High-pressure fluid injection in deep georeservoirs can induce earthquakes. Recent observations suggest that cyclic injections might trigger less seismicity than monotonic injections. Here, we report triaxial laboratory experiments conducted on faulted quartz-rich sandstone that provide new insight into the physics of fault-fluid interactions subjected to cyclic fluid pressure variations. The experiments were performed at 30 and 45 MPa confining pressure, imposing constant or sinusoidal fluid pressure oscillations of amplitudes ranging from 0 to 8 MPa in addition to a far-field constant loading rate (10 −4 and 10 −3 mm s −1 ). The results show that (i) in agreement with the Mohr-Coulomb theory, faults reactivate at the static friction criterion, which is generally reached at the maximum fluid pressure during oscillations. (ii) Oscillating fluid pressure perturbations promote seismic behavior rather than aseismic slip, and (iii) increasing the oscillation's amplitude enhances the onset of seismic activity along the fault. We demonstrate that this behavior is caused by slip rate variations resulting from the fluid pressure oscillations. Without fluid pressure oscillations, increasing the far-field loading rate also promotes seismic activity. Our experiments demonstrate that the seismicity intensification due to cyclic fluid injections could be promoted at shallow depth, where confining pressure is relatively low, resulting in large strain rate perturbations.

In tectonically active regions, natural seismicity is often correlated with the seasonal hydrology, suggesting that cyclic loading variations might trigger seismicity. Moreover, recent field observations suggest that cyclic fluid injection strategies into geological reservoirs could produce less seismicity than monotonic injections. Here we present 10 brittle creep laboratory triaxial experiments that bring new constraints on fluid-rock interactions during cyclic pore fluid variations. The experiments were performed on Fontainebleau sandstone with various pore fluid pressure conditions: (i) with constant pore fluid pressure levels from 1 to 10 MPa, at constant Terzaghi effective pressure (P c − P f = 30 MPa); and (ii) with cyclic (sinusoidal) pore fluid pressure oscillations of varying amplitudes (from 0 to 8 MPa) and periods (from 30 to 3,000 s) around a mean value of 5 MPa. During deformation, the rock's mechanical properties and the high-frequency acoustic emission signals were monitored to investigate the physics underlying the rupture processes. Under macroscopically drained conditions, rather than their amplitude, the period of the oscillations appeared to strongly affect the rock sample strength, time-to-failure, and dilatancy behavior. Moreover, even for small variations of pore fluid amplitude, and at all pore fluid pressure period, pore fluid pressure and acoustic emissions were strongly correlated. Our experiments demonstrate that pore fluid pressure oscillations may strongly affect rocks mechanical behavior and associated seismic activity.

In the Earth's crust, irreversible deformations are usually accommodated by two distinct modes: brittle and ductile. Within the upper part of the crust, irreversible deformations are brittle and mainly localized along shear fractures (i.e., faults), limiting the rock strength to frictional and fracture motions (

Large earthquakes mainly take place on faults that have accumulated large shear displacement (e.g., Manighetti et al., 2007;Scholz, 2019). For faults in the shallow crust, shear occurs within a zone of finite width that is composed of non-cohesive rock wear products, often referred to as gouge (Sibson, 1977). Fault gouge thickness increases with cumulative fault shear displacement (e.g.,

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<p>Tectonic fault zones are subject to normal stress variations with a wide range of spatiotemporal scales, resulting in stress field alteration. These perturbations can spread over a wide range of frequencies and amplitudes from the high frequency passage of seismic waves generated by earthquakes, to the low frequency of solid earth tides and underground fluid injection cycles. As a result of these normal stress perturbations, critically stressed faults can be reactivated. The resulting slip mode is then controlled by fault friction and elastic properties of the surrounding rock. Existing works show that complex behaviors may arise from the interplay between friction changes with slip and slip rate and stress perturbations.</p><p>To shed light on the mechanics of fault dynamic triggering we performed experiments in a Biaxial Apparatus in a Double Direct Shear configuration under critically stable stiffness conditions (K/Kc~1). We used powdered quartz gouge (Min-U-Sil 40) as starting material, and conducted experiments at reference normal stress of σ<sub>n</sub> = 10-13.5 MPa. After shearing the material and reaching a steady state sliding, normal stress oscillations were applied with various amplitudes, varying from A = 0.5-2 MPa, and periods, T = 0.5-50 s. In addition, we used the laboratory derived friction parameters as input for forward modeling using Rate-and-State friction laws in order to assess if these laws can explain our data. Our results show that creeping faults, under critical stiffness conditions, are sensitive to normal stress perturbations showing a variety of slip behaviors depending on amplitude and frequency of the oscillations:</p><ul><li>Oscillation frequency has a major effect on fault stability. Low and high frequencies cause a Coulomb-like response of the shear stress, that is accompanied by a complex frictional response with slow events and period doubling. At the critical frequency predicted by the Rate-and-State friction, we observe dynamic weakening resulting in regular stick-slip events.</li> <li>Oscillation amplitude also plays a role with the main effect depending on the magnitude of the perturbation.</li> <li>Using a modified Rate-and-State equation (Linker and Dieterich, 1992), we are able to accurately model the laboratory data.</li> </ul><p>Our results show that normal stress perturbation on a laboratory creeping fault, at critical stiffness condition, can reproduce the entire spectrum of fault slip behavior depending on the oscillation properties.</p>

