Table of Content

30 December 2010, Volume 32 Issue 4

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Brief Report

A STUDY ON CHARACTERISTICS OF SOURCE PARAMETERS IN THREE GORGES RESERVOIR AREA

HUA Wei, CHEN Zhang-li, ZHENG Si-hua, YAN Chun-Qing

2010, 32(4):  533-542.  DOI: 10.3969/j.issn.0253-4967.2010.04.001

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Digital seismic data recorded by 26 temporary stations from March to November in 2009 in the Three Gorges reservoir area were used to study the characteristics of anelastic attenuation in this region using the Atkinson's method and to obtain the site responses of 26 temporary stations using the Moya's method.Moreover,source parameters of 1020 earthquakes with magnitude bigger than 0.1 were calculated and used to study the scaling relations among them.Our results show that(1)The anelastic attenuation in Three Gorges reservoir is Q(f)=112.0f ^0.918,with a smaller value of Q₀ and bigger value of η.Site responses of most temporary stations are 1～10 times except a few stations.(2)Seismic moment M₀ increases with increasing magnitude M_L and their statistical relationship is LogM₀=1.07M_L+9.76.Relationship between stress drop and magnitude is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model,with a statistical relationship LogΔσ=0.66M_L-3.08.(3)Radiated seismic energy and apparent stress in Three Gorges reservoir all increase with increasing magnitude,indicating that a big earthquake is a more efficient radiator of seismic energy than a small earthquake.(4)Stress drops of reservoir induced earthquake are apparently smaller than that of tectonic earthquake with same magnitude,about 10 times smaller.These results may be due to the increase of pore pressure that leads to the occurrence of earthquake with a lower tectonic stress.

STUDY OF CRUSTAL THICKNESS VARIATION IN CHONGQING SECTION OF THREE GORGES RESERVOIR AREA FROM TELESEISMIC RECEIVER FUNCTION METHOD

WANG Xiao-long, NI Si-dao, LIU Yuan-yuan, YU Guo-zheng, LI Ke-chang

2010, 32(4):  543-551.  DOI: 10.3969/j.issn.0253-4967.2010.04.002

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Based on teleseimic data for the period of 2007 to 2010 acquired from the Three Gorges reservoir(Chongqing section)seismic network and Chongqing regional seismic network,we obtained the crustal thickness and Possion's ratio of the area through receiver function method.The results show that the crustal thickness ranges from 38.9km to 50.9km.Station CHK in northeastern Chongqing has the largest thickness which is about 50.9km;the thinnest crust is 38.9km under the station ROC in the west of Chongqing,and Poisson's ratio is about 0.27.The results show that the maximum Posson's ratio is beneath station JIZ in the east of the Chongqing section of Three Gorges Reservoir area,and the minimum Possion's ratio is beneath the station WUL,which is 0.228.In comparison with the Poisson medium(0.25),its maximum deviates up to 20.8%,and the minimum deviates by-8.80%.The Bouguer gravity anomaly often reflects subsurface density and crustal thickness variation.In this paper,the two agree with each other.The largest negative Bouguer anomaly is between CHK(50.9km)and WUX(49.7km),where the crust is the thickest.The smallest negative Bouguer anomaly is between ROC(38.9km)and YUM(41km),where the crust is the thinnest.

INTENSIVE OBSERVATION OF RESERVOIR-INDUCED EARTHQUAKE AND PRELIMINARY ANALYSIS ON THE CAUSES OF EARTHQUAKES IN THREE GORGES RESERVOIR

MA Wen-tao, XU Chang-peng, LI Hai-ou, YUAN Jing-li, XU Xi-wei, ZHANG Xin-dong, ZHANG Lan-feng

