Dynamic Stress Drop Research Articles

Shallow Moonquake Mechanisms Illuminated by Rheologic Characteristics of Basaltic Gouges

AbstractThe projected evolutionary history of the Moon and observed occurrence of moonquakes suggest that brittle faulting is present in the shallow lunar crust. The main component of the lunar crust, plagioclase, shows velocity‐strengthening behavior in the range of crustal temperatures. Chang'e 5 samples of lunar regolith show a mineral composition almost identical to basaltic bedrock. We measured the friction‐stability characteristics of dry synthetic gouges representative of basaltic faults assumed to be present in the lunar crust. Frictional strengths are ∼0.7 and exhibit an overall velocity‐strengthening response but transition to velocity‐weakening at intermediate temperatures (∼200–300°C) and stresses (∼25–100 MPa). Bounding temperature profiles representative of the lunar crust suggest that moonquakes are feasible in the lunar crust. The rheological heterogeneity of mineral fragments in basalt is a potential cause of unstable sliding on faults with the related steady‐state stress drop close to the minimum of the estimated dynamic stress drop. This suggests that some events with small stress drops are associated with the instability of mature basalt faults. However, observations of shallow moonquakes with high stress drop but merely moderate magnitude suggest that high degrees of healing on immature faults, small seismic nucleation lengths, or the failure of intact crust are present. We emphasize that moonquakes may arise from stress transfer and accumulation due to processes such as cooling contraction.

Fluid-driven fault nucleation, rupture processes, and permeability evolution in oshima granite — Preliminary results and acoustic emission datasets

This study investigated the fault nucleation and rupture processes driven by stress and fluid pressure in fine-grained granite by monitoring acoustic emissions (AEs). Through detailed analysis of the spatiotemporal distribution of the AE hypocenter, P-wave velocity, stress-strain, and other experimental observation data under different confining pressures for stress-driven fractures and under different water injection conditions for fluid-driven fractures, it was found that fluid has the following effects: 1) complicating the fault nucleation process, 2) exhibiting episodic AE activity corresponding to fault branching and the formation of multiple faults, 3) extending the spatiotemporal scale of nucleation processes and pre-slip, and 4) reducing the dynamic rupture velocity and stress drop. The experiments also show that 1) during the fault nucleation process, the b-value for AEs changes from 1 to 1.3 to 0.5 before dynamic rupture, and then rapidly recovers to around 1–1.2 during aftershock activity and 2) the hydraulic diffusivity gradually increases from an initial pre-rupture order of 0.1 ​m2/s to 10–100 ​m2/s after dynamic rupture. These results provide a reasonable fault pre-slip model, indicating that hydraulic fracturing promotes shear slip before dynamic rupture, as well as laboratory-scale insights into ensuring the safety and effectiveness of hydraulic fracturing operations related to activities such as geothermal development, evaluating the seismic risk induced by water injection, and further researching the precursory preparation process for deep fluid-driven or fluid-involved natural earthquakes. The publicly available dataset is expected to be used for various purposes, including 1) as training data for artificial intelligence related to microseismic data processing and analysis, 2) predicting the remaining time before rock fractures, and 3) establishing models and assessment methods for the relationship between microseismic characteristics and rock hydraulic properties, which will deepen our understanding of the interaction mechanisms between fluid migration and rock deformation and fracture.

Fractal Spatial Distributions of Initial Shear Stress and Frictional Properties on Faults and Their Impact on Dynamic Earthquake Rupture

