Laboratory Investigations Of Earthquakes:Nucleation And Rupture Propagation On Straight Faults

Earthquakes are one of most destructive natural hazards.Thorough understanding of rupture process is critical to the prediction of earthquakes and hazards mitigation.Recent success in this field has shown us that earthquake can be viewed as the rupture process of a seismogenic fault,including the initiation,propagation and arrest.However,several issues pertained to rupture process are still in debates.In this study,we performed experiments in order to further our understanding of rupture process.The laboratory fault is composed of two polymethy methacrylate(PMMA)plates.The roughness of the fault plane was well controlled.Using the biaxially loading machine,we studied the behaviors of faults with different roughness under different stress conditions.Based on the ultra-high speed photographing technique,the digital image correlation method and the multichannel strain acquisition technique,we developed an optical-electro joint observation method to probe the rupture process.The full-field displacement field and multipoint shear strain were recorded simultaneously,revealing the source process from nucleation to dynamic rupture propagation.Based on this approach,we investigated several unsettled issues,including the relation between the nucleation phase and the final event,the mechanism for the stable sub-Eshelby supershear and sub-Rayleigh rupture,the reason for the scale dependence of D_c,and the mechanism for the delayed dynamic triggering.The main results of this thesis are summarized as follows:(1)Characterization of nucleation during spontaneous earthquakes and the underlying mechanisms:The extent of instability is modulated by the macro static friction coefficient that is positively associated with the roughness of the faults.The instabilities on the rougher fault are more violent,with high stress drop.Besides,both higher normal stress and rougher fault complicate the nucleation process,even leading to multiple preslip zones.Such a nucleation process is related to the heterogeneous friction parameters along faults.In general,the preslip of major events is negligible.Thus,if the natural earthquakes did initiate by preslip model,it may be difficult to detect the premonitory preslip signals.(2)The governing factor and the stability of the rupture velocity for spontaneous earthquakes:The spontaneous ruptures could immediately attain a stable rupture velocity after the nucleation phase.Rougher fault and higher stress level facilitate supershear rupture.The Burridge-Andrews mechanism may not be responsible for the supershear transition of spontaneous ruptures.The rupture velocity is exactly associated with the dynamic stress drop.The self-similar dynamic rupture model matches well with the spontaneous laboratory earthquakes.Following this model,we theoretically analyzed the stability of rupture velocity.It has been shown that not only the sub-Eshelby supershear regime but also the sub-Rayleigh regime can be stable.Therefore,such a model can provide a reasonable explanation for the observed natural events.(3)Coseismic friction evolutionary relationships of faulting and the mechanisms for scale dependent D_c:The evolutionary relationship of coseismic friction depends on the fault roughness and stress state.The critical slip weakening distance D_c is larger for rougher faults.Repetitive ruptures were only observed on rough faults.The repetitive ruptures lead to a nonlinearity slip weakening behavior,containing several secondary restrengthening-reweakening phases.Such a process is responsible for extremely large D_c.For the cases where repetitive ruptures are missing on the rough faults,D_c is comparative with that of smooth faults.Therefore,the fault topographic characteristic is the primary inducement for the scale dependence of D_c,and the effect of slip weakening history cannot be neglected.(4)The rupture model of the near-field dynamic triggered earthquakes and the mechanism for time delay:We provided a direct laboratory evidence for the time delay in earthquake dynamic triggering.The triggered rupture behavior can be predicted using the disturbed nucleation model in combination with the loading condition in laboratory settings.The dynamically triggered event has a particular nucleation process,which is responsible for the time delayed.The physical process governing the near-field dynamic triggering involves the fault zone damage(alternation of contact)and the aseismic slip(disturbed nucleation).The experimental results can help explain near-field dynamic triggering with relatively short time delays,and provide a simple theoretical framework for far-field dynamic triggering.The results of this thesis have significant implications on the source process of natural earthquakes,including the nucleation and dynamic rupture propagation.