Table of Content

30 December 2013, Volume 35 Issue 4

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MICROSTRUCTURAL FEATURES AND MINERALOGY OF CLAY-RICH FAULT GOUGE AT THE NORTHERN SEGMENT OF THE YINGXIU-BEICHUAN FAULT, CHINA

YUAN Ren-mao, ZHANG Bing-liang, XU Xi-wei, LIN Chuan-yong

2013, 35(4):  685-700.  DOI: 10.3969/j.issn.0253-4967.2013.04.001

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Thin-layered clay-rich fault gouges were observed on the near-surface fault plane of the Yingxiu-Beichuan Fault during the 2008 M_S 8.0 Wenchuan earthquake. In this paper,the authors investigate the microstructural features and mineralogy of the clay-rich fault gouge samples based on the stereoscopic and optical microscopic observation and X-ray diffraction analysis. These fault gouge samples were collected from the trench excavated at the Beichuan segment of the Yingxiu-Beichuan Fault,where the largest vertical offsets were observed at the earthquake surface ruptures. The results show that the typical microstructures of localized brittle deformation are well developed in the co-seismic fault gouge,including Y-shears,R1-shears(the angle between Y-shears and R1-shears is larger than 14°),R2-shears,P-shears,tension cracks and stepped fragmented grains. All these microstructures are commonly accepted as the result of seismic slip event. In addition,there are also some microstructures similar to those representative of the distributed ductile deformations developed well within the gouges,including P-foliation,elongated clast grains and asymmetric trails of elongated clastic grains. However,all these microstructures are developed only in the area between two parallel Y-shears,indicating that they were induced by the same slip event along the two shears. The microstructures mentioned above indicate the thrust movement of the fault during the Wenchuan earthquake. Field investigation shows that the new fault gouge is very narrow with the thickness of about 3mm,which indicates that the slip movement of the fault was constrained in a very narrow slip zone. The results of analysis show that the contents of feldspar and quartz in the gouges are lower than those in the wall rocks,while the content of clay in the gouges is higher than that in the wall rock. This may indicate that feldspar and quartz were converted partly into clay or clay-particle sized materials because of the friction energy induced by fault slip. Moreover,the frictional heating produced by fault slip might cause the conversion of the I/S mixed layer in the clay-rich gouge into the illite(probably including some chemical reactions of solution),which made the differences of the contents of the illite or the I/S mixed layer in the gouge and in the wall rocks. These results present some criterions for identification of the features of the fault movement.

TECTONIC STRESS CHANGES AROUND THE EPICENTER BEFORE AND AFTER THE 2008 WENCHUAN EARTHQUAKE

FENG Mei, ZHANG Ji-zhong, AN Mei-jian

2013, 35(4):  701-720.  DOI: 10.3969/j.issn.0253-4967.2013.04.002

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The 2008 Wenchuan earthquake started at the intersection of the south and central segments of the Longmenshan Fault. Its aftershocks extended northward to the whole central and north segments of the Longmenshan Fault. The seismicity of the aftershocks along the central and north segments of Longmenshan Fault zone decreased very quickly. Within one year after the Wenchuan earthquake,the number of aftershocks per month with magnitude >3.5 is greater than 10,indicating that there was a strong tectonic stress adjustment around the epicenter region during this period. However,after April of 2009,the number of aftershocks per month with magnitude >3.5 is less than 10,indicating that the tectonic stress adjustment became more stable. Even after the Ya'an Lushan earthquake occurring in the south segment of the Longmenshan Fault in April,2013,the seismicity in the central and north segments does not increase obviously. In the present study,we firstly calculated focal mechanisms of micro-earthquakes from about one year of seismic observations recorded by 26 stations located around the main-shock epicenter. We then made an analysis on the tectonic stress changes before and after the main shock,based on our calculated focal mechanism solutions and previous results of focal mechanisms and in-situ stress measurements. The analysis results show that the maximum principal stress direction close to the epicenter changes from near E-W before the main shock to near N-S after the main shock. The focal mechanisms indicate that the Dujiangyan-Yingxiu-Gengda Fault,separating the middle segment with the south segment of the Longmenshan Fault zone,and the region to its north have a trend of northeastward movement. The northeastward movement may be one of the reasons triggering the Ya'an Lushan earthquake which occurred in the middle of the south segment of the Longmenshan Fault zone in 2013.

