From Stress Chains to Acoustic Emission

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From Stress Chains to Acoustic Emission

Ke Gao, Robert Guyer, Esteban Rougier, Christopher X. Ren, and Paul A. Johnson

Phys. Rev. Lett. 123, 048003 – Published 26 July 2019

Abstract

A numerical scheme using the combined finite-discrete element method is employed to study a model of an earthquake system comprising a granular layer embedded in a formation. When the formation is driven so as to shear the granular layer, a system of stress chains emerges. The stress chains endow the layer with resistance to shear and on failure launch broadcasts into the formation. These broadcasts, received as acoustic emission, provide a remote monitor of the state of the granular layer of the earthquake system.

Received 20 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.048003

Physics Subject Headings (PhySH)

Earthquakes Geophysics Granular avalanches

Granular chains Granular materials

Particles & FieldsStatistical Physics & ThermodynamicsInterdisciplinary PhysicsCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Ke Gao ^1,*, Robert Guyer^1,2, Esteban Rougier¹, Christopher X. Ren¹, and Paul A. Johnson¹

¹Geophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²Department of Physics, University of Nevada, Reno, Nevada 89557, USA

^*kegao@lanl.gov

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Issue

Vol. 123, Iss. 4 — 26 July 2019

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Images

Figure 1
Model system. The formation and the interior of the particles are described with a FEM scheme. The contacts of gouge particles with one another and with the formation are described with a DEM scheme. The formation has a rough surface profile (lower right). Sensors in the formation surface, adjacent to the gouge, are spaced by 2.4 mm. The elastic properties of gouge and formation are listed in the Supplemental Material [17].
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Figure 2
Primary outputs. The coefficient of friction (black line) as a function of time, left-hand scale. The average velocity of the top ( $T$ , blue line) and bottom ( $B$ , red line) sensors as a function of time, Eq. (1), right-hand scale. The green lines, all of the same slope, show the average elasticity of the gouge. The stress drop near 11000 msec (marked by red line) is examined in Fig. 3.
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Figure 3
Spectrum of velocity dipole strengths $D$ . (a) The spectrum of velocity dipole strengths, for all space-time points, Eq. (2), is broadly distributed from $10^{- 8}$ to approximately 0.2. The space-time points $(n, t)$ with large $D$ [in the 3 intervals red, green, blue on the lower right in (a)] are used to identify events. The stress near 11 000 msec in Fig. 2 is shown in detail in (b). Time is measured from 11 000 msec, where $μ = 0.45$ ; i.e., time 0 msec here corresponds to time 11 000 msec in Fig. 2. The velocity dipole strength at $(n, t)$ for this time interval is shown in (c), where the time axis is the same as in (b) and $n$ is on the vertical axis. The velocity dipole strength is shown with the color coding from (a).
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Figure 4
Details of a small, simple event. (a) Friction coefficient versus time for 3 adjacent small events that have stress drop a few percent of a major event such as the one in Fig. 3. These events are near time 25 500 msec, where $μ = 0.47$ . (b) The velocity dipole strengths associated with the third event in (a), on sensors 1–35, color coded from Fig. 3, for 9 msec measured from time 25 545 msec. (c) Velocity dipole composition during the 9 msec interval in (b): the velocity of the top sensor set (blue line) and the velocity of the bottom sensor set (red line), displaced to the right for clarity.
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Figure 5
Probability distribution functions (PDF). Probability distribution function of $r_{F}$ for the separation of the two components of the velocity dipole field (small events), blue. Probability distribution function of $r_{G}$ for isolated stress chains, red.
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Figure 6
Stress chains. (a) The spectrum of maximum principal stress of all elements in the gouge. (b) Elements in the FEM description of the particles, with maximum principal stress greater than 0.4 MPa. Each black dot represents the centroid position of the corresponding element. Two chains of elements at the right, 1 and 2, are judged to be isolated as is the chain 3 on the left. These chains are among the chains used to form the PDF of $r_{G}$ in Fig. 5.
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