In a high‐pressure injection fault activation experiment conducted at the Mont Terri underground research laboratory in Switzerland, the transmissivity of the Opalinus Clay fault significantly increased due to opening and shearing. The fluid injection, spanning a few hours, generated a 10 m radius fault activation patch. Subsequent pressure pulse tests conducted bi‐weekly for a year revealed the gradual return of fault transmissivity to its initial state. The study utilized fluid pressure decay analysis, optical fiber monitoring, continuous active source seismic measurements and borehole displacement sensors for measuring fault displacements. The fault zone exhibited a dilation of approximately 1.4 mm, associated with both normal and tangential movements during activation, resulting in a sudden transmissivity increase from 1 × 10 −12 to 3.2 × 10 −7 m 2 /s. Early post‐activation, transient compaction and the subsequent slow compaction were observed, transitioning to an extension regime. The pressure pulse tests demonstrated a rapid transmissivity drop by more than two orders of magnitude within the first 10 days, followed by a gradual and less pronounced decrease. Plastic shear and compaction dominated the transmissivity evolution until 70 days after injection ended, followed by a period where additional factors, such as clay mineral swelling, influenced the behavior. Extrapolation suggested a sealing process taking at least 50 years after the initial activation.

Evaluating and predicting the seismic hazard induced by fluid injection in enhanced geothermal systems (EGSs) is critical for safe and effective operations. This study compares the Gonghe project, a pioneering EGS initiative in China, with the well-studied Pohang EGS in South Korea, within a broader context of global fluid injection practices. We assessed the potential seismic hazard at these two sites based on their seismogenic indices (E). We find that E of the Gonghe EGS generally decreases from 0.4 to -0.7, consistent with the typical ranges of E in EGS sites, including Pohang. Our results indicate that real-time E is a more reliable measure for assessing seismic hazard in Gonghe because it offers insights into the maximum magnitude, exceedance probabilities, and expected numbers of earthquakes. Conversely, in Pohang, maximum E proves more effective for seismic hazard assessment. However, predicting the seismic hazard after the Mw 3.2 earthquake in Pohang remains challenging, particularly for the runaway rupture associated with the subsequent Mw 5.5 earthquake, highlighting the complexities involved. This study suggests that the use of real-time E is viable for assessing seismic hazard in EGS reservoirs characterized by descending E and seismic injection efficiency. Conversely, for reservoirs with ascending E and seismic injection efficiency, such as Pohang, maximum E could offer better insights into seismic hazard assessment, although precise earthquake magnitude constraints may be elusive due to dominant tectonic influences.

The conventional understanding of tectonic faults primarily categorizes them based on frictional behavior: stable due to velocity-strengthening (VS) behavior, or unstable owing to velocity-weakening (VW) that lead to seismic ruptures. This classification has traditionally led to the assumption that VS faults are unlikely candidates for earthquake nucleation. However, emerging evidence from recent laboratory experiments and field studies is challenging this simplistic view, pointing towards a more complex mechanism. In this study, we utilize a hydro-mechanically coupled fault model, which integrates VS friction governed by rate-and-state friction laws with dynamic weakening influenced by poroelastic effects. A key aspect of our findings is the impact of fluid injection on the mechanical state of the fault. This process decreases the effective normal stress and frictional resistance, initially paving the way for the propagation of an aseismic, slow-slip event. The transition from aseismic to seismic slip on VS faults hinges on the balance between shear-induced dilation and compaction. These opposing mechanisms respectively lead to a decrease and an increase in pore-fluid pressure, dictating the balance between fault stability or instability. Our results show that when the effect of compaction-induced pressurization surpasses the initial dilatancy phase, it enables the propagation of dynamic rupture as a solitary pore-pressure wave. Conversely, when dilation predominates over compaction, an aseismic slow-slip event propagates through the fault, maintaining stability and preventing rapid seismic activity. These findings advance our understanding of seismic risk associated with VS faults. They are especially relevant in the context of fluid injection practices in geothermal energy production and CO2 storage, demonstrating how such activities might activate faults that are considered nominally stable. Additionally, our results underscore the critical need for more experimental and theoretical investigations into shear-induced compaction as an efficient mechanism for fault self-pressurization, which plays a key role in leading to seismic instabilities.

