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Injection-induced seismicity and aseismic slip often involve the reactivation of long-dormant faults, which may have extremely low permeability prior to slip. In contrast, most previous models of fluid-driven aseismic slip have assumed linear pressure diffusion in a fault zone of constant permeability and porosity. Slip occurs within a frictional shear crack whose edge can either lag or lead pressure diffusion, depending on the dimensionless stress-injection parameter that quantifies the prestress and injection conditions. Here, we extend this foundational work by accounting for permeability enhancement and dilatancy, assumed to occur instantaneously upon the onset of slip. The fault zone ahead of the crack is assumed to be impermeable, so fluid flow and pressure diffusion are confined to the interior, slipped part of the crack. The confinement of flow increases the pressurization rate and reduction of fault strength, facilitating crack growth even for severely understressed faults. Suctions from dilatancy slow crack growth, preventing propagation beyond the hydraulic diffusion length. Our new two-dimensional and three-dimensional solutions can facilitate the interpretation of induced seismicity data sets. They are especially relevant for faults in initially low permeability formations, such as shale layers serving as caprock seals for geologic carbon storage, or for hydraulic stimulation of geothermal reservoirs. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

The Superstition Hills Fault (SHF) exhibits a rich spectrum of slip modes, including M 6+ earthquakes, afterslip, quasi‐steady creep, and both triggered and spontaneous slow slip events (SSEs). Following 13 years of quiescence, creepmeters recorded 25 mm of slip during 16–19 May 2023. Additional sub‐events brought the total slip to 41 mm. The event nucleated on the northern SHF in early‐May and propagated bi‐laterally at rates on the order of kilometers per day. Surface offsets reveal a bi‐modal slip distribution, with slip on the northern section of the fault being less localized and lower amplitude compared to the southern section. Kinematic slip models confirm systematic variations in the slip distribution along‐strike and with depth and suggest that slip is largely confined to the shallow sedimentary layer. Observations and models of the 2023 SSE bear a strong similarity to previous slip episodes in 1999, 2006, and 2010, suggesting a characteristic behavior. Plain Language Summary: Studying the mechanical properties and behavior of faults is essential for understanding earthquake ruptures. In this study, we investigate a recent slip event on the Superstition Hills Fault (SHF), which has a well‐documented record of slip. A notable aspect of the SHF is that it periodically undergoes "slow slip events" (SSEs), where the fault slips and releases energy without any accompanied ground shaking. During May‐July 2023, the SHF experienced a major SSE for the first time in 13 years. Our analysis shows that it was the largest documented SSE on the SHF and released equivalent energy to a magnitude 4.5 earthquake. We also find that the spatial pattern of fault slip is very similar to several previous slip events in 1999, 2006, and 2010, suggesting that the SHF has a tendency to slip in a characteristic manner. Key Points: We document a recent spontaneous slow slip event (SSE) on the Superstition Hills Fault using creepmeter, Interferometric Synthetic Aperture Radar, Global Navigation Satellite System, and field measurementsOver 41 mm of slip occurred from mid‐May to mid‐July 2023, with moment release corresponding to a Mw 4.5 earthquakeThe kinematics of the 2023 event are remarkably similar to several previous SSEs, suggesting a characteristic rupture process [ABSTRACT FROM AUTHOR]

The 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. Plain Language Summary: On 2 October 2020, the Alex storm caused extreme rainfall, exceeding 600 mm in less than 24 hr, in southern France. Subsequently, 188 earthquakes were recorded, and 78 of them were precisely relocated at depths of 3–4 km beneath the Tinée valley in the 100 days following the storm. Such increased seismic activity is unusual in this region characterized by low deformation (<1 mm). Using seismic data, we identified two directions of seismicity migration marked by dual velocities within swarms–one indicative of aseismic slip, and the other corresponding to fluid pressure diffusion. A purely fluid‐driven model fails to accurately reproduce the temporal evolution of seismicity, while observed data are effectively modeled based on intermittent stress changes resulting from a slow aseismic slip. Our investigations lead us to attribute earthquake activity to aseismic deformation, acting as the driving force for the swarms, coupled with the invasion of pressurized fluid due to infiltrating precipitation. Key Points: Multiple activation of small earthquake swarms after an extreme rainstorm in southern French AlpsBi‐directional swarm migration velocities reveal an interplay between fluid diffusion and aseismic stress perturbationsA stress‐driven model explains the overall evolution of seismicity and provides insights into the triggering mechanisms [ABSTRACT FROM AUTHOR]

Unprecedented levels of seismicity have been seen in southern Sichuan, China, since the large‐scale exploitation of shale gas. Fluid and pore pressure transported through hydrological channel are thought as pivotal elements in the induction of earthquakes. Our high‐resolution tomography results reveal two inclined seismic anomalies featured by low Vs and high Vp/Vs at different depth range. The deeper anomaly extends 15 km from NE to SE and connects the well g048 from 3 km depth to the vicinity of the Ms 4.7 Gongxian earthquake 5.4 km deep, which is hinted to be a hydrological channel inferred from the high fluid overpressure of 28 Mpa calculated from focal mechanism solution. The injection operation of multiple shale gas wells along the channel may potentially accumulate the pore pressure and cause the fault near the end of the channel to reach critical stress state through various mechanisms. Plain Language Summary: Increasing seismicity in southern Sichuan is suspected to be linked to hydraulic fracturing stimulation for shale gas extraction. Induced earthquakes resulting from fracturing operations at nearby wells can be verified using in‐situ fracturing data, while remote‐induced factors are hard to prove. In this study, a seismic anomaly of low Vs and high Vp/Vs inferred to be a long‐distance hydrological channel connecting the 2021 Ms 4.7 Gongxian earthquake to the active shale gas well 15 km away. Our results indicate that pore fluid pressure generated by hydraulic fracturing in multiple wells may continuously accumulated at the end of the channel, ultimately resulting in high pore pressure and triggering the fault rupture. Therefore, it is urgent to plug this channel and prevent the filtration loss. Key Points: Two long‐distance hydrological channels were observed in southern SichuanThe injection operation of multiple wells along the channel may increase the pore pressure in the hydrological channelThe accumulation of pore pressure at the end of the hydrological channel was confirmed by the occurrence of Ms 4.7 Gongxian earthquake [ABSTRACT FROM AUTHOR]