ABSTRACT We investigate the influence of the heterogeneous slip-weakening distance (DC) in dynamic rupture simulations, in which DC is proportional to the fault irregularities. Specifically, we compare a heterogeneous fractal DC distribution to a uniform DC over the entire fault when the initial shear stress is also heterogeneous. We find that even small changes in the average value of DC (<1 mm) can lead to significant differences in the rupture evolution; that is, the average DC and the way DC is distributed determines if the rupture is a runaway, self-arrested, or nonpropagating. We find that the self-arrested ruptures differ from runaway ruptures in the amount of area characterized by large slips (asperities). Self-arrested ruptures match the Somerville et al. (1999) asperity criteria in which ∼25% of ruptured area radiate ∼45% of the total seismic moment. This criterion is not satisfied for runaway ruptures. For runaway ruptures, ∼50% of the ruptured area radiates about 70% of the seismic moment, indicating that the ruptured area is not linearly proportional to the seismic moment. Self-arrested ruptures are characterized by dynamic shear stress drops (SDs) in the range ∼2.9–5.5 MPa, whereas for runaway ruptures the dynamic SDs increase to values between ∼12 and 20 MPa. Self-arrested ruptures generated by fractal distributed DC resemble the rupture properties of observed earthquakes. In addition, results show that the conditions for self-arrested ruptures are connected to the decrease of residual energy at rupture boundaries.

Сейсмотектонические деформации и сброшенные напряжения землетрясений Центрального Тянь-Шаня

Based on the seismic moment tensor (SMT) data of 270 earthquakes in the Central Tien Shan, which occurred from 1978 to 2021 and include 63 events from the Global Centroid Moment Tensor catalog and 207 events from the works of Aleksander D. Kostyuk and Naylya A. Sycheva, the focal parameters (seismotectonic deformations (STD), kinematic and dynamic parameters, source radius, and stress drop) were calculated. The STD calculation was based on the approaches proposed in the studies of Yuriy V. Riznichenko and Sergei L. Yunga. The area under consideration is characterized by such deformation modes as thrust, transpression, underpression, and oblique. The distribution of the Lode–Nadai coefficient was calculated and plotted. A significant part of the study area is characterized by simple compression deformation, dominated by simple compression and simple shear. To calculate the stress drop , the values of the scalar seismic moment M0, which are determined in the SMT calculation, and the source radii r, which are calculated on the basis of theoretical and experimental models of the dependence of the source radius on the moment magnitude, were used. The radii r and stress drops  were calculated for two models of earthquake sources: the Brune model and the Madariaga–Kaneko–Shearer model. A catalog of dynamic parameters was compiled. The comparison of the kinematic and dynamic parameters of earthquakes was carried out, and the relationship of the dropped stresses with the type of movement in the source, as well as with the Lode–Nadai coefficient, was established. The results obtained in the study can be useful for specialists in other fields of knowledge – geodesy, geology, and geophysics.

Stochastic Ground-Motion Simulation of the 2021 Mw 5.9 Woods Point Earthquake: Facilitating Local Probabilistic Seismic Hazard Analysis in Australia

ABSTRACT The 2021 Mw 5.9 Woods Point event is the largest onshore earthquake that has occurred in the recorded history of southeastern Australia since European settlement. To study its source and ground-motion characteristics and to extract information for local seismic hazard analysis, we employ a stochastic finite-fault simulation approach to simulate ground motions for this event based on the observations collected from 36 onshore stations. We determine the regional distance-dependent attenuation parameters using the horizontal Fourier acceleration amplitude spectrum in the frequency range of 0.1–20 Hz. We parameterize path parameters using different models to consider uncertainties and sensitivities. To investigate local site effects, we construct a VS30-based site amplification model. Source parameters are then determined by fitting the theoretical Brune’s ω2 model with a reference Fourier source spectrum at 1.0 km. The κ0 value for the reference rock site is estimated as κ0=0.01 s, and dynamic stress drop is found to be 41.0 MPa by minimizing the overall absolute residual of 5% damped pseudospectral acceleration. We validate the simulations by comparing simulated and observed ground motions in terms of various intensity measurements; analyses of residuals show that the simulations are in good agreement with observations (average residual is close to 0). To facilitate future probabilistic seismic hazard analysis, six selected ground-motion models are ranked using the deviance information criteria based on an independent data set consisting of field observations and simulated ground motions.

