The rock mass in fault zones is frequently subjected to cyclic loading and unloading during deep resource exploitation and tunnel excavation.Research on the mechanical and hydraulic characteristics of fault rock durin...The rock mass in fault zones is frequently subjected to cyclic loading and unloading during deep resource exploitation and tunnel excavation.Research on the mechanical and hydraulic characteristics of fault rock during the cyclic loading and unloading is of great signifcance for revealing the formation mechanism of water-conducting pathways in fault and preventing water inrush disasters.In this study,the mechanical and seepage tests of fault rock under the multi-stage cyclic loading and unloading of axial compression were carried out by using the fuid–solid coupling triaxial experimental device.The hysteresis loop of the stress–strain curve,peak strain rate,secant Young's modulus,and permeability of fault rock were obtained,and the evolution law of the dissipated energy of fault rock with the cyclic number of load and unloading was discussed.The experimental results show that with an increase in the cyclic number of loading and unloading,several changes occur.The hysteresis loop of the stress–strain curve of the fault rock shifts towards higher levels of strain.Additionally,both the peak strain rate and the secant Young's modulus of the fault rock increase,resulting in an increase in the secant Young's modulus of the fault rock mass.However,the growth rate of the secant Young's modulus gradually slows down with the increase of cyclic number of loading and unloading.The permeability evolution of fault rock under the multi-stage cyclic loading and unloading of axial compression can be divided into three stages:steady increase stage,cyclic decrease stage,and rapid increase stage.Besides,the calculation model of dissipated energy of fault rock considering the efective stress was established.The calculation results show that the relationship between the dissipated energy of fault rock and the cyclic number of loading and unloading conforms to an exponential function.展开更多
The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new ins...The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new insights into the ongoing debate about the frictional strength of brittle fault(Haines and van der Pluijm,2012).However,neither the conditions nor the processes which展开更多
The permeabilities of fault rocks from the rupture of Wenchuan earthquake were measured by using nitrogen gas and distilled water as pore fluids under the confining pressure ranging from 20 to 180 MPa at room temperat...The permeabilities of fault rocks from the rupture of Wenchuan earthquake were measured by using nitrogen gas and distilled water as pore fluids under the confining pressure ranging from 20 to 180 MPa at room temperature. Experimental results indi- cate that both gas and water permeabilities decrease with increasing confining pressure, described by power law relationship, i.e., b = 0.2x10-3kl-0.557. The water permeability is about one order less than gas permeability and also half order smaller than the permeability corrected by the Klinkenberg effect, so-called intrinsic permeability. The differences in the permeabilies imply that the reduction of effective pore size caused by the adhesion of water molecules to clay particle surface and water-swelling of expandable clay minerals contributes to lessening the water permeability besides the Klinkenberg effect. Hence, the liquid permeability of fault rocks cannot be deduced by gas permeability by the Klinkenberg correction reliably and accurately, and it is necessary to use liquid as pore media to measure their transport property directly.展开更多
The Koktokay Ertai fault zone was developed on the basis of a former ductile shear zone (mylonite zone). The mylonites were formed in the brittle ductile transition zone at 10~15 km depth within the crust. The rock...The Koktokay Ertai fault zone was developed on the basis of a former ductile shear zone (mylonite zone). The mylonites were formed in the brittle ductile transition zone at 10~15 km depth within the crust. The rocks had experienced multi period deformation processes, including ductile deformation, ductile instability and brittle seismic faulting which had gave rise to the formation of Koktokay Ertai fault zone and the development of pseudotachylytes. The pseudotachylytes are distributed within an area about 60 km in length and 300 m in width, which may represent the scale of the paleo earthquake source. The physical conditions of the paleo earthquake source was similar to those for the formation of the mylonites along this zone.展开更多
Terrestrial faulted lacustrine basin is considered as a favorable place for the development of source rocks,especially the fault basins in eastern China.Based on molecular composition study of saturated and aromatic h...Terrestrial faulted lacustrine basin is considered as a favorable place for the development of source rocks,especially the fault basins in eastern China.Based on molecular composition study of saturated and aromatic hydrocarbons in the extracts of source rocks of the Yingcheng and Shahezi formations in the Lishu Fault Depression,it is revealed that the extracts of source rocks are provided with low ratio of Pr/Ph,low content of C24 tetracyclic terpanes,high content of tricyclic terpanes and gammacerane,relatively high content of C27 steranes in the sag belt and its periphery;relatively high ratio of Pr/Ph,relatively high content of tetracyclic terpanes and gammacerane,low content of C27 steranes and obvious advantages of C29 steranes in its gentle slope belt;with high ratio of Pr/Ph,high content of C19,C20 tricyclic and C24 tetracyclic terpanes,very low content of gammacerane and C27 steranes,and high content of C29 steranes in the edge of fault depression.According to the organic matter input and sedimentary environment,three types of source rocks have been established,which,respectively,are the mode of mixing organic matter input in deep and semi-deep lake,the advantage of terrigenous input in shallow lake and terrigenous input in shore-shallow lacustrine.The first mode is supposed to generate oil and gas,while the second one tends to generate oil.The distribution characteristics of oil and gas in the Lishu Fault Depression are very likely to be controlled by hydrocarbon generating characteristic of different source rock types.展开更多
