SEISMOLOGY AND GEOLOGY

EARTHQUAKE LOCATION METHOD WITH ARRIVAL TIME UNCERTAINTY CONSIDERED AND ITS APPLICATION TO LOCATION OF EARTHQUAKES FROM 2001 TO 2008 IN SICHUAN AREA

WAN Yong-ge, SHENG Shu-zhong, CHENG Wan-zheng, ZHANG Zhi-wei, WU Ye, ZHAO Xiao-yan, BU Yu-fei, XUE Zhi-fang, LIU Jin-li

2012, 34(1): 1-10. DOI: 10.3969/j.issn.0253-4967.2012.01.001

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The precisely located earthquake catalogue is important to seismicity, seismic tomography and crustal stress inversion studies. It also has great application value in rapid report of an earthquake that just occurred. By considering the arrival time uncertainty, and the constraints on station elevation and seismic depth, we propose a relatively accurate method to estimate hypocentral location and its uncertainty based on inversion theory. Our method can combine the arrival times of Pg wave, Sg wave, Pn wave and Sn wave in hypocenter location, so it increases the location accuracy by involving more data; and it can be also used in local and regional earthquake location simultaneously. In order to test our location method, we located earthquakes by using the simulated data with different uncertainty of Pg,Sg,Pn,Sn arrivals. The result shows that the location determined by using our method is more accurate than that by using other method. We apply it to earthquakes occurring in the period from 2001 to 2008 in Sichuan area, and obtained a more clustered hypocentral distribution convergent to the fault zones. The result provides a solid foundation for studies of seismicity, geometry of the active faults and seismic tomography in Sichuan region. It is also helpful to study the seismicity precursors before the Wenchuan earthquake.

STRESS STATE OF WENCHUAN AREA BEFORETHE 2008 STRONG EARTHQUAKE

WANG Kai-ying, Yu. Rebetsky, DIAO Gui-ling, MA Sheng-li, MA Jin

2012, 34(1): 11-16. DOI: 10.3969/j.issn.0253-4967.2012.01.002

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Using the cataclastic analysis method, this paper tries to make an analysis on the focal mechanism data of 486 small earthquakes that occurred at the epicenter of the Wenchuan earthquake and its surrounding areas in more than three years before the 2008 Wenchuan earthquake. The result shows that obvious stress change occurred at the seismic source and its surrounding areas around June 2007 before the Wenchuan earthquake, manifested in two high numerical value areas of abnormal stress state. Meanwhile, the formation process of the above areas was accompanied by the drop of stress level of the Longmenshan central fault. The ultimate strong earthquake occurred on the stress gradient belt between the high stress area and the low stress area. The evolution process of stress level before the Wenchuan earthquake indicates that earthquake nucleation phenomenon turned up before the strong earthquake. One result can be inferred that there was an abnormal process of accelerated movement of the whole Bayankala block before the Wenchuan strong earthquake.

X-RAY DIFFRACTION ANALYSIS RESULT OF CO-SEISMIC FAULT GOUGE IN CARBON MUDSTONE AT OUTCROPS OF BAJIAOMIAO AND SHENXIGOU IN HONGKOU

DANG Jia-xiang, ZHOU Yong-sheng, HAN Liang, HE Chang-rong, CHEN Jian-ye, DANG Xin-zeng, YANG Xiao-song