THE STUDY ON THE FINITE ELEMENT MODEL OFDEFORMATION AND STRESS FIELDS DUE TO THE STRUCTURAL MOTION OF INVERSE FAULT—A CASE OF YINGXIU-QINGCHUAN FAULT

BAI Yu-zhu, ZHOU Qing

2013, 35(4):  721-730.  DOI: 10.3969/j.issn.0253-4967.2013.04.003

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This paper mainly studies the distribution of structural deformation and stress under the motion of thrust fault due to the effect of stress normal to the strike direction. As an example,we build the finite element model of central fault of Longmen Shan Fault zone,the Yingxiu-Qingchuan Fault. The finite element model is the model of single inverse fault. During the study,we compute the variations of surface deformation and stress fields near to the fault due to the thrust motion of Yingxiu-Qingchuan Fault in the process of Wenchuan earthquake. In the computation,we apply the frictional contact element and curved surface geometry to simulating the fault plane to obtain the variations of stress components and displacement field which are normal to strike direction and ground surface. In order to get the stress value of the hanging fall of the Yingxiu-Qingchuan Fault,we also apply the static dislocation model to the computation. Through the computation,we find that the displacement field along the strike direction can exist even if there is no initial stress load along the strike,but the value of this deformation is tiny. So we believe the displacement field along the strike may stem from the transverse deformation due to the extrusion of thrust fault; The structure deformation normal to the strike direction resulting from the thrust motion of inverse fault does not reach to maximum value at the zone near the fault,but the structure deformation normal to the ground surface can reach the maximum at the zone near to the fault; Generally,under the compression stress effect,the amount of crustal shortening is much bigger than that of uplift deformation at the zone near to the inverse fault,and the value of compression stress is much bigger than that of stress normal to ground surface along the fault plane. Furthermore,variation of stress on the inverse fault plane occurs mainly at the area where the dip angle of fault plane changes.

CONTENTS

NEOTECTONICS AND KINEMETICS ALONG THE YABRAI RANGE-FRONT FAULT IN THE SOUTH ALASHAN BLOCK AND ITS IMPLICATIONS FOR REGIONAL TECTONICS

YU Jing-xing, ZHENG Wen-jun, LEI Qi-yun, SHAO Yan-xiu, GE Wei-peng, MA Yan, LI You-juan

2013, 35(4):  731-744.  DOI: 10.3969/j.issn.0253-4967.2013.04.004

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The Yabrai range-front fault is a normal fault,which is about 120km long,trends N60°E and distributes along the southeast margin of the Alashan block. In this paper,we focus on the geomorphology and kinematics of the Yabrai range-front fault,and discuss the implications of the fault for the regional tectonics.
This fault consists of three segments and the most active one is located in the southwest,which has a length of about 35km. The about 1～2m-high scarp,stretching almost the full segment,might be the result of the latest earthquake event. Fresh free surface indicates that the elapsed time of the last event should not be long.
The middle segment is about 31km in length. The results suggest that just a single fault is developed along the piedmont of the Yabrai Shan,and there is no evidence of recent activity on this fault. In contrast to the simple geometric structure of the middle segment,the northeast segment consists of several faults. The scarps of the most recent earthquake event,which are clear but discontinuous,are about 0.5～1.5m high and some are up to 2m. Although the scarps along the southwest and northeast segments of the fault are similar,it is difficult to suggest they are caused by the same earthquake without precise dating.
The seismic reflection profile suggests that the Yabrai range-front fault came into being as a normal fault in Cretaceous,when the Tibetan plateau did not emerge at that time. Therefore,we conclude that the Yabrai range-front fault is not the consequence of the Indo-Asian collision. But this region plays a great role in constraining the tectonic evolution of the Alashan block and therefore,the Tibetan plateau.

