Experimental Studies On Intermediate-to High-velocity Frictional Properties Of Fault Gouges From The Longmenshan Fault Zone

In order to understand frictional properties of the Longmenshan fault zone in thecoseismic sliding during the2008Wenchuan earthquake, we conducted intermediate-to high-velocity frictional experiments on fault gouges collected from theLongmenshan fault zone at surface outcrops and from the WFSD-1borehole (a smallamount). The mainly concerned issues in this thesis include the high-velocityfrictional properties and their heterogeneities of the Longmenshan fault zone, thepost-seismic strength recovery of the seismic fault, possible dominant slip-weakeningmechanisms during the high-velocity sliding of the seismic fault and thevelocity-dependence of fault friction over a wide velocity regime.Based on detailed analysis of the fault-zone structures and fault rocks, weselected four kinds of fault gouges from the fault zone at the Kuangpingzi outcrop andthe Bajiaomiao outcrop. High-velocity experiments conducted on these four gougesand other three WFSD-1gouges revealed typical slip-weakening behaviors of anexponential-decay type, except that a kind of black gouge from the PingxiKuangpingzi outcrop showed peculiar frictional behavior which was characterized bya sharp initial weakening, subsequent slight strengthening to the second peak friction,and slip weakening towards steady-state friction. Microstructure observations andresults of some comparison experiments suggested that the special microstructureevolutions were the causes for the rapid initial weakening and subsequentstrengthening of the black gouge. Based on several empirical formulas and furtherderivation, we selected six constitutive parameters that can quantitatively describe thedependences of steady-state friction coefficient, slip-weakening distance and specificfracture energy on the slip rate and normal stress. We compared the high-velocityfrictional properties of six kinds of gouges from four surface outcrops studied in thiswork and former published work. The results show that the heterogeneity do exist forthose six constitutive parameters. But the Longmenshan fault zone is quite uniform interms of steady-state friction coefficient at high slip rates, e.g. less than0.2almost for all gouges at a slip rate of1.4m/s under dry conditions and much smaller under wetconditions. The observed dramatic high-velocity weakening must have promoteddynamic rupture propagation during the Wenchuan earthquake, and also can explainthe very small temperature anomalies measured in the drilling borehole.Slide-hold-slide (SHS) tests conducted on fault gouges at high velocity revealedvery rapid strength recovery by more than0.4in friction coefficient in less than5~10sfrom the former high-velocity slip weakening, and the healing rate (increase in frictioncoefficient divided by the logarithm of hold time) was as high as0.154-0.188duringthis rapid recovery stage. Then the fault strength recovered slowly in proportion to thelogarithm of time with a healing rate of0.015-0.016. Temperature calculationindicated that initial rapid strength recovery was related to rapid temperature drop.The time scale of the initial rapid strength recovery is about5-10s, which is less thanthe common rise time of large earthquakes; thus the observed results supportself-healing slip pulses in earthquake rupture. Also, rapid strength recovery of thefault after high-velocity sliding may play an essential role in stabilizing the fault andreducing aftershocks.Comparison experiments conducted on very thin (0.1-0.14mm) andcommon-width gouge layers (1.0-1.4mm) at intermediate to high velocitydemonstrated that shearing thin gouge layers could weaken faults more rapidly, andthe characteristic weakening velocity was also much lower for the thin gouge layercase. Microstructure observations suggested this was probably due to much notableflash heating in thin gouge experiments. The difference of high-velocity frictionalbehaviors between the fairly thin and relatively thick gouge tests suggest thatrelatively thin gouge layers in a natural fault zone may weaken the fault more rapidlyduring the seismic slip and can probably promote the propagation of a seismic rupturemore effectively. Comparison experiments using gabbro and brass host blocksrevealed that only slight slip weakening was observed in the latter tests. Thetemperature calculation showed that this might result from the restrained temperaturerise due to high thermal conductivity of brass host blocks. This phenomenon alsohighlights the importance of the thermally-related mechanisms, flash heating and thermal pressurization, on fault weakening. High-velocity experiments conducted onMgO nano-particles using brass host blocks revealed that even the pure nano-particlescould not weaken the fault effectively when the temperature rise was restricted. Theseresults suggest that rolling lubrication of nano-particles may not be one of the mainhigh-velocity weakening mechanism.Low-to high-velocity friction experiments on fault gouges from theLongmenshan fault zone revealed possible strong velocity strengthening in theintermediate velocity regime. But we also observed notable strain hardening (slipstrengthening) at slip rates less than hundreds of μm/s under large shearstrain(e.g.>1000), which may be related to material or structural change of the testedgouges under large shear strain. We will fully consider this effect in the study ofvelocity-dependence of friction over wide velocity range in the future.