Rise time of the 2018 MW 6.4 Hualien earthquake revealed by source time functions: A restrictive estimation of static stress drop

Using the ω−2 source model, the rise time of the 2018 Mw 6.4 Hualien earthquake is estimated from the corner frequency of the Fourier spectrum of the source time function. Two source parameters are also determined: average dynamic stress drop (∆σd) and static stress drop (∆σs). The results show that the rise time is 2.52 s, which is approximately 0.23 times the value of the entire source duration and then leads to a ∆σd of 3.03 MPa. In this study, we propose a method to confine the estimation of ∆σs through ∆σd. Because the radiated seismic energy exists, this leads to ∆σs<2∆σd−2μEgM0 (Eg is the fracture energy, M0 is the seismic moment, and μ is the rigidity) on average. Hence, we suggest that ∆σs for the 2018 Hualien earthquake should be less than 4.81 MPa. Because of ∆σs > ∆σd, the earthquake's rupture can be interpreted in a frictional overshoot model; then, the radiation efficiency near 0.5 suggests that 50% of the available energy is released to extend the fault. Furthermore, the lower fracture energy density (G) for the Hualien earthquake is probably due to the fact that an ML 5.9 foreshock disturbs the fault system beforehand.

A Source Physics Interpretation of Nonself-Similar Double-Corner-Frequency Source Spectral Model JA19_2S

AbstractWe investigate the relation between the kinematic double-corner-frequency source spectral model JA19_2S (Ji and Archuleta, 2020) and static fault geometry scaling relations proposed by Leonard (2010). We find that the nonself-similar low-corner-frequency scaling relation of JA19_2S model can be explained using the fault length scaling relation of Leonard’s model combined with an average rupture velocity ∼70% of shear-wave speed for earthquakes 5.3 &amp;lt; M&amp;lt; 6.9. Earthquakes consistent with both models have magnitude-independent average static stress drop and average dynamic stress drop around 3 MPa. Their scaled energy e˜ is not a constant. The decrease of e˜ with magnitude can be fully explained by the magnitude dependence of the fault aspect ratio. The high-frequency source radiation is generally controlled by seismic moment, static stress drop, and dynamic stress drop but is further modulated by the fault aspect ratio and the relative location of the hypocenter. Based on these two models, the commonly quoted average rupture velocity of 70%–80% of shear-wave speed implies predominantly unilateral rupture.

Assessing Margin-Wide Rupture Behaviors Along the Cascadia Megathrust With 3-D Dynamic Rupture Simulations.

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference.

Dynamic early warning of rockburst using microseismic multi-parameters based on Bayesian network

Rockburst is a phenomenon where the elastic deformation may be emitted suddenly because of rock fragmentation, ejection, launching, and even earthquake. This may result in casualties, failure or deformation of the support structure, and damage to field equipment. Therefore, early warning of rockburst is significant. In this paper, a dynamic early warning model of rockburst using microseismic multi-parameters based on Bayesian network is proposed. Taking the moment magnitude, seismic energy, source radius, apparent stress, and dynamic stress drop as Bayesian network parameters input. 114 sets of parameters required for Bayesian network structure learning are obtained by pre-processing the historical data of rockburst, and belief update is performed by the Junction Tree algorithm. Besides, the model passes self-validation, 6-fold cross-validation, ROC curve analysis and using new historical data to real-time early warning analysis. The results indicate that the proposed model has good precision and can effectively realize early warning of rockbursts. Furthermore, through the strength of the influence of parent–child nodes and sensitivity analysis, it can find that moment magnitude and seismic energy are the most influential parameters. They can as a significant reference for the early warning of rockburst.

Parametric analysis of the elastohydrodynamic lubrication efficiency on induced seismicity

SUMMARY During reservoir stimulations, the injection of fluids with variable viscosities can trigger seismicity. Several fault lubrication mechanisms have been invoked to explain the dynamic stress drop occurring during those seismic events. Here, we perform a parametric analysis of the elastohydrodynamic fault lubrication mechanism to assess its efficiency during fluid-induced earthquakes. The efficiency of the mechanism is measured with the dimensionless Sommerfeld number S. Accordingly, we analysed eight well-documented cases of induced seismicity associated with the injection of fluids whose viscosities range from 1 mPa s (water) to 100 mPa s (proppant). We collected information related to the in situ stress field, fault orientation and geometry, moment of magnitude and static stress drop of the events. These parameters allow us to analyse the variation in the Sommerfeld number. Our results show that the estimated dynamic friction on the fault during the event is compatible with the fault weakening predicted by the elastohydrodynamic lubrication theory, particularly for highly viscous fluids.

