| Earthquakes are the vibrations of the surface and the underground rock stratum of the Earth,by their extraordinary power,they greatly reshape the looks of the Earth,as well as bring tremendous calamities to human beings(especially for the shallow-focus earthquakes).However,the causes of earthquakes have not been scientifically explained for a long time,from the modern mechanical point of view,earthquakes result from sudden destabilization of rock.The phenomenon of stick-slip,which was originally used to describe the unstable sliding of metal materials,has greatly aroused the interest of geophysical investigators since it was proposed as a mechanism of the shallow focus earthquakes in the 60's of last century,and has achieved terrific triumph in explaining seismic phenomena.Just as the laboratory repetitive stick-slip sequence,some tectonic earthquakes in the field also frequently rupture on the same active faults.However,in their long evolutionary histories,these active faults may suffer variable loading conditions,such as variable tectonic movement,the variable in situ stress condition,meanwhile,the friction properties of the contact surface may change too.It is of great significance to explore how these internal and external changes affect the sliding mode of active faults,the recurrence interval of earthquakes and the stress drop of earthquakes.In this paper,the author experimentally investigates the effects of shear loading rate,fault surface roughness and normal stress level on the behaviors of repetitive stick-slip by step changing shear loading rate and normal stress on three kinds of roughness of fault surface.The following results are found:(1)Shear loading rate is a primary controlling parameter on the stick-slip recurrence interval,and the correlation between them can be nicely expressed by a power law.In addition,the stick-slip recurrence interval can influence the stress drop of stick-slip via two different tendencies: at a given shear loading rate,the shear stress drop increases linearly with the recurrence interval;while the combined data from all shear rate tests show that the shear stress drop increases logarithmically with the recurrence time.The phenomenon that stress drop decreases with increasing the shear loading rate is used to deduce there is a critical shear loading rate,beyond which,the sliding mode of the friction system will transform from unstable sliding to smooth sliding.(2)In the investigated range of fault roughness,we found that surface roughness conditions strikingly affect the sliding mode,the spatial frequency of stick-slip and friction drop.In detail,the two smoother fault systems side with regular stick-slip events;whereas the roughest fault system slides with chaotic stick-slip when the normal stresses are not sufficiently high,the chaotic stick-slip is also an unstable sliding,but its shear stress-time curve has high-frequency and low amplitude variations.In addition,at a given sliding distance,the smoothest fault system generates more stick-slip than the second rougher fault system.The smoother the fault surface is the more favorable to accumulate seismic energy.The test results also show that a fixed sliding distance,the spatial frequency of stick-slip and the friction drop are mutually-constrained,and the correlation between them can be approximately described by an inverse relation.(3)Normal stress can affect the sliding mode and stress drop.At lower normal stresses,the roughest fault system(fault system 60)generates chaotic stick-slip,while at higher normal stress,it slides with regular stick-slip.This two-stage sliding feature indicates that higher normal stresses enhance the tendency of unstable sliding.Furthermore,the test result suggests that shear stress drop increases nonlinearly with the increase of normal stress. |
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