Dynamically modeling fault step overs using various friction laws

Earthquakes can rupture geometrically complex fault systems by breaching fault step overs. Quantifying the likelihood of rupture jump across step overs is important to evaluate earthquake hazard and to understand the interactions between dynamic rupture and fault growth processes. Here we investigate the role of seismogenic depth and background stress on physical limits of earthquake rupture across fault step overs. Our computational and theoretical study is focused on the canonical case of two parallel strike‐slip faults with large aspect ratio, uniform prestress and friction properties. We conduct a systematic set of 3‐D dynamic rupture simulations with different seismogenic depth, step over distance, and initial stresses. We find that the maximum step over distance Hc that a rupture can jump depends on seismogenic depth W and strength excess to stress drop ratio S, commonly used to evaluate probable rupture velocity, as Hc0.3em∝W/Sn, where n = 2 when Hc/W < 0.2 (or S > 1.5) and n = 1 otherwise. The critical nucleation size, largely controlled by frictional properties, has a second‐order effect on Hc. Rupture on the secondary fault is mainly triggered by the stopping phase emanated from the rupture end on the primary fault. Asymptotic analysis of the peak amplitude of stopping phases sheds light on the mechanical origin of the relations between Hc, W, and S, and leads to the scaling regime with n = 1 in far field and n = 2 in near field. The results suggest that strike‐slip earthquakes on faults with large seismogenic depth or operating at high shear stresses can jump wider step overs than observed so far in continental interplate earthquakes.

Section: Simulation Resultssupporting

confidence: 57%

Effect of Seismogenic Depth and Background Stress on Physical Limits of Earthquake Rupture Across Fault Step Overs

Bai

Ampuero

2017

“…[] also noticed the occurrence of supershear rupture on an extensional step over. Based on 2‐D numerical simulations, Ryan and Oglesby [] found that ruptures across step overs could lead to a supershear transition and showed that the minimum step width required for sustained supershear ruptures on compressional regimes obeying linear slip‐weakening law was 0.6 km if the initial S value was 2.6 on the step overs. However, according to the critical jump distance study by Hu et al .…”

Section: Introductionmentioning

confidence: 99%

Supershear transition mechanism induced by step over geometry

Zhang

et al. 2016

Few studies have focused on the supershear transition mechanism induced by fault step overs, although seismic observations suggest that rupture speed transitions occur at geometrical complexities on faults. Based on dynamic rupture simulations on fault systems with step overs in a 3‐D full space where the initial stresses preclude a supershear transition on a single buried fault according to the Burridge‐Andrews mechanism, we show that rupture speeds can transit from subshear on the primary fault to supershear on the secondary fault. The low normal stress zone and the high shear stress zone beyond the fault step, which radiate from the end of the primary fault if its rupture arrest is sudden, determine the supershear rupture occurrence on the secondary fault. However, a low shear stress zone traveling at the shear wave speed is also radiated, making the rupture speed return to subshear in most cases. Sustained supershear ruptures are also possible on compressional step overs under certain conditions. Self‐arresting ruptures are observed in the overlap area on the secondary fault. In a half‐space model where supershear rupture is induced by the free surface on the primary fault, the rupture speed on the secondary fault rapidly transits to subshear near the fault step if its width exceeds a critical value.

“…However, earthquake ruptures involve high slip rates at which microscopic asperities in the thin slip zone may experience transient heating that thermally weakens the frictional strength. A modified model based on rate‐ and state‐friction to better fit this flash heating phenomenon is proposed (Beeler et al, ; Rice, , ) and applied in rupture modeling (e.g., Rojas et al, ; Ryan & Oglesby, ). In this model, the steady state friction coefficient f ss defined in the RS‐S law is retained for low slip rate condition and is renamed low‐velocity steady state friction coefficient f LV .…”

Section: Functional Forms Of Various Friction Lawsmentioning

confidence: 99%

“…The stress‐slip curves for the RS‐S and RS‐FH laws, however, show an exponentially decaying pattern as the shear stress gradually evolves from the yield strength to the dynamic friction with respect to the slip. One way to compare this group with the linearly weakening group is to compare fracture energy among all the friction laws (Ampuero & Rubin, ; Ryan & Oglesby, ), since the fracture energy is critical for self‐sustaining rupture propagation and determines rupture speed (Fukuyama & Madariaga, ). As a lumped parameter representing all kinds of energy dissipated in the cohesive zone, the fracture energy is defined as the work done against the friction excess [ τ ( δ ) − τ d ] over the critical distance d 0 :

g_{c} = \int_{0}^{d_{0}} ([], τ ((), δ) - τ_{d}) italicdδ

where τ ( δ ) is the friction as a function of slip.…”

Section: Comparing Frictional Behaviors Of Various Friction Lawsmentioning

confidence: 99%

Dynamics of Nonplanar Thrust Faults Governed by Various Friction Laws

Luo

Duan

2018

Large‐scale irregular geometry of a fault surface, such as seamounts and subducted oceanic plateaus, is a prevailing factor that introduces stress heterogeneity and affects rupture dynamics. However, dynamic ruptures are also controlled by the constitutive laws that describe how fault friction evolves. In this study, we explicitly incorporate nonplanar thrust fault geometry into a three‐dimensional finite element model to numerically simulate spontaneous dynamic rupture and explore how dynamic ruptures would behave on a nonplanar fault surface under the influence of different friction laws. Our results show that a subducted oceanic relief (a bump) could act as a rupture barrier with its high strength area being unfavorable for rupture propagation, and such a geometrical effect varies with the dimension of a bump. When a bump is too high, the dynamic rupture is completely arrested. When the bump is too low, the rupture is barely affected. When the bump is of an intermediate height, the rupture is obstructed by the bump and splits into two parts, which circumvent the high strength area of the bump and then converge on the other side, triggering a strong slip velocity pulse. The relation between these rupture behaviors and bump geometry under a given prestress condition varies with the specific forms of friction law, which determines how fast a rupture releases energy and how strong a bump can be as a barrier.