Table of Content

30 June 2012, Volume 34 Issue 2

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Expert's forum

KEY TECHNIQUES AND SEVERAL CASES ANALYSIS IN PALEOSEISMIC STUDIES IN MAINLAND CHINA(1): TRENCHING SITES,LAYOUTS AND PALEOSEISMIC INDICATORS ON ACTIVE STRIKE-SLIP FAULTS

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

2012, (2):  197-210.  DOI: 10.3969/j.issn.0253-4967.2012.02.001

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The main purpose of paleoseismic study is to distinguish or reveal deformation evidence of large earthquakes recorded by geologic and geomorphic features,and obtain corresponding seismic parameters such as timing,recurrence behavior,and coseismic displacement of large earthquakes. To achieve the aforementioned scientific aim,whether a trenching site preserves evidence of a complete paleoseismic sequence since late Quaternary and contains multiple measurable samples or not,and whether it can accurately identify paleoseismic events and collect well-constrained samples on events or not,all of these problems are directly responsible for reliabilities on assessment of future large earthquake hazard. Due to special displacement styles on strike-slip faults,good trenching sites are not widespread. Through comprehensive analysis on characteristics of coseismic surface ruptures and influencing factors on several trenching cases,we suggest micro-landforms such as depressions,basins,troughs,sag ponds,successive-offset channels,continuous scarp-derived deposit and multiple geomorphic surfaces are likely to be good trenching sites for paleoseismic studies on strike-slip faults. Multiple trenching or three-dimensional trenching should be the primary layout on strike-slip faults. Offsets of micro-landform across a fault,young stratigraphic units overlying on faulted units,locally distributed scarp-derived colluvial deposits,filled fissures,abrupt increases or decreases in displacement of different stratigraphic units on a fault,warping in different degrees,and multiple periodic paleo-sag ponds accumulation,all of these deformation evidences are good indicators for identifying paleoseismic events. To narrow uncertainties of paleoseismic studies,we should base on an organized research process and make a technical proposal and sophisticatedly conduct trenching work. Conclusions need to be repeatedly checked and widespread discussed,and also we should pay much attention on details and use more evidence to support or supplement analysis.

Research Paper

TEXTUAL RESEARCH OF THE QI-CHU EARTHQUAKE OF 179 BC AND DISCUSSION ON ITS SEISMOGENIC STRUCTURE

CHAO Hong-tai, DIAO Shou-zhong, WANG Zhi-cai

2012, (2):  211-227.  DOI: 10.3969/j.issn.0253-4967.2012.02.002

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The Qi-Chu earthquake,occurring in 179 BC(the first year of Hanwen Emperor,Han Dynasty),is a contentious historical earthquake event. Its basic parameters have not been determined. According to former researches on administrative division in Han Dynasty and analysis on historical earthquake records,the paper presents a discussion on the basic parameters of this earthquake event. A comprehensive method is developed to determine the epicenter,focusing on the border area between Qi and Chu States and the direct line of their capitals,and also considering other conditions from historical data. Moreover,based on results of seismic and geological investigation,the paper verifies the rationality of location of the epicenter from the seismotectonic point of view.Firstly,considering the frequent regime changes in Han Dynasty,the paper tries to recover the then administrative divisions and the boundary between Qi State and Chu State around the first year of Hanwen Emperor. Secondly,we analyzed carefully the range of the region indicated by historical earthquake records as "29 mountains collapsed and water burst during the Qi-Chu earthquake",and compared the spatial distribution and intensity of damage caused by other historical earthquakes to obtain important constraints for determining the location and magnitude of this event. It is suggested by textual analysis that the Qi-Chu earthquake ought to be the magnitude 7 earthquake occurring in the southern area of Pingyi County(N35.2?,E117.6?)on June 6,179 BC.Thirdly,geological structures and active faults around the epicenter area of Qi-Chu earthquake are studied by analyzing satellite image and geological data. As the earthquake was ruled out from the Tanlu Fault zone,it is considered that the Cangni active fault probably is the seismogenic structure of the Qi-Chu earthquake. Nearby the epicenter,the Cangni active fault with clear landforms and greatest displacement has controlled the development of the Baiyan Basin and offset the late Quaternary sediments.

