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1. Modeling the 3D dynamic rupture of microearthquakes induced by fluid injection

   https://doi.org/10.22541/essoar.171926490.05456326/v1  

   MOSCONI, Francesco; Tinti, Elisa; Casarotti, Emanuele; Gabriel, Alice-Agnes; Rinaldi, Antonio Pio; Zilio, Luca Dal; Cocco, Massimo (June 2024, ESS Open Archive)

   Understanding the dynamics of microearthquakes is a timely challengewith the potential to address current paradoxes in earthquake mechanics,and to better understand earthquake ruptures induced by fluid injection.We perform fully 3D dynamic rupture simulations caused by fluidinjection on a target fault for FEAR experiments generating Mw ≤ 1earthquakes. We investigate the dynamics of rupture propagation withspatially variable stress drop caused by pore pressure changes andassuming different constitutive parameters. We show that the spontaneousarrest of propagating ruptures is possible by assuming a high faultstrength parameter S, that is, a high ratio between strength excess anddynamic stress drop. In faults with high S values (low rupturingpotential), even minor variations in Dc (from 0.45 to 0.6 mm) have asubstantial effect on the rupture propagation and the ultimateearthquake size. Our results show that modest spatial variations ofdynamic stress drop determine the rupture mode, distinguishingself-arresting from run-away ruptures. Our results suggest that severalcharacteristics inferred for accelerating dynamic ruptures differ fromthose observed during rupture deceleration of a self-arrestingearthquake. During deceleration, a decrease of peak slip velocity isassociated with a nearly constant cohesive zone size. Moreover, theresidual slip velocity value (asymptotic value for a crack-like rupture)decreases to nearly zero. This means that an initially crack-likerupture becomes a pulse-like rupture during spontaneous arrest. Insummary, our findings highlight the complex dynamics of smallearthquakes, which are partially contrasting with established crack-likemodels of earthquake rupture. 

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2. The earthquake arrest zone

   https://doi.org/10.1093/gji/ggaa386  

   Ke, Chun-Yu; McLaskey, Gregory C; Kammer, David S (November 2020, Geophysical Journal International)

   null (Ed.)

   SUMMARY Earthquake ruptures are generally considered to be cracks that propagate as fracture or frictional slip on pre-existing faults. Crack models have been used to describe the spatial distribution of fault offset and the associated static stress changes along a fault, and have implications for friction evolution and the underlying physics of rupture processes. However, field measurements that could help refine idealized crack models are rare. Here, we describe large-scale laboratory earthquake experiments, where all rupture processes were contained within a 3-m long saw-cut granite fault, and we propose an analytical crack model that fits our measurements. Similar to natural earthquakes, laboratory measurements show coseismic slip that gradually tapers near the rupture tips. Measured stress changes show roughly constant stress drop in the centre of the ruptured region, a maximum stress increase near the rupture tips and a smooth transition in between, in a region we describe as the earthquake arrest zone. The proposed model generalizes the widely used elliptical crack model by adding gradually tapered slip at the ends of the rupture. Different from the cohesive zone described by fracture mechanics, we propose that the transition in stress changes and the corresponding linear taper observed in the earthquake arrest zone are the result of rupture termination conditions primarily controlled by the initial stress distribution. It is the heterogeneous initial stress distribution that controls the arrest of laboratory earthquakes, and the features of static stress changes. We also performed dynamic rupture simulations that confirm how arrest conditions can affect slip taper and static stress changes. If applicable to larger natural earthquakes, this distinction between an earthquake arrest zone (that depends on stress conditions) and a cohesive zone (that depends primarily on strength evolution) has important implications for how seismic observations of earthquake fracture energy should be interpreted. 

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3. Back‐Propagating Earthquakes on Simple Faults

   https://doi.org/10.1029/2025AV001649  

   Sun, Yudong; Cattania, Camilla (February 2026, AGU Advances)

   Back‐propagating earthquakes, characterized by a secondary front reversing into previously ruptured areas, challenge conventional models of rupture dynamics and have been increasingly documented in recent years. The conditions for this phenomenon remain poorly understood: while rupture complexity is often attributed to fault heterogeneity, back‐propagating fronts have also been reported on simple faults. Here, we employ earthquake simulations with slip‐rate and state‐dependent friction to reveal that back‐propagating fronts arise spontaneously during unilateral rupture propagation on frictionally homogeneous faults, when the rupture exceeds a critical length about 100 times larger than the nucleation dimension. We propose simple theoretical arguments suggesting that back‐propagating fronts are an intrinsic feature of unilateral ruptures under velocity‐weakening friction. These conditions preclude self‐similar crack‐like solutions with constant stress drop; instead, ruptures alternate between pulse and crack‐like modes, with back propagation generated at pulse‐to‐crack transitions. The crack‐to‐pulse transition is driven by restrengthening and happens when the local slip rate decreases below a specified threshold. An analytical criterion for the extent of back propagation shows that they are enhanced by low rupture velocities and stress drops, and observational examples of back propagation on simple faults are consistent with these requirements. Our study presents a simple, fundamental model for back‐propagating earthquakes, suggesting that they may be more prevalent than previously recognized. 

