Observations And Analyses To Transient Processes Of Unstable Slip On The Fault By Laboratory Experiments

It is widely accepted that an earthquake results from rock failure and unstablefrictional sliding on the fault plane. Thus fault propagation and instability is anfundamental issue in earthquake science. Observations to characteristics of physicalfield evolution during fault deformation and establishment of instability models are ofgreat significance for understanding earthquake mechanisms.The earthquake process can be simply divided into three stages: seismicgestation, earthquake occurrence, and post-seismic adjustment. Of them, there are fewrecords to the earthquake process, in particular little is known on the near-fieldmechanical process when earthquake happens in an instant. The laboratoryexperiments on the transient process of fault unstable sliding make it possible toconduct a detailed study on such transient process which would be helpful tointerpretations of field observations of earthquakes.In this work, three typical stick-slip models and fault propagation models havebeen selected as the objects of experimental research. A specially designed dynamicstrain observation system was employed to acquire data continuously with a samplingrate of3,400Hz. A velocity observation system with the sampling frequency of96KHz and an acoustic emission system with the sample rate of1MHz were used tomonitor the slip rate of stick-slip and failure signals. A series of simulationexperiments on granodiorite samples have been conducted at a biaxial horizontalloading apparatus with the electro-hydraulic servo control system. Firstly, thestructural parameters during the unstable slip failure were described, such as localstrain along the fault, acoustic emission signals, fault macroscopic slip rate. And theevolution features of these mechanical parameters during the transient process of faultunstable slip were analyzed. Secondly, how the fault rupture initiates, propagates andstops is studied, and the interaction and relationship between the parameters wasdiscussed, which provide insights into the physical nature of fault unstable slip on amicrocosmic scale and from the transient field. Next, the regional stress path and localstrain path during the whole earthquake process were described in the stress space andstrain space. The strain deformation stage on spontaneous earthquakes and inducedearthquakes was discussed, which can provide a new perspective for the in-situobservations and regional stress analysis. Finally, the calculation method of maximumdisplacement was discussed for estimating the moment magnitude of stick-slip events.The relationship of stick-slip models and stress drops and magnitude was alsoanalyzed in this thesis, which provides the basis for studying the relation betweennatural earthquakes and stick-slip friction.The research content and the main conclusions are summarized below.1) The experimental results on the three-dimensional fault propagation modelshow that the coalescence of the bridge area occurs at the last stage and is a rapidprocess. The crack initiation occurs at several points which combine each otherrandomly. There is a stable period present between the fully propagation and coalescence and sample failure, and the duration time is several tens milliseconds.2) The experiments on three typical stick-slip model show that the evolution ofthe fault stick-slip unstable process has a relatively stable feature characterized bythree typical phases (precursory slip, rapid slip incorporated with high-frequencystrain vibration and terminal adjustment), which is independent of dynamical loadingconditions and is the inherent feature of fault unstable slip. Main energy release takesplace at rapid slip with high-frequency oscillation. Stress drop is not the same as theearthquake, and the earthquake happens in the rapid slip with high-frequencyoscillation. When a stick-slip event occurs, the rock deformation enters the precursorslip phase, which means the fault enters the irreversible and unstable process, thestrain along the fault will go through the three phases in order.3) The experiment results of three typical stick-slip models indicate that theseveral high frequency fluctuation events are included in a stress drop process and canbe considered as a single stick-slip event, double stick-slip event and triple stick-slipevent. There are only single stick-slip events in the double shear model experiments.Double stick-slip events and triple stick-slip events are recorded in the both inclinedplane shear model and5°-bending model in which multi-point dislocation happens.FFT spectrum of slip rate shows that three dominant frequencies are present in doubleshear model, double dominant frequency is included in the inclined plane shear modeland5°bending model has only one dominant frequency. The durations of highfrequency oscillation are40ms in the inclined plane shear model,70ms in doubleshear model and5°bending model, respectively. The high-frequency oscillationduration of the inclined plane shear model is shorter than others. From the substick-slip event intervals, several sub events in inclined plane shear model happen oneby one, while there are tens of milliseconds "quiet period" between two sub stick-slipevents in the5°bending model. According to the acoustic emission positioning, thesources of the sub stick-slip events appear at different locations on the fault.4) The fault simulated by three stick-slip model experiments is nearly straightand flat in geometry. But there is a complicated and non-uniform fluctuation strainfield along the fault and the release of strain energy in the rock along the fault isuneven during the stick-slip process. The spatial distribution of precursor slip is notuniform before the fault instability with local shear strain concentration. When thestrain energy or strain intensity is largely unchanged, instantaneous rotation of localprincipal strain axes can cause the change of the distribution of the normal strain andshear strain along the fault, and then unstable slip starts from a small area first andmakes nearby area stress increase, which causes fault failure and unstable. Theprincipal strain axis rotation process can be used to express the mechanicalmechanism of the elastic rebound model.5) Double shear stick-slip model experiments were conducted to simulate theregional loading process of spontaneous earthquakes and induced earthquakes, and astrain observation system was employed to acquire data continuously to monitor thelocal strain changes near the fault under the loading process. Regional stress path andlocal strain path during the whole earthquake process was described in stress spaceand strain space. Research results show that the far-field dynamic process or the direction and amplitude of the regional loading process are difficult to speculatedirectly from the near fault strain observations. Deformation of the fault zone ismainly controlled by the tectonic position. Each location has different strain paths.Although the morphology of a local strain path is different greatly from themacroscopic stress path, there is a certain mapping relation on the correspondingtransformation stages between the stress path and strain path. The average strain pathcan be used to speculate the regional loading stage, but there is deviation between themain loading direction and average strain direction. The evolution trend of the localstrain path indicates the possible stage of the fault deformation. The strain path ofspontaneous earthquakes can be divided into three parts: strain accumulation, lineardeviation of shear strain and unstable slip. The strain path of induced earthquakesincludes four stages: strain accumulation with positive slope, steady state slip withnegative slope, metastable strain stalemate and unstable slip under a disturbance.Spontaneous earthquakes and induced earthquakes have their own inherent and steadypath modes, so that the fault stability and the possible earthquake types could bejudged according to the special strain path.6) Experiments on three typical stick-slip models were conducted for simulatingearthquake processes, and a specially designed velocity observation system wasemployed to acquire data continuously to monitor the slip rate of stick-slip. Thecalculation method of maximum displacement was discussed for estimating themoment magnitude of stick-slip events. The relationship of stick-slip models andstress drop and magnitude has been analyzed. The data show that the magnitude rangeof stick-slip under laboratory experiments is-4.4～-3. The magnitude of laboratoryearthquakes depends on the different fault structures and the macroscopic loadingprocess. By comparison of laboratory quakes, mine micro earthquakes, inducedearthquakes, and small quake swarms, this work determined that the stress drop is notsignificantly correlated with the value of the magnitude in a range of small-scalerupture. And the earthquake magnitude is primarily determined by the volume of theseismic source.