FRICTIONAL PROPERTIES ASSOCIATED WITH ACOUSTIC EMISSION CHARACTERISTICS OF SIMULATED GRANITE FAULT GOUGES:EFFECTS OF NORMAL STRESS AND WATER CONTENT
The Xinfengjiang Water Reservoir in Guangdong,China,is one of the reservoirs that has triggered earthquakes of magnitudes greater than 6.Numerous earthquakes have occurred since the reservoir's impoundment,making it one of the most active seismic zones in Guangdong.Disturbance in effective stress and water content in existing fault zones upon the water reservoir may play a critical role in induced earthquakes.Here,we report 17 friction experiments performed on simulated granite fault gouges(28％quartz,25％albite,and 44％microcline,particle size＜0.25mm,2mm thickness)collected from fault zones near Xinfengjiang Reservoir to investigate the effect of effective normal stress and water content.This was achieved using a direct shear apparatus associated with two acoustic emission(AE)sensors.Microstructure observation was also performed on post-deformed samples to discuss the possible mechanism of reservoir-induced earthquakes.Regarding the analysis method of acoustic emission data,we adopt a crack cumulative summation curve method,following the RA-AF crack classification method,which can successfully inverse and distinguish the possible different microcracking processes in deformed rock from the pure tensile microcrack development(k=1)to the pure shear microcrack development(k=-1)according to the variance of curve slope k value(i.e.,-1～1).The velocity stepping(1-10-50～100µm/s)and slide-hold-slide(at a constant sliding velocity of 1 mm/s with hold intervals of 10-30-100-300-1 000-3 000 seconds)experiments were conducted at the fixed normal stresses of 0.5～20MPa(10％water content)effective normal stress and 0％～25％(10MPa effective normal stress)water content under drained conditions at room temperature.Acoustic emission signals were also recorded.The shear displacement of both types of experiments is～7mm.(1)Velocity stepping experiments show that the wet samples with 10％water content exhibit a transition from velocity weakening to velocity strengthening at the normal stress of 10MPa,and the apparent dilatancy was observed for the samples showing velocity strengthening utilizing the changes in axial displacement during sliding.At the normal stress of 10MPa,the room dry sample also shows velocity strengthening accompanied by dilatancy.Still,the wet sample with 25％water content shows velocity weakening,though dilatancy was also observed.This may indicate the negative effect of water on velocity strengthening.In addition,AE analysis indicates the development of tensile microcracks(60％～80％)dominated for velocity weakening and the appearance of strongly developed shear microcracks dominated for velocity strengthening,which was consistent with the microstructural observation performed on the deformed samples.It clearly illustrates that the tensile fracture(T)zones developed in the samples showed velocity weakening,while the Ridel shear fracture(R)zones developed in the samples showed velocity strengthening.(2)Slide-hold-slide experiments show a nonlinear relation between transient peak healing(Δμpk)and the logarithm of hold time(log(th)),or"non-Dieterich"healing behavior.By contrast,postpeak weakening(Δμw,a stress drop measured as the difference between the peak frictional strength and the steady-state friction of reshearing)shows a direct,near-linear relation with log(th).In general,Δμw increased with increasing log(th)and water content but reduced with increasing normal stress.This may indicate the lower normal stress,the higher water content and the longer hold time the more likely for the unsteady slip of the faults.AE analysis showed that the stress drop is positively correlated with the proportion of tensile cracks.Based on the effects of the effective normal stress and water content on frictional properties,we suggest that the effect of hydrochemical weakening and increase of stress drop after healing may be the mechanisms responsible for a transition from velocity weakening to velocity weakening for granite gouges.The CNS model can well explain the"non-Dieterich"healing behavior of granite gouges.Combined with reservoir-induced earthquakes,it is indicated that the fault zone under Xinfengjiang Reservoir may have unstable slip below 5MPa effective normal stress and 25％water content,resulting in reservoir-induced earthquakes.Importantly,the weak rate weakening behavior under high water content is conducive to generating slow slip seismic events.The experimental results have certain indicative significance for reservoir-induced earthquakes.
万方数据
维普
