors. There exists a significant difference between them in understanding the importance of the indicator of basic environmental background.