Numerous studies have reported the occurrence of aseismic slip or slow slip events along faults induced by fluid injection. However, the underlying physical mechanism and its impact on induced seismicity remain unclear. In this study, we develop a numerical model that incorporates fluid injection on a fault governed by rate‐and‐state friction to simulate the coupled processes of pore‐pressure diffusion, aseismic slip, and dynamic rupture. We establish a field‐scale model to emulate the source characteristics of induced seismicity near the Dallas‐Fort Worth Airport (DFWA), Texas, where events with lower‐stress drops have been observed. Our numerical calculations reveal that the diffusion of fluid pressure modifies fault criticality and induces aseismic slip with lower stress drop values (<1 MPa), which further influence the timing and source properties of subsequent seismic ruptures. We observe that the level of pore‐pressure perturbation exhibits a positive correlation with aseismic‐stress drops but a reversed trend with seismic‐stress drops. Simulations encompassing diverse injection operations and fault frictional parameters generate a wide spectrum of slip modes, with the scaling relationship of moment (M0) with ruptured radius (r0) following an unusual trend, M0∝r04.4 ${M}_{0}\propto {r}_{0}^{4.4}$, similar to M0∝r04.7 ${M}_{0}\propto {r}_{0}^{4.7}$ observed in the DFWA sequence. Based on the consistent scaling, we hypothesize that the lower‐stress‐drop events in the DFWA may imply less dynamic ruptures in the transition from aseismic to seismic slip, located in the middle of the broad slip spectrum, as illustrated in our simulations. Plain Language Summary: Injection‐induced earthquakes have presented significant obstacles to developing energy resources related to fluid injection, such as enhanced geothermal systems and shale gas development. Despite their prevalence, the causes and impact of these earthquakes are not fully understood. Aseismic slip, characterized by slower velocities and longer durations than typical earthquakes, has been observed in induced earthquake studies. In this study, we use a numerical model to investigate how fluid pressures influence the slip properties of induced seismicity near the Dallas‐Fort Worth airport (DFWA), Texas. Our model shows that elevated fluid pressure induces aseismic slip and advances or delays fast slip (i.e., earthquakes). The pore‐pressure perturbation alters the source characteristics of both aseismic‐slip events and seismic ruptures, enhancing aseismic‐stress release while diminishing seismic‐stress release. Simulations involving various fault frictional properties reveal a wide spectrum of slip modes, ranging from slow to rapid slip, which are different from the stress‐release processes that drive globally observed natural earthquakes, but exhibit similarities to observations in the DFWA. Consequently, we infer that the DFWA events may exhibit reduced dynamic characteristics akin to slow slip events positioned in the middle of the broad spectrum generated in our modeling. Key Points: Pore‐pressure change induces aseismic slip with lower stress drops, either advancing or delaying seismic rupturesPore‐pressure perturbation exhibits a positive correlation with aseismic‐stress drops but a reversed trend with seismic‐stress dropsSimulations show a wide spectrum of induced‐slip behavior, exhibiting a similar source scaling to observations [ABSTRACT FROM AUTHOR]

Abstract The 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 The Superstition Hills Fault (SHF) exhibits a rich spectrum of slip modes, including M 6+ earthquakes, afterslip, quasi‐steady creep, and both triggered and spontaneous slow slip events (SSEs). Following 13 years of quiescence, creepmeters recorded 25 mm of slip during 16–19 May 2023. Additional sub‐events brought the total slip to 41 mm. The event nucleated on the northern SHF in early‐May and propagated bi‐laterally at rates on the order of kilometers per day. Surface offsets reveal a bi‐modal slip distribution, with slip on the northern section of the fault being less localized and lower amplitude compared to the southern section. Kinematic slip models confirm systematic variations in the slip distribution along‐strike and with depth and suggest that slip is largely confined to the shallow sedimentary layer. Observations and models of the 2023 SSE bear a strong similarity to previous slip episodes in 1999, 2006, and 2010, suggesting a characteristic behavior.

Surface roughness ubiquitously prevails in natural faults across various length scales. Despite extensive studies highlighting the important role of fault geometry in the dynamics of tectonic earthquakes, whether and how fault roughness affects fluid-induced seismicity remains elusive. Here, we investigate the effects of fault geometry and stress heterogeneity on fluid-induced fault slip and associated seismicity characteristics using laboratory experiments and numerical modeling. We perform fluid injection experiments on quartz-rich sandstone samples containing either a smooth or a rough fault. We find that geometrical roughness slows down injection-induced fault slip and reduces macroscopic slip velocities and fault slip-weakening rates. Stress heterogeneity and roughness control hypocenter distribution, frequency-magnitude characteristics, and source mechanisms of injection-induced acoustic emissions (AEs) (analogous to natural seismicity). In contrast to smooth faults where injection-induced AEs are uniformly distributed, slip on rough faults produces spatially localized AEs with pronounced non-double- couple source mechanisms. We demonstrate that these clustered AEs occur around highly stressed asperities where induced local slip rates are higher, accompanied by lower Gutenberg-Richter b-values. Our findings suggest that real-time monitoring of induced microseismicity during fluid injection may allow identifying progressive localization of seismic activity and improve forecasting of runaway events. [ABSTRACT FROM AUTHOR]

Abstract Unprecedented levels of seismicity have been seen in southern Sichuan, China, since the large‐scale exploitation of shale gas. Fluid and pore pressure transported through hydrological channel are thought as pivotal elements in the induction of earthquakes. Our high‐resolution tomography results reveal two inclined seismic anomalies featured by low Vs and high Vp/Vs at different depth range. The deeper anomaly extends 15 km from NE to SE and connects the well g048 from 3 km depth to the vicinity of the Ms 4.7 Gongxian earthquake 5.4 km deep, which is hinted to be a hydrological channel inferred from the high fluid overpressure of 28 Mpa calculated from focal mechanism solution. The injection operation of multiple shale gas wells along the channel may potentially accumulate the pore pressure and cause the fault near the end of the channel to reach critical stress state through various mechanisms.

