Frictional instabilities in clay illuminate the origin of slow earthquakes

The shallowest regions of subduction megathrusts mainly deform aseismically, but they can sporadically host slow-slip events (SSEs) and tsunami earthquakes, thus representing a severe hazard. However, the mechanisms behind these remain enigmatic because the frictional properties of shallow subduction zones, usually rich in clay, do not allow earthquake slip according to standard friction theory. We present experimental data showing that clay-rich faults with bulk rate-strengthening behavior and null healing rate, typically associated with aseismic creep, can contemporaneously creep and nucleate SSE. Our experiments document slow ruptures occurring within thin shear zones, driven by structural and stress heterogeneities of the experimental faults. We propose that bulk rate-strengthening frictional behavior promotes long-term aseismic creep, whereas localized frictional shear allows slow rupture nucleation and quasi-dynamic propagation typical of rate-weakening behavior. Our results provide additional understanding of fault friction and illustrate the complex behavior of clay-rich faults, providing an alternative paradigm for interpretation of the spectrum of fault slip including SSEs and tsunami earthquakes.

Self-healing solitonic slip pulses in frictional systems

A prominent spatiotemporal failure mode of frictional systems is self-healing slip pulses, which are propagating solitonic structures that feature a characteristic length. Here, we numerically derive a family of steady state slip pulse solutions along generic and realistic rate-and-state dependent frictional interfaces, separating large deformable bodies in contact. Such nonlinear interfaces feature a nonmonotonic frictional strength as a function of the slip velocity, with a local minimum. The solutions exhibit a diverging length and strongly inertial propagation velocities, when the driving stress approaches the frictional strength characterizing the local minimum from above, and change their character when it is away from it. An approximate scaling theory quantitatively explains these observations. The derived pulse solutions also exhibit significant spatially-extended dissipation in excess of the edge-localized dissipation (the effective fracture energy) and an unconventional edge singularity. The relevance of our findings for available observations is discussed.

Minimal model for the onset of slip pulses in frictional rupture

We present a minimal one-dimensional continuum model for the transition from cracklike to pulselike propagation of frictional rupture. In its nondimensional form, the model depends on only two free parameters: the nondimensional prestress and an elasticity ratio that accounts for the finite height of the system. The model predicts stable slip pulse solutions for slip boundary conditions, and unstable slip pulse solutions for stress boundary conditions. The results demonstrate that a mechanism based solely on elastic relaxation and redistribution of initial prestress can cause pulselike rupture, without any particular rate or slip dependences of dynamic friction. This means that pulselike propagation along frictional interfaces is likely a generic feature that can occur in systems of finite thickness over a wide range of friction constitutive laws.

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.

The onset of the frictional motion of dissimilar materials

Frictional motion between contacting bodies is governed by propagating rupture fronts that are essentially earthquakes. These fronts break the contacts composing the interface separating the bodies to enable their relative motion. The most general type of frictional motion takes place when the two bodies are not identical. Within these so-called bimaterial interfaces, the onset of frictional motion is often mediated by highly localized rupture fronts, called slip pulses. Here, we show how this unique rupture mode develops, evolves, and changes the character of the interface's behavior. Bimaterial slip pulses initiate as "subshear" cracks (slower than shear waves) that transition to developed slip pulses where normal stresses almost vanish at their leading edge. The observed slip pulses propagate solely within a narrow range of "transonic" velocities, bounded between the shear wave velocity of the softer material and a limiting velocity. We derive analytic solutions for both subshear cracks and the leading edge of slip pulses. These solutions both provide an excellent description of our experimental measurements and quantitatively explain slip pulses' limiting velocities. We furthermore find that frictional coupling between local normal stress variations and frictional resistance actually promotes the interface separation that is critical for slip-pulse localization. These results provide a full picture of slip-pulse formation and structure that is important for our fundamental understanding of both earthquake motion and the most general types of frictional processes.