2012, 34(1): 17-27. DOI: 10.3969/j.issn.0253-4967.2012.01.003

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The co-seismic surface ruptures of the May 12,2008 Wenchuan earthquake in Bajiaomiao and Shengxigou were developed mainly at the outcrops of carbon mudstones of Xujiahe formation of Triassic system. The black color and textures of co-seismic gouge are similar to old gouge and bed rock. We excavated trenches along the surface ruptures and collected samples of wall rock, fault breccia, old fault gouge and co-seismic gouge. All samples were analyzed quantitatively by X-ray diffraction. The main rock-forming minerals and clay minerals of co-seismic gouge are similar to old gouge, but their content is different, which shows the co-seismic gouge was formed based on old gouge. The wall rock and fault breccias adjacent to co-seismic gouge are carbon mudstones. The mineral composition and texture of the fault zone are obviously simpler than that of the northern part of the surface ruptures of Yinxiu-Beichuan Fault. The major minerals of co-seismic gouge are quartz and clay minerals, containing a few amount of feldspar, without calcite; a small amount of dolomite was found in co-seismic gouge at Shenxigou, and the content of dolomite is much lower than that in bed rock and old gouge. The marked character of new gouge is abundant in clay minerals, and the content of clay minerals decreases in turn from co-seismic gouge to old fault gouge, fault breccia and wall rock. The main clay minerals are illite and illite/smectite (I/S) mixed layer, containing a few amount of chlorite; a few kaolinite was found in co-seismic gouge of Shenxigou, the bed rock and gouge of Bjiaomiao did not contain kaolinite. Mineral characters of co-seismic gouge are different from old gouge. The old gouge contains calcite and dolomite, and the co-seismic gouge contains a few amount of dolomite and without calcite; the old gouge does not contain illite. However, for co-seismic gouge, mineral characters are different between black and white gouges, the content of illite in black gouge is higher than that in white gouge. In this study, the main clay minerals are I/S mixed layer, illite and chlorite, which is similar with San Andreas Fault and Chelunpu Fault. However, kaolinite content is extremely low in this fault, only trace kaolinite was found in the co-seismic gouge of Shenxigou. The high content of I/S in co-seismic gouge shows that the rich K+fluid participated in the seismic fault slip. All of these characters show that minerals of co-seismic fault gouge in this study are somewhat different with that of San Andreas Fault and Chelunpu Fault.

THE CRUSTAL DENSITY STRUCTURE UNDERNEATH LONGMENSHAN FAULT ZONE

TANG Xin-gong, YOU Shuang-shuang, HU Wen-bao, YAN Liang-jun

2012, 34(1): 28-38. DOI: 10.3969/j.issn.0253-4967.2012.01.004

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It is of importance to study the crustal density structure of Longmenshan Fault zone for understanding the seismicity of this area. Using commercial software of Geosoft, gravity inversion for 6 lines of Bouguer gravity anomaly data was carried out under the constraint of deep seismic sounding (DSS) data obtained in the last two decades. The depth and distribution of sediments, crustal thickness and Conrad discontinuity of Longmenshan tectonic zone and its adjacent area(100°～105°E, 28°～33°N)are obtained. The inverted results indicate that the crustal density structure of the Longmenshan Fault zone is obviously different between the two sides. The sedimentary layer in western plateau is thinner and base rocks outcrop almost in most parts; while the thickness of sedimentary layer in eastern Sichuan Basin is much larger, in most parts, it is larger than 6km. The Conrad and Moho discontinuities are relatively smooth in both sides of the fault and the depths of them increase gradually from the eastern 24km and 42km to 35km and 67km under Tibetan Plateau, respectively. The whole fault of Longmenshan displays as a SN-direction gravity gradient zone, with each of its interfaces in the crust under the faults being offset. The displacements of the Conrad and Moho discontinuities are 6～7km and 3～5km, respectively. The sharp change in crustal thickness is maybe one of the causes of strong earthquakes in this area.

TERRESTRIAL LIDAR-BASED 3D MODELING ANALYSIS OF SURFACE RUPTURE CAUSED BY YUSHU EARTHUAKE

YUAN Xiao-xiang, WANG Xiao-qing, DOU Ai-xia, DONG Yan-fang, WANG Long, JIN Ding-jian

