Rupture Front Research Articles

Predictive performance of current ground motion models for recorded strong motions in 2020 Samos Earthquake

The 2020 M7.0 Samos earthquake had occurred on the north of Samos Island; however, structural damage was observed in İzmir-Bayraklı, which is located approximately 65 km away from the epicenter. Strong ground motions recorded in İzmir Bay showed unique site amplifications, mostly due to the interaction between the basin and deep alluvial deposit response. The objective of this study is to evaluate the predictive performance of current ground motion models (GMMs) for estimating the recorded strong motions, especially the recordings over or near the Bayraklı-Bornova basin. 66 strong motion stations from Turkey with rupture distance (RRUP) < 200 km are used in the residual analysis, considering the ambiguities in the magnitude and depth to the top of the rupture estimations. Event terms of the earthquake for tested GMMs are found to be small and lie within the expected scatter, except for T = 0.5–1.5 s spectral accelerations. Event-specific distance attenuation for RRUP<100 km is consistent with the median predictions of current GMMs; however, the distance scaling for 100 km<RRUP<200 km are significantly different at high frequencies, indicating faster attenuation for Southwestern Anatolia. Relatively long period (0.5–1.5 s) spectral energy is present in both soft sites on the Bayraklı-Bornova basin and rock/stiff-soil sites on the basin edge and these stations contribute significantly to the positive event terms at T = 0.5–1.5 s. For sites within the basin or close to the basin edge, factors such as the direction of the rupture front, basin width and depth, and the seismic structure of the basin contribute to the ground motion variability.

Energy Partitioning, Dynamic Fragmentation, and Off‐Fault Damage in the Earthquake Source Volume

AbstractSeismological fracture or breakdown energy represents energy expended in a volume surrounding the advancing rupture front and the slipping fault surface. Estimates are commonly obtained by inverting ground motions and using the results to model slip on the fault surface. However, this practice cannot identify contributions from different energy‐consumption processes, so our understanding of the importance of these processes comes largely from field‐ and laboratory‐based studies. Here, we use garnet fragment size data to estimate surface‐area energy density with distance from the fault core in the damage zone of a deeply exhumed strike‐slip fault/shear zone. Estimated energy densities per fragmentation event range from 2.87 × 103 to 2.72 × 105 J/m3 in the outer and inner portions of the dynamic damage zone, respectively, with the dynamic zone being inferred from the fractal dimensions of fragment size distributions and other indicators. Integrating over the ∼105 m width of the dynamic damage zone gives fracture surface‐area energy per unit fault area ranging from a lower bound of 6.63 × 105 J/m2 to an upper bound of 1.63 × 107 J/m2 per event. This range overlaps with most geological, theoretical, and kinematic slip‐model estimates of energy expenditure in the source volume for earthquakes characterized by seismic moments &gt;1017 N·m. We employ physics‐based fragmentation models to estimate equivalent tensile strain rates associated with garnet fragmentation, which range from 5.42 × 102 to 1.04 × 104 s−1 per earthquake in the outer and inner portions of the dynamic damage zone, respectively. Our results suggest that surface‐energy generation is a nonnegligible component of the earthquake energy budget.

The effect of along-strike variation in dip on rupture propagation on strike-slip faults

AbstractStrike-slip faults can be nonplanar in both their strike and dip dimensions. While a large body of work has investigated the effects of changes in strike on earthquake rupture and arrest, no previous studies have investigated the role of along-strike variations in dip on strike-slip ruptures. Here, I use the three-dimensional finite-element method to conduct dynamic simulations of ruptures on strike-slip faults with linear surface traces and changes in dip along strike. I experiment with the amount of dip change as well as the abruptness of that change under a variety of initial stress conditions. In all of my initial stress cases, I find that a change in dip along strike can cause rupture to stop, and that larger dip changes over shorter distances are more likely to do so. This is largely due to the change in strike at depth that inherently comes from changing the dip; the majority of these behaviors are a result of the rupture front being forced to change direction mid-rupture. While some dip-slip movement does occur on the nonvertical parts of the model fault, it does not have a significant effect on rupture extent. However, linear-surface-trace, nonvertical-dip faults do produce different surface slip, stress, and ground motion patterns compared to corresponding nonlinear-strike, vertical-dip faults. Together, my results show that changes in dip along strike-slip faults do considerably impact the rupture process, suggesting that this type of geometrical complexity should be considered in rupture forecasts and hazard assessments.

