Nonlinear softening of unconsolidated granular earth materials. Issue 9 (9th September 2017)

• Record Type: Journal Article
• Authors: Lieou, Charles K. C.; Daub, Eric G.; Guyer, Robert A.; Johnson, Paul A.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 9(2017)
• Page Start: 6998
• Page End: 7008
• Publication Date: 2017-09-09
• Subjects: granular materials -- softening -- wave motion -- resonance -- nonlinear elasticity -- failure; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1002/2017JB014498
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Unconsolidated granular earth materials exhibit softening behavior due to external perturbations such as seismic waves, namely, the wave speed and elastic modulus decrease upon increasing the strain amplitude above dynamics strains of about 10 −6 under near‐surface conditions. In this letter, we describe a theoretical model for such behavior. The model is based on the idea that shear transformation zones—clusters of grains that are loose and susceptible to contact changes, particle displacement, and rearrangement—are responsible for plastic deformation and softening of the material. We apply the theory to experiments on simulated fault gouge composed of glass beads and demonstrate that the theory predicts nonlinear resonance shifts, reduction of the P wave modulus, and attenuation, in agreement with experiments. The theory thus offers insights on the nature of nonlinear elastic properties of a granular medium and potentially into phenomena such as triggering on earthquake faults. Key Points: We describe a theoretical model for softening of granular earth materials, based on the idea of soft spots in the granular medium The model predicts nonlinear resonance shifts, reduction of elastic modulus, and attenuation, in agreement with experiments The model offers insights on nonlinear elastic properties and the critical state prior to failure on earthquake faults