Numerical Study on Earthquake Energy Partitioning: Relationships Among Radiated Energy, Seismic Moment, and Stress Drop

AbstractThis paper aims at mechanistically studying some aspects of earthquake energy partitioning with a focus on the radiated energy (Es) and seismic moment (Mo) relationships for investigating differences between estimates of average dynamic stress drop ( ) and static stress drop ( ). We evaluate to what extent a relatively simple but analytically verified faulting simulation can explain such differences. We adopt a numerical methodology developed in the 2‐D Universal Distinct Element Code to simulate fault slip with slip‐weakening responses. A method is introduced for recording the ground reaction to slip and from which we discuss the energy partitioning in ideal cases of rupture. We examine a shallow strike‐slip fault model where a locally peaked stress is gradually developed on the fault by applying tectonic stresses away from the fault surface until a rupture is initiated locally and propagated outward. The rupture is terminated when the available energy is exhausted by the fracture energy and friction work especially as the rupture is followed by a creep. With investigating roles of the available energy for rupture and the fracture energy, we display limited cases where Mo does not, at least proportionately, scale with Es. Results show this is because Mo, compared to Es, does not fully represent the energy available for initiating a rupture and the fracture energy consumed during its propagation. That is why 2μEs/Mo and 2μEs/ηRMo estimates, with the radiation ratio ηR and rigidity μ, may significantly differ from and .

Origin of High‐Frequency Radiation During Laboratory Earthquakes

AbstractWe monitor dynamic rupture propagation during laboratory stick‐slip experiments performed on saw‐cut Westerly granite under upper crustal conditions (10–90 MPa). Spectral analysis of high‐frequency acoustic waveforms provided evidence that energy radiation is enhanced with stress conditions and rupture velocity. Using acoustic recordings band‐pass filtered to 400–800 kHz (7–14 mm wavelength) and high‐pass filtered above 800 kHz, we back projected high‐frequency energy generated during rupture propagation. Our results show that the high‐frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggest that the origin of high‐frequency radiation lies in the fast dynamic stress‐drop in the breakdown zone together with off‐fault coseismic damage propagating behind the rupture tip. The application of the back‐projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high‐frequency radiation.

How the Transition Region Along the Cascadia Megathrust Influences Coseismic Behavior: Insights From 2‐D Dynamic Rupture Simulations

AbstractThere is a strong need to model potential rupture behaviors for the next Cascadia megathrust earthquake. However, there exists significant uncertainty regarding the extent of downdip rupture and rupture speed. To address this problem, we study how the transition region (i.e., the gap), which separates the locked from slow‐slip regions, influences coseismic rupture propagation using 2‐D dynamic rupture simulations governed by a slip‐weakening friction law. We show that rupture propagation through the gap is strongly controlled by the amount of accumulated tectonic initial shear stress and gap friction level. A large amplitude negative dynamic stress drop is needed to arrest downdip rupture. We also observe downdip supershear rupture when the gradient in effective normal stress from the locked to slow‐slip regions is dramatic. Our results justify kinematic rupture models that extend below the gap and suggests the possibility of high‐frequency energy radiation during the next Cascadia megathrust earthquake.

Seismic b-Value for Foreshock AE Events Preceding Repeated Stick-Slips of Pre-Cut Faults in Granite

In this study, the b-values for acoustic emission (AE) events during stick-slip cycles of pre-cut faults in granite (as an analogue of unfavorably oriented immature faults) under triaxial compression (confining pressure: 40 MPa) are investigated. Using a multi-channel AE waveform recording system and two peak detectors, we recorded AE waveforms at 16 bits and at a sampling rate of 25 MHz, as well as the maximum amplitude of AE events with a dynamic range of 55 dB. For stick-slip events, the b-value decreases from 1.2 to 1.5 to approximately 0.6 as the shear stress increases, and then quickly jumps back to 1.0 to 1.3 immediately prior to the dynamic stress drop. The minimum b-value coincides with the maximum event rate and a stress level of 70 to 95% of the shear strength. It is also observed that the AE activity during each cycle was linked with the pre-failure fault slip, which accounts for 30% of the dynamic slip. Our results on b-value evaluation preceding repeated stick-slips can be used as an indicator of the degree of fault maturity and shear stress acting on the fault, which is important in seismic hazard assessment and earthquake prediction, especially for the injection-induced seismicity for fields in which reactivated shear rupture of unfavorable and immature faults or tensile fractures is important.