Asymmetric Topography Causes Normal Stress Perturbations at the Rupture Front: The Case of the Cajon Pass

AbstractWe use 3D dynamic rupture simulations to discover a previously un‐described effect of asymmetric topography on the earthquake process. With the Cajon Pass along the San Andreas Fault as an example, we find that asymmetric topography generates an alternating normal stress perturbation around the rupture front, near the free surface. When topography lies to the right of the propagating right‐lateral front, the normal stress perturbation is clamping ahead of the rupture front and unclamping behind. When topography alternates to the left, the perturbation reverses sign. The process is analogous to the normal stress variations on dip‐slip faults. While this effect does not strongly affect rupture propagation and slip in our current parametrization, it requires explanation and exploration. An understanding of the normal stress perturbation due to asymmetric topography will help prevent its misattribution to other sources and lead to a better understanding of the interplay of multiple processes during earthquakes.

Thermally activated intermittent dynamics of creeping crack fronts along disordered interfaces

We present a subcritical fracture growth model, coupled with the elastic redistribution of the acting mechanical stress along rugous rupture fronts. We show the ability of this model to quantitatively reproduce the intermittent dynamics of cracks propagating along weak disordered interfaces. To this end, we assume that the fracture energy of such interfaces (in the sense of a critical energy release rate) follows a spatially correlated normal distribution. We compare various statistical features from the obtained fracture dynamics to that from cracks propagating in sintered polymethylmethacrylate (PMMA) interfaces. In previous works, it has been demonstrated that such an approach could reproduce the mean advance of fractures and their local front velocity distribution. Here, we go further by showing that the proposed model also quantitatively accounts for the complex self-affine scaling morphology of crack fronts and their temporal evolution, for the spatial and temporal correlations of the local velocity fields and for the avalanches size distribution of the intermittent growth dynamics. We thus provide new evidence that an Arrhenius-like subcritical growth is particularly suitable for the description of creeping cracks.

Self-similar fault slip in response to fluid injection

There is scientific and industrial interest in understanding how geologic faults respond to transient sources of fluid. Natural and artificial sources can elevate pore fluid pressure on the fault frictional interface, which may induce slip. We consider a simple boundary value problem to provide an elementary model of the physical process and to provide a benchmark for numerical solution procedures. We examine the slip of a fault that is an interface of two elastic half-spaces. Injection is modelled as a line source at constant pressure and fluid pressure is assumed to diffuse along the interface. The resulting problem is an integro-differential equation governing fault slip, which has a single dimensionless parameter. The expansion of slip is self-similar and the rupture front propagates at a factor$\lambda$of the diffusive length scale$\sqrt {\alpha t}$. We identify two asymptotic regimes corresponding to$\lambda$being small or large and perform a perturbation expansion in each limit. For large$\lambda$, in the regime of a so-called critically stressed fault, a boundary layer emerges on the diffusive length scale, which lags far behind the rupture front. We demonstrate higher-order matched asymptotics for the integro-differential equation, and in doing so, we derive a multipole expansion to capture successive orders of influence on the outer problem for fault slip for a driving force that is small relative to the crack dimensions. Asymptotic expansions are compared with accurate numerical solutions to the full problem, which are tabulated to high precision.

Estimating Rupture Front of Large Earthquakes Using a Novel Multi-Array Back-Projection Method

A ruptured front obtained from high-frequency energy radiation is the key to understand the complex source. It is commonly observed that rupture fronts derived from different arrays often show some variations due to the obvious difference of the positioning accuracy of the far-field array between the azimuth and the epicentral distance. We developed a new multi-array back-projection method based on the classical back-projection method and applied the method to the 2015 MW7.8 Nepal earthquake. The back azimuth information with small error is separated from the classical back-projection results, and the azimuth intersection of multiple arrays is used to obtain more accurate spatial and temporal distribution information of the source rupture fronts.

Coseismic fault lubrication by viscous deformation

Despite the hazard posed by earthquakes, we still lack fundamental understanding of the processes that control fault lubrication behind a propagating rupture front and enhance ground acceleration. Laboratory experiments show that fault materials dramatically weaken when sheared at seismic velocities (>0.1 m s−1). Several mechanisms, triggered by shear heating, have been proposed to explain the coseismic weakening of faults, but none of these mechanisms can account for experimental and seismological evidence of weakening. Here we show that, in laboratory experiments, weakening correlates with local temperatures attained during seismic slip in simulated faults for diverse rock-forming minerals. The fault strength evolves according to a simple, material-dependent Arrhenius-type law. Microstructures support this observation by showing the development of a principal slip zone with textures typical of sub-solidus viscous flow. We show evidence that viscous deformation (at either sub- or super-solidus temperatures) is an important, widespread and quantifiable coseismic lubrication process. The operation of these highly effective fault lubrication processes means that more energy is then available for rupture propagation and the radiation of hazardous seismic waves. Viscous deformation is a potentially prevalent mechanism of fault lubrication during earthquakes, according to laboratory experiments that simulate seismic faulting of various rock-forming minerals.