STUDY ON LATE-QUATERNARY ACTIVITY AND DISPLACEMENT OF DRAINAGE SYSTEMS ALONG THE WANDING FAULT

CHANG Zu-feng, AN Xiao-wen, ZHANG Yan-feng

2012, (2):  228-239.  DOI: 10.3969/j.issn.0253-4967.2012.02.003

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Wanding Fault lies along the border of west Yunnan,China and Myanmar,striking near EW on the west and NE on the east of Bengdong,dipping to N and NW,with a length of 170km. Recent studies show that there are neotectonic landforms along the fault,represented as clear fault facets,straight fault troughs,fault scarps,linear ridges,multi-level waterfalls,and etc. The fault has played an obvious control role in the development of Quaternary basins,such as Wanding,Manhai,Menglianba,Jiucheng and Wandian Basins etc. ,which show a beaded distribution along the fault. The Quaternary faults near the Longzhen bridge and the displacement of the alluvial fans at Gaijiazhai reveal that the Wanding Fault has offset the late Pleistocene deposits with TL age of(17.60±1.49)ka BP to(38.24±3.25)ka BP,indicating that the fault was obviously active in late Quaternary. The Nujiang River extends along the fault in the Bengdong area,the main river course was offset left-laterally about 9.5km,forming a hairpin-like bend. The Gongyanghe River,Zhongshanhe River,Wanmahe River,Manxinhe River,Mengguhe River,Wandinghe River and their tributaries all run along the Wanding Fault,and there are clear evidences that these river segments are partially or entirely dominated by the fault,suggesting that these segments were formed by tracking erosion along the fault fracture zone after the faults latest activity. Synchronous left-lateral displacement of drainage systems was observed at many parts along the Wanding Fault,such as between Pingziting to Gongyanghe,where 11 streams were synchronously displaced left-laterally,and more than 10 streams between Zhongshan to Wanmahe displaced. The displacements can be divided into 5 levels,i.e.40～50m,90～100m,200～250m,300～400m and 600～1100m. Comb-form rivers are developed along the fault. The synchronous left-lateral displacement of drainage systems and the displacement of terraces and alluvial fans suggest that the Wanding Fault was dominated by horizontal left-lateral slip in late Quaternary,with an average rate of 1.7～2.2mm/yr. Several moderate earthquakes occurred along the fault,which are assumed to be the aftershocks of the May 29,1976 Longling M_S7.3 and M_S7.4 earthquakes,e.g.the January 26,1946 Myanmar M_S6.0 earthquake,and the June 1,1976 Anding M_S6.0 earthquake,and the July 4,1976 M_S6.0 earthquake at east of Mengbang,Yunnan,etc. These aftershocks are not distributed in a band-like pattern along the Longling-Ruiling Fault-the seismogenic fault,but concentrated in an area of 55km in length and 32km in width between the Longling-Ruili Fault and the Wanding Fault. So,it is inferred that the extensive distribution of the aftershocks is the result of the successive activities of the Longling-Ruili Fault and Wanding Fault.