ACOUSTIC SURVEYING OF THE BURIED FAULTS IN TIANJIN QIKOU DEPRESSION

YAN Cheng-guo, CHEN Yu-kun, GAO Wu-ping, YANG Fei

2013, 35(4):  745-753.  DOI: 10.3969/j.issn.0253-4967.2013.04.005

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Qikou depression is an important tectonic unit in the eastern part of the Huanghua depression,which constitutes the Cenozoic depocenter,with the Cenozoic maximum deposition thickness reaching to 8000～10,000m. Several profiles of acoustic survey have been conducted across the buried faults in Tianjin Qikou depression. The surveying results show that the high-angle near EW trending normal faults are well developed. The latest upper break points of these faults are 35m from the seabed,being very shallow,and the dislocated stratums are very new.
Combined with regional stratigraphic analysis,the buried faults should be late Pleistocene active faults. But the deep seismic profiles reveal that these faults in shallow strata appear as negative flower structures,which form a compound graben structure,and gradually integrate into the Qibei Fault and Haihe Fault in the deep part,as branch faults of Qibei Fault and Haihe Fault. So,these faults don't have great tectonic significance,as the normal faults associated with the coseismic slip at the shallow strata when earthquakes occurred at the deep part of Haihe Fault. The earthquake risk assessment of these faults should be different from other active faults. We can't think they have a high seismic risk,just because they are late Pleistocene active faults. But,there is a high seismic risk for the Haihe Fault.
The deep seismic profiles show that Haihe Fault and Haiyi Fault are boundary faults of Qikou depression,Qibei Fault and Qizhong Fault are the secondary faults within Qikou depression; Haihe Fault is shown as a listric normal fault on the seismic profile, and in its shallow part,multiple shallow branch faults are developed,which control the Cenozoic sedimentary development of Qikou depression. The Qibei Fault is the main branch of Haihe Fault,and as shown on the acoustic shallow stratigraphic profiles,it is a late Pleistocene active fault,though it is smaller in size relative to Haihe Fault,we can't ignore the seismic risk of Qibei Fault.

LATE QUATERNARY ACTIVITIES OF THE WESTERN SEGMENT OF THE CANGSHAN-NISHAN FAULT

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

2013, 35(4):  754-764.  DOI: 10.3969/j.issn.0253-4967.2013.04.006

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Many NW-trending faults are developed in West Shandong. Cangshan-Nishan Fault,about 130km long,striking 310°～340°,dipping to SW and NE with dip angle 70°～80°,is the largest one among these faults. According to geomorphological characteristics and relationship between fault and Quaternary deposits,Cangshan-Nishan Fault can be divided into three segments: the western segment(Fangshan-Tianhuang segment),about 30km long,controlling the western margin of Qufu Basin; the middle segment in the bedrock area(Tianhuang-Ganlin segment),about 80km long,forming a valley and controlling evolution of Baiyan River; and the eastern segment(Ganlin-Cangshan segment),buried in the Quaternary basin,about 20km long.
The western segment(Fangshan-Tianhuang segment)appears as a linear scarp in the satellite images. Field investigation shows that the linear scarp is mainly composed of rock with 2～5m high in topography. On the northeast side of the scarp is mountains composed of Archaeozoic Taishan group gneiss,and on the south-west side is late Pleistocene alluvial fan. A lot of profiles reveal that the late-Pleistocene deposits(the thermoluminescence dating results)are dislocated by the fault. The fault cross sections near the Qufu city show it is a normal fault with high scarps. The highest scarp is 4.7m high and the normal vertical slip rate is 0.07mm/a. However,the fault cross sections near the Tianhuang Town show it is a reverse fault with high dip angle. The highest scarp is about 1.5m high, lower than that near the Qufu city. All these information indicate that the fault,from west to east,is changed gradually from normal feature to reverse feature,and the height of fault scarp is decreased gradually from west to east.
Based on reported results in this area,Cangshan-Nishan Fault is a left-lateral strike-slip hinge fault. The results presented in this paper suggest that the western segment is dominated by normal dip-slip with left-lateral strike-slip component,the middle segment is dominated by left-lateral strike-slip with reverse dip-slip component. As the axes of hinge fault,the middle segment is the most active segment of Cangshan-Nishan Fault. Besides Cangshan-Nishan Fault,a series of NW-trending faults are developed in West Shandong with weak activity since late-Pleistocene. Many moderate-strong earthquakes are related to these NW-trending faults. We thus think these NW-trending faults have capability of generating moderate-sized quakes.

