4:00 PM - 4:15 PM
[SSS10-25] Slow to fast earthquake transition in slip behavior statistics: Granular shear experiments under various friction and rigidity conditions
Keywords:slow earthquake, brittle ductile transition, fluid rock interaction, soft matter, exponential distribution, Gutenberg Richter law
What is the cause of the difference in statistical behaviors between slow and regular earthquakes? In this study, we experimentally demonstrate that the seismic moment statistics of slow earthquakes are realized by the breakdown of fractality. The reason for the greater decay of the moment-frequency distribution in slow earthquakes compared to the Gutenberg-Richter law (power-law distribution) in regular earthquakes [1] is the inhibition of networking of force chains (locally stress-concentrated particle arrays) by low interparticle friction. The reason for the linearity of the moment-duration scaling in slow earthquakes differs from the cubic law in regular earthquakes [2] is the collapse of the fractal structure of slip planes by low elasticity of particles.
We conducted stick-slip experiments on a quasi-two-dimensional granular layer. The layer consists of approximately 4000 spherical particles with a diameter of 4 mm and is floated on a heavy liquid surface. We rotationally sheared the layer at 0.6°/s. We investigated the effects of friction, elasticity, and number of the particles on the statistics of slip events, using hydrogel polymer particles (friction coefficient ≦ 0.1, Young's modulus of a few kPa) and glass beads (friction coefficient > 0.2, Young's modulus of a few 10 GPa). Torque was measured with a viscometer and the motion of particles was recorded in situ from the container's bottom.
The experimental results can be summarized in the following three points:
1. The cumulative frequencies of torque drop and slip moment follow exponential distributions regardless of the experimental conditions. Since slip events of dry particles under confining pressure follow power-law distributions [3-4], fluid lubrication and friction reduction are related to the transition between exponential and power-law distributions. Inside the stressed granular material, the average stress in each force chain follows an exponential distribution [5]. Thus, low interparticle friction causes force chains to yield easily and suppresses the cascading up of slip avalanches.
2. Event duration T is approximately proportional to torque drop and slip moment M0 for hydrogel particles (M0 ∝ T ), while M0 ∝ T 2 for glass beads. For regular earthquake faults, the self-similarity (fractal) holds in that slip amount is proportional to fault length, resulting in the scaling M0 ∝ T 3. Therefore, when particles become softer, the fractality of slip planes reduces, transitioning to M0 ∝ T scaling.
3. Decreasing the porosity of the particle layer increases the means of moment M0 and moment rate M0/T. Furthermore, shear band localization is enhanced. Considering that the width and pressure of the shear band are determined by the porosity of the surrounding particles, the porosity dependence of moment M0 and moment rate M0/T are semi-quantitatively explained by the two effects of shear zone localization and pressure enhancement with decreasing porosity.
Based on the above results [6], it is presumed that interparticle friction determines the moment-frequency distribution, while particle viscoelasticity determines the moment-duration scaling. When we consider slow and regular earthquakes as corresponding to low and high frictional-viscoelastic phenomena, respectively, another type of seismic phenomena that does not belong to either category could be suggested. Therefore, to further investigate the effects of interparticle friction and particle viscoelasticity on the strength of the particle layer supported by force chains, we additionally performed comparative experiments and simulations using the discrete element method (DEM).
[1] Chestler & Creager (2017) JGR
[2] Ide & Beroza (2023) PNAS
[3] Korkolis et al (2021) JGR
[4] Geller et al (2015) PRE
[5] Zhang et al (2014) PRE
[6] Sasaki & Katsuragi (2025). Origin of slow earthquake statistics in low-friction soft granular shear. arXiv. https://arxiv.org/abs/2502.0135
We conducted stick-slip experiments on a quasi-two-dimensional granular layer. The layer consists of approximately 4000 spherical particles with a diameter of 4 mm and is floated on a heavy liquid surface. We rotationally sheared the layer at 0.6°/s. We investigated the effects of friction, elasticity, and number of the particles on the statistics of slip events, using hydrogel polymer particles (friction coefficient ≦ 0.1, Young's modulus of a few kPa) and glass beads (friction coefficient > 0.2, Young's modulus of a few 10 GPa). Torque was measured with a viscometer and the motion of particles was recorded in situ from the container's bottom.
The experimental results can be summarized in the following three points:
1. The cumulative frequencies of torque drop and slip moment follow exponential distributions regardless of the experimental conditions. Since slip events of dry particles under confining pressure follow power-law distributions [3-4], fluid lubrication and friction reduction are related to the transition between exponential and power-law distributions. Inside the stressed granular material, the average stress in each force chain follows an exponential distribution [5]. Thus, low interparticle friction causes force chains to yield easily and suppresses the cascading up of slip avalanches.
2. Event duration T is approximately proportional to torque drop and slip moment M0 for hydrogel particles (M0 ∝ T ), while M0 ∝ T 2 for glass beads. For regular earthquake faults, the self-similarity (fractal) holds in that slip amount is proportional to fault length, resulting in the scaling M0 ∝ T 3. Therefore, when particles become softer, the fractality of slip planes reduces, transitioning to M0 ∝ T scaling.
3. Decreasing the porosity of the particle layer increases the means of moment M0 and moment rate M0/T. Furthermore, shear band localization is enhanced. Considering that the width and pressure of the shear band are determined by the porosity of the surrounding particles, the porosity dependence of moment M0 and moment rate M0/T are semi-quantitatively explained by the two effects of shear zone localization and pressure enhancement with decreasing porosity.
Based on the above results [6], it is presumed that interparticle friction determines the moment-frequency distribution, while particle viscoelasticity determines the moment-duration scaling. When we consider slow and regular earthquakes as corresponding to low and high frictional-viscoelastic phenomena, respectively, another type of seismic phenomena that does not belong to either category could be suggested. Therefore, to further investigate the effects of interparticle friction and particle viscoelasticity on the strength of the particle layer supported by force chains, we additionally performed comparative experiments and simulations using the discrete element method (DEM).
[1] Chestler & Creager (2017) JGR
[2] Ide & Beroza (2023) PNAS
[3] Korkolis et al (2021) JGR
[4] Geller et al (2015) PRE
[5] Zhang et al (2014) PRE
[6] Sasaki & Katsuragi (2025). Origin of slow earthquake statistics in low-friction soft granular shear. arXiv. https://arxiv.org/abs/2502.0135