QUATERNARY DEFORMATION OF THE HEJING THRUST-FOLD BELT ON NORTHERN MARGIN OF THE YANQI BASIN, SOUTHERN TIANSHAN

LI An, YANG Xiao-ping, HUANG Wei-liang, Yiliyaer

2012, (2):  240-253.  DOI: 10.3969/j.issn.0253-4967.2012.02.004

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The Yanqi Basin is an important depression area of the Tianshan Mountains near the northeast margin of the Tarim Basin. The Hejing thrust-fold belt,a renascent thrust-fold belt with intense tectonic activity,is located on the north margin of the Yanqi Basin. We calculated the shortening and uplifting of the Hejing thrust-fold belt since Quaternary to analyze the tectonic activities of this region,then,made a preliminary estimation of the shortening and uplifting rate in the Yanqi Basin and compared the result with other areas to demonstrate the role of the Yanqi Basin in the tectonic deformation of Tianshan in late Cenozoic. With few seismic data but abundant attitude of stratum and fault,we restituted the geometry of the fold to calculate the shortening and uplifting of the fold and slip of the fault on the Hejing thrust-fold belt which has a simple and intact structural pattern,and obtained the shortening amount of 1.79km,0.88km and 26m,respectively since early Pleistocene(1.8Ma),middle Pleistocene(780ka) and late Pleistocene(80ka). The respective shortening rate is estimated as 0.99mm/yr,1.13mm/yr and 0.33mm/yr. The intensity of tectonic activity is not constant on the Hejing thrust-fold belt since the beginning of deformation. Compared with the result of crustal deformation,Yanqi Basin,as a major depression on southeastern Tianshan Mountains,absorbs most of crustal shortening of this area(86°～88°E)and exhibits strong deformation of the newborn thrust-fold belt on the northern margin of the basin.

DISTRIBUTION OF THE KNICKPOINTS IN FLUVIAL GULLIES IN RESPONSE TO THE EVENTS OF HOLOCENE FAULT ACTIVITY ——A CASE STUDY OF THE JIAOCHENG FAULT IN SHANXI

SUN Chang-bin, XIE Xin-sheng, JIANG Wa-li

2012, (2):  254-268.  DOI: 10.3969/j.issn.0253-4967.2012.02.005

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Jiaocheng Fault zone is located in central Shanxi downfaulted system,and controlls the western margin of the Jinzhong Basin. It starts at Nitun town,Yangqu County in the north and ends to the west of Fenyang city in the south,about 125km long,striking NE and dipping SE.Based on the survey and research of 41 gullies and 124 knickpoints along Jiaocheng Fault zone,this paper discusses the distribution characteristics of knickpoints of the gullies along Jiaocheng Fault zone: ①The number of knickpoint series of upstream gully is positively proportional to the length of upstream gully; ②The distance of knickpoints is positively proportional to the length of the gully in each class knickpoints.③The distance of different class knickpoints is close to equal in about 1/3 gullies,and it reflects equal interval in time of the fault activity with vertical displacement; ④Bedrock knickpoints and knickpoints in loose sediments have commonness,there isnt obvious difference between them; ⑤ 1～5m high knickpoints accounted for 72%of the total 124 knickpoints in the survey area,the height of the main knickpoints reflects the vertical displacement of the fault activity; ⑥With the comparison of the paleoearthquake events revealed by the excavation of trenches along the fault zone,the latest first,second,third class knickpoints in survey area are corresponding with three faulting events occurring respectively 3.06～3.74ka,5.91ka and 8.53～8.56ka BP; ⑦taking the time of the paleoearthquake events as a reference,the headward erosion rates of the first knickpoints of a-d grade gullies are 0.6～0.7cm/yr,2.7～3.3cm/yr,5.6～6.8cm/yr and 8.4～10.2cm/yr,respectively. The rates of the second class knickpoints of b-d grade gullies are 2.6cm/yr,4.6cm/yr and 8.4cm/yr,respectively; and ⑧ the average heights of the 3 events are 2.8m,3.0m and 2.6m,respectively,which match the dislocations of the 3 paleoearthquake events revealed by previous trenches. Some factors which may influence knickpoint series and knickpoint distance in the investigation of knickpoint are discussed in the end.