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4. Dynamic restrengthening and fault heterogeneity explain megathrust earthquake complexity

   https://doi.org/10.1038/s41467-026-71722-3  

   Wong, Jeremy_Wing Ching; Gabriel, Alice-Agnes; Fan, Wenyuan (April 2026, Nature Communications)

   Abstract Megathrusts host Earth’s largest earthquakes. Understanding the physical conditions controlling their rupture dynamics is critical for assessing seismic and tsunami hazards. These earthquakes often display complex rupture dynamics, exemplified by the 2011 Tohoku-Oki earthquake, which exhibited multiple rupture episodes, depth-dependent seismic radiation, and substantial tsunamigenic slip near the trench. However, how such complexity arises from preexisting physical conditions remains uncertain. Here, we demonstrate that the observed rupture complexity of the Tohoku-Oki earthquake can spontaneously and self-consistently emerge, driven by rapid coseismic frictional restrengthening and data-informed fault heterogeneity. We use an ensemble of 3D dynamic rupture simulations to identify that mixed downdip pulse-like and updip crack-like rupture are driven by dynamic stress redistribution with episodic rupture reactivation. By featuring low fault strength compared to its dynamic stress drop, a preferred model can consistently reproduce the observed complex depth-dependent propagation speeds, multiple rupture fronts as imaged by back-projection, and large tsunamigenic slip at the trench. Our findings demonstrate that preexisting fault heterogeneity conjointly with dynamic frictional weakening and restrengthening drives seemingly unexpected megathrust rupture complexity, highlighting the need to include dynamic effects into physics-based seismic and tsunami hazard assessments of future earthquakes. 

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5. Ground-Motion Characteristics of Cascading Earthquakes in a Multiscale Fracture Network

   https://doi.org/10.1785/0120240281  

   Palgunadi, Kadek Hendrawan; Gabriel, Alice-Agnes; Garagash, Dmitry Igor; Ulrich, Thomas; Schliwa, Nico; Mai, P Martin (June 2025, Bulletin of the Seismological Society of America)

   ABSTRACT Fault zones exhibit geometrical complexity and are often surrounded by multiscale fracture networks within their damage zones, potentially influencing rupture dynamics and near-field ground motions. In this study, we investigate the ground-motion characteristics of cascading ruptures across damage zone fracture networks of moderate-size earthquakes (Mw 5.5–6.0) using high-resolution 3D dynamic rupture simulations. Our models feature a listric normal fault surrounded by more than 800 fractures, emulating a major fault and its associated damage zone. We analyze three cases: a cascading rupture propagating within the fracture network (Mw 5.5), a non-cascading main-fault rupture with off-fault fracture slip (Mw 6.0), and a main-fault rupture without a fracture network (Mw 6.0). Cascading ruptures within the fracture network produce distinct ground-motion signatures with enriched high-frequency content, arising from simultaneous slip of multiple fractures and parts of the main fault, resembling source coda-wave-like signatures. This case shows elevated near-field characteristic frequency (fc) and stress drop, approximately an order of magnitude higher than the estimation directly on the fault of the dynamic rupture simulation. The inferred fc of the modeled vertical ground-motion components reflects the complexity of the radiation pattern and rupture directivity of fracture-network cascading earthquakes. We show that this is consistent with observations of strong azimuthal dependence of corner frequency in the 2009–2016 central Apennines, Italy, earthquake, sequence. Simulated ground motions from fracture-network cascading ruptures also show pronounced azimuthal variations in peak ground acceleration (PGA), peak ground velocity, and pseudospectral acceleration, with average PGA nearly double that of the non-cascading cases. Cascading ruptures radiate high-frequency seismic energy, yield nontypical ground-motion characteristics including coda-wave-like signatures, and may result in a significantly higher seismologically inferred stress drop and PGA. Such outcomes emphasize the critical role of fault-zone complexity in affecting rupture dynamics and seismic radiation and have important implications for physics-based seismic hazard assessment. 

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