STUDY ON THE SLIP RATE OF CHANGMA FAULT IN QILIAN MOUNTAINS SINCE LATE PLEISTOCENE

LUO Hao, HE Wen-gui, WANG Ding-wei, YUAN Dao-yang, SHAO Yan-xiu

2013, 35(4):  765-777.  DOI: 10.3969/j.issn.0253-4967.2013.04.007

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The Hexi Corridor-Qilian Fault systems,the Altyn Tagh Fault and the Haiyuan Fault jointly control the north boundary and deformation of the Qinghai-Tibet plateau. The Changma Fault,as one of the Hexi Corridor-Qilian Fault systems,is a highly active strike-slip fault,and connects the Altyn Tagh Fault and the Haiyuan Fault. Based on the active characteristics and geometrical distribution,this fault is divided into four segments. We obtain the left-lateral strike-slip rates of three segments,which are 1.33±0.39mm/yr at the west segment,3.11±0.31mm/yr at the middle-west segment,and 3.68±0.41mm/ya at the middle-east segment,respectively,and the shortening rate at the west segment(0.70±0.20mm/yr).The result shows that the sinistral slip rate of the fault is significantly increased from west segment to east segment. The activity of Changma Fault accommodates～30%reduction of Altyn Tagh Fault slip rate. The studies in this paper confirm that the sinistral slip and shortening of Changma Fault and other secondary faults in accompany with deformation inside the basin absorb most of displacement of east segment of Altyn Tagh Fault,and this structural change mode supports the hypotheses that the northeastern margin of Qinghai-Tibet plateau has a continuous crust thickening mode with lateral displacement.

A STUDY ON THE ACTIVITY OF THE FAULTS AROUND AND THE LATE QUATERNARY TECTONIC FEATURES IN THE HANZHONG BASIN

WANG Ming-ming, ZHOU Ben-gang, YANG Xiao-ping, LI Jiang-yi

2013, 35(4):  778-792.  DOI: 10.3969/j.issn.0253-4967.2013.04.008

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Since the Wenchuan earthquake,the seismic hazard of the northeastern segment of the Longmenshan Fault zone as well as the Hanzhong Basin has drawn more and more concerns. However,the essential data needed for further analysis on the seismic hazard in this region is scarce at present time,hence there is an urgent need for an in-depth study on the activities of faults around the basin. The faults around the Hanzhong Basin include five main faults,namely,the northern margin fault of Hanzhong Basin,the southern margin fault of Hanzhong Basin,the Qingchuan Fault,the Chaba-Lin'ansi Fault and the southern margin fault of Liangshan. Based on several detailed field investigations on the geometric distribution,movement nature and active ages of the five faults,and with consideration of previous work,our study shows that the late-Quaternary tectonic activity in the basin is relatively intense in west and weak in east. The west section of the north-margin fault of the Hanzhong Basin(east of Baohe)was active in early late Pleistocene,while its eastern section(west of Baohe)was active in middle Pleistocene. The south-margin fault of the basin was also active in middle Pleistocene. And the three faults in the southwest of the basin were all active in late Pleistocene. This activity pattern of high in the west and low in the east is also demonstrated by the difference in thickness of Quaternary system and the distribution of small earthquakes.

EXPERIMENTAL STUDY ON EROSION RATE OF MAN-MADE SLOPES IN LOESS AREA

YAN Ji-ming, WEI Zhan-yu, HE Hong-lin

2013, 35(4):  793-804.  DOI: 10.3969/j.issn.0253-4967.2013.04.009

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The degradation of fault scarps in unconsolidated deposits can be accurately simulated. Under the appropriate correction,high-accuracy digital models can be built for the fault scarps evolution. These models provide the basis for estimating the age of faulting. In this paper we measured a series of artificial fault scarps with different slope angles in frequency of once a month,and found that the artificial fault scarps with slope angle above 30° are in an unstable stage and a group of parameters are acquired. There are obvious differences in erosion thicknesses,daily erosion rates and annual recession rates of artificial fault scarps. Besides the greatest erosion thickness of 6.13mm on the slope of 80°,there is a peak value of 5.24mm at the scarp of 50°,and the erosion thicknesses then gradually reduce for other slopes. There are the similar distribution characters in the daily erosion rates and the annual recession rate. Besides the recession rate of (6.74±0.26)mm/a on the slope of 80°,the greatest value of (7.41±0.84)mm/a is observed at the scarp with slope angle of 50°.But there is an abnormally high value of (8.19±1.16)mm/a in recession rate for scarps with a slope of 30°,which may result from the low angle. As the angle reduces,the deviation of the erosion thickness from the recession distance becomes bigger. The average recession rate of 5.8mm/a can be used as the correction value of the evolution of the normal fault scarps in Shanxi rift system,though it is based on the measurement of one year and has larger limitations. In addition,there is an obvious difference between field observations and ideal model. The accumulation amounts at the foot of scarps are not equal to the erosion amounts of scarp surface,due to that part of weathering material was washed away by the rainwater. The experimental observations show that there are not obvious accumulations during the rainy season(until October)due to rushing of rainwater,but the loess accumulations appear at the foot of scarp after the snow melts in winter. According to the measurements of one year,41.6% of total erosion amount is washed away by rainwater at the scarp with slope angle of 80°,and for others scarps the results are 52.4%,47.6%,50.6%,60.5% at slope angles of 70°,60°,40°,respectively.