2012, 34(1): 39-46. DOI: 10.3969/j.issn.0253-4967.2012.01.005

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Yushu M_S 7.1 earthquake, occurring on April 14,2010, has caused a series of co-seismic surface ruptures. The surface deformation and ruptures caused by earthquake can easily be changed due to natural environment changes and from human activities. Moreover, the complex topography and severe natural environment in the Tibet Plateau make it difficult to acquire original features of surface rupture wholly via traditional surveying methods. However, as a new type of remote sensing technique, due to its characteristics of non-touching, high-resolution, rapid acquisition, terrestrial LIDAR technique can obtain the 3D information of surface rupture quickly and effectively, and provide the comprehensive scientific data for the further quantitative analysis. This paper introduces the characteristic of terrestrial laser scanner-Trimble GX 3D, and based on which the finite point clouds of the typical co-seismic surface rupture located on the southeast segment of Ganzi-Yushu Fault zone near Changu Temple were acquired. Based on the point cloud data prepossessing of registration, segmentation, smoothing and so on, the different principles and methods for surface modelling of co-seismic surface rupture were analysed, and the modelling experiment was conducted on surface rupture. After that, combining with the high resolution texture photos obtained on site, which had been precisely matched with point cloud data, the surface rupture 3D model was created based on the non projection mode. Also, the profiles from different perspectives were chosen to measure and analyse the displacement of the rupture exactly in the 3D view. The result shows that the average vertical displacement is about 74cm and the average left-lateral strike-slip displacement is about 10cm. Finally, the characteristics of Ganzi-Yushu Fault and characteristic of its rupture were analyzed and discussed.

STUDY ON THE CHARACTERISTICS,MECHANISM, AND SPATIAL DISTRIBUTION OF YUSHU EARTHQUAKE TRIGGERED LANDSLIDES

XU Chong, XU Xi-wei, YU Gui-hua

2012, 34(1): 47-62. DOI: 10.3969/j.issn.0253-4967.2012.01.006

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On April 14,2010 at 07:49 (Beijing time), a catastrophic earthquake with M_S 7.1 struck Yushu County, Qinghai Province, China. About 2036 landslides, covering an area of about 1.194km², were interpreted from aerial photographs and remote sensing imageries and verified by field check. And based on the above, the spatial distribution of the Yushu earthquake triggered landslides is presented in this paper. The distribution of the landslides was strongly dominated by main surface ruptures, and their types are varied, with the collapse-type landslide as the dominant. There are five genetic mechanisms of Yushu earthquake triggered landslides, they are: the slope-toe excavation type, the surface water infiltration induced slope slip type, the fault dislocation type, the shaking type, and post-quake snow melting and rainfall penetration type. Besides the main seismic surface ruptures, there are many slope fissures developed mainly on the SE end of the surface rupture zone on the SW wall, an area undergoing intensive compression in the earthquake.

EXPERIMENTAL STUDY ON THE INFLUENCE OF MUSCOVITE ON VELOCITY-DEPENDENCE TRANSITION

MIAO A-li, MA Sheng-li

2012, 34(1): 63-75. DOI: 10.3969/j.issn.0253-4967.2012.01.007

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In order to better understand the effect of phyllosilicate on fault strength, velocity-dependence of friction and seismicity, the velocity-dependence of friction for dry and wet muscovite-bearing halite gouge was studied by using biaxial friction configuration. Acoustic emission produced during the frictional sliding was recorded, and the microstructure of gouge zone was observed. The experiments show that dry gouge behaves stick-slip and velocity weakening at velocities of 0.1～100μm/s; Increasing σ₂ can enhance the transition to velocity strengthening and velocity-dependence transition occurs at velocity of 0.1μm/s, where fault behaves either stable sliding or stick-slip with much longer time than that in the velocity weakening region. Wet gouge behaves velocity strengthening at velocities of 0.05～0.01μm/s, velocity weakening at velocities of 0.1～10μm/s, and velocity strengthening again at velocities of 50～100μm/s. Each stick-slip event corresponds to one or a cluster of AE events for dry gouge, while there is no AE event corresponding to stable sliding for wet gouge. The microscope observation indicates that brittle fracturing and localized slip are predominant in the velocity weakening region and the velocity strengthening is controlled by distributed fracturing of halite under dry condition. While under wet condition, the two velocity strengthening regions correspond to different mechanisms. At higher velocities, the deformation of fault may be controlled by frictional sliding on the network developed by muscovite and uniform fragmentation of halite, and at lower velocities, the deformation of fault is also controlled by pressure solution of halite. Comparing to the results of halite gouge, it can be seen that the existence of muscovite has no effect on sliding mode and velocity-dependence for dry halite gouge, while it enhances the transition to stable sliding for wet halite gouge. The results we got provide basis for analyzing strength and stability of phyllosilicate-bearing faults. The stick-slip with longer time at transitional region confirms what observed in frictional experiments of halite gouge, which is significant for understanding mechanism of slow earthquakes.

