Earthquakes normally occur as frictional stick-slip instabilities, resulting in catastrophic failure and seismic rupture. Tectonic faults also fail in slow earthquakes with rupture durations of ...months or more, yet their origin is poorly understood. Here, we present laboratory observations of repetitive, slow stick-slip in serpentinite fault zones and mechanical evidence for their origin. We document a transition from unstable to stable frictional behavior with increasing slip velocity, providing a mechanism to limit the speed of slow earthquakes. We also document reduction of P-wave speed within the active shear zone before stick-slip evnets. If similar mechanisms operate in nature, our results suggest that higher-resolution studies of elastic properties in tectonic fault zones may aid in the search for reliable earthquake precursors.
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.
It is widely recognized that the significant increase of M > 3.0 earthquakes in Western Canada and the Central United States is related to underground fluid injection. Following injection, fluid ...overpressure lubricates the fault and reduces the effective normal stress that holds the fault in place, promoting slip. Although, this basic physical mechanism for earthquake triggering and fault slip is well understood, there are many open questions related to induced seismicity. Models of earthquake nucleation based on rate- and state-friction predict that fluid overpressure should stabilize fault slip rather than trigger earthquakes. To address this controversy, we conducted laboratory creep experiments to monitor fault slip evolution at constant shear stress while the effective normal stress was systematically reduced via increasing fluid pressure. We sheared layers of carbonate-bearing fault gouge in a double direct shear configuration within a true-triaxial pressure vessel. We show that fault slip evolution is controlled by the stress state acting on the fault and that fluid pressurization can trigger dynamic instability even in cases of rate strengthening friction, which should favor aseismic creep. During fluid pressurization, when shear and effective normal stresses reach the failure condition, accelerated creep occurs in association with fault dilation; further pressurization leads to an exponential acceleration with fault compaction and slip localization. Our work indicates that fault weakening induced by fluid pressurization can overcome rate strengthening friction resulting in fast acceleration and earthquake slip. Our work points to modifications of the standard model for earthquake nucleation to account for the effect of fluid overpressure and to accurately predict the seismic risk associated with fluid injection.
•The conditions for fault reactivation due to fluid overpressure are tested.•First example of creep experiments on fault gouge under fluid overpressure.•Fault weakening by fluid overpressure overcome rate strengthening behavior.•Inform fault deformation with microstructural analysis.•Short term fluid overpressure causes an energy unbalance that trigger dynamic slip.
We investigate the evolution of the frequency-magnitude b-value during stable and unstable frictional sliding experiments. Using a biaxial shear configuration, we record broadband acoustic emissions ...(AE) while shearing layers of simulated granular fault gouge under normal stresses of 2–8 MPa and shearing velocity of 11 μm/s. AE event amplitude ranges over 3–4 orders of magnitude and we find an inverse correlation between b and shear stress. The reduction of b occurs systematically as shear stress rises prior to stick–slip failure and indicates a greater proportion of large events when faults are more highly stressed. For quasi-periodic stick–slip events, the temporal evolution of b has a characteristic saw-tooth pattern: it slowly drops as shear stress increases and quickly jumps back up at the time of failure. The rate of decrease during the inter-seismic period is independent of normal stress but the average value of b decreases systematically with normal stress. For stable sliding, b is roughly constant during shear, however it exhibits large variability. During irregular stick–slip, we see a mix of both behaviors: b decreases during the interseismic period between events and then remains constant when shear stress stabilizes, until the next event where a co-seismic increase is observed. Our results will help improve seismic hazard assessment and, ultimately, could aid earthquake prediction efforts by providing a process-based understanding of temporal changes in b-value during the seismic cycle.
•We explore the evolution of b-value during stable and unstable frictional sliding.•We find an inverse correlation between b and shear stress.•Rate of decrease during the inter-seismic period is independent of normal stress.•The average value of b decreases systematically with normal stress.
Frictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of ...quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double‐direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer‐normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay‐rich faults—together with rate‐neutral to rate‐strengthening friction—should promote frequent, small failures or stable sliding.
Plain Language Summary
Healing is a time‐dependent strengthening process that makes it possible for faults to regain strength after an earthquake and fail again in future events. Faults are commonly hosted in clay‐rich, fine‐grained gouges. While there is a wealth of data on the frictional behavior of these gouges, there is a relative lack of data regarding their healing behavior. Here, we measure the change in frictional healing with increasing clay content using experimental deformation of synthetic fault gouges. We find that the amount and rates of healing decrease with clay content. This decrease is likely caused by the tendency for clay grains, which are elongate and plate‐shaped, to align and limit the ability for the fault gouge to increase the area of contact between grains, thus limiting healing. This small healing, together with their low strength and tendency toward stable behavior, may promote small frequent earthquakes—or inhibit earthquakes—on faults hosted in clay‐rich gouges.