PROSPECTING UNDERGROUND CONDITION FOR BOREHOLE STRAIN-MONITORING LOCATION BY MEANS OF RESISTIVITY IMAGING SURVEY

QIU Ze-hua, ZHANG Guo-hong, ZHANG Bao-hong, TANG Lei, SONG Nai-bo, GUO Yan-ping, ZHOU Long-shou, ZHAO Jing-yi, SU Wen-hao

2013, 35(4):  805-816.  DOI: 10.3969/j.issn.0253-4967.2013.04.010

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Borehole strain monitoring is an important geodetic means with a wide range of use in geodynamics research. One of the main reasons for the slow development of this kind of observation is that the establishment of a borehole strain monitoring site is costly and the success rate is not very high. Some sites fail due to the unfavorable borehole conditions,that is,rocks at the depth where the sensor is embedded are not intact but fractured. Sometimes even if the rocks were found not as good as required,the instrument had to be installed because of the expansive cost in drilling the hole. To solve the problem,it is necessary to prospect the rock condition of the site before drilling. Fractured rocks usually contain ground water in the fractures,which lower the rocks' resistivity. Resistivity imaging survey can be applied to the investigation of underground condition and give local distribution of resistivity with relatively high resolution. Three experiments have been carried out in Shanxi Province,in which single profiling is done at Shanghuangzhuang,cross profilings at Dongmafang and at Jiaokou,respectively. Three boreholes at Shanghuangzhuang and one at Dongmafang and at Jiaokou each were drilled for comparison of different types of instruments. Results of rocks strength experiments and instrument installations for the five boreholes agree well with results of the surveys. It suggests that resistivity imaging survey is an effective method to predict the underground condition of rocks. Instrument installation should avoid low-resistivity zones indicated by the profiling to prevent putting the sensor into fractured rocks.

ISOSTATIC GRAVITY ANOMALY AND DENSITY STRUCTURE BENEATH POTASSIC VOLCANIC REGION IN NORTHERN TIBET

LIU Guo-cheng, QIN Zun-li, HE Ri-zheng, WU Yan-gang

2013, 35(4):  817-832.  DOI: 10.3969/j.issn.0253-4967.2013.04.011

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This paper uses two techniques of isostatic correction and two-dimensional density fitting to analyze Bouguer gravity anomaly in Tibet,and obtains the regional and residual isostatic gravity anomaly in Tibet and two-dimensional density structure along 88.5°E. The results show that the crust and upper mantle beneath northern Tibet are in a deficit state with a lower density than other regions in Tibet. Based on the existing data of regional geology,tectonics,geophysics and geochemical characteristics of magmatic rocks in the northern Tibet,those great eruptions of potassic volcanoes in northern Tibet are the result of break-off of the front of lithosphere mantle of Asia under on-going northward subduction of the Indian lithosphere mantle since～65Ma.