Dynamic modeling of sequences of earthquakes and aseismic slip (SEAS) provides a self-consistent, physics-based framework to connect, interpret, and predict diverse geophysical observations across spatial and temporal scales. Amid growing applications of SEAS models, numerical code verification is essential to ensure reliable simulation results but is often infeasible due to the lack of analytical solutions. Here, we develop two benchmarks for three-dimensional (3D) SEAS problems to compare and verify numerical codes based on boundary-element, finite-element, and finite-difference methods, in a community initiative. Our benchmarks consider a planar vertical strike-slip fault obeying a rate- and state-dependent friction law, in a 3D homogeneous, linear elastic whole-space or half-space, where spontaneous earthquakes and slow slip arise due to tectonic-like loading. We use a suite of quasi-dynamic simulations from 10 modeling groups to assess the agreement during all phases of multiple seismic cycles. We find excellent quantitative agreement among simulated outputs for sufficiently large model domains and small grid spacings. However, discrepancies in rupture fronts of the initial event are influenced by the free surface and various computational factors. The recurrence intervals and nucleation phase of later earthquakes are particularly sensitive to numerical resolution and domain-size-dependent loading. Despite such variability, key properties of individual earthquakes, including rupture style, duration, total slip, peak slip rate, and stress drop, are comparable among even marginally resolved simulations. Our benchmark efforts offer a community-based example to improve numerical simulations and reveal sensitivities of model observables, which are important for advancing SEAS models to better understand earthquake system dynamics.

An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

We investigate the problem of fluid injection at constant pressure in a 2D Discrete Fracture Network (DFN) with randomly oriented and uniformly distributed frictionally-stable fractures. We show that this problem shares similarities with the simpler scenario of injection in a single planar shear fracture, investigated by Bhattacharya and Viesca (2019); Viesca (2021) and whose results are here extended to include closed form solutions for aseismic moment as function of injected volume V inj . Notably, we demonstrate that the hydro-mechanical response of the fractured rock mass is at first order governed by a single dimensionless parameter T associated with favourably oriented fractures: low values of T (critically stressed conditions) lead to fast migration of aseismic slip from injection point due to elastic stress transfer on critically stressed fractures. In this case, therefore, there is no effect of the DFN percolation number on the spatio-temporal evolution of aseismic slip. On the other hand, in marginally pressurized conditions ( T ≳ 1 ), the slipping patch lags behind the pressurized region and hence the percolation number affects to a first order the response of the medium. Furthermore, we show that the aseismic moment scales ∝ V inj 2 in both limiting conditions, similarly to the case of a single planar fracture subjected to the same injection condition. The factor of proportionality, however, depends on the DFN characteristics in marginally pressurized conditions, while it appears to be only mildly dependent on the DFN properties in critically stressed conditions. Highlights: The problem of injection at constant pressure in a two-dimensional Discrete Fracture Network shares similarities with the simpler case of injection in a planar shear fracture. Injection-induced aseismic slip propagation is at first order governed by a single dimensionless parameter associated with favorably oriented fractures. In critically stressed conditions, the aseismic slipping patch migration from injection point is driven by elastic stress transfer and outpaces pore-fluid diffusion. In marginally pressurized conditions, the aseismic slipping patch lags the pressurization region, and the percolation number affects the response of the medium. The aseismic moment scales quadratically to the injected fluid volume, both in critically stressed and marginally pressurized conditions. [ABSTRACT FROM AUTHOR]

Due to the lack of recognized creep on dip‐slip faults, the nature of fault creep and its role in the generation of large earthquakes is largely unexplored. The Chihshang fault in Taiwan serves as one of the best examples of a thrust creeping fault in the world, exhibiting a fast creep rate of 2–3 cm/year and the capability of producing magnitude six earthquakes. Land‐based geodetic measurements are less sensitive to fault slip with depth, especially for the deeper portion of the seaward dipping fault. Taking advantage of slip rates inferred from repeating earthquake sequences (RESs) at greater depths (up to 30 km), we present a method that embeds RES‐derived slip rates into the neighboring fault patch for geodetic data inversion. Using the geodetic and seismological data from 2007 to 2011, we achieve a higher resolution of inter‐seismic slip rate distribution below a depth of 10 km. The inferred low coupling ratio establishes an extensive creeping area that coincides with a location of abundant RES and earthquake swarms. The inferred high coupling ratio, on the other hand, delineates a locked area corresponding to the two co‐seismic slip zones of the 2003 Mw 6.8 Chengkung and 2022 Mw 6.7 Yuli earthquakes. The transition zone marking the gradual change from fully locked to shallow creeping areas appears to coincide with the post‐seismic slip area of the 2003 Chengkung event. Using the fault coupling model, the moment rate deficit measured in the two locked areas is capable of generating a Mw 6.8 event every 78 years to the south and a Mw 6.7 event every 28 years to the north. This moment rate deficit is decreased by 10% when the closely located Central Range fault is considered in a two‐fault model, leading to the slightly longer recurrence interval on the Chihshang fault. The two‐fault model potentially contributes to a better understanding of the regional inter‐seismic deformation, which is recommended to be adopted in the future studies of seismic hazard assessment. Plain Language Summary: Large earthquakes can occur on the creeping faults that slowly slip at the surface but accumulate energy at greater depth. How much different portions of the fault slip annually is the key question for seismic hazard assessment. However, for a seaward‐dipping thrust fault like the Chihshang fault in Taiwan, finding the answer is difficult because the loss of resolution at the greater depth (>10 km) due to a lack of offshore data. Taking advantage of the slip rates inferred from repeating earthquakes that mainly occurred at 10–30 km depth, we present a method that embeds repeating events data into the geodetic inversion for higher resolution of inter‐seismic slip model. We found that the areas slipped faster during the inter‐seismic period (higher inter‐seismic slip rate) coincides with the location of abundant repeating earthquakes and swarm events; The areas with lower inter‐seismic slip rate on the other hand, correspond to the zones that slipped during the 2003 Mw 6.8 Chengkung and 2022 Mw 6.7 Yuli earthquakes. By calculating how much and where the seismic energy was released through inter‐seismic slip rate, we are able to identify two separate zones on the Chihshang fault that is capable of generating a ∼Mw 6 event every 28 to the north and 78 years to the south of the fault, respectively. Key Points: We present a method that embeds repeaters‐derived slip rates into the neighboring fault patch for geodetic data inversionWe achieve a higher resolution of inter‐seismic slip rate distribution below a depth of 10 km, on a seaward‐dipping thrust faultThe inferred low coupling ratio establishes an extensive creeping area, coinciding with the location of active repeaters and swarm events [ABSTRACT FROM AUTHOR]

