Experimental Study On The Spatiotemporal Evolution Of Deformation Fields Of Faults In The Meta-instability Stage

Laboratory experiments suggest that the evolution of the differential stress of loading vesus time during stick-slip on a fault can be devided into four stages: linearity, depature from linearity, meta-instability, and instability. The meta-instability stage (MIS) refers to a transition that the differential stress of loading evolves from the peak stress into the onset of sudent stress drop. The MIS is also the final stage before the fault reaches instability. In the laboratory, both the stress state of the sample and the sample surface deformation can be observed. Thus, experiments can be conducted to study the characteristics of the spatiotemporal evolution of the fault deformation field in the MIS and search for identifiers of the MIS. Such experimental results can help to analyze the stress state of faults in the field condition and estimate the time that a major earthquake is impeding. In this work, two granite samples were loaded by a biaxial sevo-control press mathine under different loading rates. The spatiotemporal evolution of deformation fields of the samples was observed via a high speed camera and the digital image correlation method. Multiple physical field measurements, such as strain gage and accorstic emission, were also performed to aid studing the MIS. In addition, the evolution of deformation fields of a cover layer under a basement fault was simulated on a clay model, which provides the basis for studying the MIS of the basement fault via the deformation of the cover layer. The main results of this thesis are summarized below:(1) The MIS can be identified as the accelerated growth of the local pre-slip areas. The sample releases stress via local pre-slip of the fault. The local pre-slip areas begin to appear during the depature from linearity stage as the stress buildup is dominant relative to the stress release. The local pre-slip areas begin to grow distinctly and acceleratedly during the MIS as the stress release of the sample is dominant and the stress state shows a transition from steady to accelerated release. The growth of local pre-slip areas can be quantatively expressed as the change of the normalized length of local pre-slip areas (W) versus time. It is found that the time of the peak curvature of Wean be used as an approximate mark of the MIS.(2) Another identifier of the MIS is the accelerated coherent tendency of the fault displacement directions of different fault segments. The disorder of the spatial distribution of fault displacement directions can be quantatively described as the normalized information entropy of fault displacement directions (5) changes versus time. The time of peak curvature of S can also be used to approximately identify the MIS.(3) The striped distribution of strain and its dynamic transmition across the sample are observed in the late MIS. This finding was obtained under a full-field and high sampling rate monitoring of the sample strain field (with spatial and temporal resolutions of 0.15mm and 1ms, respectively). Combining the present monitoring and the previous measurements of strain gages, it is found that this kind of strain waves transmit towards the focal area and its frequency and amplitude increase as it gets close to the instability of the fault. Advanced studies on this phynomenon may help to understand the precursory waves which are observed prior earthquakes and widely discussed in previous studies.(4) The deformation of the cover layer is controlled by the activity of the basement fault. Basin systems form in the cover layer due to the activity of the basement fault. The deformation process of the basins can be divided into two successive phases:An initial independent propagation phase followed by an interaction phase between adjacent basins. Studying the activity of the basement fault via the deformation of its cover layer can help to identify the MIS of the basement fault in further research.