THE PRELIMINARY RESEARCH ON RUPTURE AND FACTORS AFFECTING RUPTRUING OF HOH SAI HU SEGMENT IN EASTERN KUNLUNSHAN

BAI Yu-zhu, XU Jie, ZHOU Qing, ZHOU Ben-gang

2012, (2):  269-281.  DOI: 10.3969/j.issn.0253-4967.2012.02.006

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On 14 November 2001,an extraordinarily large earthquake(M_S 8.1)occurred on the Hoh Sai Hu segment of the eastern Kunlun Fault,northern Tibetan Plateau. The seismogenic fault,Hoh Sai Hu segment,is a left-lateral fault with a high slip rate in the geological history,and the average slip rate reaches(14.8±2.8)mm/a since the late Pleistocene. Different slip rates of Hoh Sai Hu segment can affect the fault motion in the future. So,the paper analyzes the effect of different slip rates and different initial friction coefficients on the fault surface of the Hoh Sai Hu segment of eastern Kunlun Fault on the rupture behaviors of the fault. In the research,we apply the single degree of spring block model controlled by the rate- and state-dependent frictional constitutive laws. Using the fault dislocation model and based on ancient earthquake researches,historical earthquakes data and the achievements of previous researchers,we obtained the parameters of the model. Through the numerical simulation of rupturing motion of the Hoh Sai Hu segment in the future 6500 years under different slip rates,we find that a faster annual slip rate will shorten the recurrence interval of earthquake. For example,the earthquake recurrence interval is 2100a at a slip rate of 0.014m/a,which agrees with previous research results,but,the recurrence interval will be 1000～1500a and 2100～2500a,corresponding to the slip rates of 0.018m/a and 0.008m/a,respectively. Slip rate of fault has no regular effect on the coseismic slip rate and displacement of fault in an earthquake. The initial friction coefficient on the fault surface has effect on earthquake recurrence interval. A smaller initial friction coefficient will lengthen earthquake recurrence interval. At the same time,the smaller initial friction coefficient will lead to larger slip rate and displacement of fault when earthquake occurs.

SLIP RATE ON THE SOUTH SEGMENT OF DALIANGSHAN FAULT ZONE

WEI Zhan-yu, HE Hong-lin, SHI Feng, XU Yue-ren, BI Li-si, SUN Hao-yue

2012, (2):  282-293.  DOI: 10.3969/j.issn.0253-4967.2012.02.007

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Daliangshan Fault zone constitutes an important part of the eastern boundary of Sichuan-yunnan active block. The studies of slip rate along the fault is not only significant to the crust movement and deformation pattern on the southeast edge of Tibetan Plateau,but also has great value in seismic hazard assessment and mid-and long-term forecasting of earthquake of the Daliangshan region. Through detailed field work along the south segment of Daliangshan Fault zone,namely the Butuo Fault and the Jiaojihe Fault,and based on accurate RTK(GPS)survey for the alluvial fans and activity dating,we suggest that left-lateral slip rate of the south segment of the fault zone is between 2.5～4.5mm/a,and the slip rate of Jiaojihe Fault is slightly higher than that of the Butuo Fault. Due to partitioning of part of the strike-slip component on the Daliangshan Fault zone,there is an obvious deficit in the displacement and slip rate on the Anninghe-Zemuhe Fault,compared to the Xianshuihe and Xiaojiang Faults. Comparing to the slip rates between Daliangshan Fault and Anninghe-Zemuhe Fault,it is found that they have similar horizontal slip rate,indicating the seismicity level of the Daliangshan Fault will not be lower than that of Anninghe-Zemuhe Fault. As the Daliangshan Fault gradually replaces the role of Anninghe-Zemuhe Fault in the Xianshuihe-Xiaojiang Fault system,the seismicity on the Daliangshan Fault zone will increase,and the Dalianghan region will have a higher risk of earthquake damage.