Fault is a complex dynamic system controlled by the coupling of rock texture, reaction, fluid flow,stress, and rock deformation mechanism. A coupled reaction-transport- mechanical dynamic model for fault system is est...Fault is a complex dynamic system controlled by the coupling of rock texture, reaction, fluid flow,stress, and rock deformation mechanism. A coupled reaction-transport- mechanical dynamic model for fault system is established and described in this paper. An example is presented for the Shuikoushan deposit, Hunan. The results of dynamic simulation indicate that the evolution and magnitude of fracture permeability of different rocks are different, and that faulting can enhance the spatial heterogeneity of rock permeability and facilitate fluid flow and mineralization in local fault zone. The pressure for a fault usually shows a variation mode of aperiodic oscillation with time, which reflects the chaotic behavior of the evolution of a fault.展开更多
Near-fault ground motions, potentially with large amplitude and typical velocity pulses, may significantly impact the performance of a wide range of structures. The current study is aimed at evaluating the safety impl...Near-fault ground motions, potentially with large amplitude and typical velocity pulses, may significantly impact the performance of a wide range of structures. The current study is aimed at evaluating the safety implications of the near-fault effect on nuclear power plant facilities designed according to the Chinese code. To this end, a set of near- fault ground motions at rock sites with typical forward-directivity effect is examined with special emphasis on several key parameters and response spectra. Spectral comparison of the selected records with the Chinese and other code design spectra was conducted. The bi-normalized response spectra in terms of different comer periods are utilized to derive nuclear design spectra. It is concluded that nuclear design spectra on rock sites derived from typical rupture directivity records are significantly influenced both by the earthquake magnitude and the rupture distance. The nuclear design spectra specified in the code needs to be adjusted to reflect the near-fault directivity effect of large earthquakes.展开更多
The Xujiaweizi Fault Depression is located in the northern Songliao Basin,Northeast China.The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formatio...The Xujiaweizi Fault Depression is located in the northern Songliao Basin,Northeast China.The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation(K 1 yc).Based on seismic interpretation,drill cores and the results of previous research,we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K 1 yc,and studied the relationship of volcanic activities and main faults.Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones.The volcanic activities strongly eroded the faults during the eruption process,which resulted in the structural traces in the seismic section being diffuse and unclear.The tectonic activities weakened in the study area in the depression stage.The analysis of seismic interpretation,thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities,which greatly improved the physical properties of volcanic reservoirs,and made the volcanic rocks of K 1 yc be favorable natural gas reservoirs.The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults,the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks.展开更多
This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) at the south Longman-Shan fault (LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.I...This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) at the south Longman-Shan fault (LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.Immediately after the mainshock,two Wenchuan Fault Scientific Drilling (WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault (YBF),the middle fault strand of the south LSF zone.Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m,respectively.The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge,breccia,cataclasite and fractures.Close to WFSD-1 site,the nearly-vertical slip of ~4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake.A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks.Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths.The zone is several hundred meters wide along the principal slip,within which seismic velocities are reduced by ~30-55% from wall-rock velocities and with the maximum velocity reduction in the ~200-m-wide rupture core zone at shallow depth.The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes.We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake.We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ~10% due to severe damage of fault rocks during the M8 mainshock.Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault (AGF),two strands of the south LSF at shallow depth.A combination of seismic,petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics,and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.展开更多