Abstract Slow slip events (SSEs) lasting for approximately 1 year occur every 6–8 years around the Bungo Channel in the southwest Japan subduction zone. The slip time evolution of the latest Bungo Channel SSE that occurred in 2018–2019 has been studied; however, the detailed spatial and temporal relationship between the slip process and other nearby phenomena, such as tectonic tremors and SSEs, is not well understood. Moreover, the migration of such long-term SSEs from Hyuganada to Shikoku through the Bungo Channel has been suggested, but a slip process connecting the SSEs has not been observed. In this study, we utilized 21 continuous global navigation satellite system (GNSS) stations around the Bungo Channel and Hyuganada that have been installed by us since 2014 in addition to GNSS Earth Observation System (GEONET) stations to improve the spatial resolution of such interplate slip. Based on these data, we estimate the spatial and temporal slip evolutions of the major SSE in 2018–2019, which was accompanied by tremor activity in the deep episodic tremor and slip (ETS) zone, and a smaller SSE in 2015–2016 without tremor activity. We show that the slip area of the major SSE overlaps the ETS zone, whereas that of the smaller SSE does not. This strongly suggests that synchronized tremor activity with an SSE requires a slip close to or overlapping the ETS zone. We also show two distinct slip propagation paths from the Oita area during the 2018–2019 sequence: one is a southward propagation to the Miyazaki area, leading to an SSE around the Miyazaki Plain, and the other is an eastward propagation to an area close to Cape Ashizuri, where “invading slip” is proposed to propagate from the ETS zone to a shallower megathrust source area. These slip propagations may be two of fundamental slip modes that connect slow-slip patch-like areas around the Bungo Channel and Hyuganada. Graphic Abstract

Abstract An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms.

The 2011 Tohoku-Oki great earthquake increased the difficulty of evaluating the long-term probability of seismic activity along the Japan Trench because of the unknown impact of the unprecedentedly large slip. In this study, the long-term activity of "Miyagi-ken-Oki earthquakes", an M > 7 earthquake sequence off Miyagi Prefecture, located at the edge of the source area of the Tohoku-Oki earthquake was simulated. We conducted numerical simulations of earthquake generation cycles based on the rate- and state-dependent friction law representing the stress accumulation and release process on the plate interface. We also validated the results based on analyses of observational data, including time intervals and afterslip distributions for the previous Miyagi-ken-Oki earthquakes. The simulation results were then compared with repeating-earthquake-derived interplate slip observations over 30 years. The results showed that the spatial and temporal characteristics of aseismic slips on the plate interface near the M > 7 Miyagi-ken-Oki source were qualitatively reproduced. The time interval between the M ~ 9 earthquake and the first M > 7 earthquake is shorter than the average recurrence interval of M > 7 earthquakes during the latter stage of each M ~ 9 earthquake cycle. In contrast, the interval between the first and the second M > 7 earthquakes is the longest in each M ~ 9 earthquake cycle. The time intervals of the M > 7 earthquakes fluctuated in the early stage compared to those in the latter stage of the M ~ 9 earthquake cycle. These characteristics were associated with the extent of the locked and afterslip areas in and around the source. Hence, monitoring the spatio-temporal distribution of the slip rate in and around the source area during the preparation process of earthquakes occurring in the downdip area off Miyagi Prefecture is very important to assess whether the next M > 7 earthquake is approaching. Furthermore, earthquake cycle simulations combined with earthquake and slow slip monitoring can provide meaningful insights for long-term assessments of great interplate earthquakes. [ABSTRACT FROM AUTHOR]

Slow, aseismic slip plays a crucial role in the initiation, propagation, and arrest of large earthquakes along active faults. In addition, aseismic slip controls the budget of elastic strain in the crust, hence the amount of energy available for upcoming earthquakes. The conditions for slow slip include specific material properties of the fault zone, pore fluid pressure, and geometrical complexities of the fault plane. Fine scale descriptions of aseismic slip at the surface and at depth are key to determine the factors controlling the occurrence of slow, aseismic versus rapid, seismic fault slip. We focus on the spatial and temporal distribution of aseismic slip along the North Anatolian Fault, the plate boundary accommodating the 2 cm/yr of relative motion between Anatolia and Eurasia. Along the eastern termination of the rupture trace of the 1944 M7.3 Bolu‐Gerede earthquake lies a segment that slips aseismically since at least the 1950s. We use Sentinel 1 time series of displacement and GNSS data to provide a spatio‐temporal description of the kinematics of fault slip. We show that aseismic slip observed at the surface is coincident with a shallow locking depth and that slow slip events with a return period of 2.5 years are restricted to a specific section of the fault. In the light of historical measurements, we discuss potential rheological implications of our results and propose a simple alternative model to explain the local occurrence of shallow aseismic slip at this location. Plain Language Summary: Earthquakes are the manifestation of the rapid release of elastic energy stored in the crust under the action of moving tectonic plates on either sides of a fault system. Interestingly, some faults release energy under the form of aseismic slip, which is slow and harmless. The conditions for slow slip, as opposed to earthquakes, are not fully understood and it appears of higher importance to study high‐resolution, small scale features to grow our understanding. We analyze satellite Radar imagery and GNSS data to build a movie of ground motion in the vicinity of the North Anatolian Fault in Türkiye over a section that was recognized to slip aseismically in the 70s. We show that aseismic slip there is made of slow slip events repeating every 2.5 years embedded within a larger region that slips steadily. Using these data, we model the distribution of slip rates at depth on the fault and show that aseismic slip extends until 5–8 km depth. Below, the fault is locked, accumulating energy for upcoming earthquakes. In the light of past measurements and based on our high‐resolution data set, we discuss potential physical models explaining the occurrence of slow slip in this region. Key Points: We image the spatio‐temporal variations of aseismic slip along the central section of the North Anatolian Fault with Interferometric Synthetic Aperture Radar and GNSS dataSlow slip extends over 70 km, reaches 1 cm/yr and coincides with shallow locking depth along the faultSlow slip events do not occur along the whole creeping section but have been detected since, at least, the 1980s [ABSTRACT FROM AUTHOR]

