Experimental Study On The Spatial And Temporal Patterns Of The Super-dynamic Deformation Of Unstable Sliding Of Fault

As a basic physical model of tectonic earthquakes,it is a fundamental problem whether the fault instability can be simply divided into two parts:the strain accumulation of the stick phase and the rapid release of the co-seismic slip phase,and if it can be explained by the slider-spring model.In recent decades,the study of the mechanical state of fault before earthquake has indicated that many researchers have realized the complexity of the fault instability.Specific objects such as nuclear phase,critical scale,and fault localization have been studied and reported.The research of the imminent earthquake has been promoted by the progress of the study on meta-instability model,which points out the temporal and spatial evolution of various physical quantities at the meta-instability stage is characteristic,and the mechanical process of the earthquake source controls it.Therefore,a new type of experiment recording system is needed,which should be capable of high-speed,multi-channel,continuous recording,and is compatible for multi-parameter measurement.With this system,we hope to record the whole deformation process,including the relative slow deformation a few seconds before stress drop,the fast deformation a few micron seconds before stress drop,and especially the transient process of co-seismic slip.By doing so,we can record as many details as possible of the transient process of unstable sliding,and try to analysis and interpret of the relevant mechanics problems of earthquake source.We have developed a super dynamic deformation measurement system,which features 64-channel,16-bit resolution,4MHz sampling frequency,and parallel continuous acquisition.It is the first system that can record strain signal and acoustic emission continuously and synchronously.From a technical point of view,this system is one of the best among similar laboratories all over the world.Several experiments have been finished with this new system,a total of 209 unsteady sliding events were obtained,and about 42TB strain and acoustic emission experimental data were recorded.It has been proved that this system can work stably.It provides a brand new technology platform for the research on pre-slip,meta-instability,and the process of dynamic acceleration and unsteady sliding.Based on the new system,the whole process of fault instability sliding is observed.Detailed information of the deformation field of stick-slip has been collected,especially the Quasi-dynamic part of meta-instability stage and the transient process of the stress drop of unstable sliding.The main results of this thesis are summarized below:1.Acriterion for entering the quasi-static phase of the meta–instability stage--the acceleration of strain localization accompanied with unloading.In the oretical model,the beginning of the meta-instability stage is marked by loading stress curve passing the peak value.With the help of the strain observation along the fault,it can be identified that the overall stress dropping after peak is corresponding to unloading of individual segments.That is to say,deformation has transited from stress accumulation contributed by stable deformation over the whole filed to a partial unloading stage--the quasi-static phase of the meta-instability stage.During this stage the loading and unloading interaction between different segments of fault can enhance the strain localization.This feature can be used as a microscopic?near field?criterion for entering the quasi-static phase of the meta-instability stage.2.Reciprocating transfer of the strain pause in quasi-dynamic stage.The quasi-dynamic stage is divided into three sub-stages?AA₁,A₁A₂ and A₂A₃?,and each sub-phase corresponds to a strain pulse transmission event.In the first sub-stage AA₁,the strain pulse initiates at the middle part of the fault and transmits to the upper end of fault where a high strain region is locking the fault,during this process,strain energy is cumulated and released pointwise.In the second sub-stage A₁A₂,a new strain pulse appears where last pulse transfer disappear and transmits backwards to the lower end of the fault;in the third sub-stage A₂A₃,similar to the previous one,the third strain pulse emerge at the lower end of the fault where the second pulse vanish,and transmits backwards to the upper end of the fault.The last strain pulse passes through the entire fault and reaches the high strain zone at the upper part of the fault,resulting in the cumulative strain at the upper corner reaching the local shear strength.Eventually,starting from the high strain zone,the strain around the entire fault zone is released nearly simultaneously?at speed of kilometer per second scale?,causing the earthquake.In the three sub-stages,the durations of strain pulse transmission get shorter and shorter;the speed gets faster and faster.At the first stage,it takes the pulse 7 seconds to transmit half of the fault length at average speed of 16mm/s;at the second stage,the pulse transmitting time is about 0.3s and the average velocity is about 920 mm/s;at the third stage,the pulse transmission time is about 0.017s,the average speed is about17600mm/s?17.6m/s?the pulse corss almost the entire fault length.3.The short strain recovery period after each sub-stage of the quasi-dynamic stage.At the quasi-dynamic stage,there is always a transient strain recovery period after the transitive pulse transfer of each sub-phase is completed.The next strain pulse transfer process can not proceed until the recovery period is over.After the strain transfer of the first sub-stage is completed,there is a strain recovery period of about100 ms in the segment of fault where strain pulse transfer ends,and the recovery period is followed by the second sub-stage of the strain pulse transmission.Similarly,after the second strain pulse transfer,there is a second strain recovery period of about 25ms in the transfer ending segment,followed by a strain pulse transfer in the third sub-stage;finally,after the strain pulse transfer of the third sub-stage ends,there is a recovery period of about 3ms in the transfer ending segment,and then"earthquake"occurs,the strain of entire fault is released rapidly.The possible reason why recovery period occurs is that the strain released by the strain pulse delivered to the ending segment is not sufficient enough to cause the whole fault to slip,and the fault needs to regain strain for the next pulse event.The occurrence of strain recovery period,especially the transient recovery period before the fault instability,may provide the basis for short-term and impending earthquake prediction.4.The co-seismic,high-frequency strain oscillations at the beginning of transient unstable sliding.At the beginning of fault transient instability,there are several high-frequency strain oscillations of frequency about 2KHz,the upper limit of the frequency of the oscillations is about 15KHz,and the period of each strain oscillation is about 0.5ms.Each high-frequency oscillation is accompanied by a laboratory earthquake?acoustic emission?event,so we call them the co-seimic high-frequency oscillation.There is certain amount of time difference between the strain oscillations at different sites,but we don't think the relationship between the waveforms of different sites can be explained by the propagation of strain wave because:1)the travel time difference are quite different even if the observation points are equally spaced;2)one backward propagation was recorded;3)the wave amplitudes of different sites are roughly the same,we did not observer amplitude attenuation from the source to the sides.5.Co-seismic sub-events at millisecond scale.At the stage of fault instability,if a"double earthquake"occurs,each"sub-earthquake"corresponds to one or several high-frequency co-seismic oscillations.And the location of the high-frequency oscillations may be different.This is similar to the multi-point source observed by the intermediate frequency system at 3.4 KHz,which means that multi-point sources can be observed at different time scales.6.Reverse pulse before sliding stops.After several high-frequency oscillations,a relative low-frequency reverse strain pulse starts from the lower end of the fault which is far from the source and transmits back to the starting point of the first pulse.The fourth pulse?i.e.the reverse pulse?is obviously different from the previous three that it is not accompanied by any AE event.After that,the co-seismic stage of the fault ends;and each segment begins to perform a high-frequency damping oscillations based on their own strain levels.This reversal of the strain pulse may contain important information about the stop of earthquake and the stop the slip of fault.7.Tri-axial test in shallow source environmentAt the same time,in order to verify whether the meta-instability stage and the unstable transient process in the pressure environment of the shallow earthquake are consistent with similar laws,we have completed several experiments in the tri-axal confining pressure vessel.The experiments show that the meta-instability stage still exist under the shallow-source pressure environment,and its process is consistent with the observation from the bi-axial loading system.