Dynamic Stress Drop for Selected Seismic Events at Rudna Copper Mine, Poland

In this paper, we report on an analysis of rupture processes of mining-induced events using the Empirical Green Function approach. The basic goal of this analysis is to estimate the dynamic stress drop—the quantity describing frictional forces at a slipping fault plane during the unstable part of an earthquake. The presented results cover 40 selected events with magnitude M_{rm{w}} ranging between 2.1 and 3.6 occurring in the Rudna copper mine (Poland) between 1996 and 2006. The spatial extent of the seismic network operated by the mine enabled us to estimate not only the dynamic stress drop but also the rupture velocities for most of the events studied. The results are analyzed in terms of correlations of the ratio of static to dynamic stress drop with other parameters characterizing the rupture processes. We also pose a question about the conditions of occurrence of over- and undershooting stress drop mechanisms. For all but two events, the estimated dynamic stress drop ranges between 0.1 and 4 MPa and is dominated by lower values. There are only two exceptions when it reaches the level of the order of 10 MPa. The reliability of these two exceptional cases is low, however. The rupture velocity, needed for dynamic stress drop calculation, was estimated from observed directivity effects. The obtained values ranged between 0.2 and almost 0.9 shear wave velocity, with dominating low velocities. The ratio of static to dynamic stress drop does not clearly correlate with the seismic scalar moment, source radius estimated according to Madariaga’s model, or static stress drop. It only positively correlates with the rupture velocity and ranges from values lower than 1 (undershooting mechanisms), mostly for slow events, up to values larger than 1 (overshooting mechanisms) for the majority of fast events. For all events with overshooting-type mechanisms, the estimated rupture velocity V_{rm{r}} was larger than half of the S-wave velocity V_{rm{s}}. On the other hand, for the undershooting-type events the associated rupture velocity was in most cases smaller than 0.6 V_{rm{s}} and large rupture speed was observed only in a few cases for this type of events.

From slow to fast faulting: recent challenges in earthquake fault mechanics.

Faults-thin zones of highly localized shear deformation in the Earth-accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 104 s-1) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

Prediction of the area affected by earthquake-induced landsliding based on seismological parameters

Abstract. We present an analytical, seismologically consistent expression for the surface area of the region within which most landslides triggered by an earthquake are located (landslide distribution area). This expression is based on scaling laws relating seismic moment, source depth, and focal mechanism with ground shaking and fault rupture length and assumes a globally constant threshold of acceleration for onset of systematic mass wasting. The seismological assumptions are identical to those recently used to propose a seismologically consistent expression for the total volume and area of landslides triggered by an earthquake. To test the accuracy of the model we gathered geophysical information and estimates of the landslide distribution area for 83 earthquakes. To reduce uncertainties and inconsistencies in the estimation of the landslide distribution area, we propose an objective definition based on the shortest distance from the seismic wave emission line containing 95 % of the total landslide area. Without any empirical calibration the model explains 56 % of the variance in our dataset, and predicts 35 to 49 out of 83 cases within a factor of 2, depending on how we account for uncertainties on the seismic source depth. For most cases with comprehensive landslide inventories we show that our prediction compares well with the smallest region around the fault containing 95 % of the total landslide area. Aspects ignored by the model that could explain the residuals include local variations of the threshold of acceleration and processes modulating the surface ground shaking, such as the distribution of seismic energy release on the fault plane, the dynamic stress drop, and rupture directivity. Nevertheless, its simplicity and first-order accuracy suggest that the model can yield plausible and useful estimates of the landslide distribution area in near-real time, with earthquake parameters issued by standard detection routines.