TECTONIC LANDFORM AND LOCATION OF THE NORTHERN END OF XIANNVSHAN FAULT AT THE THREE GORGES AREA

BI Min-feng, CHU Quan-zhi, DENG Zhi-hui, PAN Bo, ZHANG Chang-hou, ZHOU Qing

2012, (2):  294-302.  DOI: 10.3969/j.issn.0253-4967.2012.02.008

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The Xiannüshan Fault zone,lying along the western margin of Huangling anticline,is one of the most important fault zones in Three Gorges reservoir area. The fault experienced strong activities during Cenozoic. Whether the fault zone crosses the Yangtze River is one of the key problems in previous studies,as it has significant influence on the assessment of geological hazards and earthquake stability in the reservoir area. Based on tectonic and geomorphic observations along this fault zone around Baixianchi in Changyang County,Huangkou in Zigui County,together with the comparisons between the geology in Guizhou and Quyuan Town in the north bank of Yangtze River and the Xiannüshan Fault zone,it is suggested that the north end of this fault zone locates around Huangkou village and doesnt traverse the Yangtze River northward.
The details are as follows: ① On the basis of field data collection,it is found that the Xiannüshan Fault zone,which stretches 80km,underwent thrust movement in Cenozoic,resulting in ravines and fault scarps,topographically. Whereas,on the northern bank of Yangtze River,faults are rarely found,and most of the faults are developed in the Jurassic strata,without topographical effects. Therefore,the Xiannüshan Fault zone has not stretched to the north bank of Yangtze River.②Fault gouge and tectonite zone were found developed on Xiannüshan Fault zone at Baixianchi village,but only tectonite zone was found at Zhouping village. There are also some branch faults close to the northern end of the fault zone. So,the activity of the fault zone weakened from south to north in Cenozoic. The fault zone extends northward and dies out at Huangkou. It doesnt stretch forward any longer as indicated by continuous strata,sparse joints,and small folds,etc.

NEW INSIGHTS INTO THE SEISMOGENIC STRUCTURES OF THE TWO DESTRUCTIVE EARTHQUAKES IN LIYANG, JIANGSU PROVINCE

HOU Kang-ming, XIONG Zhen, LI Li-mei

2012, (2):  303-312.  DOI: 10.3969/j.issn.0253-4967.2012.02.009

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Two destructive earthquakes occurred in the southern Liyang region,Jiangsu Province in 1974 and 1979,with magnitude 5.5 and 6,respectively. The 1979 Liyang earthquake with magnitude 6 is the largest earthquake in the land area in the province in modern times. The interval between the two earthquakes is 5 years,and their meizoseismal areas overlap basically,indicating that the seismogenic structures of the two earthquakes belong to a same active fault. Based on field investigations,many scholars believed that the seismogenic structures of these two earthquakes were related to the NNE-trending Maodong Fault. In recent years,more in-depth studies were conducted in this region,e.g.shallow seismic prospecting,high density electric prospecting,drilling exploration and seismic geological investigation. And with the accumulation of data and the precise location of aftershocks in the earthquake region,a clearer understanding has been gained of the seismogenic fault of the Liyang earthquakes. The main shocks and the aftershocks of the two earthquakes are all distributed along the Jintan-Nandu Fault located 2km east of the Maodong Fault. Both main shocks are located at the intersection of the southern segment of Jintan-Nandu Fault and the near-EW-trending Liyang-Nandu Fault. Thus,the authors conclude that the Jintan-Nandu Fault is the major seismogenic fault for both Liyang earthquakes.

FEATURE AND FORMATION MECHANISM OF THE QUATERNARY FAULT PLANE AT XILINGANG,FOSHAN CITY

DONG Hao-gang, HUANG Chang-sheng, ZENG Min, ZHAO Xin-wen, GUO Liang-tian, WANG Ping

2012, (2):  313-324.  DOI: 10.3969/j.issn.0253-4967.2012.02.010

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A Quaternary fault plane was found located at Chencun Town of Foshan City,which is thought as a late Quaternary active fault by many researchers. To explain the formation mechanism of this fault plane,a series of work was carried out,such as geomorphologic survey,validation by exploratory trench,shallow seismic reflection and dating of Quaternary system,etc. According to the analysis of the geomorphologic features,the contrast between bedrock fault and Quaternary fault,and the characteristics of shallow seismic reflection,there are few geomorphologic signs of recent tectonic activities. The displacement of Quaternary bottom and the rupture in the rocks were not formed in the same tectonic stress field,and the bedrock fault didnt offset the Quaternary formation according to the shallow seismic reflection characteristics. Thus,a conclusion is drawn that the Xilingang Fault plane wasnt generated by tectonic movement but the gravity inequilibrium.