Minimal model for slow, sub-Rayleigh, supershear, and unsteady rupture propagation along homogeneously loaded frictional interfaces

In nature and experiments, a large variety of rupture speeds and front modes along frictional interfaces are observed. Here, we introduce a minimal model for the rupture of homogeneously loaded interfaces with velocity strengthening dynamic friction, containing only two dimensionless parameters; τ[over ¯], which governs the prestress, and α[over ¯], which is set by the interfacial viscosity. This model contains a large variety of front types, including slow fronts, sub-Rayleigh fronts, supershear fronts, slip pulses, cracks, arresting fronts, and fronts that alternate between arresting and propagating phases. Our results indicate that this wide range of front types is an inherent property of frictional systems with velocity strengthening branches.

Impulsive Source of the 2017 MW=7.3 Ezgeleh, Iran, Earthquake

On 12 November 2017, a M W=7.3 earthquake struck near the Iranian town of Ezgeleh, at the Iran-Iraq border. This event was located within the Zagros fold and thrust belt which delimits the continental collision between the Arabian and Eurasian Plates. Despite a high seismic risk, the seismogenic behavior of the complex network of active faults is not well documented in this area due to the long recurrence interval of large earthquakes. In this study, we jointly invert interferometric synthetic aperture radar and near-field strong motions to infer a kinematic slip model of the rupture. The incorporation of these near-field observations enables a fine resolution of the kinematic rupture process. It reveals an impulsive seismic source with a strong southward rupture directivity, consistent with significant damage south of the epicenter. We also show that the slip direction does not match plate convergence, implying that some of the accumulated strain must be partitioned onto other faults.

Rupture, waves and earthquakes

Normally, an earthquake is considered as a phenomenon of wave energy radiation by rupture (fracture) of solid Earth. However, the physics of dynamic process around seismic sources, which may play a crucial role in the occurrence of earthquakes and generation of strong waves, has not been fully understood yet. Instead, much of former investigation in seismology evaluated earthquake characteristics in terms of kinematics that does not directly treat such dynamic aspects and usually excludes the influence of high-frequency wave components over 1 Hz. There are countless valuable research outcomes obtained through this kinematics-based approach, but "extraordinary" phenomena that are difficult to be explained by this conventional description have been found, for instance, on the occasion of the 1995 Hyogo-ken Nanbu, Japan, earthquake, and more detailed study on rupture and wave dynamics, namely, possible mechanical characteristics of (1) rupture development around seismic sources, (2) earthquake-induced structural failures and (3) wave interaction that connects rupture (1) and failures (2), would be indispensable.

The structure of slip-pulses and supershear ruptures driving slip in bimaterial friction

The most general frictional motion in nature involves bimaterial interfaces, when contacting bodies possess different elastic properties. Frictional motion occurs when the contacts composing the interface separating these bodies detach via propagating rupture fronts. Coupling between slip and normal stress variations is unique to bimaterial interfaces. Here we use high speed simultaneous measurements of slip velocities, real contact area and stresses to explicitly reveal this bimaterial coupling and its role in determining different classes of rupture modes and their structures. We directly observe slip-pulses, highly localized slip accompanied by large local reduction of the normal stress near the rupture tip. These pulses propagate in the direction of motion of the softer material at a selected (maximal) velocity and continuously evolve while propagating. In the opposite direction bimaterial coupling favors crack-like ‘supershear' fronts. The robustness of these structures shows the importance of bimaterial coupling to frictional motion and modes of frictional dissipation.

Friction commonly involves different material types (bimaterials) at their sliding interface. Here, in laboratory experiments Shlomai and Fineberg reveal effects uniquely due to biomaterial coupling, with slip-pulses and crack-like supershear fronts dominating opposing propagation directions.

Pulse-like and crack-like ruptures in experiments mimicking crustal earthquakes

Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatiotemporal complexity of fault slip. Early theoretical models treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated. This study presents experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes; reveals how different rupture modes are selected based on the level of fault prestress; demonstrates that both rupture modes can transition to supershear speeds; and advocates, based on comparison with theoretical studies, the importance of velocity-weakening friction for earthquake dynamics.