The spatial and temporal distributions of acoustic emission (AE) during the deformation of samples containing an inhomogeneous fault have been studied under biaxial compression. The results show that the fault stren...The spatial and temporal distributions of acoustic emission (AE) during the deformation of samples containing an inhomogeneous fault have been studied under biaxial compression. The results show that the fault strength and the duration from loading to failure increase and the failure mode changes from abrupt instability to gradual failure with increase of lateral stress σ2. The pre-setting fault and its heterogeneity play an important role in controlling AE spatial distribution during the deformation. The basic pattern of AE spatial distribution is controlled by the pre-setting fault, especially the parts with inhomogeneous strength and the strong segment, and the localization of fracturing starts from the positions with inhomogeneous strength, With increase of σ5, the dense AE distribution area spreads from the positions with inhomogeneous strength to the whole strong segment gradually. AE temporal sequence is significantly affected by σ2. The fault shows abrupt instability in final failure, which occurs on a background of "enhancement-quiescence" in microfracturing activity at lower σ2. At higher σ5, the fault shows gradual failure, and AE occurrence rate increases continuously and AER increase exponentially before and after the failure, The effect of σ2 on b-value is also remarkable, b-value shows precursory decrease in the weakening stage when fault behavior is abrupt instability, but it shows balanced change when fault behavior is gradual failure,展开更多
The age and evolutional history of the Altyn Tagh fault and its role in the formation and uplift of the Qinghai—Tibetan plateau have been focused for years. Many geologists believe that the formation of the fault is ...The age and evolutional history of the Altyn Tagh fault and its role in the formation and uplift of the Qinghai—Tibetan plateau have been focused for years. Many geologists believe that the formation of the fault is a result of the collision between India and Asia. Some people thought that it should have formed earlier than Cenozoic but have no critical evidence. Here we report a preliminary result from our recent investigation on the volcanic rocks at the north end of the Altyn Tagh fault.1 The volcanic rocks on the north end of the Altyn Tagh fault The volcanic rocks are located on the north end of the Altyn Tagh fault, northern Qinghai—Tibetan plateau. The investigated volcano occurs in the Jiuxi basin, a Cretaceous and Tertiary depositional basin. It is about 300m×100m in size and form about a 100m high cliff above the folded Cretaceous strata. It likes relic neck of a volcano rather than a kind of widely distributed lava flow commonly seen in the northern Tibet. The country rocks are Cretaceous sandstone, silt and fine\|grained conglomerate. The cliff formed most likely due to the differing erosion between the hard volcanic rocks and soft rocks.展开更多
The spatio-temporal characteristics of acoustic emission (AE) during the deformation of rock samples with compressional and extensional en-echelon faults have been studied. The results show that the pre-existing struc...The spatio-temporal characteristics of acoustic emission (AE) during the deformation of rock samples with compressional and extensional en-echelon faults have been studied. The results show that the pre-existing structure can significantly influence the patterns of AE spatial distribution. With increasing of differential stress, AE events firstly cluster around the two ends of pre-existing faults inside the jog and then along the line joining the two ends. The biggish AE events often occur around one end repeatedly. The image of AE clusters indicates the direction and the area of the fracture propagation. The direction of the macroscopic fracture in extensional and compressional jogs is perpendicular and parallel to the direction of axial stress, respectively. The weakening process before the fracturing of jog area is remarkable, and one of the typical precursors for the instability is that the cumulative frequency of AE events increases exponentially. After the fracturing of the jog the frequency and releasing strain energy of AE events decrease gradually. During the friction period, there is no precursory increasing of AE activity before the big stick-slip events. The change of b value in jog shows a typical change of decreasing tendentiously returning quickly before the instability. The decrease of b value occurs in the process of stress increasing and sometime goes down to the weakening stage, and the quick increase b values appears in a short time just before the instability. The comparative analysis shows that the difference in b value due to the different structures is larger than b value variation caused by increase of the differential stress. For the same sample, the temporal sequence of AE is strongly affected by the mechanical state, and the high loading velocity corresponds to the high release rate of strain energy and low b value. Due to its lower failure strength, the broken area is sensitive to small changes in differential stress. Therefore, it offers a potential explanation for the phenomena of precursory window or sensitive point and separation of seismic source and precursors.展开更多
基金supported by the National Science Fund for Excellent Young researchers of Science China(52122404)the National Natural Science Foundation of China(41977238).