3D Ground‐Motion Simulations of M w 7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault Zone: Variability of Long‐Period ( T ≥1 s) Ground Motions and Sensitivity to Kinematic Rupture Parameters

Abstract We examine the variability of long‐period ( T ≥1 s) earthquake ground motions from 3D simulations of M w 7 earthquakes on the Salt Lake City segment of the Wasatch fault zone, Utah, from a set of 96 rupture models with varying slip distributions, rupture speeds, slip velocities, and hypocenter locations. Earthquake ruptures were prescribed on a 3D fault representation that satisfies geologic constraints and maintained distinct strands for the Warm Springs and for the East Bench and Cottonwood faults. Response spectral accelerations (SA; 1.5–10 s; 5% damping) were measured, and average distance scaling was well fit by a simple functional form that depends on the near‐source intensity level SA 0 ( T ) and a corner distance R c :SA( R , T )=SA 0 ( T )(1+( R / R c )) −1 . Period‐dependent hanging‐wall effects manifested and increased the ground motions by factors of about 2–3, though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault. Comparisons with modern ground‐motion prediction equations (GMPEs) found that the simulated ground motions were generally consistent, except within deep sedimentary basins, where simulated ground motions were greatly underpredicted. Ground‐motion variability exhibited strong lateral variations and, at some sites, exceeded the ground‐motion variability indicated by GMPEs. The effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors: distance to high‐slip subevents, dynamic stress drop, and changes in the contributions from directivity. These results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3D ground‐motion simulations employed in probabilistic seismic‐hazard analyses. Electronic Supplement: Description of seismic‐velocity model and wave propagation code and interpretation of directivity effects, and figures of fault geometry, Z1 and Z2.5 depths, effects of rupture speed variations on directivity effects and amplifications.

Dynamic rupture processes inferred from laboratory microearthquakes

AbstractWe report macroscopic stick‐slip events in saw‐cut Westerly granite samples deformed under controlled upper crustal stress conditions in the laboratory. Experiments were conducted under triaxial loading (σ1&gt;σ2=σ3) at confining pressures (σ3) ranging from 10 to 100 MPa. A high‐frequency acoustic monitoring array recorded particle acceleration during macroscopic stick‐slip events allowing us to estimate rupture speed. In addition, we record the stress drop dynamically and we show that the dynamic stress drop measured locally close to the fault plane is almost total in the breakdown zone (for normal stress &gt;75 MPa), while the friction f recovers to values of f &gt; 0.4 within only a few hundred microseconds. Enhanced dynamic weakening is observed to be linked to the melting of asperities which can be well explained by flash heating theory in agreement with our postmortem microstructural analysis. Relationships between initial state of stress, rupture velocities, stress drop, and energy budget suggest that at high normal stress (leading to supershear rupture velocities), the rupture processes are more dissipative. Our observations question the current dichotomy between the fracture energy and the frictional energy in terms of rupture processes. A power law scaling of the fracture energy with final slip is observed over 8 orders of magnitude in slip, from a few microns to tens of meters.

Properties of the shear stress peak radiated ahead of rapidly accelerating rupture fronts that mediate frictional slip

We study rapidly accelerating rupture fronts at the onset of frictional motion by performing high-temporal-resolution measurements of both the real contact area and the strain fields surrounding the propagating rupture tip. We observe large-amplitude and localized shear stress peaks that precede rupture fronts and propagate at the shear-wave speed. These localized stress waves, which retain a well-defined form, are initiated during the rapid rupture acceleration phase. They transport considerable energy and are capable of nucleating a secondary supershear rupture. The amplitude of these localized waves roughly scales with the dynamic stress drop and does not decrease as long as the rupture front driving it continues to propagate. Only upon rupture arrest does decay initiate, although the stress wave both continues to propagate and retains its characteristic form. These experimental results are qualitatively described by a self-similar model: a simplified analytical solution of a suddenly expanding shear crack. Quantitative agreement with experiment is provided by realistic finite-element simulations that demonstrate that the radiated stress waves are strongly focused in the direction of the rupture front propagation and describe both their amplitude growth and spatial scaling. Our results demonstrate the extensive applicability of brittle fracture theory to fundamental understanding of friction. Implications for earthquake dynamics are discussed.