Frictional Stability of Metamorphic Epidote in Granitoid Faults Under Hydrothermal Conditions and Implications for Injection‐Induced Seismicity

Abstract

The presence of metamorphic epidote on faults has been implicated in the transition from stable to unstable slip and the nucleation of earthquakes. We present structured laboratory observations of mixed epidote and simulated Pohang granodiorite (analogous to the EGS‐enhanced geothermal system site) gouges to evaluate the impact of heterogeneity and contiguity of epidote‐patch structure on frictional instability. Experiments are at a confining pressure of 110 MPa, pore fluid pressures of 42–63 MPa, temperatures 100–250°C and epidote percentages of 0–100 vol.%. The simulated Pohang granodiorite gouge is frictionally strong (friction coefficient ∼0.71) but transits from velocity‐strengthening to velocity‐weakening at temperatures >150°C. This velocity‐weakening effect is amplified in approximate proportion to increasing epidote content. Modes of epidote precipitation likely control the size and contiguity of the epidote‐only patches and this in turn changes the response of 50:50 epidote‐granodiorite mixed gouges for different geometric configurations. However, 50:50 epidote‐granodiorite mixtures that are variously homogeneously mixed, encapsulated and checkerboarded in their structures are insensitive to their geometries – all reflect the high frictional strength and strong velocity‐weakening response of 100:0 pure epidote. This suggests that the epidote present as thin coatings on fractures/faults can enhance velocity‐weakening behavior, independent of individual patch size and can thereby support the potential seismic reactivation of faults. Considering the frictional and stability properties of epidote at conditions typical of shallow depths, the presence of low‐grade metamorphism exerts a potentially important control on fault stability in granitoids with relevance as a marker mineral for susceptibility to injection‐induced seismicity.

Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting

“…Whereas epidote is frictionally strong and relatively conducive to earthquake slip, growth of micas may cause substantial weakening and preference for aseismic creep (An et al, 2022;Fagereng & Ikari, 2020). The amount of retrograde mineral growth is more substantial in the granodioritic composition than the granite, but much less pronounced than in metasediments studied elsewhere (e.g., faults where retrograde phyllosilicate growth may potentially create interconnected weak faults; Fagereng & Diener, 2011;Imber et al, 1997).…”

Microphysical Modeling of Fault Slip and Stability Transition in Hydrothermal Conditions

“…An et al. (2022): $$\bar{\sigma }=\ 110\ \text{MPa},\ {P}_{f}=42-63\ \text{MPa},\ {V}_{L}=0.0488-1.22\ \mu \text{m/s}$$. Lockner et al.…”

“…Extensive laboratory studies have documented the temperature dependence of frictional properties and associated physical processes for a variety of representative rocks in different tectonic faults, such as pure gouges of granite (An et al, 2022;Blanpied et al, 1991Blanpied et al, , 1995Kolawole et al, 2019), quartz (Chester, 1994;Chester & Higgs, 1992), basalt (Okuda et al, 2023), and blueschist (Sawai et al, 2016), mixed gouges of quartz-biotite (Lu & He, 2018), illite-quartz (Den Hartog et al, 2012), muscovite-quartz (Den Hartog & Spiers, 2013), and actinolite-chlorite (Okamoto et al, 2020), as well as some natural gouges from the Alpine Fault (Niemeijer et al, 2016), Nankai subduction zone (Den Hartog et al, 2022), and Changning Block (An et al, 2020). The experimental results provide important constraints on physical models of fault friction by showing how temperature, effective stress, and lithology affect frictional strength and frictional parameters, for example, friction coefficient, direct friction effect, and evolutionary friction effect.…”

Thermally Induced Microcracks in Granite and Their Effect on the Macroscale Mechanical Behavior

“…Thermally induced cracking has been identified as the cause of the reduced strength and enhanced permeability of rock at high temperatures (Nasseri et al., 2009). Furthermore, the reduction of friction coefficient (Du et al., 2022) at elevated temperatures and pore fluid pressures may potentially promote fault destabilization or failure under conditions typifying a geothermal reservoir (An et al., 2021, 2022). Therefore, an in‐depth understanding of the strength evolution of granite is important for the safe and economical design of deep underground engineering (Wong et al., 2020).…”

“…or failure under conditions typifying a geothermal reservoir (An et al, 2021(An et al, , 2022. Therefore, an in-depth understanding of the strength evolution of granite is important for the safe and economical design of deep underground engineering (Wong et al, 2020).…”