Mechanochemical Changes Of Fault-zone Materials During Shear Deformation: Implications For Dynamic Fault Weakening

In the past three decades,high-velocity friction experiments conducted on rocks have demonstrated a significant reduction in fault frictional strength during coseismic sliding,indicating the occurrence of dynamic weakening phenomenon.The dynamic weakening of a fault exerts significant influence on the propagation,stress drop,and coseismic displacement of an earthquake rupture.The velocity of coseismic slip on faults during large earthquakes typically reaches speeds of approximately meters per second.Rapid frictional slip induces significant shear deformation and generates substantial frictional heating effects within the shear zone.The intricate physical and chemical processes occurring within the slip zone,triggered by frictional heating and shear deformation,are intimately linked to the phenomenon of dynamic fault weakening.A plethora of prior studies have extensively explored the physical mechanism of thermal activation in dynamic fault weakening,yet scant attention has been devoted to whether shear deformation impacts the process of thermal activation-induced weakening.The investigation of this matter is critical as frictional heating and shear deformation are closely linked physical processes that take place within a fault zone during an earthquake.Improving our comprehension of how shear deformation affects thermal weakening in faults can offer significant insights into the co-seismic mechanics of fault rupture.In this study,the dynamic weakening process of kaolinite,serpentine and calcite was investigated through low-velocity friction experiments,simultaneous thermal analyses,X-ray diffraction analyses,microstructural observations and numerical simulations.The primary mechanisms of dynamic weakening that are of concern include flash heating and thermal（thermochemical）pressurization.In the study,low-velocity friction experiments were conducted to induce shear deformation in the sample,while simultaneous thermal analyses were employed to simulate the significant temperature rise experienced by the sample during frictional sliding.Therefore,it is possible to differentiate between the processes of"deformation"and"warming"in order to evaluate the impact of shear deformation on mineral thermodynamic processes and thermal activation weakening mechanisms.The friction experiments primarily assessed the impact of various factors,including effective normal stress,displacement,fluid environment,mixed minerals and other influential parameters.With the increase of shear deformation degree,the thermal decomposition starting temperature₍（90） of both kaolinite and serpentine after deformation decreases obviously（the thermal decomposition initiation temperature of kaolinite decreases from 460℃to 162℃in extreme cases and the thermal decomposition characteristic temperature of serpentine decreases from 563℃to 176℃in extreme cases）and the thermal decomposition reaction rate increases significantly;In contrast to kaolinite and serpentine,the initial thermal decomposition temperature of deformed calcite exhibited only a slight decrease（<50℃）.Microstructure and X-ray diffraction analyses reveal that the shear deformation of kaolinite,serpentine,and calcite results in a reduction in grain size,distortion of crystal structure,and transformation of mineral crystals into amorphous structures.The kinetic analysis of the thermal decomposition reaction of kaolinite reveals a significant reduction in activation energy(_（6）subsequent to shear deformation.Numerical simulation indicates that by taking the thermal decomposition starting temperature₍（90）,which significantly reduces the deformability of kaolinite or serpentine,as the characteristic temperatureleading to thermal softening of asperity contacts,the efficiency of flash heating and weakening of concavite can be greatly enhanced.Lowering the activation energy₍（6）facilitates the thermal decomposition and fluid release of kaolinite and serpentine at lower temperatures,thereby further promoting the role of thermal（chemical）pressurization mechanism.This study has significant implications for the co-seismic mechanical behavior of faults rich in layered silicate minerals,such as clay-rich fault zones near the surface and plate boundary faults produced by serpentine subduction zones,during large earthquakes.Shear slip of these faults,whether in pre-slip or accelerated slip,can alter the thermodynamic properties of phyllosilicate minerals（such as reducing their thermal decomposition temperature and accelerating their thermal decomposition rate）,thereby promoting dynamic weakening of the faults and facilitating earthquake rupture propagation.In addition,the thermal decomposition process and characteristic temperature of calcite exhibit low susceptibility to shear deformation.Therefore,for seismic events occurring in limestone strata,the spatial variation of calcite content after earthquakes can be utilized to constrain the temperature of fault slip zones and co-seismic friction intensity.