Bubbles form and grow as volatiles dissolved in magma exsolve in response to depressurization during magma ascent in the crust (Coumans et al., 2020;Gardner et al., 1999;Sparks, 1978;Toramaru, 1995). If magma ascent is sufficiently rapid, and if these exsolved volatiles cannot escape the system through permeable networks of bubbles and cracks, the gas pressure within the bubbles can increase (Melnik et al., 2005). In that scenario, magma can fragment if the gas overpressure exceeds a critical threshold value (Koyaguchi et al., 2008;Spieler et al., 2004;Zhang, 1999), a process that is thought to be the origin of explosive eruptive behavior. As a result, the permeability of a volcanic system (the magma and the surrounding host-rock) is thought to exert influence on eruption style, effusive or explosive. High pore pressures associated with permeability reductions in the volcanic edifice and lava dome are also thought to promote volcanic instability (

<p>Large earthquakes take place on mature faults with hundreds of meters to kilometres of cumulative slip. At shallow depths, the fault zone is generally composed of non-cohesive rock wear products, often referred to as gouge. Seismic and aseismic slip occur in this fault gouge and fracture/brecciation of the wall rock and damage zone can add to the fault gouge as part of the wear process. Gouge thickness generally increases linearly with the cumulative fault shear displacement and laboratory work shows that gouge tends to stabilize fault frictional stability. Previous works show that frictional stability of simulated fault gouge varies as a function of shear displacement. The stability evolution is interpreted as a consequence of the degree of shear localisation within the simulated fault gouge: the more the deformation is localized, the more the fault slip is unstable. This implies that for bare rock surfaces, unstable behaviour is expected as the deformations are forced to be localized at the interface between the two sheared surfaces.</p><p>On natural faults at large shear displacement (or for faults having a high gouge production rate), a competition must take place between 1) the localization of the deformation at rock-on-rock surfaces, 2) the delocalization of deformation due to gouge production and wall rock brecciation, 3) fault zone lithification and frictional healing and 4) shear localization within the gouge and wear material. The competition and interaction between these phenomena are modulated by cumulative fault slip, temperature and fluid chemistry. In turn, this competition may influence the frictional stability of faults with increasing shear displacement, and thus, their potential seismic activity.</p><p>To characterise the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to large displacement. Two initially intact rocks were chosen as starting material: a high porosity Fontainebleau sandstone and a low porosity quartzite. These samples represent very different resistances to abrasion (i.e., wear production with slip) for the same initial mineral composition (< 95% quartz), which allows us to investigate wear and wear rate on fault stability. Additionally, simulated quartz gouge was tested for comparison. Mechanical data are analysed within the rate-and-state framework, and post-mortem microscopic analyses of the sample were performed. For initially bare surface experiments a threshold shear displacement is required to transition from stable to unstable sliding. Stick-slip events (laboratory earthquakes) evolve systematically as a function of fault zone shear displacement. The inversion of the rate-and-state parameters shows that shear displacement has a dominant influence on both (<em>a</em>-<em>b</em>) and <em>D</em><sub>c</sub>. For all the faults tested, (<em>a</em>-<em>b</em>) decreases with increasing shear displacement. For high wear rates and simulated gouge, <em>D</em><sub>c</sub> decreases with increasing shear displacement. However, for low wear rate faults, <em>D</em><sub>c</sub> is constant within the tested shear displacement. These results demonstrate that, under the tested boundary conditions, fault stability varies systematically with fault maturity and in particular that shear displacement and strain localization are the dominant parameters controlling fault slip stability.</p>