Abstract Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area.

Abstract We carried out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional tomographic studies suggest that a low-velocity anomaly exists at depths of 50–150 km around the Noto peninsula that is probably interpreted as a fluid-rich region. We infer that fluids have been supplied from the uppermost mantle to the lower crust over a geological time scale and a large volume of fluids have accumulated below the seismogenic zone beneath the Noto peninsula. A further upward migration of fluids to the upper crust, which may have suddenly started in December 2020, probably triggers numerous earthquakes at depths of 10–15 km. Since major active faults exist at shallower extensions of the hypocenters of the Noto earthquake swarm, we consider that the earthquake swarm occurs along pre-existing and weak fault planes. Dense temporary seismic observations will highlight a smaller-scale (5–10 km) 3D seismic velocity model and finer hypocenter distribution, which provide additional information for better understanding of the generation mechanisms of the Noto earthquake swarm. Graphical Abstract

Slow slip events (SSEs) lasting for approximately 1 year occur every 6–8 years around the Bungo Channel in the southwest Japan subduction zone. The slip time evolution of the latest Bungo Channel SSE that occurred in 2018–2019 has been studied; however, the detailed spatial and temporal relationship between the slip process and other nearby phenomena, such as tectonic tremors and SSEs, is not well understood. Moreover, the migration of such long-term SSEs from Hyuganada to Shikoku through the Bungo Channel has been suggested, but a slip process connecting the SSEs has not been observed. In this study, we utilized 21 continuous global navigation satellite system (GNSS) stations around the Bungo Channel and Hyuganada that have been installed by us since 2014 in addition to GNSS Earth Observation System (GEONET) stations to improve the spatial resolution of such interplate slip. Based on these data, we estimate the spatial and temporal slip evolutions of the major SSE in 2018–2019, which was accompanied by tremor activity in the deep episodic tremor and slip (ETS) zone, and a smaller SSE in 2015–2016 without tremor activity. We show that the slip area of the major SSE overlaps the ETS zone, whereas that of the smaller SSE does not. This strongly suggests that synchronized tremor activity with an SSE requires a slip close to or overlapping the ETS zone. We also show two distinct slip propagation paths from the Oita area during the 2018–2019 sequence: one is a southward propagation to the Miyazaki area, leading to an SSE around the Miyazaki Plain, and the other is an eastward propagation to an area close to Cape Ashizuri, where "invading slip" is proposed to propagate from the ETS zone to a shallower megathrust source area. These slip propagations may be two of fundamental slip modes that connect slow-slip patch-like areas around the Bungo Channel and Hyuganada. [ABSTRACT FROM AUTHOR]

One controversial issue associated with enhanced geothermal systems is induced seismicity, which limits their broader application. As induced seismicity has a strong correlation with the activation of pre-existing faults, understanding processes controlling the potential fault activation during geothermal heat extraction is of critical importance. In this study, a coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method to investigate the triggering mechanisms of potential fault activation during geothermal heat extraction. The developed thermal and hydraulic solving frameworks account for the heat transfer of various kinds and fracture fluid migration in the rock mass, respectively. The fully coupled THM scheme is established by pairwise coupling between thermal, hydraulic, and mechanical solving frameworks, and is then progressively verified against analytical solutions by five validation examples. Finally, cold-fluid injection-induced potential activation of a fault during geothermal heat extraction is investigated. The results demonstrate that the gradual aseismic opening and slip of the fault at low injection pressure are caused by the thermal contraction-induced reduction of the imposed normal stress on the fault surface due to convective cooling. The thermally induced aseismic slip weakens the fault and contributes to triggering sudden (unstable) fault slips. The convective thermal transfer coefficient is found to greatly affect the onset time of the fault aseismic slip. The effects of the injection pressure and temperature as well as the in-situ stress field are also discussed. The findings justify thermal effects in controlling the potential fault slip behavior and illuminate the triggering mechanism of unexpected seismic activities during geothermal heat extraction even at injection pressure below the safety threshold value. Highlights: A fully coupled thermal–hydraulic–mechanical (THM) scheme is formulated based on the combined finite–discrete-element method (FDEM) to investigate the potential fault activation during geothermal heat extraction. Fault aseismic slip caused by thermal contraction due to convective cooling at low injection pressure contributes to triggering unstable fault slip. The convective thermal transfer coefficient greatly affects the onset time of the fault aseismic slip. The injection pressure affects the time at which the fault aseismic slip becomes stable and the in-situ stress heavily affects the aseismic slip amount. Thermal effects are justified in triggering seismic activities during geothermal heat extraction at injection pressure below the safety threshold. [ABSTRACT FROM AUTHOR]

Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut‐in. Understanding the extent and duration of the post‐injection microseismicity is critical to limit the risk of fluid‐induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut‐in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone toward induced seismicity mitigation strategies. Plain Language Summary: Injection of fluid in the subsurface for energy and storage applications can lead to the onset of microseismicity, and possibly to major induced seismic events. Fluid pressurization decreases the shear strength of surrounding faults and slip occurs when the in situ shear stress on a fault reaches its shear strength. The nature of slip (aseismic or seismic) depends on the rate at which it occurs and thus on the stability of the deformation. Understanding the mechanics controlling the onset and arrest of aseismic slip and the transition to seismic slip is therefore key to design mitigation strategies for the safe utilization of the subsurface. In this contribution, we investigate using theoretical and numerical techniques how aseismic slip on a fault plane nucleates and evolves in response to fluid injection and how it stops after injection shut‐in when fluid pressure relaxes. We demonstrate that critically stressed faults prior to injection can slip for a longer time after injection shut‐in than during injection and that the extent of rupture can double in size after the end of injection. These results help to quantify the duration and sphere of influence of fluid injection where microseismicity can occur during and after injection. Key Points: Quantification of fault aseismic slip that continues to propagate away from the former injection point after injection shut‐inArrest time of aseismic slip is proportional to the pressurization durationCritically stressed faults slip significantly longer and at a greater distance than marginally pressurized faults after injection shut‐in [ABSTRACT FROM AUTHOR]

The strength and sliding behavior of faults in the crust is largely controlled by friction and effective stress, which is itself modulated by fluid pressure. Most earthquake models assume a fixed pore fluid pressure despite widespread evidence that is varies strongly in time due to changes in permeability. Here we explore how dynamic changes in pore pressure influence the properties of earthquakes in the upper crust. To study this problem we develop a two dimensional model that incorporates slow tectonic loading and fluid pressure generation during the interseismic period with frictional sliding on a thrust fault whose permeability evolves with slip. We find that the presence of relatively modest fluid overpressures tends to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity and the earthquake recurrence time compared to models without fluids. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to the presence of high pore pressures prior to rupture. The models also show evidence for aftershocks that are driven by fluid transfer along the fault plane after the mainshock. Overall, this study shows that fluids can exert an important influence on earthquakes in the crust, which is mostly due to modulation of the effective stress and variations in permeability, and to a lesser extent to poro‐elastic coupling. Plain Language Summary: In this study we use a numerical model to investigate how fluid pressures vary over the seismic cycle and how they interact with and influence the properties of earthquakes that occur in the upper crust. In the model, fluid overpressures are generated slowly during the interseismic period by phenomena such as dehydration reactions while they are episodically released during earthquakes due to fracturing and a dramatic increase in permeability. The models show that the presence of high fluid pressures has an important influence on earthquakes. High fluid pressures favor smaller, more frequent earthquakes. Also high fluid pressures may sometimes be responsible for aftershocks and for anomalously slow earthquakes that involve slip over several months rather than several seconds. Overall, we show that the presence of fluids in the crust plays an integral part in the earthquake process. Key Points: Properties of earthquakes are markedly influenced by the presence of fluid overpressuresRuptures in wet models have relatively low stress drops, sliding speed and rupture velocitySlow slip events and aftershocks might be a fingerprint of fluid overpressures [ABSTRACT FROM AUTHOR]

Aseismic slip is a stable fault slip, which allows strain to be relieved smoothly. Aseismic slip prevents the earthquake propagation, but it could nucleate an earthquake elsewhere. Understanding the mechanism of aseismic slip is promising in revealing the seismic cycle. Experimental evidence showed clay-rich fault gouge bears a low-friction strength, and the friction is strengthened with slip velocity (velocity-strengthening), which was thought to support aseismic slip. Clay minerals are comprised of platy crystalline layers with water intercalated between them, which may act as a lubricant. Sliding between clay layers was suspected to support aseismic slip but lacked a clarified mechanistic insight. We use non-equilibrium molecular dynamics simulations to show that shear-induced interlayer sliding is frictionally weak and velocity-strengthening, which evidences the role of clay minerals in aseismic slip. We find that interlayer water is a viscous fluid at most times, which explains the shear response of interlayer sliding. Depending on temperature and pressure conditions, intercalated water can be monolayer or bilayer, fluidic or ice like. Shear induces icelike water to transform into fluidic water, which happens as a stick-slip phenomenon reflecting a first-order transition. Increased pore fluid pressure leads to the transformation from monolayer to bilayer intercalated water, resulting in a lower friction strength and enhanced velocity-strengthening behavior. Our work suggests that disclosing the hydration state of a clay mineral is preliminary when studying fault mechanics. [ABSTRACT FROM AUTHOR]

Temporal seismic observations from pop-up type ocean-bottom seismometers were used to detect tectonic tremors immediately following the 2011 Tohoku-Oki earthquake in the northern periphery of the aftershock area. Near-field observations clearly distinguished tremors from regular earthquakes based on their spectral shape in the frequency band of 1–4 Hz. In addition to tremors accompanied by very low-frequency earthquakes (VLFEs), we detected 130 tremors without known VLFE activity during April–October 2011. The newly detected tremors were in the vicinity of a sequence of small repeating earthquakes, indicating a mixed distribution of tremors and regular interplate earthquakes in the region. Tremor activity was high immediately after the deployment of seismometers and gradually decreased. In addition, the tremor activity fluctuated with two activations with an interval of approximately 90 days, similar to the intervals between tremor bursts after 2016. The results of the study suggest that the observed tremors occurred under the influence of aseismic slip caused by the decaying afterslip of the preceding Tohoku-Oki and Mw 7.4 interplate earthquakes and episodic accelerations with a quasi-periodicity unique to the area. [ABSTRACT FROM AUTHOR]