ANALYSIS OF HOLOCENE FAULTING PHENOMENA REVEALED IN THE THREE TRENCHES ALONG THE NORTHERN AND CENTRAL JIAOCHENG FAULT,SHANXI

GUO Hui, JIANG Wa-li, XIE Xin-sheng

2012, 34(1): 76-92. DOI: 10.3969/j.issn.0253-4967.2012.01.008

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Several trenches were excavated in the previous work along the Jiaocheng Fault zone, which revealed three paleoearthquake events. But the northern boundary of the Jiaocheng Fault zone cannot be confirmed by the previous studies. The paper discussed the fault activities revealed in the three trenches excavated in the northern and central Jiaocheng Fault. Among them, Longwanggou trench is a combination which includes several trenches excavated in western Nitun Basin, Yangqu County. The paper introduced the trench profiles which cross the two right-stepped faults. As is shown in Longwanggou trench, the range of Holocene activity of the Jiaocheng Fault is extended 20km northward. The Wayaogou and Xinmin trenches are located in the central segment of the fault. One is at the fore-platform margin on the east of Wayaogou. The other is in front of the terrace T1 on the west of Shierkougou. Loam and mud layers of mid Holocene (with ¹⁴C ages of 5～6ka BP) and multi-parallel fault planes were observed in all the three trenches. In conclusion, the three Holocene faulting events as revealed in the three trenches are synchronous, which is consistent with previous trenching results. The time of the three events is 3.06～3.53ka, about 5.32ka or 6.14ka, and about 8.36ka B.P. The recurrence interval of the three events is about 2.02～2.84ka and about 3.04～2.22ka. The coseismic vertical displacement of these events is 1.5～4.7m, which shows surface faulting earthquake activity with magnitude greater than 7.0. This paper concludes with a discussion of the effect of ¹⁴C dating samples collected from the trenches.

PREHISTORIC EARTHQUAKES IN CHISHAN SEGMENT OF TANCHENG-LUJIANG FAULT ZONE DURING MID-LATE PLEISTOCENE

YAO Da-quan, TANG You-biao, SHEN Xiao-qi, CHEN An-guo, ZHENG Hai-gang, ZHAO Peng, ZHENG Ying-ping, LI Guang, XIAO Wei-peng

2012, 34(1): 93-99. DOI: 10.3969/j.issn.0253-4967.2012.01.009

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Chishan is located in the northeastern Sixian county, Anhui Province. The western branch fault of Tangcheng-Lujiang Fault zone passes through this area. According to the previous researches, the Chishan segment of Tancheng-Lujiang Fault zone is obviously active since Quaternary. Trenches excavated perpendicular to the Chishan segment of Thancheng-Lujiang Fault zone for this study have revealed many prehistoric earthquake remains-the multi-phase reverse faulting colluvial wedges, represented as the western brick-red sandstone of the late Cretaceous or maize gravel stratum of the mid Pleistocene of the hanging wall of the fault overlapping eastward the mid-late Pleistocene brown clay. In the base of the wedges, steep NW-dipping faults were found, and upward, the steep fault planes turned to gently dipping collapse planes. As revealed by the trenches, the connection line of the breaking points strikes NNE in general, heaving landforms are preserved at most part of the tailing edge of the hanging wall in the places the fault passes through, and some EW-trending gullies were offset by right-lateral faulting. Evidences of collapse under west-to-east extrusion are revealed unanimously in several trenches. Among them, Tc1-Tc4 show that the brick-red limestone (K₂) overthrust and collapsed on the yellow-brown clay containing ferro-manganese nodules (Q_p^2-3); Tc5 reveals that the yellowish-white gravel (Q_p²) and the limestone (K₂) overthrust and collapsed on the aforementioned clay. Reverse faulting colluvial wedges are found on both walls of each of the 8 trenches, but the number of wedges revealed in different trenches is different: there is 1 wedge in some trenches, and 2 wedges in Tc1 and Tc3, 3 wedges in Tc2, Tc4 and Tc5, and in individual trenches, few wedges are revealed, this may be related to the trench's location, depth and height of the remaining denudation. From the analysis of the trenches and the thermoluminescence dating results, we can conclude preliminarily that multiple large-scale reverse faulting stick-slip events took place on the Chishan segment of the Tancheng-Lujiang Fault zone during mid-late Pleistocene, that is to say, many pre-historic strong earthquakes occurred.

