François Xavier Thibault Passelègue

From Sub-Rayleigh to Supershear Ruptures During Stick-Slip Experiments on Crustal Rocks

Sonic Boom from Below Seismic shear waves released by an earthquake typically far outpace motion along the fault surface. Occasionally, however, earthquakes along strike-slip faults appear to propagate so that the rupture velocity is faster than shear waves, creating a sort of sonic boom along the fault surface. Passelègue et al. (p. 1208) were able to reproduce and measure these so-called supershear ruptures in stick-slip experiments with two pieces of granite under high applied normal stress. Much like during a sonic boom when a plane travels faster than the speed of sound, the ruptures created a shock wave in the form of a Mach cone around the rupture front. Rupture fronts propagate faster than shear waves following experimental microearthquake nucleation. Supershear earthquake ruptures propagate faster than the shear wave velocity. Although there is evidence that this occurs in nature, it has not been experimentally demonstrated with the use of crustal rocks. We performed stick-slip experiments with Westerly granite under controlled upper-crustal stress conditions. Supershear ruptures systematically occur when the normal stress exceeds 43 megapascals (MPa) with resulting stress drops on the order of 3 to 25 MPa, comparable to the stress drops inferred by seismology for crustal earthquakes. In our experiments, the sub-Rayleigh–to–supershear transition length is a few centimeters at most, suggesting that the rupture of asperities along a fault may propagate locally at supershear velocities. In turn, these sudden accelerations and decelerations could play an important role in the generation of high-frequency radiation and the overall rupture-energy budget.

Dynamic weakening and amorphization in serpentinite during laboratory earthquakes

The mechanical properties of serpentinites are key factors in our understanding of the dynamics of earthquake ruptures in subduction zones, especially intermediate-depth earthquakes. Here, we performed shear rupture experiments on natural antigorite serpentinite, which showed that friction reaches near-zero values during spontaneous dynamic rupture propagation. Rapid coseismic slip (>1 m/s), although it occurs over short distances (<1 mm), induces significant overheating of microscale asperities along the sliding surface, sufficient to produce surface amorphization and likely some melting. Antigorite dehydration occurs in the fault walls, which leaves a partially amorphized material. The water generated potentially contributes to the production of a low-viscosity pressurized melt, explaining the near-zero dynamic friction levels observed in some events. The rapid and dramatic dynamic weakening in serpentinite might be a key process facilitating the propagation of earthquakes at intermediate depths in subduction zones.

Dynamic rupture processes inferred from laboratory microearthquakes

We report macroscopic stick-slip events in saw-cut Westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory. Experiments were conducted under triaxial loading (σ1>σ2=σ3) at confining pressures (σ3) ranging from 10 to 100 MPa. A high frequency acoustic monitoring array recorded particle acceleration during macroscopic stick-slip events allowing us to estimate rupture speed. In addition, we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane, is almost total in the breakdown zone (for normal stress > 75 MPa), while the friction f recovers to values of f > 0.4 within only a few hundred microseconds. Enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our post-mortem microstructural analysis. Relationships between initial state of stress, rupture velocities, stress drop and energy budget suggest that at high normal stress (leading to supershear rupture velocities), the rupture processes are more dissipative. Our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes. A power law scaling of the fracture energy with final slip is observed over eight orders of magnitude in slip, from a few microns to tens of meters.

Radon emanation from brittle fracturing in granites under upper crustal conditions

Radon-222, a radioactive gas naturally produced in the Earths crust, informs us about the migration of fluids and is sometimes considered as a potential earthquake precursor. Here we investigate the effects of mechanical and thermal damage on the radon emanation from various granites representative of the upper crust. Radon concentration measurements performed under triaxial stress and pore fluid pressure show that mechanical damage resulting from cycles of differential stress intensifies radon release up to 170 ± 22% when the sample ruptures. This radon peak is transient and results from the connection of isolated micropores to the permeable network rather than new crack surface creation per se. Heating to 850°C shows that thermal fracturing irreversibly decreases emanation by 59–97% due to the amorphization of biotites hosting radon sources. This study, and the developed protocols, shed light on the relation between radon emanation of crustal rocks, deformation, and pressure-temperature conditions.