文摘The rock mass in fault zones is frequently subjected to cyclic loading and unloading during deep resource exploitation and tunnel excavation.Research on the mechanical and hydraulic characteristics of fault rock during the cyclic loading and unloading is of great signifcance for revealing the formation mechanism of water-conducting pathways in fault and preventing water inrush disasters.In this study,the mechanical and seepage tests of fault rock under the multi-stage cyclic loading and unloading of axial compression were carried out by using the fuid–solid coupling triaxial experimental device.The hysteresis loop of the stress–strain curve,peak strain rate,secant Young's modulus,and permeability of fault rock were obtained,and the evolution law of the dissipated energy of fault rock with the cyclic number of load and unloading was discussed.The experimental results show that with an increase in the cyclic number of loading and unloading,several changes occur.The hysteresis loop of the stress–strain curve of the fault rock shifts towards higher levels of strain.Additionally,both the peak strain rate and the secant Young's modulus of the fault rock increase,resulting in an increase in the secant Young's modulus of the fault rock mass.However,the growth rate of the secant Young's modulus gradually slows down with the increase of cyclic number of loading and unloading.The permeability evolution of fault rock under the multi-stage cyclic loading and unloading of axial compression can be divided into three stages:steady increase stage,cyclic decrease stage,and rapid increase stage.Besides,the calculation model of dissipated energy of fault rock considering the efective stress was established.The calculation results show that the relationship between the dissipated energy of fault rock and the cyclic number of loading and unloading conforms to an exponential function.
基金financed by the National Youth Sciences Foundation of China (No. 41502044)
文摘The role of authigenic clay growth in clay gouge is increasingly recognized as a key to understanding the mechanics of berittle faulting and fault zone processes,including creep and seismogenesis,and providing new insights into the ongoing debate about the frictional strength of brittle fault(Haines and van der Pluijm,2012).However,neither the conditions nor the processes which
基金supported by Wenchuan Fault Scientific Drilling Program(Grant No.WFSD-09)the Foundation of State Key Laboratory of Earthquake Dynamics(Grant No.LED2010A03)+1 种基金the National Natural Science Foundation of China(Grant No.41372202)the Foundation of Earthquake Sciences of China Earthquake Administration(Grant No.2008419012)
文摘The permeabilities of fault rocks from the rupture of Wenchuan earthquake were measured by using nitrogen gas and distilled water as pore fluids under the confining pressure ranging from 20 to 180 MPa at room temperature. Experimental results indi- cate that both gas and water permeabilities decrease with increasing confining pressure, described by power law relationship, i.e., b = 0.2x10-3kl-0.557. The water permeability is about one order less than gas permeability and also half order smaller than the permeability corrected by the Klinkenberg effect, so-called intrinsic permeability. The differences in the permeabilies imply that the reduction of effective pore size caused by the adhesion of water molecules to clay particle surface and water-swelling of expandable clay minerals contributes to lessening the water permeability besides the Klinkenberg effect. Hence, the liquid permeability of fault rocks cannot be deduced by gas permeability by the Klinkenberg correction reliably and accurately, and it is necessary to use liquid as pore media to measure their transport property directly.
文摘The Koktokay Ertai fault zone was developed on the basis of a former ductile shear zone (mylonite zone). The mylonites were formed in the brittle ductile transition zone at 10~15 km depth within the crust. The rocks had experienced multi period deformation processes, including ductile deformation, ductile instability and brittle seismic faulting which had gave rise to the formation of Koktokay Ertai fault zone and the development of pseudotachylytes. The pseudotachylytes are distributed within an area about 60 km in length and 300 m in width, which may represent the scale of the paleo earthquake source. The physical conditions of the paleo earthquake source was similar to those for the formation of the mylonites along this zone.