To better understand the mechanics of injection-induced seismicity, we developed a 2-D numerical code to simulate both seismic and aseismic slip on non-planar faults and fault networks driven by fluid diffusion along permeable faults, in an impervious host rock. Our approach integrates a boundary element method to model fault slip governed by rate-and-state friction with a finite-volume method to simulate fluid diffusion along fault networks. We demonstrate the capabilities of the method with two illustrative examples: (1) Fluid injection inducing slow slip on a primary rough, rate-strengthening fault, which subsequently triggers microseismicity on nearby secondary, smaller faults, and (2) fluid injection on a single fault in a network of intersecting faults, leading to fluid diffusion and reactivation of slip throughout the network. This work highlights the importance of distinguishing between mechanical and hydrological processes in the analysis of induced seismicity, providing a powerful tool for improving our understanding of fault behaviour in response to fluid injection, in particular when a network of geometrically complex faults is involved.

Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have rapidly progressed to address fundamental problems in fault mechanics and provide self‐consistent, physics‐based frameworks to interpret and predict geophysical observations across spatial and temporal scales. To advance SEAS simulations with rigor and reproducibility, we pursue community efforts to verify numerical codes in an expanding suite of benchmarks. Here we present code comparison results from a new set of quasi‐dynamic benchmark problems BP6‐QD‐A/S/C that consider an aseismic slip transient induced by changes in pore fluid pressure consistent with fluid injection and diffusion in fault models with different treatments of fault friction. Ten modeling groups participated in problems BP6‐QD‐A and BP6‐QD‐S considering rate‐and‐state fault models using the aging (‐A) and slip (‐S) law formulations for frictional state evolution, respectively, allowing us to better understand how various computational factors across codes affect the simulated evolution of pore pressure and aseismic slip. Comparisons of problems using the aging versus slip law, and a constant friction coefficient (‐C), illustrate how aseismic slip models can differ in the timing and amount of slip achieved with different treatments of fault friction given the same perturbations in pore fluid pressure. We achieve excellent quantitative agreement across participating codes, with further agreement attained by ensuring sufficiently fine time‐stepping and consistent treatment of boundary conditions. Our benchmark efforts offer a community‐based example to reveal sensitivities of numerical modeling results, which is essential for advancing multi‐physics SEAS models to better understand and construct reliable predictive models of fault dynamics.

Slip transients cover a wide range of scales in length, duration, moment, slip among others. They also exhibit a rich spectrum of behaviors like slow aseismic slip in subduction zones or transform faults, or faster and potentially devastating ruptures in the case of earthquakes. Earthquake swarms, either occurring in natural tectonic context or due to anthropogenic fluid injections at depth, have been found to exhibit another peculiar behavior: they show a global migration, sometimes accompanied by faster bursts, and could result from the interplay between fluid processes, aseismic slip and seismicity. In this study, we synthesize findings from the literature on slow slip events and earthquakes, integrating insights from our research on earthquake swarms. We examine how swarms conform to or deviate from established scaling relations for seismic phenomena. Specifically, we compare earthquake swarms, slow slip events, and earthquakes in terms of moment and duration, and analyze the migration or rupture velocities of swarms and slow slip events relative to moment. We highlight two different but parallel behaviors among these sequences: one linked to slow-slips, with elevated migration velocities and moments, and the other related to fluid-induced processes, featuring lower velocities and moments. These results provide metrics for distinguishing between the drivers of earthquake swarms —both natural and injection-induced— and their connections to fast seismic transients and foreshocks. This work also highlights promising directions for instrumentation and the study of slow and aseismic slip transients.