Nucleation of frictional sliding by coalescence of microslip

The onset of frictional motion is mediated by the dynamic propagation of a rupture front, analogous to a shear crack. The rupture front nucleates quasi-statically in a localized region of the frictional interface and slowly increases in size. When it reaches a critical nucleation length it becomes unstable, propagates dynamically and eventually breaks the entire interface, leading to macroscopic sliding. The nucleation process is particularly important because it determines the stress level at which the frictional interface fails, and therefore, the macroscopic friction strength. However, the mechanisms governing nucleation of frictional rupture fronts are still not well understood. Specifically, our knowledge of the nucleation process along a heterogeneous interface remains incomplete. Here, we study the nucleation of localized slip patches on linear slip-weakening interfaces with deterministic and stochastic heterogeneous friction properties. Using numerical simulations, we analyze the process leading to a slip patch of critical size for systems with varying correlation lengths of the local friction strength. Our deterministic interface model reveals that the growth of the critical nucleation patch at interfaces with small correlation lengths is non smooth due to the coalescence of neighboring slip patches. Existing analytical solutions do not account for this effect, which leads to an overestimation of global interface strength. Conversely, when the correlation length is large, the growth of the slip patch is continuous and our simulations match the analytical solution. Furthermore, nucleation by coalescence is also observed on stochastic interfaces with small correlation length. In this case, the applied load for a given slip patch size is a random variable. We show that its expectation follows a logistic function, which allows us to predict the strength of the interface well before failure occurs. Our model and observations provide new understanding of the nucleation process and its effect on the static frictional strength.

Coseismic slickenlines record the emergence of multiple rupture fronts during a surface-breaking earthquake

Coseismic changes in slip direction recorded by curved slickenlines on fault surfaces are commonly observed following surface-breaking earthquakes. Such observations represent a dynamic record of seismic slip and may provide a new set of constraints on the evolution of propagating rupture and hence earthquake dynamics. We test this hypothesis by conducting dynamic rupture simulations of the 2011 Mw 6.6 Fukushima-Hamadori Earthquake (Japan). These simulations aim to reproduce the well-documented field observations of curved slickenlines that formed during coseismic fault displacement at the ground surface. We consider relatively simple dynamic rupture models with a dipping fault embedded into a homogeneous or layered elastic halfspace. Among a wide range of model parameters tested, we find a model with shallow (<1.5km) low-velocity layers derived from a regional 3D velocity model combined with a depth-dependent prestress result in curved slip trajectories that closely match slickenline observations. This same model also generates a slip distribution and rupture propagation direction consistent with published inversions constrained by seismological and geodetic data. Furthermore, the characteristics of slickenline curvature are consistent with theoretically predicted earthquake rupture direction. Unlike previous theoretical studies, coseismic changes in rake angle occur due to the emergence of multiple slip fronts within the shallow low-velocity medium. Our results indicate that on-fault geological observations can supplement seismological studies of earthquake rupture evolution beyond traditional datasets.

SIMULATION OF BEHAVIOR OF FUNCTIONALLY - HETEROGENEOUS MATERIALS UNDER TEMPERATURE LOADS

The paper considers the issue of nonlinear mathematical modeling of functionally inhomogeneous materials at temperature loads. The proposed model makes it possible to describe the thermo-pseudo-plastic behavior of the material at the point. The diagram of pseudo-elastic material consisting of three curvilinear sections is used. This approach leads to an unstable stress-strain diagram, and to describe the thermo-mechanical behavior of samples of different shapes, it is necessary to have a solution of the boundary value problem taking into account the development of the deformation front of the phase transformation. This takes into account not only the ambient temperature, but also the heat released at the point during the phase transition. A numerical procedure for calculating a material diagram has been developed, which is a curve enveloping a family of material diagrams constructed for certain laws of change in the velocity of the deformation rupture front. An integrated diagram of the material under the influence of a complex load is constructed.Keywords: phenomenological model, nonlinear material model, materials with shape memory, thermo-pseudo-plasticity, numerical procedure for calculating the diagram.