2010, 32(4):  552-563.  DOI: 10.3969/j.issn.0253-4967.2010.04.003

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In accordance with the requirements of "Eleventh Five-Year" national scientific and technological support projects,an intensive seismic observation network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009(21 short-period seismographs,5 broadband digital seismographs).From March to December,2009,2995 M_L-0.8～2.9 earthquakes were observed during the trial impounding of the Three Gorges Reservoir(water level rose from 145m to 172.8m).Using the double difference earthquake location algorithm,2837 earthquakes were relocated precisely.The results show: The pattern of small local earthquake swarms in Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution,the mass-like clusters of events were generally within the distance of 5km from waterfront,and the linear distribution of the earthquakes could be extended to a distance of 16 kilometers away from waterfront.In Hubei section of Three Gorges Reservoir,earthquakes were mainly concentrated in the northern end of Xiannüfeng and Jiuwanxi Fault in the vicinity of the Xiangxihe river,and along the banks of the Yangtze river at the east of Xietan village and at Shenlongxi area on the northern bank in Badong region;the focal depths were less than 10km,and 4km in average.Earthquake frequency of the reservoir region had a positive correlation with reservoir water level fluctuation,indicating that the seismicity belonged to the reservoir induced earthquakes.Along the Shenlong river in the reservoir area,earthquakes showed three linear distributions in the northern Badong County,which was related to karst distribution.There are underground rivers along carbonate strata.When reservoir was impounded,water permeated into the underground rivers,thus inducing earthquakes.Earthquakes in the areas on the cross-river segment of Xiannüfeng Fault,the Jiuwanxi Fault and at the areas west of Xietan,Shazhen and Xizhen,may be related to the softening of discontinuities,such as the Nukou Fault,the Xiannüfeng Fault or the bedding joints,etc.,which would lead to failure of rock masses,thus,inducing earthquakes.However,convincing conclusions about the triggering mechanism still need further study.In addition,in the areas south of Wenhua and Yanglin of Zigui County and at Rangkou town east of Badong county,there are mining-induced earthquakes at the mines nearby.And on the shores of the reservoir,there are some collapse earthquakes.

RELOCATION AND GENETIC ANALYSIS OF EARTHQUAKES IN XIETAN AREA OF THREE GORGES RESERVIOR

LI Hai-ou, MA Wen-tao, XU Xi-wei, YUAN Jing-li, XU Chang-peng

2010, 32(4):  564-569.  DOI: 10.3969/j.issn.0253-4967.2010.04.004

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136 earthquakes,taking place in the west of Xietan area,recorded by portable stations deployed in the Three Georges reservoir area were relocated using the double difference algorithm.The relocations show that the root-mean-square shifts of the relocations in the directions of E-W,N-S and U-D are 0.38km,0.33km and 0.98km,respectively.The earthquakes in clasolite area with the focal depths about 4～5km take on linear distributions from shallow to deep part.These earthquakes were deduced as reservoir induced earthquake of fault fracture type.In the contrast,the earthquakes in limestone pavement with the focal depths about 2～3km take on slightly divergent distribution and have the characters of reservoir induced earthquake of the karts collapse type.

ANALYSIS OF EFFECT OF RESERVOIR WATER SEEPAGE AND LOADING ON THE OCCURRENCE OF EARTHQUAKES

LIU Yuan-zheng, MA Jin, JIANG Tong, PAN Ji-shun

2010, 32(4):  570-585.  DOI: 10.3969/j.issn.0253-4967.2010.04.005

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So far,although many reservoirs have been built,few of them have triggered seismicity.In order to study the mechanism of triggered seismicity so as to provide useful reference data for site selection of dams,we analyze the effects of hydro-geological structures and gravity loads in reservoir areas,and draw some conclusions:1)Rapid-response type seismicity may occur when there are some abandoned mines or karst caves;2)Based on the results of numerical calculation,Coulomb stress on different faults changes differently when the gravity load of the reservoir increases or decreases.The calculation results change with the stress field,the fault parameters and the relative position of the fault and reservoir.When the reservoir is on the down-thrown block and the fault dip is large,the fault may be more unstable because of the gravity load.In this case,Coulomb stress may increase locally on normal dip-slip faults,but on whole reverse dip-slip fault.Under the same fault occurrences,the Coulomb stress change of a normal dip-slip fault is larger than that of the reverse dip-slip fault.When the coefficient of friction is 0.6,the quantity of Coulomb stress change induced by gravity load is about 1/4 as much as that induced by pore water pressure in the fracture of the saturated karst cave.So,more attention should be paid to pore water pressure in the fracture.