Coastal areas can be tremendously biodiverse and host a substantial part of the world’s population and critical infrastructure. However, there are often fragile environments that face various hazards such as flooding, coastal erosion, land salinization or pollution, earthquake-induced land motion, or anthropogenic processes. In this article, we investigate the stability of the Nice Côte d’Azur Airport, which has been built on reclaimed land in the Var River delta (French Riviera, France). This infrastructure, as well as the ongoing subsidence of the airport runways, has been a permanent concern since the partial collapse of the platform in 1979. Moreover, using InSAR data between 2003 and 2011, Cavalié et al. (2015) showed that parts of the airport platform were subsiding up to 10 mm/yr.Understanding the mechanism and thus the evolution of sediment compaction is essential to evaluate the danger caused by the coastal subsidence. Therefore, in this study, we extended the observation period of InSAR measurements to better analyze the temporal evolution of the ground displacement on the Nice Côte d’Azur Airport platform in the hope of capturing the non-linear component of the deformation. Indeed, the relatively short period of observation (2003-2011) of the previous study (Cavalié et al., 2015) impeded the accurate detection of non-linearity in the surface displacement and thus to understand its dynamic. So, we used here the complete archive of SAR images acquired by ESA over a much longer period of time (28 years from 1992 to 2020).Extending the observation window to study the long-term subsidence leads to substantial improvements in the understanding of the ongoing mechanisms along this coastal area. Indeed, the new analysis reveals a notable deceleration in the maximum downward motion rate, decreasing from 16 mm/yr in the 1990s to 8 mm/yr in the present day (for the fastest subsidence area). We then used a simple analytical Burgers creep model to constrain the mechanisms and rheology at play. The data are properly explained by the phases of primary and secondary creep, highlighting a slow viscoelastic deformation at multiyear timescales. Our study thus proves that the long-term InSAR data can improve our understanding of the surface processes and the subsurface material properties. Although the subsidence rate decelerates, at least for 28 years, our results show that the compaction of the sediment is still active and its future evolution is uncertain and still at stake. Indeed, if compaction bands are developing under the airport platform, creep processes could potentially lead accumulated material damage to failure.This study underscores the critical role of remote monitoring in comprehending coastal land motion. We show here that employing advanced InSAR techniques offers a better understanding of actual hazards posed by the airport built on reclaimed lands. The findings advocate for ongoing monitoring initiatives to mitigate risks and enhance the resilience of coastal infrastructure.

Numerical modeling of Discrete Fracture Networks (DFNs) is commonly used to assess the behavior and properties of hydraulic diffusion and seismicity in the Earth’s crust within a network of fractures and faults, and to study the hydromechanical evolution of fractured reservoirs stimulated by hydraulic injection and production. The modelling of such fractures is typically carried out under a quasi-static approximation, and occasionally accounting for elasto-dynamics in single-rupture studies that assume a slip-weakening friction law. In this work, we develop a 2D DFN model to simulate fluid-induced seismicity that couples hydraulic diffusion and slip governed by rate-and-state friction on multiple interacting faults. The main goal of this numerical model is to establish a connection between laboratory derived friction parameters and field observations, enabling the inference of the long-term evolution of fractured reservoirs and crustal fault systems undergoing multiple earthquakes and (slow) slip events induced by fluid pressure perturbations.In the model, the elastic interactions are computed with a boundary element method, accelerated by the hierarchical matrix method. We assessed the convergence of the method at fracture junctions and verified it does not create unphysical singularities. The use of rate-and-state friction makes it possible to model several seismic events over the injection duration.The simulations will be later used to fit measurements of permeability and friction collected in laboratory experiments, in-situ observations of fault slip and opening from fluid injection experiments at decametric scale, and finally, induced seismicity at reservoir scale.

Enhanced Geothermal Systems (EGS) employ fluid injection for both reservoir stimulation and circulation, a practice that may sometimes induce earthquakes. Paradoxically, numerous EGS projects have observed the occurrence of the largest-magnitude earthquake (LME, 1.6 < M < 5.5) after injection cessation, posing a significant challenge for the development of geothermal power plants. This paper aims to examine the cases of post-injection LME observed in EGS, and provide a detailed understanding of the coupled mechanisms responsible for seismic activity during and after fluid injection. We report the mechanisms that have been hypothesised for different cases of LME emerging in the post-injection phase of EGS. The analysis of post-injection LME cases reveals two types of seismicity, early and late, differentiated by the time at which pore pressure changes become negligible, i.e., a post-injection time longer than the injection time. Notably, we remark that, for most cases of early post-injection high-magnitude earthquakes, the primary triggering mechanisms involve pore pressure diffusion, thermal effects, poromechanical stress relaxation and static stress transfer resulting from seismic fault slip. Conversely, prolonged fluid injections could lead to the gradual accumulation of stress on faults, mainly due to aseismic slip, causing late post-injection LME. Furthermore, recent studies have also demonstrated the importance of the initial fault stress criticality in influencing the post-injection behaviour of fault aseismic slip and trailing seismicity. Finally, we present recent efforts devoted to develop effective methodologies for mitigating induced seismicity, and the current research perspectives aimed at forecasting and minimizing seismic activity associated with EGS.