Fault‐Zone Damage Promotes Pulse‐Like Rupture and Back‐Propagating Fronts via Quasi‐Static Effects

AbstractDamage zones are ubiquitous components of faults that may affect earthquake rupture. Simulations show that pulse‐like rupture can be induced by the dynamic effect of waves reflected by sharp fault zone boundaries. Here we show that pulses can appear in a highly damaged fault zone even in the absence of reflected waves. We use quasi‐static scaling arguments and quasi‐dynamic earthquake cycle simulations to show that a crack turns into a pulse after the rupture has grown larger than the fault zone thickness. Accompanying the pulses, we find complex rupture patterns involving back‐propagating fronts that emerge from the primary rupture front. Our model provides a mechanism for back‐propagating fronts recently observed during large earthquakes. Moreover, we find that slow‐slip simulations in a highly compliant fault zone also produce back‐propagating fronts, suggesting a new mechanism for the rapid tremor reversals observed in Cascadia and Japan.

Surface Rupture Kinematics and Coseismic Slip Distribution during the 2019 Mw7.1 Ridgecrest, California Earthquake Sequence Revealed by SAR and Optical Images

The 2019 Ridgecrest, California earthquake sequence ruptured along a complex fault system and triggered seismic and aseismic slips on intersecting faults. To characterize the surface rupture kinematics and fault slip distribution, we used optical images and Interferometric Synthetic Aperture Radar (InSAR) observations to reconstruct the displacement caused by the earthquake sequence. We further calculated curl and divergence from the north-south and east-west components, to effectively identify the surface rupture traces. The results show that the major seismogenic fault had a length of ~55 km and strike of 320° and consisted of five secondary faults. On the basis of the determined multiple-fault geometries, we inverted the coseismic slip distributions by InSAR measurements, which indicates that the Mw7.1 mainshock was dominated by the right-lateral strike-slip (maximum strike-slip of ~5.8 m at the depth of ~7.5 km), with a small dip-slip component (peaking at ~1.8 m) on an east-dipping fault. The Mw6.4 foreshock was dominated by the left-lateral strike-slip on a north-dipping fault. These earthquakes triggered obvious aseismic creep along the Garlock fault (117.3° W–117.5° W). These results are consistent with the rupture process of the earthquake sequence, which featured a complicated cascading rupture rather than a single continuous rupture front propagating along multiple faults.

Initial effective stress controls the nature of earthquakes

Modern geophysics highlights that the slip behaviour response of faults is variable in space and time and can result in slow or fast ruptures. However, the origin of this variation of the rupture velocity in nature as well as the physics behind it is still debated. Here, we first highlight how the different types of fault slip observed in nature appear to stem from the same physical mechanism. Second, we reproduce at the scale of the laboratory the complete spectrum of rupture velocities observed in nature. Our results show that the rupture velocity can range from a few millimetres to kilometres per second, depending on the available energy at the onset of slip, in agreement with theoretical predictions. This combined set of observations bring a new explanation of the dominance of slow rupture fronts in the shallow part of the crust or in areas suspected to present large fluid pressure.

Parameter interdependence of dynamic self-similar crack with distance-weakening friction

SUMMARY In several analytical and numerical studies, the slip rate function and energy release rate for dynamic self-similar crack growth have been investigated, and the results obtained have contributed to a theoretical understanding and estimation of on-fault energetics. However, the relationships among physical parameters, including stress state, process zone size, rupture velocity, peak slip rate and energy release rate, are still unclear. Therefore, the aim of this study is to derive an analytical solution of the slip rate distribution of antiplane self-similar crack growth under distance-weakening friction that mimics slip-weakening friction. To satisfy the condition that the slip rate starts from zero at the rupture front, a trade-off relationship among rupture velocity, process zone size and breakdown stress-to-stress drop ratio is proposed. The peak slip rate, slip-weakening distance and fracture energy obtained using the proposed model provide a possible mechanism for the determination of the rupture velocity and the estimation of the fracture energy of the self-similar crack growth, based on the seismic observables.