Frictional evolution, acoustic emissions activity, and off‐fault damage in simulated faults sheared at seismic slip rates

We present a series of high-velocity friction tests conducted on Westerly granite, using the Slow to HIgh Velocity Apparatus (SHIVA) installed at Istituto Nazionale di Geofisica e Vulcanologia Roma with acoustic emissions (AEs) monitored at high frequency (4 MHz). Both atmospheric humidity and pore fluid (water) pressure conditions were tested, under effective normal stress sigma(eff)(n) in the range 5-20 MPa and at target sliding velocities V-s in the range 0.003-3 m/s. Under atmospheric humidity two consecutive friction drops were observed. The first one is related to flash weakening, and the second one to the formation and growth of a continuous layer of melt in the slip zone. In the presence of fluid, a single drop in friction was observed. Average values of fracture energy are independent of effective normal stress and sliding velocity. However, measurements of elastic wave velocities on the sheared samples suggested that larger damage was induced for 0.1 < V-s < 0.3 m/s. This observation is supported by AEs recorded during the test, most of which were detected after the initiation of the second friction drop, once the fault surface temperature was high. Some AEs were detected up to a few seconds after the end of the experiments, indicating thermal rather than mechanical cracking. In addition, the presence of pore water delayed the onset of AEs by cooling effects and by reducing of the heat produced, supporting the link between AEs and the production and diffusion of heat during sliding. Using a thermoelastic crack model developed by Fredrich and Wong (1986), we confirm that damage may be induced by heat diffusion. Indeed, our theoretical results predict accurately the amount of shortening and shortening rate, supporting the idea that gouge production and gouge comminution are in fact largely controlled by thermal cracking. Finally, we discuss the contribution of thermal cracking in the seismic energy balance. In fact, while a dichotomy exists in the literature regarding the partitioning between fracture and heat energy, the experimental evidence reported here suggests that both contribute to fault weakening and off-fault damage.

Dynamic weakening during earthquakes controlled by fluid thermodynamics

Earthquakes result from weakening of faults (transient decrease in friction) during co-seismic slip. Dry faults weaken due to degradation of fault asperities by frictional heating (e.g. flash heating). In the presence of fluids, theoretical models predict faults to weaken by thermal pressurization of fault fluid. However, experimental evidence of rock/fluid interactions during dynamic rupture under realistic stress conditions remains poorly documented. Here we demonstrate that the relative contribution of thermal pressurization and flash heating to fault weakening depends on fluid thermodynamic properties. Our dynamic records of laboratory earthquakes demonstrate that flash heating drives strength loss under dry and low (1 MPa) fluid pressure conditions. Conversely, flash heating is inhibited at high fluid pressure (25 MPa) because water’s liquid–supercritical phase transition buffers frictional heat. Our results are supported by flash-heating theory modified for pressurized fluids and by numerical modelling of thermal pressurization. The heat buffer effect has maximum efficiency at mid-crustal depths (~2–5 km), where many anthropogenic earthquakes nucleate.Understanding the physics of fault lubrication during earthquake propagation can help assess seismic hazard. In this study, by replicating earthquakes in the laboratory at upper-crustal conditions, the authors show that fluid thermodynamics control fault lubrication, specifically at man-made earthquake depths.

Development and Recovery of Stress‐Induced Elastic Anisotropy During Cyclic Loading Experiment on Westerly Granite

In the upper crust, where brittle deformation mechanisms dominate, the development of crack networks subject to anisotropic stress fields generates stress-induced elastic anisotropy. Here a rock specimen of Westerly granite was submitted to differential stress cycles (i.e., loading and unloading) of increasing amplitudes, up to failure and under upper crustal conditions. Combined records of strains, acoustic emissions, and P and S elastic wave anisotropies demonstrate that increasing differential stress promotes crack opening, sliding, and propagation subparallel to the main compressive stress orientation. However, the significant elastic anisotropies observed during loading (≥20%) almost vanish upon stress removal, demonstrating that in the absence of stress, crack-related elastic anisotropy remains limited (≤10%). As a consequence, (i) crack-related elastic anisotropies measured in the crust will likely be a strong function of the level of differential stress, and consequently (ii) continuous monitoring of elastic wave velocity anisotropy along faults could shed light on the mechanism of stress accumulation during interseismic loading. Plain Language Summary In the upper crust, large strains are accommodated by brittle deformation mechanisms, leading to macroscopic faults embedded within a substantially damaged rock medium. The development of crack damage affects both the strength and the elastic and transport properties of rocks. Nowadays, the evolution of rock elastic properties is commonly used to estimate the direction of the maximum stress along faults and evaluate seismic hazard of seismogenic area. Up to now, stress-induced anisotropy was expected to be irreversible and observable by geophysics method even after unloading or exhumation of the rocks. In this study, we demonstrate for the first time that unloading induces an almost complete recovery of both stress-induced anisotropy and stress-induced damage. Our results suggest that elastic properties estimated from wave velocity measurement could then underestimate both damage and anisotropy of the crust under shallow depth conditions.