TECTONIC STRESS ANALYSIS IN GANSU AND ITS ADJACENT AREAS

BU Yu-fei, WAN Yong-ge, ZHANG Yuan-sheng

2013, 35(4):  833-841.  DOI: 10.3969/j.issn.0253-4967.2013.04.012

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Gansu and its adjacent areas are on the northeastern margin of the Qinghai-Tibet plateau. The study area locates at the junction of Alashan,Qilian,Qaidam and Ordos blocks. The tectonic structure is complicated in this area. The research of tectonic stress field in the study area has an important scientific significance for earthquake prediction of Gansu. Characteristics of present-day tectonic stress field is studied based on 245 M_L≥3.5 focal mechanism solutions since January 2001 to June 2012 in Gansu and its adjacent areas.
For solving the stress field,we follow Wan's(2000)method to solve the rake angle according to two nodal planes' strikes and dips of the focal mechanisms. We treat the stress tensor solution of each fault plane as the optimal solution of the area when the slip on the fault plane minus the angle between shear stress direction and strike is minimal. We use F test to get the confidence interval of the four parameters, as well as the confidence interval of the principal compressive and tensile stress axes. We divide the whole area into 1°×1°grids. In order to cover the whole study region and get the smoothed stress field,we select the data within the square areas of 2°×2° centered at the grid point. The direction and the relative size of the tectonic stress field in 2°×2° grid in Gansu and its adjacent areas are presented for the first time. These results show that the dip angles of maximum principal compressive stress axes of the whole region are generally small. And the dip angles of maximum principal tensile stress axes vary widely,which are small in the western part,and relatively larger in the east. It indicates that the tectonic stress field with horizontal action of compression acts as the major in the west. Many deep large strike-slip faults in Qilian seismic belt correspond with the results. The geological structure of southeastern area of Gansu is complex,and the characteristics of stress field indicate that strike-slip reverse faulting is likely to occur. Result of the inverted tectonic stresses also shows that maximum principal compressive stress axis strikes near NE on the plate margin of Tibet plateau. This is caused by Qinghai-Tibet plateau moving to the northeast. At the outer edge of the Tibet plateau plate,its strikes show a radiated pattern,i.e. NS in west part,NNE in the middle section,and NNW in the east. The reason is that Qinghai-Tibet plateau,when moving to the northeast,is encountered with stable Ordos and Alashan blocks,therefore the material moves to southeast. The stress directions show that the source of the stress field in Gansu and its adjacent areas is extrusion of the India plate to Eurasian plate. Our inversion results are similar with the previous results,indicating the correctness of the approach. The relative stress values are more than 0.5 in the western areas and less than 0.5 in the eastern areas. According to Wan's research(2011), the maximum principle compressive stress solutions are more reliable in the western area,while the maximum principle tensile stress solutions are more reliable in the eastern areas.
The results indicate a non-uniform stress field spatial distribution in Gansu area,and have certain reference significance for explaining the seismo-geological background and earthquake prediction research.

DISCUSSION ON THE CHARACTERISTICS AND SEISMO-TECTONICS OF THE GAIZHOU M_L 4.7 EARTHQUAKE SWARM SEQUENCE ON FEBRUARY 2, 2012

CAO Feng-juan, WANG Liang, JIAO Ming-ruo, ZHAI Li-na, WANG Yan

2013, 35(4):  842-852.  DOI: 10.3969/j.issn.0253-4967.2013.04.013

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During the Gaizhou M_L4.7 earthquake swarm sequence on February 2,2012,the energy release weakened gradually with time,accompanied with two obviously accelerated release processes. The double-difference location results show that the spatial distribution of Gaizhou earthquake swarm is mainly concentrated in the area between the Jinzhou Fault and the Yingkou-Haicheng Fault,spreading roughly in NW direction. The sequence showed a trend of migrating from Jinzhou Fault to Yingkou-Haicheng Fault,and the focal depths are distributed from 6km to 9km.
The P-axis directions of focal mechanism of the two M_L 4 earthquakes occurring on February 2 are distributed from 94 to 120 degrees,which coincide with the previous studies on the regional tectonic stress field. We could infer that Gaizhou M_L 4.7 earthquake sequence on February 2 was caused by the strike-slip faulting of Jinzhou Fault under the action of NWW or near-EW regional stress field. But the P-axis direction of the two M_L 4 earthquakes occurring on July 12 and November 1 turned to near NS,and according to the precise relocations,these two earthquakes had the trend of migrating to Yingkou-Haicheng Fault. It is inferred,therefore,that the two obvious stress releases are not only the result of slip of Jinzhou Fault,the Yingkou-Haicheng Fault also played a certain role.
Combining with acoustic emission results,it can be further found that seismogenic structure of the M_L 4.7 earthquake swarm in Gaizhou is the two en echelon faults,the NE-trending Jinzhou Fault and the Yingkou-Haicheng Fault. This earthquake swarm could be caused by joint action of the two faults. Meanwhile,it also indicates that there are signs of activity of both Jinzhou and Yingkou-Haicheng Faults under the action of the regional stress field.

STUDY ON THE CRUSTAL THICKNESS BENEATH STATIONS OF SEISMIC NETWORK IN ANHUI PROVINCE BY TELESEISMIC RECEIVER FUNCTION

HONG De-quan, WANG Xing-zhou, LI Jun-hui, NI Si-dao

2013, 35(4):  853-863.  DOI: 10.3969/j.issn.0253-4967.2013.04.014

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In this paper,we calculate receiver functions of body wave under the 18 stations in Anhui Province from 3-component digital waveform data of teleseismic earthquake events and obtain the thickness and V_P/V_S ratio in the crust of this area through H-Kappa stacking. Our result is consistent with previous studies. Combining the tectonics data and our result,we consider that Anhui area can be divided into three areas according to the crustal thickness. The first one is the Dabie Shan area in the southwest of Anhui Province,the crustal thickness is about 35 to 38km; the second one is the southeast of Anhui Province,the crustal thickness is about 34km; and the last one is middle and northeast of Anhui,the crustal thickness is about 31 to 32km. The V_P/V_S ratio in Anhui area does not vary obviously spatially,but there are three stations LAN,SCH and JSA,under which the crustal velocity ratio is obviously higher,up to 1.79,1.80 and 1.80,respectively. In fact,the station LAN and SCH are located at the border between the North China block and the Yangtze block,and the JSA station is on the Tan-Lu Fault zone and beside the Sulu UHP metamorphic belt. Therefore,we infer that the high crustal velocity ratio under LAN,SCH and JSA stations are probably attributable to the deep large faults beneath the stations.