Ground motion signatures of supershear ruptures in the Burridge-Andrews and free-surface-induced mechanisms

We analyze the ground motions of supershear ruptures in the Burridge-Andrews (BAM) and free-surface-induced (FSI) mechanisms. BAM supershear ruptures require higher initial shear stress than FSI supershear ruptures, thus cause faster rupture speed. The supershear slip pulse initiates at the free surface or upper boundary of a fault for FSI and at the hypocenter depth in the in-plane direction for BAM in homogeneous models. Both BAM and FSI supershear ruptures can generate Mach waves and the Rayleigh wave field. The BAM and the faster FSI supershear ruptures can make stronger shear Mach waves. The shear Mach waves show a larger amplitude of negative component for a faster FSI supershear rupture. The FSI supershear ruptures make the strongest dilatational wave next to the fault while it is at a distance away from the fault for the BAM supershear ruptures. The different shapes of supershear rupture front, locations of supershear triggered and the rupture speeds contribute to these contrasts. And we find that the BAM and the faster FSI supershear ruptures can trigger more high-frequency radiation due to the sharper and stronger shear Mach waves. Finally, the comparisons between the simulated seismograms and the records of PS10 indicate that the 2002 Denali earthquake may represent a supershear one with the FSI mechanism and a slow supershear rupture speed.

Inchworm-like source evolution through a geometrically complex fault fueled persistent supershear rupture during the 2018 Palu Indonesia earthquake

How does fault slip follow an earthquake rupture front propagating faster than the local shear-wave velocity (i.e., at supershear speed)? How does a supershear rupture front pass through a geometrically complex fault system? Resolving the evolution of such complex earthquake ruptures is fundamental to our understanding of earthquake-source physics, but these events have not been well captured by conventional waveform inversions of observational data. We applied a new framework of finite-fault inversion to globally observed teleseismic waveforms and resolved both the spatiotemporal evolution of slip and the fault geometry of the 2018 Palu earthquake (moment magnitude 7.6) in Sulawesi, Indonesia. We show that supershear rupture propagation for this event was sustained by transient slip stagnation and advancement as the rupture front passed through the geometrically complex fault system. This peculiar inchworm-like slip evolution was caused by the rupture front encountering fault bends with favorable and unfavorable orientations for rupture propagation. Our analysis also identified the possible existence of a fault junction beneath Palu Bay connecting an unmapped primary fault in northern Sulawesi with the Palu-Koro fault in the south.

Stress Perturbation From Aseismic Slip Drives the Seismic Front During Fluid Injection in a Permeable Fault

AbstractFluid pressure changes affect fault stability and can promote the initiation of earthquakes and aseismic slip. However, the relationship between seismic and aseismic fault slip during fluid injection remains poorly understood. Here, we investigate, through 3‐D hydromechanical modeling, the spatiotemporal evolution of seismicity and aseismic slip on a permeable, slip‐weakening fault subjected to a local injection of fluid, under different prestress conditions. The model results in an expanding aseismic slip region, which concentrates shear stress at its edge and triggers seismicity. The aseismic slip dominates the slip budget, whatever the initial fault stress. We find that the seismicity is collocated with the aseismic rupture front rather than with the fluid pressure diffusion front. On faults initially far from failure, the aseismic rupture front is located behind or at the pressure front. On faults initially closer to failure, the model predicts that both the rupture front and the seismicity outpace the pressurized zone, resulting in a sharp increase of the migration velocity and released moment of the seismicity. Insights gained from this modeling study exhibit various features that are observed in sequences of induced earthquakes in both field experiments and natural reservoir systems and can help guide the interpretation of past and future observations of induced seismicity.

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.

Effects of Barriers on Fault Rupture Process and Strong Ground Motion Based on Various Friction Laws

A barrier may induce a supershear rupture on a fault. This paper focuses on two questions: One is whether the existence of a barrier accelerates the propagation speed of a whole fault rupture, and the other is what are the effects of friction laws and strength of a barrier on the rupture propagation process. For these purposes, classical slip-weakening, rate-state, and modified slip-weakening friction laws are employed to simulate the effect of a barrier on the fault rupture process. The simulation results showed that the rupture speed of the fault obviously decreases when the rupture front propagates to the barriers, and the rupture speed obviously increases when the rupture front leaves barriers. It was also found that a barrier on a fault may induce a supershear rupture via the rate-state friction law. The simulation results also showed that with the increase of barrier strength, the rupture speed near barriers fluctuates more and more; when the barrier strength exceeds a certain level, a supershear rupture area appears on the fault; with the increase of barrier strength, the propagation distance of the rupture at supershear wave velocity correspondingly increases. In addition, with the increase of barrier strength, the overall rupture duration of the fault slightly increases. This indicates that a barrier cannot shorten the total duration of a fault rupture. Though a barrier will lead to a supershear rupture, it just regulates the distribution of the rupture speed on the fault surface. Moreover, with the increase of barrier strength, the peak ground acceleration caused by rupture through the barrier also increases, indicating that the existence of a barrier may lead to the intensification of seismic hazards.