The Thermal Efficiency Of Unstable Slip On The Fault By Laboratory Experiments

The majority of the elastic energy released by earthquakes,according to seismic theory,is transformed into heat;some even assert that the seismic efficiency has a maximum（6%）.The thermal efficiency of fault slip instability may offer significant limits for the distribution of earthquake energy.Given that it is so difficult to evaluate coseismic slip heating in the field,laboratory measurements are a crucial part of determining thermal efficiency.However,the thermal efficiency reported in the known experimental results for different experiments is significantly variable,ranging from 50%to 90%.It is objectively challenging to quantify the temperature or heat within the fault plane because of the fault plane’s exceptional thinness.Whether the discreteness of the aforementioned thermal efficiency is a true physical characteristic or a mistake brought on by the limitations of measurement technology is not entirely known.This could be the cause of the lack of thermal efficiency studies.To investigate the heating and thermal efficiency characteristics of stick-slip events under various stress conditions,various fault materials,and various experimental designs,a series of stick-slip experiments were conducted under normal stress conditions of 10/20/30/40/50 MPa,with or without fault gouge,PMMA or granite,single shear or double shear.The main results obtained include:（1）The measurement of friction heat was successful in overcoming the effects of lateral non-uniformity.Two strategies were developed in this work to counteract the impact of lateral non-uniformity:（ⅰ）using a 3D theoretical inversion method with multiple sensors along the fault plane to directly calculate the heat distribution and magnitude within the fault zone;（ⅱ）embedding high thermal conductivity materials near the fault zone to homogenize the non-uniform heat.（2）There are two different forms of temperature change brought on by stick-slip:friction heat and stress temperature.Among them,the stress temperature change in PMMA surrounding rock is about-28～7 mK,and the peak temperature of friction heating near the fault surface is 2～40 mK.The results of the calculations demonstrate that neglecting the change in stress temperature will result in a 10～20%reduction in the computation of thermal efficiency（3）Frictional heat has a significant scaling relationship with normal stress and displacement.The frictional heat calculated in this study ranges from 309 to 6191 J/m2,and the experimental results fill the gap between the frictional heat difference under low normal stress and high normal stress for pre-cut faults,finding that there is a powerlaw relationship between the heat and normal stress of Q ∝ σ_n^1.13.From the scale dependence of co-seismic displacement,the relationship between frictional heat and earthquake shear displacement is Q ∝ D^1.3～3.3,and the scale dependence difference is related to stress conditions or loading devices.（4）The thermal efficiency of stick-slip events is not constant.and it ranges from 47%to 98%in this study.Moreover,the heat efficiency is unrelated to the existence or absence of fault gouge and has no direct correlation with normal stress,coseismic displacement,or normal stress.Thermal efficiency is negatively linearly correlated with the fraction of stress drop（Δ_τ/（τ₂+τ₁））,which satisfies R=1+（2b-1Δ_τ/（τ₂+τ₁））,where b is the overshoot coefficient of the loading system.In conclusion,it is conventional wisdom that heat accounts for more than 90%of the earthquake process,which is only the result of a stress condition.Or rather,considering that the fracture energy accounts for a small proportion,from the perspective of energy conservation,the radiation energy ratio may reach as high as 50%.This value is much higher than the current understanding:that is,only a small part of the elastic energy is converted into seismic waves（≤6%）.