Understanding the physical mechanisms controlling fluid-injection-induced seismicity in a gouge-filled fault is crucial in the assessment and mitigation of induced seismicity, especially in the context of subsurface technologies like wastewater disposal, hydraulic fracturing, oil and gas production, geothermal energy extraction, and geologic carbon storage. Here, we study the seismic behavior of fault gouge induced by fluid injection with a novel ring shear experimental apparatus. We designed this experimental apparatus to enable large shear strain, shear load stiffness control, normal stress and pore pressure control, and acoustic emission measurement. We use fine soda-lime spheres (∼50 microns) as analog material for weakly-consolidated fault gouge. We load the gouge material to its critical state with the shear load stiffness controlled by a torsion spring. The fluid is then injected with different pressurization rates to induce slip in the gouge. Our experimental results are capable of quantitatively reproducing the main statistical laws describing seismicity. The results also provide qualitative experimental validation to the recent study on the pressurization rate dependence of fault slip: for the same amount of injected volume, lower injection rate builds up lower pore pressure, and triggers less slip and less seismic events.

1. Introduction

Fault gouge is crushed and ground-up rock produced by friction between the two sides when a fault moves (Scholz, 2019). Most natural faults are filled with fault gouge of various grain sizes depending on their stress and slip history. The frictional behavior of fault gouge controls the response of the fault formation to natural and industrial activities such as tectonic movements, groundwater level change, oil and gas production, water injection, and CO2 storage (National Research Council, 2013). The response is observed to be aseismic in some sites (Davis and Pennington, 1989) and seismic in others (Healy et al., 1968), with events of various magnitudes (McGarr et al, 2020). These responses could be aseismic or seismic, with various magnitudes of seismic events. Therefore, it is important to understand the physical mechanisms controlling the fluid-injection-induced slip response in a gouge-filled fault.

There are many experimental and theoretical studies on the seismic behavior of faults (Marone, 1998). The focus of the studies have gradually shifted from studying the rate-and-state friction of the gouge material itself to studying the gouge with its mechanical and flow boundary conditions as a system (Marone et al., 1990; Linker and Dieterich, 1992; Segall and Rice, 1995; Ikari et al., 2009; Ougier-Simonin and Zhu, 2013; French et al., 2016; Leeman et al., 2016; Scuderi et al., 2017; Scuderi and Collettini, 2018, Cappa et al., 2019; Im et al., 2020; Wang et al., 2020a, 2020b). The experimental system has also evolved from simple two-axis mechanical control and measurements to the coupling with controlled shear stiffness, controlled pore pressure, and acoustic emission (AE) measurements. These studies provide valuable insights into the seismic and aseismic behavior of fault gouge coupled with fluid.

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