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1. Fault valving and pore pressure evolution in simulations of earthquake sequences and aseismic slip

   https://doi.org/10.1038/s41467-020-18598-z  

   Zhu, Weiqiang; Allison, Kali L.; Dunham, Eric M.; Yang, Yuyun (December 2020, Nature Communications)

   null (Ed.)

   Abstract Fault-zone fluids control effective normal stress and fault strength. While most earthquake models assume a fixed pore fluid pressure distribution, geologists have documented fault valving behavior, that is, cyclic changes in pressure and unsteady fluid migration along faults. Here we quantify fault valving through 2-D antiplane shear simulations of earthquake sequences on a strike-slip fault with rate-and-state friction, upward Darcy flow along a permeable fault zone, and permeability evolution. Fluid overpressure develops during the interseismic period, when healing/sealing reduces fault permeability, and is released after earthquakes enhance permeability. Coupling between fluid flow, permeability and pressure evolution, and slip produces fluid-driven aseismic slip near the base of the seismogenic zone and earthquake swarms within the seismogenic zone, as ascending fluids pressurize and weaken the fault. This model might explain observations of late interseismic fault unlocking, slow slip and creep transients, swarm seismicity, and rapid pressure/stress transmission in induced seismicity sequences. 

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2. Cascading foreshocks, aftershocks and earthquake swarms in a discrete fault network

   https://doi.org/10.1093/gji/ggad278  

   Im, Kyungjae; Avouac, Jean-Philippe (May 2023, Geophysical Journal International)

   SUMMARY Earthquakes come in clusters formed of mostly aftershock sequences, swarms and occasional foreshock sequences. This clustering is thought to result either from stress transfer among faults, a process referred to as cascading, or from transient loading by aseismic slip (pre-slip, afterslip or slow slip events). The ETAS statistical model is often used to quantify the fraction of clustering due to stress transfer and to assess the eventual need for aseismic slip to explain foreshocks or swarms. Another popular model of clustering relies on the earthquake nucleation model derived from experimental rate-and-state friction. According to this model, earthquakes cluster because they are time-advanced by the stress change imparted by the mainshock. This model ignores stress interactions among aftershocks and cannot explain foreshocks or swarms in the absence of transient loading. Here, we analyse foreshock, swarm and aftershock sequences resulting from cascades in a Discrete Fault Network model governed by rate-and-state friction. We show that the model produces realistic swarms, foreshocks and aftershocks. The Omori law, characterizing the temporal decay of aftershocks, emerges in all simulations independently of the assumed initial condition. In our simulations, the Omori law results from the earthquake nucleation process due to rate and state friction and from the heterogeneous stress changes due to the coseismic stress transfers. By contrast, the inverse Omori law, which characterizes the accelerating rate of foreshocks, emerges only in the simulations with a dense enough fault system. A high-density complex fault zone favours fault interactions and the emergence of an accelerating sequence of foreshocks. Seismicity catalogues generated with our discrete fault network model can generally be fitted with the ETAS model but with some material differences. In the discrete fault network simulations, fault interactions are weaker in aftershock sequences because they occur in a broader zone of lower fault density and because of the depletion of critically stressed faults. The productivity of the cascading process is, therefore, significantly higher in foreshocks than in aftershocks if fault zone complexity is high. This effect is not captured by the ETAS model of fault interactions. It follows that a foreshock acceleration stronger than expected from ETAS statistics does not necessarily require aseismic slip preceding the mainshock (pre-slip). It can be a manifestation of a cascading process enhanced by the topological properties of the fault network. Similarly, earthquake swarms might not always imply transient loading by aseismic slip, as they can emerge from stress interactions. 