THE RESISTIVITY VARIATION IN WATER LOADING OF LONGTAN RESERVOIR IN GUANGXI,CHINA

WANG Li-feng, ZHAN Yan, ZHAO Guo-ze, WANG Ji-jun, XIAO Qi-bin, MO Qing-yun, GAO Peng-fei, ZHANG Yuan-han, WEI Yong-fu

2010, 32(4):  586-594.  DOI: 10.3969/j.issn.0253-4967.2010.04.006

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Three times of magnetotelluric measurements were carried out during loading and unloading of the Longtan reservoir in Guangxi Zhuang Autonomous Region from 2009 to early 2010.The surveys were conducted at 4 sites with a frequency band 320Hz～1000s.The observational results show that the electricity of deep subsurface changed in different manners during the loading and unloading of the reservoir,depending on the specific structures and rock properties.At the 3 sites where the exposed rocks are of same lithology,the measured electric resistivity exhibited considerable variations at the band less than 1Hz.When the water level was the highest,the apparent resistivity was the lowest.As the water was declining,the apparent resistivity reached the maximum.And this value was the intermediate when the water level was the lowest.These measurements indicate that at different stages of loading and unloading of the reservoir,the rock properties,area of the reservoir and change of reservoir weight can pose a major influence on interconnection and seepage of underground water,causing variations of electricity of the subsurface at a certain depth.

STUDY ON THE S-WAVE SPLITTING CHARACTERISTICS IN LONGTAN RESERVOIR AREA,GUANGXI,CHINA

SHI Hai-xia, ZHAO Cui-ping

2010, 32(4):  595-606.  DOI: 10.3969/j.issn.0253-4967.2010.04.007

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In this paper,the shear wave splitting features in Longtan reservoir area are studied by adopting the traditional cross-correlation coefficient method and polarization analysis,using the data recorded by the seismic network founded by a project under the National Science and Technology Pillar Program from April 2009 to April 2010.We found that most of polarization directions at seismic stations are consistent with the direction of the overall regional stress field,but local structures and faults may control or influence the fast shear-wave polarization direction.The time-delay normalized to source-station path is between 10 to 25ms/km,and among them,the time-delay is about 10ms/km at the stations LIL and XIL,which are farther away from the dam.The water depth is relatively shallow and seismic activity relatively weak after water storage,indicating the effect of reservoir water penetration or loading on the state of cracks in the reservoir area.We also found that the delay time changes consistently with the water level at stations DPD and GAL.It may be related to crack expansion and water penetration caused by the reservoir impoundment.

DISTRIBUTION CHARACTERS OF WENCHUAN EARTHQUAKE AFTERSHOCKS IN ZIPINGPU RESERVIOR REGION

LI Hai-ou, MA Wen-tao, XU Xi-wei, XIE Rong-hua, YUAN Jing-li, XU Chang-peng

2010, 32(4):  607-613.  DOI: 10.3969/j.issn.0253-4967.2010.04.008

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815 aftershocks of Wenchuan earthquake recorded by 12 seismic stations in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the epicenters of aftershocks concentrate relatively in three regions.Focal depth distribution characters of different concentration regions take on obvious difference.This means earthquakes in different concentration regions may have different causes.Compared with relocations of earthquakes taking place before the Wenchuan earthquake done by other researchers,locations of the seismic concentration regions in the reservoir area changed noticeably after the Wenchuan earthquake.These variations are related with the stress adjustment of local part in the reservoir area and may also be related with the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.