文摘Terrestrial faulted lacustrine basin is considered as a favorable place for the development of source rocks,especially the fault basins in eastern China.Based on molecular composition study of saturated and aromatic hydrocarbons in the extracts of source rocks of the Yingcheng and Shahezi formations in the Lishu Fault Depression,it is revealed that the extracts of source rocks are provided with low ratio of Pr/Ph,low content of C24 tetracyclic terpanes,high content of tricyclic terpanes and gammacerane,relatively high content of C27 steranes in the sag belt and its periphery;relatively high ratio of Pr/Ph,relatively high content of tetracyclic terpanes and gammacerane,low content of C27 steranes and obvious advantages of C29 steranes in its gentle slope belt;with high ratio of Pr/Ph,high content of C19,C20 tricyclic and C24 tetracyclic terpanes,very low content of gammacerane and C27 steranes,and high content of C29 steranes in the edge of fault depression.According to the organic matter input and sedimentary environment,three types of source rocks have been established,which,respectively,are the mode of mixing organic matter input in deep and semi-deep lake,the advantage of terrigenous input in shallow lake and terrigenous input in shore-shallow lacustrine.The first mode is supposed to generate oil and gas,while the second one tends to generate oil.The distribution characteristics of oil and gas in the Lishu Fault Depression are very likely to be controlled by hydrocarbon generating characteristic of different source rock types.
基金supported by the National Natural Science Foundation of China(Gramt No.70171057 and No.49702024)a Key Project of the Ninth Five-Year Plan of the Chinese Academof Sciences(Grant No.KZ952-S1-402).
文摘Fault is a complex dynamic system controlled by the coupling of rock texture, reaction, fluid flow,stress, and rock deformation mechanism. A coupled reaction-transport- mechanical dynamic model for fault system is established and described in this paper. An example is presented for the Shuikoushan deposit, Hunan. The results of dynamic simulation indicate that the evolution and magnitude of fracture permeability of different rocks are different, and that faulting can enhance the spatial heterogeneity of rock permeability and facilitate fluid flow and mineralization in local fault zone. The pressure for a fault usually shows a variation mode of aperiodic oscillation with time, which reflects the chaotic behavior of the evolution of a fault.
基金National Natural Science Foundation of China Under Grant No.50808168Ministry of Science and Technology of Weihai Under Grant No.2008087Beijing Natural Science Foundation Under Grant No.8092029
文摘Near-fault ground motions, potentially with large amplitude and typical velocity pulses, may significantly impact the performance of a wide range of structures. The current study is aimed at evaluating the safety implications of the near-fault effect on nuclear power plant facilities designed according to the Chinese code. To this end, a set of near- fault ground motions at rock sites with typical forward-directivity effect is examined with special emphasis on several key parameters and response spectra. Spectral comparison of the selected records with the Chinese and other code design spectra was conducted. The bi-normalized response spectra in terms of different comer periods are utilized to derive nuclear design spectra. It is concluded that nuclear design spectra on rock sites derived from typical rupture directivity records are significantly influenced both by the earthquake magnitude and the rupture distance. The nuclear design spectra specified in the code needs to be adjusted to reflect the near-fault directivity effect of large earthquakes.
基金supported by the Major State Basic Research Development Program of China (973 Program(No.2012CB214705))the National Natural Science Foundation of China (No. 41206035)
文摘The Xujiaweizi Fault Depression is located in the northern Songliao Basin,Northeast China.The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation(K 1 yc).Based on seismic interpretation,drill cores and the results of previous research,we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K 1 yc,and studied the relationship of volcanic activities and main faults.Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones.The volcanic activities strongly eroded the faults during the eruption process,which resulted in the structural traces in the seismic section being diffuse and unclear.The tectonic activities weakened in the study area in the depression stage.The analysis of seismic interpretation,thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities,which greatly improved the physical properties of volcanic reservoirs,and made the volcanic rocks of K 1 yc be favorable natural gas reservoirs.The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults,the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks.
基金supported by the "Wenchuan Earthquake Fault Scientific Drilling" of the National Science Foundation of China
文摘This article is to review results from scientific drilling and fault-zone trapped waves (FZTWs) at the south Longman-Shan fault (LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan,China.Immediately after the mainshock,two Wenchuan Fault Scientific Drilling (WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault (YBF),the middle fault strand of the south LSF zone.Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m,respectively.The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge,breccia,cataclasite and fractures.Close to WFSD-1 site,the nearly-vertical slip of ~4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake.A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks.Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths.The zone is several hundred meters wide along the principal slip,within which seismic velocities are reduced by ~30-55% from wall-rock velocities and with the maximum velocity reduction in the ~200-m-wide rupture core zone at shallow depth.The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes.We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake.We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ~10% due to severe damage of fault rocks during the M8 mainshock.Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault (AGF),two strands of the south LSF at shallow depth.A combination of seismic,petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics,and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.