PRIMARY RESEARCH ON THE COULOMB STRESS TRIGGERING OF THE 2011 M_W 9.0 TOHOKU EARTHQUAKE

SHENG Shu-zhong, WAN Yong-ge, CHENG Jia, LI Ying-qiu, WU Ye

2012, (2):  325-337.  DOI: 10.3969/j.issn.0253-4967.2012.02.011

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In this research,we made a primary research on the Coulomb stress triggering of the March 11,2011 M_W=9.0 Tohoku earthquake sequence by using the software of Coulomb 3.2,the earthquake rupture models obtained by Hayes and Guangfu Shao et al. , and the aftershocks data from Harvard CMT catalogue(Centroid Moment Tensor)and Japan F-net catalogue. Our results suggest that: Firstly,the M_W7.2 foreshock,which occurred on March 9,has stress triggering effect on the M_W9.0 main shock; Secondly,the statistical result of the aftershocks triggered by the main shock shows that different statistical results would be obtained when using different main shock rupture model,aftershock catalogue,equivalent friction coefficient and different nodal plane of focal mechanism as the receive plane. The minimum and maximum triggering rates of main shock to aftershock are 56.8%and 75.3%,respectively; and thirdly,when calculating the Coulomb stress by using earthquake focal mechanism,the shear stress on the two nodal planes is the same theoretically. However,in actual calculation,the shear stress would be different between the two nodal planes,due to the non-orthogonality of the two nodal planes or rounding off decimal places in the focal mechanism results. But,the difference of the shear stress on the two nodal planes is relatively small. More attention should be paid on the selection of receiver fault plane from the two nodal planes,when discussing the stress triggering of one specific earthquake or making statistics of aftershock triggering rate,because the selection of receiver fault plane would have a certain effect on the shear stress,and have greater effect on the normal stress,thus the Coulomb stress would be affected.

ANALYSIS OF INFLUENCING FACTORS IN DETERMINING THE UPPER OFFSET POINT OF ACTIVE FAULTS USING SEISMIC METHOD

GAO Jing-hua, XU Ming-cai

2012, (2):  338-347.  DOI: 10.3969/j.issn.0253-4967.2012.02.012

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Urban areas are mostly covered by thick Quaternary loose sediments in China. Many sizable hidden faults are developed in this loose layer. These hidden faults can be detected by using seismic method. Whether or not the upper offset points of the hidden faults detected by seismic method represent the real upper offset points of the faults? On the basis of the discussion on resolution of seismic records,the influence of S/N ratio of seismic records,the seismo-geological conditions and the data processing and interpretation methods on the seismic exploration results is analyzed in this paper. Study results show that appropriate technical measures are necessary for improving the S/N ratio and resolution of seismic records. For example,seismic profiles shall be laid vertical to fault strike during the data acquisition. Under conditions of guaranteed seismic records with high signal-to-noise ratio,the use of the methods with appropriate fold number and small group interval will help to get reflections from shallower wave impedance interfaces. In the course of seismic data processing and interpretation,it would be helpful to improve the effect of detecting the upper offset point of buried fault by obtaining accurate reflected wave velocity and adopting the processing technology that can enhance resolution and S/N ratio of seismic records as well as abandoning some mixed wave processing methods. It is difficult to detect with seismic method the interfaces in stratum without wave impedance difference,including the interface broken by buried fault.Even though the upper offset point of buried fault detected by seismic method is not necessarily the upper offset point in a real sense,the seismic prospecting result still provides scientific basis for the layout of drill hole profile,design of drilling depth,and the determination of fault activity.

FINE VELOCITY STRUCTURE AND RELOCATION OF THE 2010 M_L5.1 EARTHQUAKE SEQUENCE IN RONGCHANG GAS FIELD

WANG Xiao-long, MA Sheng-li, LEI Xing-lin, GUO Xin, WANG Qiang, YU Guo-zheng, GOU Xian-bin, KUWAHARA Yasuto, IMANISHI Kazutoshi, JIANG Xia-dong

2012, (2):  348-358.  DOI: 10.3969/j.issn.0253-4967.2012.02.013

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Based on data collected from a temporary seismic network,in addition to some nearby permanent stations,we investigate velocity structure and seismicity in Rongchang gas field,where significant injection-induced seismicity has been identified. First,we use receiver functions from distant earthquakes to invert for detailed 1-D velocity structures beneath typical stations. Then,we use the double-difference hypocenter location method to relocate earthquakes of the 2010 M_L5.1 earthquake sequence occurring at the region. The relocated hypocenters show that the 2010 M_L 5.1 earthquake sequence was distributed in a small area surrounding major injection wells and clustered mostly along pre-existing faults. Major earthquakes show a focal depth less than 5km with a dominant depth of～2km,a depth of major reservoirs and injection wells. We thus conclude that the 2010 M_L 5.1 earthquake sequence might be induced by deep well injection of unwanted water at a depth～3km in Rongchang gas field.