THE FOCAL MECHANISMS AND CHARACTERISTIC OF STRESS FIELD IN ZIPINGPU AREA BEFORE WENCHUAN EARTHQUAKE

ZHANG Yong-jiu, ZHANG Zhi-wei

2010, 32(4):  614-627.  DOI: 10.3969/j.issn.0253-4967.2010.04.009

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Based on waveform data recorded by Zipingpu Reservoir Digital Seismic Network and Chendu Digital Seismic Network from August 2004 to May 2008 before Wenchuan MS 8.0 earthquake,and using ratio of amplitude,we calculated the focal mechanism parameters of 486 earthquakes with magnitude above 1.6 in Zipingpu area,and analyzed the temporal-spatial variation characteristic of focal mechanisms and stress.Results show:from 2006 to 2008,there were seldom thrust-type earthquakes,and strike-slip earthquakes increased in Zipingpu reservoir water field.The ratio of thrust earthquake was high,and strike-slip earthquakes' was low in areas far away from water field.Since the impoundment of the reservoir in October,2005,the thrust earthquakes in area A reduced significantly.The seismicity did not decrease in areas B and C,but showed a trend of increase.The strike-slip faulting earthquakes increased significantly in area A,but in areas B and C,they did not show an increasing trend.Under the background of increased thrust earthquakes,strike-slip earthquakes decreased in area C.The azimuth of mean principal compression stress fields deflected and disturbed to a certain extent before the M_S 8.0 Wenchuan earthquake in all areas.From the analysis of temporal variation of mean principal compression stress in all A,B and C areas,the variation of stress field of the whole study area is similar to that of Sichuan-Qinghai block,i.e.deflection and disturbance of mean stress field occurred in 2006.The variations in area A,B and C were similar,but deflection and disturbance in areas B and C were more distinct in scope.Analyses on the ratio between the thrust,strike-slip and normal earthquakes and on the temporal variation of stress fields of areas A,B and C show that,after the impoundment,the relative change of ratio of earthquake types of area A is the smallest,that of C the biggest,and that of area B is between A and C.This may be because that area C is right in the water area of the reservoir,B is farther,and A is the farthest from reservoir waters.

FILTRATION MODEL OF INTEGRATED RISK ASSESSMENT INDEX SYSTEM OF RESERVOIR INDUCED SEISMICITY BASED ON FUZZY ANALYTIC HIERARCHY PROCESS AND ACCELERATING GENETIC ALGORITHM

ZOU Le-le, JIN Ju-liang, ZHOU Yu-liang

2010, 32(4):  628-637.  DOI: 10.3969/j.issn.0253-4967.2010.04.010

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Integrated risk assessment index system of reservoir induced seismicity reflects the complex interaction relationship between reservoir induced seismicity risk factors and the socioeconomic system of reservoir region.Establishing the index system needs comprehensive and integrated studies of seismology,sociology,economics,management and engineering science.An integrated risk system of reservoir induced seismicity consisting of four subsystems of hazard,vulnerability,disaster-reduction capability and disaster condition is presented according to the mechanism analysis of interaction relationship between reservoir induced seismicity and the socioeconomic system of reservoir region.The four subsystems and their second-class indexes make up the preliminary integrated risk assessment index system of reservoir induced seismicity.The preliminary index system is filtrated with expert advisory information and accelerating genetic algorithm based fuzzy analytic hierarchy process,and then the final integrated risk assessment index system of reservoir induced seismicity is established,which can be applied to the integrated risk assessment of reservoir induced seismicity and can provide the supporting technology for establishing early warning system of integrated risk of reservoir induced seismicity.

STUDY ON THE MEDIUM AND MEDIUM-SHORT TERM CHARACTERISTICS OF ANOMALIES OF UNDERGROUND WATER LEVEL IN HAINAN PROVINCE

GU Shen-yi, ZHANG Hui, XIE Xiao-jing, LIU Yang

2010, 32(4):  638-646.  DOI: 10.3969/j.issn.0253-4967.2010.04.011

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Based on careful selection of underground fluid observation wells(springs),and using the monthly means and subordinate function method,we select the precursory anomalies information of underground water level before the moderately strong earthquakes in Hainan and its vicinity from these wells(springs)and analyze the characteristics of these precursory anomalies.Anomalies in water level are categorized into two classes: ascending medium term anomalies and descending medium-short term anomalies.Statistics show that the average time of medium-short term anomalies in subordinate function is approximately one year before the earthquake,which can be used as reference time index for mid and short term earthquake prediction in this area.