Key Points
We conducted experiments on a suite of quartz‐smectite mixtures to investigate the role of clay in frictional healing
Increasing smectite content reduces both healing and frictional strength, interpreted to result from saturation of real area of contact
Low healing rates in clay‐rich gouges may favor more frequent and smaller failures than in quartz‐dominated gouges
Pore fluids are ubiquitous throughout the lithosphere and are commonly invoked as the cause of induced seismicity and slow earthquakes. We perform lab experiments to address these questions for ...drained fault conditions and low pore pressure. We shear simulated faults at effective normal stress σn′ $\left({\sigma }_{n}^{\prime }\right)$ of 20 MPa and pore pressures Pp from 1 to 4 MPa. We document the full range of lab earthquake behaviors from slow slip to elasto‐dynamic rupture and show that slow slip can be explained by the slip rate dependence of the critical rheologic stiffness without dilatancy hardening or other fluid effects. Our fault permeabilities ranges from 10−18 to 10−17 m2 with an initial porosity of 0.1 and estimated fluid diffusion time ≈1 s. Slow slip and quasi‐dynamic fault motion may arise from high Pp at higher pressures but dilatancy strengthening is not a general requirement.
Plain Language Summary
Earthquakes begin and propagate within the fluid‐saturated rocks of Earth's crust. Many investigators have suggested that high pore fluid pressure (Pp) is essential for slow earthquakes and tremor. These studies rely on the idea that changes in Pp can impact rupture propagation speed by dilatant volume increase during faulting with concurrent increases in fault effective normal stress. Thus, understanding the processes that produce slow‐slip versus dynamically propagating rupture is integral to seismic hazard forecasting. Here, we describe experiments on granular faults that produce the full spectrum of slip observed in nature. We measure the mechanical and hydraulic behavior of the faults and determine that frictional and fluid‐driven processes occur in conjunction. Importantly, we demonstrate that frictional processes are sufficient to explain slow‐slip when fluid migration is not inhibited. We demonstrate that for low pore fluid pressures, the full transition from slow slip to dynamic rupture events can be explained as a frictional effect via the critical rheologic stiffness.
Key Points
The frictional stability transition does not require dilatant hardening for granular fault zones sheared at low pore pressures
Slow earthquakes and quasi‐dynamic fault slip can be explained by the strain‐rate dependence of the critical fault stiffness (Kc)
For the effective normal stresses studied, pore pressure has a negligible impact on frictional stability and the mode of fault slip
Frictional heterogeneities within fault zones depend strongly on lithology as well as shear fabric and strain localization structures. Here we investigate the impact of fault rock composition and ...shear fabric on friction constitutive properties for mature High Zagros (Iran) fault rocks. We present field observations along with results from friction experiments on intact and powdered fault rock from carbonate, quartzofeldspathic and schist. We measured frictional strength and rate/state dependence over normal stresses from 25 to 100 MPa and shear slip velocities from 3 to 300 μm/s. The sliding friction coefficients of intact fault rocks are lower than their powdered equivalents. The foliated quartzofeldspathic and lensoidal carbonate rocks from the Main Zagros Reverse Fault (MZRF) exhibit velocity strengthening friction and stable sliding. In contrast, the cataclasite carbonate rocks from the younger, Main Recent Fault (MRF) exhibit potentially unstable, velocity-weakening frictional behavior, consistent with regional seismicity. Our results suggest that solution seams, cleavage processes, and development of stylolite and calcite veins in the carbonate fault rocks of the MZRF continually rejuvenate and rework shear structures, which leads to dominantly aseismic shear. In contrast, younger fault rocks of the MRF form highly localized shear zones and shear fabric that result in velocity-weakening frictional behavior and seismic slip. Our field observations and laboratory data provide insights for using friction data and fault zone structure to build predictive models of the mode of fault slip in zones of tectonic collision.
•Fault zone structures including shear localization zones play a key role in determining frictional properties•Fault rocks from the main Zagros reverse fault exhibit velocity strengthening frictional behavior and aseismic slip•Frictional properties of intact fault rocks differ from their identical powders due to shear fabric and internal structure•The sliding friction coefficients of intact fault rocks are lower than their powdered equivalents
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
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