BODY WAVE TRAVELTIME TOMOGRAPHY FOR ZHEJIANG DIGITAL SEISMIC NETWORK

ZOU Zhen-xuan, LI Jun, ZHOU Xin-min, WANG Zheng, LI Jin-long

2013, 35(4):  864-872.  DOI: 10.3969/j.issn.0253-4967.2013.04.015

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Large amounts of seismic records from Zhejiang digital seismic network are collected,and by double difference seismic tomography,fine velocity structure and local heterogeneity of upper crust in Zhejiang Province and its surrounding areas are studied,and the relationship of the crustal velocity structure to the local active faults,geological and geographical structure is discussed. Research shows that there is a large-area low velocity zone developed in northern Zhejiang at depths between 3.5km and 6.5km,and also some low velocity zones distributed sporadically in southern Zhejiang,mostly around Shanxi Reservoir in Wenzhou. The research indicates that most earthquakes happened along active faults or active fault segments,and are concentrated in zones. These zones are almost all in the transitions between low to high velocity zones,and are significantly related to the regional topographic features. The correlation relation derived by this paper between earthquake,active fault,and geological and geographical structure will be of theoretical value and practical significance to the study of seismogenesis and the prediction of future strong earthquakes.

RELOCATION OF SMALL EARTHQUAKES IN LINFEN AREA, SHANXI, CHINA

DONG Chun-li, LI Le, ZHAO Jin-quan, LI Dong-mei, HU Yu-liang, REN Li-wei, XU Zhi-guo

2013, 35(4):  873-886.  DOI: 10.3969/j.issn.0253-4967.2013.04.016

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In the paper,we relocate the small earthquakes occurring in Linfen region from 1981 to 2010 by using the double-difference earthquake location algorithm. Our result shows that the distribution of relocated earthquakes is characterized by cluster activities,and most of the them occurred at the depths of 5～11km and 15～27km,which suggests that the two main seismogenic layers in this region are located in the upper and mid-upper crust. Earthquake frequency is high along both sides of the Subu Fault zone in the northern Linfen Basin,and most of the earthquakes in the north of Subu Fault zone are deeper than that in the south of Subu Fault. To some extent,this result indicates that the regional tectonic setting plays a major control role in the distribution of earthquakes. Analysis on the distribution of earthquake focal depths reveals that there exist deep faults which cut through the crust into the mantle in the middle of the Linfen Basin and connect with the active faults around the basin in the upper crust. The dislocations along the active faults on both sides of the basin are a reflex on the surface of faulting of the deep faults beneath the central basin.

A PRELIMINARY ANALYSIS ON THE INTERFERENCE GENERATED BY URBAN RAILWAY TRANSIT IN BEIJING TO THE GEOELECTRIC FIELD OBSERVATIONS

WANG Tong-li, HU Le-ying, CUI Bo-wen, SONG Yan-ru

2013, 35(4):  887-893.  DOI: 10.3969/j.issn.0253-4967.2013.04.017

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Earthquake geoelectric field observation in Beijing area is subject to serious interference from urban rail transit. In this paper,the interferences to the geo-electrical field observation data over the years from urban rail transit in Beijing area are analyzed. According to the characteristics of rail transit operation,FFT spectral analysis is performed on the seriously disturbed geoelectric field observation data of Tongzhou station to extract the data of normal variation from the geo-electric field observations; meanwhile,methods of processing of such disturbed data are also addressed.