CALCULATION METHOD OF MONITORING CAPABILITY OF RESERVOIR SEISMIC NETWORK——BASED ON G-R RELATIONSHIP

HU Xian-ming, SHAO Yu-ping

2010, 32(4):  647-655.  DOI: 10.3969/j.issn.0253-4967.2010.04.012

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It is required to answer for reservoir seismic network whether the network's monitoring capability achieves the design goal.According to research results,the use of magnitude-frequency relationship,with the aid of frequency-magnitude diagrams,can answer the above question.In recent years,the method was applied to calculating the monitoring capability of Zipingpu,Pubugou and Wawushan reservoir networks.When studying the background seismicity of specific reservoir,the method can make a better use of regional network catalogues to the study of the background of reservoir seismic activity.When small earthquake cluster occurs in reservoir network area,the possible greatest magnitude of the follow-up earthquake predicted by this method is well consistent with the follow-up practical one.The method can be extensively applied to monitoring and prediction of reservoir-induced and natural earthquake.In characterizing the monitoring capability of a seismic network,the method solves the concern of determining the network's monitoring capability on the aspects of both design goal and measured data.

GENETIC ANALYSIS OF RESERVOIR-INDUCED EARTHQUAKE BASED ON GIS

CHEN Xiao-li, MA Wen-tao, YANG Qing-yuan

2010, 32(4):  656-665.  DOI: 10.3969/j.issn.0253-4967.2010.04.013

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Earthquakes induced by reservoirs always cause more serious damage to reservoir dam due to its epicenter location and destroying effect.Reservoir induced earthquakes have different characteristics from the natural ones,their hypocenters are always shallow,less than ten kilometers,and the magnitude is small too.Till now the biggest earthquake induced by reservoir is the MS6.1 earthquake in Xinfengjiang reservoir in 1962.The paper analyzes the effects of different factors related to reservoir induced earthquake based on GIS technology and lots of reservoir induced earthquakes data from the world.The primary results show that reservoir's local geological and hydrogeological settings have closer relations with induced earthquake than regional geological and seismic activity setting does.The reservoirs with dams higher than 50m or reservoir volume greater than 5?1010 m3 are prone to induced earthquake.Moreover,the reservoir region,where limestone,carbonatite,karst landform,or joints and high permeable structure are developed,will have a higher probability to induce earthquake.

DISCUSSION ON RELATIONSHIP BETWEEN ATMOSPHERIC PRECIPITATION AND EARTHQUAKE

WANG Ji-yi, PING Jian-jun, ZHANG Shuang-feng, FAN Xue-fang

2010, 32(4):  666-677.  DOI: 10.3969/j.issn.0253-4967.2010.04.014

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The atmospheric precipitation data of three seismic stations,Fengtai of Beijing,Dingxiang of Shanxi Province and Changli of Hebei Province,are studied using the moving average method for monthly means and the subordinate function method.The results indicate that there exist obvious medium-term and mid-short-term atmospheric precipitation precursory anomalies before the 1976 Tangshan M_S 7.8 earthquake,the 1981 M_S 5.6 Fengzhen earthquake,the 1989 Datong M_S 5.9 earthquake,the 1998 Zhangbei M_S 6.2 earthquake and the 2006 Wen'an M_S 5.1 earthquake.Furthermore,these anomalies are well consistent with precursory anomalies of water level,water radon and ground deformation.The relationship between atmospheric precipitation anomaly and earthquake can be interpreted using the "the water trigger mechanism".The atmospheric precipitation anomalies are external cause and not essential signal of earthquake.The atmospheric precipitation anomalies are related to the occurrence time of earthquake and can be used for medium and short term prediction of earthquake,its predictive period is about 4～14months.It would be practicable and necessary to try out the atmospheric precipitation anomaly method for earthquake prediction.

THOUGHTS INITIATED BY INDIAN OCEAN TSUNAMI ON EARTHQUAKE PREPAREDNESS:THINKING FROM A RETIRED VETERAN GEOLOGICAL ENGINEER

ZHU Fu-zhi

2010, 32(4):  678-680. 

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