Earthquake swarms may be driven by fluids, through hydraulic injections or natural fluid circulation, but also by slow and aseismic slip transients. Understanding the driving factors for these prolific sequences and how they can potentially develop into larger ruptures remains a challenge. A notable and almost ubiquitous feature of swarms is their hypocenters migration, which occurrence is closely related to the processes driving the observed seismicity, in a similar way as seismicity accompanies slow-slip events at subduction zones. Here, we analyze global data on migrating sequences, and identify scaling laws for migration velocity, moment and duration measured on natural and injection-induced swarms, foreshock sequences, and slow slip events. We highlight two different behaviors among these sequences: one linked to slow slips, with elevated migration velocities and moments, and the other related to fluid-induced processes, featuring lower velocities and moments. These results provide metrics for distinguishing between the drivers of earthquake swarms, fluid or slow-slip related, and prompt a reevaluation of scaling laws of fault slip transients, especially for swarms. Migrating earthquake sequences from a wide range of tectonic contexts exhibit two distinct behaviors. One is linked to slow slip and characterized by elevated migration velocities and moments, and the other is related to fluid-induced processes, with lower velocities and moments.

Seismic swarms represent clusters of seismicity without large mainshocks.While they occur naturally, they can also be induced by human activities, particularly during reservoir hydraulic stimulations.A striking feature of seismic swarms is the migration of their hypocenters.The seismic front, initially attributed to fluid diffusion, has more recently been understood as the result of the propagation of a fluid-induced aseismic slip.Close to the center of the swarm, a seismic back-front is commonly admitted after the injection end, but a low density of events is also observed during the injection period.In our investigation, based on a compilation of 22 swarms of both natural or anthropogenic origin, we aim to explore the existence and origin of a seismic back-front.Interestingly, we observe a post-injection back-front only in rare cases, where a rapid fluid pressure decrease is imposed at the injection point.Conversely, a back-front during the injection period is always observed in both types of swarms.Consequently, the back-front cannot be reliably used to infer the end of injection, as commonly done for natural swarms.Moreover, the occurrence of this back-front during injection is linked to an increase in the minimum magnitude of seismic events.We interpret the vanishing of the seismicity close to the injection point as a consequence of the increase in earthquake nucleation length with increasing fluid pressure.With a substantially enhanced capability for detecting small events, it may become feasible to use this back-front as a means of monitoring injection pressure, even in the context of natural swarms.

The evolution of fluid injection-induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid-induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well-studied injection-induced seismic sequences at the Soultz-sous-Fôret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid-induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection-induced and natural earthquake swarms.

AbstractThe 2020 Alex storm in southern France led to localized extreme rainfall exceeding 600 mm in less than 24 hr. In the 100 days following the storm, a series of small earthquakes swarm occurred beneath the Tinée valley, a region characterized by a low background deformation. To gain insight into the mechanisms controlling swarm evolution, we used an enhanced seismic catalog to detect 188 events. These events exhibited magnitudes comprised between −1.03 and 2.01, and 78 of them were relocated using relative locations at an average depth of 3–4 km. Additionally, we estimated the directions and velocities of seismicity migration. Our analyses reveal multiple episodes of hypocenter expansion and migration within a fluid‐saturated fault system. Observations provide evidence of a bi‐directional seismicity migration marked by dual velocities within a swarm. The northward seismicity migration aligns with velocities indicative of aseismic slip (∼130 m/hr), while the southward migration corresponds to velocities associated with fluid pressure diffusion (∼5 m/hr). This migration pattern underscores the interplay of multiple physical mechanisms in both triggering and driving earthquakes. A stress‐driven model based on rate‐and‐state friction successfully explains the overall evolution of observed seismicity, whereas a fluid‐driven model fails to reproduce the data. Our observations and models suggest that fluid pressure changes resulting from intense rainfall caused aseismic slip in the shallow portion of the crust. We hypothesize that aseismic deformation serves as the driving force for the earthquake swarms, coupled with the invasion of pressurized fluid due to diffusing rainfall.