THE ACTIVE CHARACTERISTICS OF FAULTS AROUND HANDAN CITY

ZHOU Jun-jie, SONG Hong-li, LIU Hai-xin, ZHANG Hong-fen, ZHOU Long-quan

2012, 34(1): 100-109. DOI: 10.3969/j.issn.0253-4967.2012.01.010

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Activity of fault zones can often lead to earthquake. On the contrary, earthquake can also help people to analyze and forecast the characteristics of faults. To know more about the characteristics of active faults around Handan, and serve better for the urban construction and planning, a joint inversion method, which is based on earthquake data from local seismic network in Handan, was employed in this paper to get the distribution characteristic of earthquake parameters in space and time on the main faults around Handan and determine the characteristics of P-wave velocity structure in the crustal medium. Integrated analysis shows: The small earthquake activities around Handan are caused mainly by Cixian Fault and the Linzhou-Wu'an Fault; There is almost no activity on the southern end of Taihang Mountains piedmont fault zone (Cixian-Wu'an section); Earthquake activities are concentrated in a range of 6～10km; Fault activity gradually evolved from the faulting of the two faults simultaneously at the beginning to that the Cixian Fault was dominant.

THE FORMATION MECHANISM OF THE LAKES IN JINJIANG CITY, FUJIAN PROVINCE

XU Qi-hao, SHI Jian-sheng

2012, 34(1): 110-128. DOI: 10.3969/j.issn.0253-4967.2012.01.011

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On the basis of field investigation of the Qiangang Lake, Longhu Lake, Huihu Lake, and Bingzhou Lake in Jinjiang City of Fujian Province, and analysis of sedimentary facies and micro-palaeontology of drill cores, especially the analysis of the cores from 78 bore holes in Longhu Lake and along its banks, the distribution of eluvial-deluvial formations and late Pleistocene sediments was obtained, then, the environment of the depression and the palaeo-river developed in the depression before the formation of the lakes was analyzed, and finally, the four above-mentioned lakes were inferred to be earthquake-dammed lakes formed in late Holocene. The evidences are as follows: 1)The four lakes spread linearly along the hidden Yakou-bingzhou fault (which controls the northwest boundary of Shenhuwan graben. 2)All are fresh water lakes developed in the depression consisting of Epipleistocene gravel-bearing sandy clay and eluvial-deluvial deposits; paleo-rivers are found existing in the depression before the formation of the lakes. 3)The bottom of the paleo-river channel in the lower reaches is about 3 meters higher than that in the upper reaches in the Longhu Lake depression. 4)The earliest age of lake facies deposits of Qiangang Lake and Lonhu Lake is about 2.4ka BP by ¹⁴C dating of drilling cores, and it is inferred that the two lakes were formed in about 2400a BP, an age being possibly in the time range of the late Holocene strong paleo-earthquake that caused the paleo-forest to be buried more deeply on the sea floor of Shenhuwan Bay. The above lakes are inferred to be earthquake-dammed lakes resulting from the titling and upwarping of the northwest block of Yakou-bingzhou fault in this earthquake, which blocked the rivers and created the lakes. The dams were widened and elevated due to aggradation at the lake dams and filling in the graben after the formation of the lakes, and connected the graben deposits into one mass of roughly the same height, thus, damming the outflow of the lakes. This is an important reason for the lakes to develop continuously and remain up to now.