POPULATION, HOUSING STATISTICS DATA SPATIALIZATION RESEARCH IN THE APPLICATION OF RAPID EARTHQUAKE LOSS ASSESSMENT—A CASE OF YILIANG EARTHQUAKE

HAN Zhen-hui, LI Zhi-qiang, CHEN Zhen-tuo, DING Wen-xiu, WANG Li-li

2013, 35(4):  894-906.  DOI: 10.3969/j.issn.0253-4967.2013.04.018

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The paper introduces the method of population and housing data spatialization. Field investigation data are used for the first time to test and verify the spatialized grid data. Based on Yiliang earthquake,the grid data and earthquake intensity map are overlaid to obtain the number of earthquake-affected population and housing in the support of GIS software. Compared with the data released by Earthquake Administration of Yunnan Province,the error of estimating earthquake direct economic loss of Yiliang earthquake in Yunnan area is minus six percent. The research shows that the map of population density and housing density can reasonably reflect the actual distribution of people and building. Using the grid data,the data of affected population and housing and the earthquake direct loss can be got directly and precisely,which can provide reliable basis for earthquake emergency rescue and decision-making.

RESEARCH ON EARTHQUAKE EMERGENCY PREPAREDNESS CAPABILITY AND HAZARD COEFFICIENT

GAO Na, NIE Gao-zhong, DENG Yan

2013, 35(4):  907-913.  DOI: 10.3969/j.issn.0253-4967.2013.04.019

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Earthquake emergency preparedness capability refers to the degree of disaster mitigation functions of a region or city against future destructive earthquake,which reflects the disaster-coping capability of a region or city in a future earthquake. To measure the size of earthquake emergency preparedness capability needs to estimate the future earthquake damage. In this paper,we introduce the hazard coefficient of earthquake emergency preparedness capability to express the future earthquake damage,and select two indicators,namely,the casualties and economic loss caused by earthquake to construct the disaster coefficient model of earthquake emergency preparedness capability. At last,using the earthquake damage forecast data in the future 20 years,the author calculate the hazard coefficients of earthquake emergency preparedness capability of the whole country,which lays a foundation for the precise calculation of earthquake emergency preparedness capability.

CONTENTS

COMPARISON AND ANALYSIS ON THE BASIC FEATURES OF RESERVOIR-INDUCED SEISMICITY

MA Wen-tao, LIN Yong, YUAN Jing-li, LI Hai-ou, XU Chang-peng, LUO Jia-hong

2013, 35(4):  914-929.  DOI: 10.3969/j.issn.0253-4967.2013.04.020

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Based on the differences of seismic activity,focal mechanism,geological structure,and hydrologic condition between reservoir induced seismicity(RIS)to natural earthquake,the basic features of reservoir induced earthquake from natural earthquake are studied statistically by the RIS database of 150 cases of reservoir induced earthquakes world-wide,and the results are shown as follows: ① Using statistical principle,we find that RIS occur in the areas near the reservoir banks in the impounding period,most of reservoir induced earthquake are distributed within a distance of 10km to the bank,with depth less than 10km,and a few events extend outwards as far as over 10km along fault zones,karst caves,and(thermal)springs; ②The frequency and magnitude of reservoir induced seismicity change with the reservoir water level. Most maximum magnitude earthquakes take place in the period before the first highest water level is reached,or in the following 2-3 impounding stages after the first highest water level,after then,a few would take place; ③The sequence of RIS is complete,usually as "foreshock-main shock-aftershock" or "foreshock-swarm-aftershock",most of them are microseisms of M_L1-3, so the b value is larger than natural earthquakes; ④ RIS differ from natural shocks in higher epicenter intensity,higher frequency of ground motion and higher peak ground acceleration,but faster attenuation with distance; ⑤The stress drop of RIS is smaller than that of natural earthquake of the same size,and the source size is larger; ⑥ The RIS occur in areas under lithological and geologic conditions of good permeability. Tensional fracture,tensional fault zone,unconsolidated fault plane,karst,and(thermal)springs are all good water penetration channels.

DEVELOPMENTS AND PERFORMANCE TESTS OF THE NEW AMS GRAPHITE TARGET LINE

YANG Xue, ZHENG Yong-gang, YIN Jin-hui

2013, 35(4):  930-934.  DOI: 10.3969/j.issn.0253-4967.2013.04.021

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A new AMS graphite target preparation line based on Fe/H₂ reduction method has been developed in the ¹⁴C laboratory at China Earthquake Administration,China. This paper demonstrates its performance on AMS-¹⁴C of internationally-accepted standards,known-age samples and blank measurements,and also discusses the reduction conditions of the graphite. The quality of this line is tested by the measurement of background and standard samples and the results show that the blank value of this line is between 0.45 and 0.49pMC,corresponding to the limited age of 45,700a,if the machine background is subtracted. We also have tested the reproducibility and memory effect of this line. Furthermore,we carried out the dendro-dated tree ring samples test and the comparison of different labs with same samples to test the reliability. These good results allow us to use the line in routine ¹⁴C dating.