Anthropogenic fluid injections at depth induce seismicity which is generally organized as swarms, clustered in time and space, with moderate magnitudes. Earthquake swarms also occur in various geological contexts such as subduction zones, mountain ranges, volcanic, and geothermal areas. While some similarities between anthropogenic and natural swarms have already been observed, whether they are driven by the same mechanism, or by different factors, is still an open question. Fluid pressure diffusion or aseismic deformation processes are often proposed to explain observations of hypocenters migration during swarms, while recent models suggest that swarm seismicity is rather triggered by fluid‐induced aseismic fault slip. Here, using a global compilation of 22 natural and anthropogenic swarms, we observe that duration, migration velocity, and total moment scale similarly for all swarms. This supports a common driving process for both natural and induced swarms. The scaling relations are similar to those found for slow slip events. These observations highlight the prevalence of fluid‐induced aseismic slip as main driver of earthquakes migration during swarms. After quantifying aseismic slip released in the swarms, we propose an approach to estimate the seismic‐to‐total moment ratio, which we then compare to a theoretical estimation that depends on the migration velocity of the swarm and the effective stress drop. Our findings lead to a generic explanation of the process driving earthquake swarms that might open new possibilities to monitor seismic swarms. Plain Language Summary: Earthquake swarms are a particular type of seismic activity, during which a sequence of many earthquakes occurs without being initiated by a larger one. Swarms can be induced by anthropic hydraulic injections at depth, like during geothermal power exploitation and massive storage of diverse fluids (i.e., wastewater, CO2) in porous reservoir formations. Natural earthquake swarms are also observed in a large variety of geological contexts. Previous works showed that natural and injection‐induced swarms share some similarities, like the migration of seismicity. However, their underlying processes remain unclear. Here, we explain the observed similarities in both types of swarms by a model in which earthquakes are triggered by the propagation of an aseismic slip transient, which in turn is induced by pressurized fluid circulation. This model reconciles a suite of independent observations made over different length and time scales and provides a generic explanation of the driving process for the migration of earthquake swarms. Key Points: Scaling laws support that injection‐induced and natural earthquake swarms have the same driving mechanismAseismic slip is a key feature of earthquake swarms, although its contribution differs from one swarm to anotherWe introduce a simple model based on fluid‐induced aseismic slip propagation to relate observables to physical parameters [ABSTRACT FROM AUTHOR]

Abstract Microseismicity associated with fluid pressurization in the subsurface occurs during fluid injection but can also be triggered after injection shut‐in. Understanding the extent and duration of the post‐injection microseismicity is critical to limit the risk of fluid‐induced seismicity and insure the safe utilization of the subsurface. Using theoretical and numerical techniques, we investigated how aseismic slip on a fault plane evolves and stops after a fluid pressurization event. We found that the locking mechanisms controlling the arrest of aseismic slip highly depend on the initial fault stress criticality and the pressurization duration. The absolute arrest time of fault aseismic slip after injection shut‐in is proportional to the pressurization duration and increases significantly with the initial fault stress criticality. Given that microseismicity can be triggered by aseismic slip, these results provide insights into the mechanics controlling the arrest of microseismicity after fluid pressurization as a milestone toward induced seismicity mitigation strategies.

Multi-source datasets, including the UAV-Lidar/Photogrammetry, InSAR, and field investigation data, have been used for revealing the complex surface displacement pattern and focal mechanism of the 2021 Maduo Mw 7.3 earthquake. First, a co-seismic surface deformation field was extracted from the Synthetic Aperture Radar (SAR) images captured by ALOS-2 and Sentinel-1 satellites. Second, the SAR pixel offset tracking results were adopted to detect the initial location of the seismogenic fault. Then, the Lidar digital elevation model with high spatial resolution and field investigation were employed to refine and verify the location of the seismogenic fault. It was found that bifurcated strike-slip rupture should account for the 2021 Maduo earthquake. As indicated by the estimated faulting model based on the InSAR data, the maximal fault slip was ~6.2 m, occurring in the southeast of the main seismogenic fault, and five remarkable slip concentrations controlled the surface displacement of the 2021 Maduo earthquake. Furthermore, the co- and post-seismic InSAR deformation, dilatation, shear strain, Coulomb failure stress, and aftershock sequence suggest that the co-seismic rupture of the two main seismogenic faults have triggered the aseismic slip along the Changmahe fault. Lastly, according to the Coulomb failure stress change due to the historical earthquakes and the 2021 Maduo earthquake, the 1937 M 7.8 earthquake predominantly controlled the Coulomb failure stress change along the Kunlun fault, and the Xidatan-Alake Lake and Maqin segments had a higher risk of future earthquake than the other segments. [ABSTRACT FROM AUTHOR]

We carried out seismic tomography study to reveal three-dimensional (3D) seismic velocity structures in the Noto peninsula, Japan, where swarm-like seismic activity started in December 2020. The obtained results reveal a highly heterogeneous structure in the crust. The most striking feature is the existence of a low-velocity anomaly in the lower crust beneath the Noto earthquake swarm. Although the data set used in this study cannot resolve the upper mantle structure, previous regional tomographic studies suggest that a low-velocity anomaly exists at depths of 50–150 km around the Noto peninsula that is probably interpreted as a fluid-rich region. We infer that fluids have been supplied from the uppermost mantle to the lower crust over a geological time scale and a large volume of fluids have accumulated below the seismogenic zone beneath the Noto peninsula. A further upward migration of fluids to the upper crust, which may have suddenly started in December 2020, probably triggers numerous earthquakes at depths of 10–15 km. Since major active faults exist at shallower extensions of the hypocenters of the Noto earthquake swarm, we consider that the earthquake swarm occurs along pre-existing and weak fault planes. Dense temporary seismic observations will highlight a smaller-scale (5–10 km) 3D seismic velocity model and finer hypocenter distribution, which provide additional information for better understanding of the generation mechanisms of the Noto earthquake swarm. [ABSTRACT FROM AUTHOR]

Accelerated creep, primarily through vertical displacement, has been occurring along 15 en-echelon faults belonging to a 15 km-long and 1.5 km-wide, N-S-trending dilational jog of the active West Valley Fault (or West Marikina Valley Fault), on the southeastern part of Metro Manila, Philippines. The much-larger-than-known tectonic slip rates had been the only reliable evidence in support of excessive groundwater withdrawal as the trigger of creep in the 1990s. Recently available groundwater extraction data (1977–2019) could more directly and consistently link groundwater withdrawal to accelerated creep in the 1990s. Twenty years (1999–2019) of precise displacement measurements could also reveal significant spatial and temporal links between slip rate changes and patterns of groundwater extraction. Our analysis shows that greater rates of vertical displacement are related to the timing of faster extraction rates. Variations in slip rates between the northern and southern measurement sites are primarily due to regional differences in groundwater extraction, which are influenced by differences in the implementation of water extraction reduction regulations. Proximity to the key source of groundwater recharge (Laguna Lake) is also an influencing factor. Although there are many unknowns inherent to this type of study, continued depressurization could induce static stress changes that could modulate the timescale of earthquake occurrence due to the natural course of stress loading driven by regional tectonics. The current and potential effects of continued depressurization in the areas with high slip rates and surrounding regions are paramount considerations in crafting and implementing tighter and extended groundwater extraction regulations.