Abstract. Coastal areas can be tremendously biodiverse and host as well a substantial part of the world population and many critical infrastructures. However, there are often fragile environments and face various hazards as flooding, coast erosion, land salinization or pollution, earthquake-induced land motions, or anthropogenic processes. In this article, we investigate the stability of the Nice-Côte d’Azur airport that has been built on reclaimed land in the Var river delta (French Riviera, France). This infrastructure is 5 a permanent concern since the partial collapse of the platform in 1979 and the on-going subsidence of the airport runways. Here, we used the full archive of ESA SAR images from 1992 to 2020 to comprehensively monitor the dynamics of the airport subsidence.We find that maximum downward motion rate is slowing down from 16 mm/yr in the 1990s to 8 mm/yr today. However, sediment compaction is still active and an acceleration phase of the continuous creep leading to a potential failure of a part of the platform cannot be excluded. Our study demonstrates the importance of remotely monitoring 10 of the platform to better understand coastal land motions, which will ultimately help evaluate and reduce associated hazards.

Coastal areas can be tremendously biodiverse and host a substantial part of the world's population and critical infrastructure. However, there are often fragile environments that face various hazards such as flooding, coastal erosion, land salinization or pollution, earthquake-induced land motion, or anthropogenic processes. In this article, we investigate the stability of the Nice Côte d'Azur Airport, which has been built on reclaimed land in the Var River delta (French Riviera, France). This infrastructure, as well as the ongoing subsidence of the airport runways, has been a permanent concern since the partial collapse of the platform in 1979. Here, we used the full archive of ESA SAR (synthetic-aperture radar) images from 1992 to 2020 to comprehensively monitor the dynamics of the airport subsidence. We found that the maximum downward motion rate has been slowing down from 16 mm yr−1 in the 1990s to 8 mm yr−1 today. However, sediment compaction is still active, and an acceleration phase of the continuous creep leading to a potential failure of a part of the platform cannot be excluded. Our study demonstrates the importance of remotely monitoring of the platform to better understand the motion of coastal land, which will ultimately help evaluate and reduce associated hazards.

SUMMARYThe Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10−19 to 10−21 m2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository project can compromise the long-term hydrological confinement, and lead to mechanical instability. Here, we have performed laboratory experiments to test the effect of relative humidity (RH), grain size (g.s.) and normal stress on rate-and-state frictional properties and stability of fault laboratory analogues corresponding to powders of OPA shaly facies. The sifted host rock powders at different grain size fractions (<63 μm and 63 < g.s. < 125 μm), at room (∼25 per cent) and 100 per cent humidity, were slid in double-direct shear configuration, under different normal stresses (5–70 MPa). We observe that peak friction, μpeak and steady-state friction, μss, depend on water vapour content and applied normal stress. Increasing relative humidity from ∼25 per cent RH (room humidity) to 100 per cent RH causes a decrease of frictional coefficient from 0.41 to 0.35. The analysis of velocity-steps in the light of rate-and-state friction framework shows that the stability parameter (a–b) is always positive (velocity-strengthening), and it increases with increasing sliding velocity and humidity. The dependence of (a–b) on slip rate is lost as normal stress increases, for each humidity condition. By monitoring the variations of the layer thickness during the velocity steps, we observe that dilation (Δh) is directly proportional to the sliding velocity, decreases with normal stress and is unaffected by humidity. Microstructural analysis shows that most of the deformation is accommodated within B-shear zones, and the increase of normal stress (σn) promotes the transition from strain localization and grain size reduction to distributed deformation on a well-developed phyllosilicate network. These results suggest that: (1) the progressive loss of velocity dependence of frictional stability parameter (a–b) at σn > 35 MPa is dictated by a transition from localized to distributed deformation and (2) water vapour content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μss), and enhances fault stability.