Ultralow friction of carbonate faults caused by thermal decomposition

High-velocity weakening of faults may drive fault motion during large earthquakes. Experiments on simulated faults in Carrara marble at slip rates up to 1.3 meters per second demonstrate that thermal decomposition of calcite due to frictional heating induces pronounced fault weakening with steady-state friction coefficients as low as 0.06. Decomposition produces particles of tens of nanometers in size, and the ultralow friction appears to be associated with the flash heating on an ultrafine decomposition product. Thus, thermal decomposition may be an important process for the dynamic weakening of faults.

Dynamic sliding of frictionally held bimaterial interfaces subjected to impact shear loading

The fast frictional sliding along an incoherent interface of a bimaterial system composed of a Homalite and a steel plate is studied experimentally in a microsecond time-scale. The plates are held together by a static uniform compressive pre-stress while dynamic sliding is initiated by asymmetric impact. The full-field technique of dynamic photoelasticity is simultaneously used with a local technique of velocimetry based on laser interferometry. In the case where the impact loading is applied to the Homalite plate, a shear Mach line originates from a disturbance propagating along the interface supersonically with respect to the dilatational wave speed of Homalite and it crosses the P-wave front. The sliding starts well behind the P-wave front in Homalite and it propagates with a supershear speed with respect to Homalite. A fast interface wave and a wrinkle-like opening pulse (detachment wave) travelling along the interface are observed. When the impact loading is applied to the steel plate, the local sliding velocity measurement reveals that sliding initiates with the arrival of the P-wave front in the steel plate.

Laboratory earthquakes along inhomogeneous faults: directionality and supershear

We report on the experimental observation of spontaneously nucleated ruptures occurring on frictionally held bimaterial interfaces with small amounts of wave speed mismatch. Rupture is always found to be asymmetric bilateral. In one direction, rupture always propagates at the generalized Rayleigh wave speed, whereas in the opposite direction it is subshear or it transitions to supershear. The lack of a preferred rupture direction and the conditions leading to supershear are discussed in relation to existing theory and to the earthquake sequence in Parkfield, California, and in North Anatolia.

Quasidislocation stoneley wave and Eshelby dislocation Scholte wave

A quasidislocation (a dislocationlike entity described here for the first time) moves at the speed of a Stoneley surface wave that travels at the interface between two different elastic solids. An Eshelby glide edge dislocation moves at the speed of a Scholte surface wave that travels at the interface between a solid and an ideal liquid. The quasidislocation and the glide edge dislocation (that moves at the Eshelby velocity) are the Green's functions of their waves. Scholte waves are planar distributions of transonic moving glide edge dislocations. They are not Stoneley waves, although often called by that name, because Stoneley waves are planar distributions of subsonic moving quasidislocations.

Detachment fronts and the onset of dynamic friction

The dynamics of friction have been studied for hundreds of years, yet many aspects of these everyday processes are not understood. One such aspect is the onset of frictional motion (slip). First described more than 200 years ago as the transition from static to dynamic friction, the onset of slip is central to fields as diverse as physics, tribology, mechanics of earthquakes and fracture. Here we show that the onset of frictional slip is governed by three different types of coherent crack-like fronts: these are observed by real-time visualization of the net contact area that forms the interface separating two blocks of like material. Two of these fronts, which propagate at subsonic and intersonic velocities, have been the subject of intensive recent interest. We show that a third type of front, which propagates an order of magnitude more slowly, is the dominant mechanism for the rupture of the interface. No overall motion (sliding) of the blocks occurs until either of the slower two fronts traverses the entire interface.

Laboratory Earthquakes: The Sub-Rayleigh-to-Supershear Rupture Transition

We report on the experimental observation of spontaneously nucleated supershear rupture and on the visualization of sub-Rayleigh-to-supershear rupture transitions in frictionally held interfaces. The laboratory experiments mimic natural earthquakes. The results suggest that under certain conditions supershear rupture propagation can be facilitated during large earthquake events.