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3. Fault Roughness Promotes Earthquake‐Like Aftershock Clustering in the Lab

   https://doi.org/10.1029/2022GL101241  

   Goebel, Thomas H.; Brodsky, Emily E.; Dresen, Georg (April 2023, Geophysical Research Letters)

   Abstract Earthquakes rarely occur in isolation but rather as complex sequences of fore, main and aftershocks. Assessing the associated seismic hazard requires a holistic view of event interactions. We conduct frictional sliding experiments on faulted Westerly Granite samples at mid‐crustal stresses to investigate fault damage and roughness effects on aftershock generation. Abrupt laboratory fault slip is followed by periods of extended stress relaxation and aftershocks. Large roughness promotes less co‐seismic slip and high aftershock activity whereas smooth faults promote high co‐seismic slip with few aftershocks. Conditions close to slip instability generate lab‐quake sequences that exhibit similar statistical distributions to natural earthquakes. Aftershock productivity in the lab is linearly related to the residual strain energy on the fault which, in turn, is controlled by the level of surface heterogeneity. We conclude that roughness and damage govern slip stability and seismic energy partitioning between fore, main and aftershocks in lab and nature. 

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4. The Roles of Shear Displacement and Normal Stress on Earthquake Nucleation in Meter‐Scale Laboratory Faults

   https://doi.org/10.1029/2025JB031696  

   Bolton, David C; Shreedharan, Srisharan; Saffer, Demian; Trugman, Daniel T (August 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Earthquake nucleation is a fundamental problem in earthquake science and has practical implications for forecasting seismic hazards. Laboratory experiments performed on large, meter‐scale fault systems offer unique insights into the nucleation process because the migration and expansion of the nucleation zone can be precisely detected, measured, and characterized using arrays of local strain and slip measurements. We report on a series of laboratory experiments conducted on a 1‐m direct shear machine. We sheared layers of quartz gouge between roughened acrylic forcing blocks over a range of normal stresses between 3 and 12 MPa, generating a spectrum of slip modes, ranging from aseismic creep to fast‐dynamic rupture. Co‐seismic slip, peak slip velocity, and high‐frequency acoustic energy content of laboratory earthquakes increases systematically with both cumulative fault slip and normal stress. Slower and smaller laboratory earthquake sequences have larger nucleation zones, creep more during their inter‐seismic period, and are deficient in high‐frequency energy compared to larger and faster rupture sequences. We find that the critical nucleation length scale,H*, scales inversely with cumulative fault slip and normal stress. A reduction inH*and an increase in event size can be explained by a decrease in the critical slip distance,D_c, or an increase in the frictional rate parameterb–aand is likely driven by shear localization. Together, our results indicate that homogeneous, mature fault zones that have undergone more cumulative fault slip are expected to have smallerH*and can more easily host dynamic instabilities, relative to immature faults. 

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5. How the changes of fault zone material properties influence earthquake nucleation and rupture

   Huang, Yihe; Thakur, Prithvi (October 2021, AGU Fall Meeting 2021)

   null (Ed.)

   The fault damage zone is a well-known structure of localized deformation around faults. Its material properties evolve over earthquake cycles due to coseismic damage accumulation and interseismic healing. We will present fully dynamic earthquake cycle simulations to show how the styles of earthquake nucleation and rupture propagation change as fault zone material properties vary temporally. First, we will focus on the influence of fault zone structural maturity quantified by near-fault seismic wave velocities in simulations. The simulations show that immature fault zones promote small and moderate subsurface earthquakes with irregular recurrence intervals, whereas mature fault zones host pulse-like earthquake rupture that can propagate to the surface, extend throughout the seismogenic zone, and occur at regular intervals. The interseismic healing in immature fault zones plays a key role in allowing the development of aseismic slip episodes including slow-slip events and creep, which can propagate into the seismogenic zone, and thus limit the sizes of subsequent earthquakes by releasing fault stress. In the second part, we will discuss how the precursory changes of seismic wave velocities of fault damage zones may affect earthquake nucleation process. Both laboratory experiments and seismic observations show that the abrupt earthquake failure can be preceded by accelerated fault deformation and the accompanying velocity reduction of near-fault rocks. We will use earthquake cycle simulations to systematically test the effects of timing and amplitudes of such precursory velocity changes. Our simulations will provide new insights into the interplay between fault zone structure and earthquake nucleation process, which can be used to guide future real-time monitoring of major fault zones. 

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