Intermittent lab earthquakes in dynamically weakening fault gouge

Large and destructive earthquakes on mature faults in Earth's crust occur as slip in a layer of a fine granular material-fault gouge-produced by comminution during sliding^1,2. A range of insights into the frictional resistance of faults-one of the main factors controlling earthquake nucleation, dynamic propagation and arrest, and hence the destructive ground shaking of earthquakes^2,3-has been obtained in experiments with spatially uniform slip imposed in small samples^2,4-21. However, how various features of gouge friction combine to determine spontaneous progression of earthquakes is difficult to study in the lab owing to substantial challenges with sample sizes and adequate imaging²². Here, using lab experiments, we show that spontaneously propagating dynamic ruptures navigate a fault region with fine rock gouge through complex, intermittent slip processes with dramatic friction evolution. These include repeated arrest of rupture propagation caused by friction strengthening at lower slip rates and dynamic earthquake re-nucleation enabled by pronounced rapid friction weakening at higher slip rates consistent with flash heating^8,12,23. The spontaneous repeated weakening and strengthening of friction in fine rock gouge highlights the fundamental dependence of friction on slip rate and associated processes, such as shear heating, localization and delocalization of shear, and dilation and compaction of the shear layer^6,7,9-21. Our findings expand experimental support^9,11 of the concept that co-seismic weakening may enable earthquake rupture to break through stable fault regions^24,25, with substantial implications for seismic hazard.

Physical characterization of fault rocks within the Opalinus Clay formation

Near-surface disposal of radioactive waste in shales is a promising option to safeguard the population and environment. However, natural faults intersecting these geological formations can potentially affect the long-term isolation of the repositories. This paper characterizes the physical properties and mineralogy of the internal fault core structure intersecting the Opalinus Clay formation, a host rock under investigation for nuclear waste storage at the Mont Terri Laboratory (Switzerland). We have performed porosity, density, microstructural and mineralogical measurements in different sections of the fault, including intact clays, scaly clays and fault gouge. Mercury intrusion porosimetry analysis reveal a gouge that has a pore network dominated by nanopores of less than 10 nm, yet a high-porosity (21%) and low grain density (2.62 g/cm³) when compared to the intact rock (14.2%, and 2.69 g/cm³). Thus, a more permeable internal fault core structure with respect to the surrounding rock is deduced. Further, we describe the OPA fault gouge as a discrete fault structure having the potential to act as a preferential, yet narrow, and localized channel for fluid-flow if compared to the surrounding rock. Since the fault gouge is limited to a millimetres-thick structure, we expect the barrier property of the geological formation is almost not affected.

Seismic Slip-Pulse Experiments Simulate Induced Earthquake Rupture in the Groningen Gas Field

Rock materials show dramatic dynamic weakening in large-displacement (m), high-velocity (∼1 m/s) friction experiments, providing a mechanism for the generation of large, natural earthquakes. However, whether such weakening occurs during induced M3-4 earthquakes (dm displacements) is unknown. We performed rotary-shear experiments on simulated fault gouges prepared from the source-, reservoir- and caprock formations present in the seismogenic Groningen gas field (Netherlands). Water-saturated gouges were subjected to a slip pulse reaching a peak circumferential velocity of 1.2-1.7 m/s and total displacements of 13-20 cm, at 2.5-20 MPa normal stress. The results show 22%-81% dynamic weakening within 5-12 cm of slip, depending on normal stress and gouge composition. At 20 MPa normal stress, dynamic weakening from peak friction coefficients of 0.4-0.9 to 0.19-0.27 was observed, probably through thermal pressurization. We infer that similar effects play a key role during induced seismic slip on faults in the Groningen and other reservoir systems.

Propagation of large earthquakes as self-healing pulses or mild cracks

Observations suggest that mature faults host large earthquakes at much lower levels of stress than their expected static strength^1-11. Potential explanations are that the faults are quasi-statically strong but experience considerable weakening during earthquakes, or that the faults are persistently weak, for example, because of fluid overpressure. Here we use numerical modelling to examine these competing theories for simulated earthquake ruptures that satisfy the well known observations of 1-10 megapascal stress drops and limited heat production. In that regime, quasi-statically strong but dynamically weak faults mainly host relatively sharp, self-healing pulse-like ruptures, with only a small portion of the fault slipping at a given time, whereas persistently weak faults host milder ruptures with more spread-out slip, which are called crack-like ruptures. We find that the sharper self-healing pulses, which exhibit larger dynamic stress changes compared to their static stress changes, result in much larger radiated energy than that inferred teleseismically for megathrust events¹². By contrast, milder crack-like ruptures on persistently weak faults, which produce comparable static and dynamic stress changes, are consistent with the seismological observations. The larger radiated energy of self-healing pulses is similar to the limited regional inferences available for crustal strike-slip faults. Our findings suggest that either large earthquakes rarely propagate as self-healing pulses, with potential differences between tectonic settings, or their radiated energy is substantially underestimated, raising questions about earthquake physics and the expected shaking from large earthquakes.