Tectonic faults are often assumed to slip either slow due to stable, velocity-strengthening frictional behavior, or fast as a result of velocity-weakening friction leading to dynamic (seismic) rupture. As a consequence, velocity-strengthening faults may be regarded as intrinsically stable as they do not spontaneously nucleate seismic events. However, recent laboratory and in-situ experiments of fluid injection challenged such assumptions. Here we present a fully coupled hydro-mechanical fault model in which stable, rate-strengthening frictional behavior is combined with dynamic weakening due to rapid poroelastic effects, allowing unstable (seismic) slip to occur on nominally stable faults. In our numerical experiments, fluid injection reduces the effective normal stress and frictional resistance, thus bringing the fault to failure. The onset of fault failure is controlled by competing mechanisms of shear-induced dilation and shear-enhanced compaction, which cause fault weakening and the propagation of a slow-slip transient from the fluid injection point. When a critical size of the slow slip patch is reached, dynamic rupture eventually nucleates at the slow-slip event front and propagates beyond the fluid pressure perturbed region. Further numerical experiments indicate that, when the fault is critically stressed, the growth of the aseismic patch – prior to dynamic rupture – occurs in a few seconds, whereas at lower stress levels, the aseismic slip phase propagates slowly over hundreds of seconds. These results predict that poroelastic compaction and fluid pressurization can cause the transition from aseismic slow-slip to fast seismic slip and the propagation of dynamic rupture on velocity-strengthening faults. In particular, they demonstrate that compaction-induced fluid pressurization can overcome the initial phase of shear-induced dilatancy, thus allowing the propagation of dynamic rupture in the form of pulse-like pore-pressure waves. The implication that earthquake rupture may nucleate on rate-strengthening faults, presently considered to be nominally stable, requires a re-evaluation of seismic hazard in many areas, particularly in the case of fluid injection in enhanced geothermal systems and CO2 storage.

SUMMARY Fault slip induced by fluid perturbation in shale formations may only lead to a sparse seismicity. However, fault slip may strongly impact the integrity of shale formations that serve as caprocks for geological reservoirs holding buoyant fluids such as CO2, natural gas or hydrogen. A better understanding of the fluid reactivation processes of fault and the seismic triggering process is therefore critical for reservoir monitoring and fault stability. Here we analyse the seismic responses of a shale fault exposed to fluid pressurization during an in situ field-scale injection experiment at ∼340 m depth in the Mont Terri underground research laboratory (Switzerland). Two main types of seismic signals are observed as the fault was activated and started to slowly slip. After an aseismic phase, we observed tremor signatures and an increase in noise amplitude, which were directly associated with the slowly propagating fault slip in response to fluid injection. These signatures were later followed by micro-earthquakes that seem to occur further away from the fluid-pressurized area. We interpret these micro-earthquakes to be triggered by stress perturbations from the main slip growth. These two classes of seismic responses therefore highlight two different processes. Tremors seem to be a more direct observation for the fluid-induced slip propagation than micro-earthquakes. Even hidden in the noise, they precede earthquake failures, thus providing a useful tool for monitoring fluid leakage activated by slow deformation on low permeable shale faults, with applications for sealing integrity of caprocks.

Seismicity migration is one of the most remarkable features of earthquake swarms because of its ubiquity and the wide range of migration durations, velocities and shapes observed. The dynamic properties of swarms, like seismic moment or number of events, are often attributed to fluid circulation, directly or indirectly. However, classical models of fluid pressure diffusion aiming at explaining seismicity triggering and migration show some limitations. An increasing body of evidence points at an important contribution of fluid-induced aseismic slip during swarms. Moreover, parameters like injection history and fault criticality are expected to intervene. In this work, we use a fracture mechanics framework to show that earthquake migration can be explained as driven by the propagation of a fluid-induced aseismic slip transient on a rate-and-state fault. This theoretical model predicts a simple linear relation between the seismic migration distance and the square root of the injected fluid volume. This relation is validated by observations in two well-studied seismic sequences induced by injections for geothermal purposes (Basel and Soultz-sous-Forêts), in which the seismicity is mainly clustered around a single surface. In addition, the model helps constrain frictional, hydraulic and structural properties of the fault hosting aseismic slip, and can be reasonably generalized to all fluid-induced earthquake swarms, natural and anthropogenic.