(NAS Colloquium) Earthquake Prediction The Scientific Challenge (1996) / Chapter Skim
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Implications of fault constitutive properties for earthquake prediction
Implications of fault constitutive properties for earthquake prediction
Pages 3787-3794
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From page 3787... ...
This formulation provides a unified representation of diverse sliding phenomena including slip weakening over a characteristic sliding distance Dc, apparent fracture energy at a rupture front, time-dependent healing after rapid slip, and various other transient and slip rate effects. Laboratory observations and theoretical models both indicate that earthquake nucleation is accompanied by long intervals of accelerating slip.
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From page 3788... ...
This results in strengthening of surfaces by the logarithm of time, which is also observed in experiments (1, 7, lie. At constant normal stress, ~ increases whenever ~ < Dc/0, which provides for the recovery of frictional strength after unstable slip events.
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From page 3789... ...
This subject is relevant to earthquake prediction because the nucleation process may be accompanied by detectable precursors and because nucleation determines the time and place of origin of earthquakes. A1SO7 it is shown below that the sensitivity of the nucleation times to stress changes may result in significant changes of earthquake probabilities after relatively small stress perturbations, such as occur in the vicinity of prior earthquakes.
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From page 3790... ...
Slip speeds for the case ~ ~ O approach the asymptote formed by the ~ = 0 case as the slip speeds become large compared to the stressing rate. Once the shear stress on the nucleation zone exceeds the steady-state friction, rss, external stressing of the fault is no longer needed to drive the acceleration of slip to instability (i.e., the nucleation process is self-driven)
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From page 3791... ...
If the seismic nucleation phase represents the transition from stable accelerating slip to dynamic rupture propagation, the dimensions of the quasi-static nucleation process may also scale by the moment of the eventual earthquake. Effect of Stress Changes on Seismic Activity It is evident from laboratory observations and the rate- and state-dependent formulation that small changes of shear stress or normal stress will result in large changes in slip speed.
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From page 3792... ...
~[18] Assuming the frequency distribution of earthquake magnitudes remains constant, the effect of stress changes on earthquake probability at different magnitude thresholds may be expressed through the background earthquake rate r -- M by using the familiar Gutenberg-Richter frequency distribution of earthquake magnitudes N 1 o (a hM)
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From page 3793... ...
Discussion The rate- and state-dependent constitutive formulation encompasses characteristics of rather disparate simplified constitutive formulations and it describes laboratory observations of fault healing and history dependence that are not represented in other formulations. Several features of the earthquake nucleation process arising as a consequence of this constitutive formulation may be relevant to earthquake prediction.
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From page 3794... ...
Alternative mechanisms for aftershocks and clustering, based on viscoelastic stress transfer or diffusion processes that alter fault stress, lead to characteristic after shock times that are insensitive to stressing rates but depend on a characteristic length of the mainshock disturbance. Both foreshock models provide comparable fits to the data.
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