STUDY ON SEISMIC ACCELERATION ATTENUATION RELATIONSHIP OF GUANZHONG PLAIN USING DIFFERENT TRANSFORMATION METHODS

FAN Wen, DENG Long-sheng, SHAO Hui-cheng, LIU Jia, HE Long-peng, WEN Yi

2012, 34(1): 129-137. DOI: 10.3969/j.issn.0253-4967.2012.01.012

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Using the attenuation relationships which consider the near-field distance and magnitude situation of seismic acceleration and the characteristic models, and by collecting a large number of seismic data of Guanzhong and its adjacent area, the attenuation relations of bedrock horizontal ground acceleration are established. Attenuation relationships of seismic ground motions in western United States are taken as reference for this study. And new attenuation relationships are obtained from different mapping transformation methods for the Guanzhong plain region. Results from different mapping transformation methods are compared and analyzed at last.

THE IMPACT OF HVDC ON GEOMAGNETIC FIELD OBSERVATION

FANG Wei, YAN Rui, SHAO Hui-cheng, ZHANG Guo-qiang

2012, 34(1): 138-144. DOI: 10.3969/j.issn.0253-4967.2012.01.013

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In December 2009, synchronous abnormal variations in geoelectric field observation and Z-component difference of geomagnetic field in central China were observed. After a careful investigation, it was confirmed that this phenomenon was caused by the trial operation of Baoji-Deyang ?500kV DC transmission project. HVDC is an important means to solve power transmission of long distance, but it can cause new interference in seismic geoelectric and geomagnetic field observations. In this paper, we analyzed the geomagnetic observation data from the stations nearby this HVDC project and calculated the impact of HVDC on geomagnetic observation under ideal conditions based on Amp theorem and Biot-Savart law. According to the calculations, the theoretical values are basically identical with the observed values, so the results are explanatory to the mechanism of the impact of HVDC on geomagnetic field observation. However, we did not take into account in the calculation the influencing factors, such as the difference between different geomagnetic observation systems and underground electrical structure, etc. so, the calculation method remains to be improved.

GEOLOGICAL STUDIES ON KELUO VOLCANIC CLUSTER, HEILONGJIANG PROVINCE

ZHANG Liu-yi, LI Ni, FAN Qi-cheng, ZHAO Yong-wei, CAO Yuan-yuan, PAN Xiao-dong

2012, 34(1): 145-159. DOI: 10.3969/j.issn.0253-4967.2012.01.014

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Keluo volcanic cluster, which covers an area of ca. 350km², is located between the Greater Khingan Mountains and the Songnen Plain, Heilongjiang Province, and in the north of the famous Wudalianchi volcanic cluster. Twenty-three Cenozoic volcanoes that primarily consist of alkaline basalts are distributed along the Keluo River. The activities and distribution of this volcanic cluster are largely controlled by the regional basement faults, and consistent with the special tectonic setting of NE-trending fault basin. The Strombolian-type central eruption is the predominant eruptive type in this area. Volcanic activities in this area can be divided into three periods, namely, Pliocene period, Pleistocene period and Holocene period. During the Pliocene, a series of central-overflow-type volcanoes, which situated on the edge of the basin, were formed along the NE-trending faults. However, except for some shield lava platforms, most of the original volcanic geomorphic features have been destroyed by weathering. The volcanic activities of early Pleistocene were relatively quiet. But the eruption activities of the Mid- and Late-Pleistocene were also controlled by the NE-trending basement faults, and the eruptive center, type and intensity were all changed as well. These volcanoes are mainly composed of alkali-basaltic scoria cones and lavas. The Holocene volcanoes, as represented by Nanshan, have similar eruptive type and composition to the former ones, but their volcanic structures are well preserved. Some micro-morphological characteristics can be easily recognized at their outcrops. On the whole, the lava covered the former sedimentary strata and changed the river flows attributed to the volcanic activities of Keluo volcanic cluster, as a result, the original geomorphy in the basin was totally altered.

