Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that ...underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed.
Fluid overpressure is one of the primary mechanisms for tectonic fault slip, because fluids lubricate the fault and fluid pressure reduces the effective normal stress that holds the fault in place. ...However, current models of earthquake nucleation, based on rate- and state- friction laws, imply that stable sliding is favoured by the increase of pore fluid pressure. Despite this controversy, currently, there are only a few studies on the role of fluid pressure under controlled, laboratory conditions. Here, we use laboratory experiments, to show that the rate- and state- friction parameters do change with increasing fluid pressure. We tested carbonate gouges from sub hydrostatic to near lithostatic fluid pressure conditions, and show that the friction rate parameter (a - b) evolves from velocity strengthening to velocity neutral behaviour. Furthermore, the critical slip distance, Dc, decreases from about 90 to 10 μm. Our data suggest that fluid overpressure plays an important role in controlling the mode of fault slip. Since fault rheology and fault stability parameters change with fluid pressure, we suggest that a comprehensive characterization of these parameters is fundamental for better assessing the role of fluid pressure in natural and human induced earthquakes.
Giant rockslides are widespread and sensitive to hydrological forcing, especially in climate change scenarios. They creep slowly for centuries and then can fail catastrophically posing major threats ...to society. However, the mechanisms regulating the slow-to-fast transition toward their catastrophic collapse remain elusive. We couple laboratory experiments on natural rockslide shear zone material and in situ observations to provide a scale-independent demonstration that short-term pore fluid pressure variations originate a full spectrum of creep styles, modulated by slip-induced undrained conditions. Shear zones respond to pore pressure increments by impulsive acceleration and dilatancy, causing spontaneous deceleration followed by sustained steady-rate creep. Increasing pore pressure results in high creep rates and eventual collapse. Laboratory experiments quantitatively capture the in situ behavior of giant rockslides and lay physically-based foundations to understand the collapse of giant rockslides.
While analysis of glacial seismicity continues to be a widely used method for interpreting glacial processes, the underlying mechanics controlling glacial stick‐slip seismicity remain speculative. ...Here, we report on laboratory shear experiments of debris‐laden ice slid over a bedrock asperity under carefully controlled conditions. By modifying the elastic loading stiffness, we generated the first laboratory icequakes. Our work represents the first comprehensive lab observations of unstable ice‐slip events and replicates several seismological field observations of glacier slip, such as slip velocity, stress drop, and the relationship between stress drop and recurrence interval. We also observe that stick‐slips initiate above a critical driving velocity and that stress drop magnitude decreases with further increases in velocity, consistent with friction theory and rock‐on‐rock friction laboratory experiments. Our results demonstrate that glacier slip behavior can be accurately predicted by the constitutive rate‐and‐state friction laws that were developed for rock friction.
Plain Language Summary
Glacier beds and tectonic faults may at first appear to be quite different, but they share important characteristics. In both cases, motion may be smooth (aseismic creep) or earthquake‐producing “stick‐slip.” A powerful physical constitutive relationship called rate‐and‐state friction has been developed to understand earthquakes and smooth slip on tectonic faults. Laboratory experiments reported here simulate glacier‐bed motion by sliding debris‐bearing ice over a rock plate under conditions that are typical for glacier beds. They produce the first laboratory icequakes. Transitions between steady and stick‐slip motions are generated by controlling shearing velocity and other conditions, as predicted by rate‐and‐state friction theory. Future studies can thus apply this physical framework to glacier slip, helping to understand ice motion and its potential to accelerate sea level rise in a warming world. Furthermore, because motion at the glacier bed is often much easier to study than tectonic faults, additional observations of glaciers may provide useful insights into earthquake behavior.
Key Points
The first laboratory “icequakes” were generated and slip stability was predicted well by rate‐and‐state friction
Laboratory icequake attributes such as peak slip velocity, stress drop and healing agree well with other laboratory and field observations
The laboratory icequake events suggest that debris‐bed contacts dominate the sudden slip mechanics
Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6<μ<0.85. In marked contrast, numerous studies have documented ...significant fault weakening induced by fluid-assisted reaction softening that generally takes place during the long-term evolution of the fault. Reaction softening promotes the replacement of strong minerals with phyllosilicates. Phyllosilicate development within foliated and interconnected fault networks has been documented at different crustal depths, in different tectonic regimes and from a great variety of rock types, nominating fluid-assisted reaction softening as a general weakening mechanism within the seismogenic crust. This weakening originates at the grain-scale and is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones resulting in a friction as low as 0.1<μ<0.3.
Collectively, geological data and results from laboratory experiments provide strong supporting evidence for structural and frictional heterogeneities within crustal faults. In these structures, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor.
•Fault friction drops from 0.6 to 0.2 for interconnected networks of phyllosilicates.•Fluid-assisted reaction softening is a general weakening mechanism.•Structural and frictional heterogeneities of crustal faults are widespread.
Fluid overpressure is one of the primary mechanisms for triggering tectonic fault slip and human‐induced seismicity. This mechanism is appealing because fluid overpressure reduces the effective ...normal stress, hence favoring fault reactivation. However, upon fault reactivation models of earthquake nucleation suggest that increased fluid pressure should favor stable sliding rather than dynamic failure. Here we describe laboratory experiments on shale fault gouge, conducted in the double direct shear configuration in a true‐triaxial machine. To characterize frictional stability and hydrological properties we performed three types of experiments: (1) stable sliding shear experiments to determine the material failure envelope and permeability, (2) velocity step experiments to determine the rate‐and‐state frictional properties, and (3) creep experiments to study fault slip evolution with increasing pore fluid pressure. The shale gouge shows low frictional strength, μ = 0.28, and permeability, k ~ 10−19 m2 together with a velocity strengthening behavior indicative of aseismic slip. During fault pressurization, we document that upon failure slip velocity remains slow (i.e., v ~ 200 μm/s), not approaching dynamic slip rates. We relate this fault slip behavior to the interplay between the fault weakening induced by fluid pressurization, the strong rate‐strengthening behavior of shales, and the evolution of fault zone structure. Our data show that fault rheology and fault stability is controlled by the coupling between fluid pressure and rate‐and‐state friction parameters.
Key Points
Laboratory experiments on shale fault gouge designed to test fault slip behavior during fluid pressurization
Shale fault gouge fails by accelerated but slow shear slip not evolving into a dynamic slip instability
Slow shear slip arises from the interplay of fault hydromechanical properties and fault structure evolution
We investigate the physics of laboratory earthquake precursors in a biaxial shear configuration. We conduct laboratory experiments at room temperature and humidity in which we shear layers of glass ...beads under applied normal loads of 2–8 MPa and with shearing rates of 5–10 µm/s. We show that above ~ 3 MPa load, acoustic emission (AE), and shear microfailure (microslip) precursors exhibit an exponential increase in rate of occurrence, culminating in stick‐slip failure. Precursors take place where the material is in a critical state—still modestly dilating, yet while the macroscopic frictional strength is no longer increasing.
Key Points
Sheared granular media exhibits precursor events to stick‐slip failure
Rate of precursor activity increases exponentially prior to stick‐slip failure
Acoustic emission and microslip precursors are correlated in time
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