The break of earthquake asperities imaged by distributed acoustic sensing

Rupture imaging of megathrust earthquakes with global seismic arrays revealed frequency-dependent rupture signatures1-4, but the role of high-frequency radiators remains unclear3-5. Similar observations of the more abundant crustal earthquakes could provide critical constraints but are rare without ultradense local arrays6,7. Here we use distributed acoustic sensing technology8,9 to image the high-frequency earthquake rupture radiators. By converting a 100-kilometre dark-fibre cable into a 10,000-channel seismic array, we image four high-frequency subevents for the 2021 Antelope Valley, California, moment-magnitude 6.0 earthquake. After comparing our results with long-period moment-release10,11 and dynamic rupture simulations, we suggest that the imaged subevents are due to the breaking of fault asperities-stronger spots or pins on the fault-that substantially modulate the overall rupture behaviour. An otherwise fading rupture propagation could be promoted by the breaking of fault asperities in a cascading sequence. This study highlights how we can use the extensive pre-existing fibre networks12 as high-frequency seismic antennas to systematically investigate the rupture process of regional moderate-sized earthquakes. Coupled with dynamic rupture modelling, it could improve our understanding of earthquake rupture dynamics.

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.

Unconventional singularities and energy balance in frictional rupture

A widespread framework for understanding frictional rupture, such as earthquakes along geological faults, invokes an analogy to ordinary cracks. A distinct feature of ordinary cracks is that their near edge fields are characterized by a square root singularity, which is intimately related to the existence of strict dissipation-related lengthscale separation and edge-localized energy balance. Yet, the interrelations between the singularity order, lengthscale separation and edge-localized energy balance in frictional rupture are not fully understood, even in physical situations in which the conventional square root singularity remains approximately valid. Here we develop a macroscopic theory that shows that the generic rate-dependent nature of friction leads to deviations from the conventional singularity, and that even if this deviation is small, significant non-edge-localized rupture-related dissipation emerges. The physical origin of the latter, which is predicted to vanish identically in the crack analogy, is the breakdown of scale separation that leads an accumulated spatially-extended dissipation, involving macroscopic scales. The non-edge-localized rupture-related dissipation is also predicted to be position dependent. The theoretical predictions are quantitatively supported by available numerical results, and their possible implications for earthquake physics are discussed.

Ordinary cracks in bulk materials feature square root singular deformation fields near their edge. Here, the authors show that rupture fronts propagating along frictional interfaces, while resembling ordinary cracks in some respects, feature edge sigularity that differs from the conventional square root one.

Breakdown of Scaling and Friction Weakening in Intermittent Granular Flow

Many materials are produced, processed and stored as grains, while granularity of matter can be crucial in triggering potentially catastrophic geological events like landslides, avalanches and earthquakes. The response of grain assemblies to shear stress is therefore of utmost relevance to both human and natural environment. At low shear rate a granular system flows intermittently by distinct avalanches. In such state the avalanche velocity in time is expected to follow a symmetrical and universal average behavior, whose dependence on the slip size reduces to a scale factor. Analyzing data from long lasting experiments, we observe a breakdown of this scaling: While in short slips velocity shows indeed a self-similar and symmetric profile, it does not in long slips. The investigation of frictional response in these different regimes evidences that this breakdown can be traced back to the onset of a friction weakening, which is of dynamical origin and can amplify instabilities exactly in this critical state, the most frequent state for natural hazards.

Unstable Slip Pulses and Earthquake Nucleation as a Nonequilibrium First-Order Phase Transition

The onset of rapid slip along initially quiescent frictional interfaces, the process of "earthquake nucleation," and dissipative spatiotemporal slippage dynamics play important roles in a broad range of physical systems. Here we first show that interfaces described by generic friction laws feature stress-dependent steady-state slip pulse solutions, which are unstable in the quasi-1D approximation of thin elastic bodies. We propose that such unstable slip pulses of linear size L^{*} and characteristic amplitude are "critical nuclei" for rapid slip in a nonequilibrium analogy to equilibrium first-order phase transitions and quantitatively support this idea by dynamical calculations. We then perform 2D numerical calculations that indicate that the nucleation length L^{*} exists also in 2D and that the existence of a fracture mechanics Griffith-like length L_{G}<L^{*} gives rise to a richer phase diagram that features also sustained slip pulses.

Understanding dynamic friction through spontaneously evolving laboratory earthquakes

Friction plays a key role in how ruptures unzip faults in the Earth's crust and release waves that cause destructive shaking. Yet dynamic friction evolution is one of the biggest uncertainties in earthquake science. Here we report on novel measurements of evolving local friction during spontaneously developing mini-earthquakes in the laboratory, enabled by our ultrahigh speed full-field imaging technique. The technique captures the evolution of displacements, velocities and stresses of dynamic ruptures, whose rupture speed range from sub-Rayleigh to supershear. The observed friction has complex evolution, featuring initial velocity strengthening followed by substantial velocity weakening. Our measurements are consistent with rate-and-state friction formulations supplemented with flash heating but not with widely used slip-weakening friction laws. This study develops a new approach for measuring local evolution of dynamic friction and has important implications for understanding earthquake hazard since laws governing frictional resistance of faults are vital ingredients in physically-based predictive models of the earthquake source.

Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, ~16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.