基金Joint Seismological Science Foundation of China (105076) and continued sub-project ″Statistic Features of After-shock Sequences and Forecast of the Large Aftershocks″ (2004BA601B01-04-02) managed by the Ministry of Science and Tech-nology of China in the 10th five-year plan.
文摘The spatial and temporal distributions of acoustic emission (AE) during the deformation of samples containing an inhomogeneous fault have been studied under biaxial compression. The results show that the fault strength and the duration from loading to failure increase and the failure mode changes from abrupt instability to gradual failure with increase of lateral stress σ2. The pre-setting fault and its heterogeneity play an important role in controlling AE spatial distribution during the deformation. The basic pattern of AE spatial distribution is controlled by the pre-setting fault, especially the parts with inhomogeneous strength and the strong segment, and the localization of fracturing starts from the positions with inhomogeneous strength, With increase of σ5, the dense AE distribution area spreads from the positions with inhomogeneous strength to the whole strong segment gradually. AE temporal sequence is significantly affected by σ2. The fault shows abrupt instability in final failure, which occurs on a background of "enhancement-quiescence" in microfracturing activity at lower σ2. At higher σ5, the fault shows gradual failure, and AE occurrence rate increases continuously and AER increase exponentially before and after the failure, The effect of σ2 on b-value is also remarkable, b-value shows precursory decrease in the weakening stage when fault behavior is abrupt instability, but it shows balanced change when fault behavior is gradual failure,
文摘The age and evolutional history of the Altyn Tagh fault and its role in the formation and uplift of the Qinghai—Tibetan plateau have been focused for years. Many geologists believe that the formation of the fault is a result of the collision between India and Asia. Some people thought that it should have formed earlier than Cenozoic but have no critical evidence. Here we report a preliminary result from our recent investigation on the volcanic rocks at the north end of the Altyn Tagh fault.1 The volcanic rocks on the north end of the Altyn Tagh fault The volcanic rocks are located on the north end of the Altyn Tagh fault, northern Qinghai—Tibetan plateau. The investigated volcano occurs in the Jiuxi basin, a Cretaceous and Tertiary depositional basin. It is about 300m×100m in size and form about a 100m high cliff above the folded Cretaceous strata. It likes relic neck of a volcano rather than a kind of widely distributed lava flow commonly seen in the northern Tibet. The country rocks are Cretaceous sandstone, silt and fine\|grained conglomerate. The cliff formed most likely due to the differing erosion between the hard volcanic rocks and soft rocks.
基金Project Study on the Short-term Forecasting Technique of Strong Earthquake (01-04-01) during the Tenth Five-Year Plan from Ministry of Science and Technology of China and Joint Seismological Science Foundation of China (102037).
文摘The spatio-temporal characteristics of acoustic emission (AE) during the deformation of rock samples with compressional and extensional en-echelon faults have been studied. The results show that the pre-existing structure can significantly influence the patterns of AE spatial distribution. With increasing of differential stress, AE events firstly cluster around the two ends of pre-existing faults inside the jog and then along the line joining the two ends. The biggish AE events often occur around one end repeatedly. The image of AE clusters indicates the direction and the area of the fracture propagation. The direction of the macroscopic fracture in extensional and compressional jogs is perpendicular and parallel to the direction of axial stress, respectively. The weakening process before the fracturing of jog area is remarkable, and one of the typical precursors for the instability is that the cumulative frequency of AE events increases exponentially. After the fracturing of the jog the frequency and releasing strain energy of AE events decrease gradually. During the friction period, there is no precursory increasing of AE activity before the big stick-slip events. The change of b value in jog shows a typical change of decreasing tendentiously returning quickly before the instability. The decrease of b value occurs in the process of stress increasing and sometime goes down to the weakening stage, and the quick increase b values appears in a short time just before the instability. The comparative analysis shows that the difference in b value due to the different structures is larger than b value variation caused by increase of the differential stress. For the same sample, the temporal sequence of AE is strongly affected by the mechanical state, and the high loading velocity corresponds to the high release rate of strain energy and low b value. Due to its lower failure strength, the broken area is sensitive to small changes in differential stress. Therefore, it offers a potential explanation for the phenomena of precursory window or sensitive point and separation of seismic source and precursors.