Transparentization And Quantification Of The Stress Field Associated With Shear Deformation Of Rough Faults Using 3D Printed Models

Earthquake induced by human activities such as energy resources development and engineering construction has become an important hidden danger threatening production construction and People’s Daily life.Studies show that the main causes of human-induced earthquakes are mining,oil and gas development,geothermal exploitation,water reservoir impoundment,CO₂ geological storage,waste liquid treatment and other engineering activities that break the original stress balance state of nearby rock faults,leading to earthquake nucleation induced by fault activation.The change of near-fault stress field is the fundamental driving force of earthquake nucleation.Therefore,the quantitative analysis of stress field during shear deformation of rock faults is of great significance for revealing and characterizing the mechanisms of fault shear slip and earthquake nucleation as well as forewarning and alleviating human-induced earthquake disasters.However,the surface of natural faults is rough,and there are asperities with varying periodic fluctuations;these asperities cause complex resistance effect of fault shear deformation.As a result,it is a great challenge to quantitatively characterize the stress field associated with shear deformation of rough faults.At present,the main methods of stress field characterization in the vicinity of faults include theoretical analysis,field monitoring,laboratory model experiment and numerical simulation.Due to the complex structure of rough faults,no theoretical method and model can accurately and quantitatively describe the full-field stress evolution associated with shear deformation of rough faults.Field monitoring is difficult to characterize the continuous evolution of full-field stress during fault activation using a limited number of stress measurement points,and as a result,some important stress change information is easily omitted;moreover,the accuracy of fault stress measurement results is also significantly affected by the measurement environment,equipment and personnel and other factors.Scholars at home and abroad try to characterize the full-field stress evolution of fault structure using laboratory model experiments,among which the representative methods are digital image correlation method and digital photoelastic method.However,the direct measurement result of digital image correlation method is the displacement field of fault structure,and it is difficult to accurately calculate the stress field of rough fault structure;moreover,its measurement accuracy is also affected by speckle size,subset size and gray interpolation and other factors.Digital photoelasticity method is a model experiment method that can directly quantify the full-field stress of fault structure.However,most current studies can only qualitatively describe the full-field stress in simple plane faults.How to accurately quantify the stress field associated with shear deformation of rough faults has always been a challenge in the field of digital photoelasticity.Numerical simulation can quantitatively characterize the full-field stress of rough faults;however,there are many disputes about the accuracy of numerical simulation results due to the difficulties in determining the mechanical parameters of model materials,boundary conditions,failure criteria,mesh optimization and contact conditions,and experimental verification.Aiming at the challenge of quantitative analysis of the stress field associated with shear deformation of rough faults,in this thesis,transparent physical models of rough rock faults with different surface roughness and geometric morphology were fabricated using 3D printing technology and light-sensitvie material based on Barton’s rough profile curve;shear deformation photoelastic tests of fault models and the comparative analysis of full-field stress evolution were performed;phase unwrapping algorithm of photoelastic fringes and quantitative analysis method of the dynamic full-field stress associated with shear deformation of rough faults were proposed;the correspoinding computational programs were written;the photoelastic experiment and quantitative analytical methods of full-field stress distribution and continuous dynamic evolution in the vicinity of rough faults were established;which achieves the quantitative analysis of full-field stress distribution and continuous dynamic evolution associated with shear deformation of rough faults.According to the quantitative analytical results of near-fault stress field,it is found that the evolution of near-fault principal stress difference（PSD）instead of shear stress can better describe and reveal the mechanisms of shear slip and earthquake nucleation of rough faults.Furthermore,the quantitative relationship model between the sequence of near-fault stress drops or the rupture sequence of asperities and the geometric characteristics of asperities was established,which provides a new method and a new way for quantitative analysis and prediction of shear slip and earthquake nucleation of rough faults.The significant findings of this study are as follows:（1）The proposed stress field analysis method based on 3D printing models and photoelastic tests can effectively obtain and quantitatively characterize the full-field stress distribution and continuous dynamic evolution in the vicinity of rough faults,providing a method and means for quantitative analysis of the mechanisms of fault shear slip and earthquake nucleation.（2）The evolution of near-fault PSD instead of shear stress can better describe and reveal the mechanisms of shear slip and earthquake nucleation of rough faults.（3）The shear slip failure of rough fault can be divided into two different stages:the first stage is the early stage of shear deformation,i.e.the internal lock and shear deformation of asperities are dominant,which is mainly manifested as the local stress concentration near the uphill section of asperities.The second stage is the competition stage between the internal lock and the wear or rupture of asperities in the middle and late stage of shear deformation.In the middle stage,the internal lock effect of asperities is dominant,which shows that asperities with higher shear strength remain internal lock,but some asperities with weaker shear strength begin to wear or rupture.In the late stage,the wear or rupture of asperities is dominant,which indicates that some asperities with larger shear strength begin to wear or rupture,and the overall shear strength of fault models decreases rapidly.（4）The wear or rupture of asperities will reduce the contact area of rough fault surfaces and further cause a sudden increase of local peak stress near the upslope section of the locked asperities,and then accelerate the wear or rupture process of the subsequent asperities until the whole fault structure is completely worn or ruptured.（5）For the same rough fault structure,asperities with lower geometric characteristic value S^u tend to rupture preferentially;for the different rough fault structures,asperities with the same geometric characteristic value S^u tend to rupture preferentially on the fault with larger surface roughness coefficient.（6）The sequence of PSD drops of rough fault is consistent with that of rupture sequence of asperities,indicating that the aseismic slip caused by the wear and rupture of the asperities with the smaller geometric characteristic value S^u may trigger a series of foreshock nucleation events,and the seismic slip caused by the wear and rupture of the asperities with the biggest geometric characteristic value S^u triggers main earthquake nucleation.In addition,it is found that a series of foreshock nucleation events prior to main earthquake nucleation are independent,and there is no obvious mutual promotion or inhibition between them.The main innovations of this thesis are as follows:（1）Based on Barton’s rough profile curves,the transparent physical models of rough rock faults with different surface roughness and geometric morphology were fabricated in batches using 3D printing technology and light-sensitvie material,and the shear slip experiments and comparative analysis were performed on fault models with the same physical property and different roughness morphology.It creates the conditions for transparent and quantitative analysis of the mechanisms of fault shear slip and earthquake nucleation using physical model experiments.（2）Phase unwrapping algorithm of photoelastic fringes and quantitative analysis method of dynamic full-field stress during shear deformation of rough faults were proposed,the correspoinding computational programs were written,and the photoelastic experiment and quantitative analytical methods of the full-field stress distribution and continuous dynamic evolution in the vicinity of rough faults were established.It provides an idea and method for transparent display and quantitative analysis of full-field stress distribution and continuous dynamic evolution associated with shear deformation of rough faults.（3）The effects of asperities on shear deformation failure modes of friction and internal lock were revealed,and the geometric characteristic values of asperities were given to quantitatively describe the wear and rupture trend of rough faults.The nucleation of foreshocks and mainshock associated with shear slip of rough faults were described using the sequence of PSD drops.The quantitative relationship between the wear or rupture sequence of asperities or the sequence of near-fault stress drops and the geometric characteristics of asperities was established,which provides the experimental basis and theoretical model for the analysis,evaluation and prediction of shear slip and seismic nucleation of rough faults.