SEISMOIONOSPHERIC ANOMALIES OBSERVED BEFORE WENCHUAN EARTHQUAKE USING GPS AND DEMETER DATA

YAN Xiang-xiang, SHAN Xin-jian, CAO Jin-bin, TANG Ji, WANG Fei-fei

2012, 34(1): 160-171. DOI: 10.3969/j.issn.0253-4967.2012.01.015

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Based on GPS TEC data and ISL detector data recorded by the French DEMETER satellite, ionospheric perturbations before the Wenchuan earthquake were analyzed. It is found that GPS TEC above the epicenter continuously decreased in the afternoon period during 6 May to 10 May but increased in the afternoon of 9 May. The spatial distribution of the TEC anomaly mainly located in the south of the epicenter, and the size of perturbation area was 1 100～1 670km and 1 600～3 700km from the epicenter in the latitudinal and longitudinal directions, respectively. It is further found that negative anomaly area located southeast on 6 May and moved to southwest on 7 May from epicenter. On 9 May, the anomaly turned to be positive and occurred in the southeast of the epicenter. The intensity of negative anomaly in the southwest or south area from epicenter was stronger than that in the southeast area, compared with the southwest area, the intensity of positive anomaly in southeast or south area was stronger. Moreover, it could be seen that the perturbation was also observed in the geomagnetic conjugate points of epicenter at the southern hemisphere. In addition, contour analysis on ionopheric plasma shows that electron density (N_e) and ion density (N_i) also reduced during 6 to 10 May. The anomalous distribution was in good conformity with TEC, which was concentrated mainly in the south of epicenter. There were two negative anomaly peaks along the earthquake longitude, which were symmetrical about the magnetic equator. Base on the above observation, we finally discuss the physical mechanism of the seismoionospheric anomaly according to Lithosphere-Atmosphere-Ionosphere Couling (LAIC) model.

THE STRENGTH AND DEFORMATION MECHANISMS OF BRITTLE-PLASTIC TRANSITION ZONE, AND THE EFFECTS OF STRAIN RATE AND FLUIDS

ZHANG Yuan-yuan, ZHOU Yong-sheng

2012, 34(1): 172-194. DOI: 10.3969/j.issn.0253-4967.2012.01.016

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Constraints provided by field observation, laboratory experiments and seismic data have lead to a general consensus that the shallow crust deforms by brittle faulting, while the lower crust deforms by crystal plastic flow. These constraints provide the basis for the dual mechanism model for the rheology of the crust and lithosphere in which the strength of the upper brittle crust is limited by Byerlee's law, while the strength of the lower ductile crust is limited by power law creep. The maximum depth of microseismic activity is controlled by the broad zone of brittle-plastic transition that lies between the two extreme brittle and plastic layers. While the dual mechanism model is so simple that overestimates the strength of rocks near the brittle-plastic transition zone. Although many studies about the deformation mechanism of brittle-plastic transition zone have been made, a 'flow law' representation, which can describe the strength for the brittle-plastic transition, has not been formulated, and there has been little research about fluid effects; In addition, research on brittle-plastic transition usually focuses on temperature effects, while the research on the aspects of strain rate and fluid are relatively weak. Studies of deformation mechanisms of minerals in faults have indicated that brittle-plastic transition of some faults occurred in the same depth (temperature and pressure) and this phenomenon, which has been considered to be relevant to synseismic loading and postseismic creep in earthquake cycles and confirmed by distribution of focal depth, is due to the strain rate. The presence of high-pressure fluid in active fault at depth is proved by analysis of characteristics of fault fluids, and these fluids, which can evolve in pressure pertaining to fracturing and sealing processes, play a key role during the seismic cycle. The formation of high-pressure fluid (cracks sealing) has several mechanisms, but researches show pressure solution deposition is one of the main mechanisms which controls crack sealing kinetics around active faults. Studies on pressure solution under the action of water can supplement and correct the crustal strength profile defined by traditional relations describing brittle/frictional behavior (Byerlee's law) and dislocation creep. As a consequence, we believe it is necessary to further study the impact of strain rate and fluid pressure on the brittle-plastic transition through deformation samples both from field and high-pressure high-temperature experiments. Simultaneously, we may establish the equation for the pressure solution to approximately estimate the strength of brittle-plastic transition zone.

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