ABNORMAL RESPONSE OF CHONGQING DAZU WELL WATER LEVEL CAUSED BY NEARBY WELL PUMPING AND THE MECHANISM

CHEN Min, TANG Xiao-yong, GUO Wei-ying, HE Man-qiu, GONG Hao-bo, LI Guang-ke, CHEN Lei, YANG Lin, CHEN Kai, TAN Jun, JIA Hong-fei

2012, (2):  359-364.  DOI: 10.3969/j.issn.0253-4967.2012.02.014

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Dazu Well is located in Shiwan Town of Dazu County,Chongqing City,the well is 108.7m deep. The observed aquifer layer is weather-fractured phreatic aquifer,of which the rock stratum is mudstone of Middle Jurassic Shaximiao Formation(J_2S)interbedded by sand. Years of observations show that whenever the nearby well,which is 14.6m away,was pumped,the water level of Dazu well rose instead of drop. This kind of anomalous response has been rarely reported both at home and abroad. By analyzing the observation data,investigating the local geological and hydrogeological conditions,field observation and theoretic analysis,etc. ,we conclude that the anomalous response of Dazu well water level to the pumping of nearby well is attributed to the special water level regime of the observation well caused by hydrodynamic pressure due to the larger flow velocity in the outer boundary of pumping-depression cone,under a specific hydrogeological condition.

Special review

RHEOLOGY OF FELSIC ROCKS AND RELATIVE INFLUENCE FACTORS

LIU Gui, ZHOU Yong-sheng

2012, (2):  365-383.  DOI: 10.3969/j.issn.0253-4967.2012.02.015

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In this paper,we summarized deformation mechanisms of crustal rocks and the progress of the rheological experiments under high temperature and pressure,and discussed the quality of rheological experiment data. Although a great amount of creep test data have been published,the data of natural felsic rock,as well as data of quartz published in early time are hard to be reduplicated. However, the creep data of quartz and feldspar published in recent years are of very high quality. Some studies show that flow laws of two-phase rocks could be fitted using the empirical and theoretical models and rheological parameters of end member minerals. However,the flow laws for felsic rocks are hard to be determined because of complicated components and special rheological properties. As a result,when rheological parameters of felsic rocks are used to estimate the strength profile of the continental crust,the stress envelope and the depth of brittle-plastic transition zone are different even in the same temperature and strain rate condition. Therefore,we suggest more creep experiments should be done in order to obtain high quality rheological data of felsic rocks.
Based on recent progresses in rheological experiment,the influence factors on rheology of felsic rocks in laboratory conditions are discussed,especially,the covering water,mineral component,grain size and fabric of the rock. Little water in the rock would have a significant weakening role on the rheological strength. The effect of the rheological strength of melt is dependent on the observations and the distribution of the melt. If grain boundaries are wetted by melt films,the weakening effect of the melt will become more obvious. The effect of mineral component on rock rheology is reflected not only by stress exponents of samples,but also transition temperature from semi-brittle state to plastic state. The grain size mainly influences the deformation mechanism of rocks. In diffusion regime,the flow stress has negative linear correlation with grain size of fine grain samples,which is not only used to estimate as stress piezometer,but also suitable to determine the rheological strength in the ductile shear zone. However,in dislocation regime,the flow stress is independent of grain size,which is the foundation in estimating the rheological strength of the crust rocks in natural condition using experimental data. Finally,the effect of rock fabric and anisotropy,which is the common phenomenon in the crust,on rheology of experimental samples is very limited,which needs further experimental studies to get more data.