5:15 PM - 6:45 PM
[SCG44-P14] Effects of packing fraction on stick-slip behavior and shear-zone structure: Analogue experiment using floated particles
Keywords:Stick slip, Shear zone, Gutenberg-Richter law, Jamming transition, Analog experiment
Shear deformation experiments were conducted using particles floated on a liquid surface to investigate the effect of porosity on the stick-slip behavior of faults. During rotationally shearing at various packing fractions of particles, we measured torque and particle motion simultaneously. The results showed that as the packing fraction increased, stick-slip behavior became dominant and the shear zone became narrower and more anisotropic.
Fault slip is strongly affected by porosity and microstructure of the fault zone (e.g., [1, 2]). In this study, we performed rotational shear of particles floated on a liquid, as analogous system to fault zone, to investigate the effects of particle packing fraction and arrangement on stick-slip behavior. We used opaque spherical gels (approximately 4 mm in diameter) for sample particles. A particle layer on the liquid surface was prepared with a thickness of a single particle and with different number of particles to control packing fraction. A rotating cylinder was vertically inserted into the particle layer for quasi-two-dimensional rotary-shear setting. The same particles used for the sample were glued in a horizontal array around the side wall of the rotating cylinder to prevent the slipping of the most inner array of particles. To float the particles, we used an aqueous solution of sodium polytungstate (specific gravity ∼3).
We conducted shear experiments under each condition with various packing fraction 0-0.9. The rotational speed was kept constant at 0.6°/s by a motor connected to the rotating cylinder via a spiral spring. We simultaneously measured the torque and filmed the particle motion in-situ during the experiments. The opaque particles let us track each particle motion by analyzing the captured images.
As a result, at a packing fraction of 0.8, torque increased and decreased intermittently and slowly, while at 0.9, slow increase and instantaneous drop of torque occurred periodically, indicating stick-slip behavior. The recurrence interval of stick-slip became shorter as the packing fraction increased. Both the mean value and the amplitude of oscillating torque at the steady state were three times larger at a packing fraction of 0.9 than at 0.8. The cumulative number of torque-drop events showed the power-law distribution as a function of the amount of torque drop. The power-law exponent was smaller at a larger packing fraction (about 0.6 for a fraction of 0.8 and about 0.3 for a fraction of 0.9). The power-law exponent can be compared with the b-value of the Gutenberg-Richter law [3] in seismology.
We also investigated the motion of particles by analyzing the in-situ images. We could recognize three distinctive zones in the radial direction from the rotating cylinder: the inner shear zone defined as the active motion of particles in the shear direction, the oscillatory zone as the radial oscillating motion of particles, and the creep zone as slow displacement of particles in the circumferential direction. As the packing fraction increased, the shear zone became narrower and more anisotropic (polygonal spread shape).
In summary, the results showed that the power-law distribution of stick-slip magnitude can be reproduced in rotational shearing of floated particles. Besides, changes in the particle packing fraction strongly affected the period, amplitude, and statistical properties of stick-slip and the development of the shear zone.
[1] Pozzi et al. (2022) The Role of Fault Rock Fabric in the Dynamics of Laboratory Faults. JGR.
[2] Noda (2020) Dynamic Earthquake Cycle Simulations Considering Changes in Dominant Deformation Mechanisms of a Fault Slip. Chigaku Zasshi.
[3] Gutenberg & Richter (1941) Seismicity of the Earth. GSA.
Fault slip is strongly affected by porosity and microstructure of the fault zone (e.g., [1, 2]). In this study, we performed rotational shear of particles floated on a liquid, as analogous system to fault zone, to investigate the effects of particle packing fraction and arrangement on stick-slip behavior. We used opaque spherical gels (approximately 4 mm in diameter) for sample particles. A particle layer on the liquid surface was prepared with a thickness of a single particle and with different number of particles to control packing fraction. A rotating cylinder was vertically inserted into the particle layer for quasi-two-dimensional rotary-shear setting. The same particles used for the sample were glued in a horizontal array around the side wall of the rotating cylinder to prevent the slipping of the most inner array of particles. To float the particles, we used an aqueous solution of sodium polytungstate (specific gravity ∼3).
We conducted shear experiments under each condition with various packing fraction 0-0.9. The rotational speed was kept constant at 0.6°/s by a motor connected to the rotating cylinder via a spiral spring. We simultaneously measured the torque and filmed the particle motion in-situ during the experiments. The opaque particles let us track each particle motion by analyzing the captured images.
As a result, at a packing fraction of 0.8, torque increased and decreased intermittently and slowly, while at 0.9, slow increase and instantaneous drop of torque occurred periodically, indicating stick-slip behavior. The recurrence interval of stick-slip became shorter as the packing fraction increased. Both the mean value and the amplitude of oscillating torque at the steady state were three times larger at a packing fraction of 0.9 than at 0.8. The cumulative number of torque-drop events showed the power-law distribution as a function of the amount of torque drop. The power-law exponent was smaller at a larger packing fraction (about 0.6 for a fraction of 0.8 and about 0.3 for a fraction of 0.9). The power-law exponent can be compared with the b-value of the Gutenberg-Richter law [3] in seismology.
We also investigated the motion of particles by analyzing the in-situ images. We could recognize three distinctive zones in the radial direction from the rotating cylinder: the inner shear zone defined as the active motion of particles in the shear direction, the oscillatory zone as the radial oscillating motion of particles, and the creep zone as slow displacement of particles in the circumferential direction. As the packing fraction increased, the shear zone became narrower and more anisotropic (polygonal spread shape).
In summary, the results showed that the power-law distribution of stick-slip magnitude can be reproduced in rotational shearing of floated particles. Besides, changes in the particle packing fraction strongly affected the period, amplitude, and statistical properties of stick-slip and the development of the shear zone.
[1] Pozzi et al. (2022) The Role of Fault Rock Fabric in the Dynamics of Laboratory Faults. JGR.
[2] Noda (2020) Dynamic Earthquake Cycle Simulations Considering Changes in Dominant Deformation Mechanisms of a Fault Slip. Chigaku Zasshi.
[3] Gutenberg & Richter (1941) Seismicity of the Earth. GSA.