Fluid injection stimulates seismicity far from active tectonic regions. However, the details of how fluids modify on‐fault stresses and initiate seismic events remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus with a 3 m saw‐cut granite fault and compared events induced at different levels of background shear stress. Water was injected at 10 mL/min and normal stress was constant at 4 MPa. In all experiments, aseismic slip initiated on the fault near the location of fluid injection and dynamic rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa‐level injection pressures were reached and the dynamic rupture propagated beyond the fluid pressure perturbed region. At lower stress levels, dynamic rupture initiated hundreds of seconds later and was limited to regions where aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We found that the initiation of slow slip was broadly consistent with a Coulomb failure stress, but that initiation of dynamic rupture required additional criteria to be met. Even high background stress levels required aseismic slip to modify on‐fault stress to meet initiation criteria. We also observed slow slip events prior to dynamic rupture. Overall, our experiments suggest that initial fault stress, relative to fault strength, is a critical factor in determining whether a fluid‐induced rupture will "runaway" or whether a fluid‐induced rupture will remain localized to the fluid pressurized region. Plain Language Summary: Humans can create earthquakes on natural faults by injecting fluids underground. However, details regarding what factors affect these earthquakes are not fully understood. We conducted laboratory experiments on 3 m blocks of rock that slip similar to a natural fault. Our experiments investigated how the initiation and overall size of earthquakes differed when fluid was pumped into a critically loaded fault (nearly ready to host an earthquake) versus a fault that was less critical. In the near‐critical case, earthquakes occurred quickly and ruptured the entire fault. These earthquakes required fluid pressure to start the earthquake, but then were sustained by energy already present in the rock rather than due to fluid pressure. However, when the fault was not critical, earthquakes could only initiate after fluid pressure caused silent slip to redistribute significant amounts of shear stress from within the fluid pressurized area to the surrounding areas. When stress had redistributed enough that the surrounding areas reached a critical state, an earthquake initiated, but it did not rupture very far and remained small. Our experiments agree with recently published computer simulations that illustrate how induced earthquakes are strongly affected by the levels of preexisting stress in the rock. Key Points: Slip under high stress levels is driven by elastic stress transfer from induced aseismic slip, ruptured beyond the fluid pressurized regionSlip under low stress levels was primarily driven by fluid injection and was limited by the extent of the fluid pressurized regionRegardless of background stress level, fluid injection first produced aseismic slip and slow slip events, then dynamic rupture [ABSTRACT FROM AUTHOR]

Improved imaging of the spatio‐temporal growth of fault slip is crucial for understanding the driving mechanisms of earthquakes and faulting. This is especially critical to properly evaluate the evolution of seismic swarms and earthquake precursory phenomena. Fault slip inversion is an ill‐posed problem and hence regularization is required to obtain stable and interpretable solutions. An analysis of compiled finite fault slip models shows that slip distributions can be approximated with a generic elliptical shape, particularly well for M ≤ 7.5 events. Therefore, we introduce a new physically informed regularization to constrain the spatial pattern of slip distribution. Our approach adapts a crack model derived from mechanical laboratory experiments and allows for complex slipping patterns by stacking multiple cracks. The new inversion method successfully recovered different simulated time‐dependent patterns of slip propagation, that is, crack‐like and pulse‐like ruptures, directly using wrapped satellite radar interferometry (InSAR) phase observations. We find that the new method reduces model parameter space, and favors simpler interpretable spatio‐temporal fault slip distributions. We apply the proposed method to the 2011 March–September normal‐faulting seismic swarm at Hawthorne (Nevada, USA), by computing ENVISAT and RADARSAT‐2 interferograms to estimate the spatio‐temporal evolution of fault slip distribution. The results show that (a) aseismic slip might play a significant role during the initial stage and (b) this shallow seismic swarm had slip rates consistent with those of slow earthquake processes. The proposed method will be useful in retrieving time‐dependent fault slip evolution and is expected to be widely applicable to studying fault mechanics, particularly in slow earthquakes. Plain Language Summary: A key earthquake science challenge is to understand when an instability on a fault will arrest or run away into a large rupture. However, the slip nucleation process seems not to produce seismic waves and hence remains hidden to most seismological methods. Geodetic methods, which can directly measure motions at Earth's surface, offer a complementary tool to improve our ability to map the fault slip. In this work, we expand an experimentally observed crack model, and propose a new inversion method for finding models of fault slip that can fit the observations of surface motions. The new method greatly reduces computation complexity respecting previous state‐of‐the‐art methods, and is validated against synthetic experiments. We apply this new method to 2011 Hawthorne earthquake swarm (Nevada, USA) and discovered an aseismic slow slip before seismicity rate increased. That preparation stage was followed by a triggered larger slip on a nearby fault, and after that, the seismicity and fault slip rate reduced rapidly. We expect that this new methodology will be applied to detect similar precursory aseismic slip during long‐lasting earthquake sequences, and allow us to retrieve detailed slip growth in space and time, which ultimately will advance our understanding of the faulting mechanics. Key Points: We invert for time‐dependent fault slip distribution from geodetic data, based on a low dimensional model for elliptical slip distributionsSignificant aseismic slip preceded the most energetic M4.6 event in the 2011 Hawthorne shallow seismic swarmAverage slip rates (lower bound) of this swarm and slow‐slip phenomena are similar, implying a notable role of aseismic processes in swarms [ABSTRACT FROM AUTHOR]

Inversions of InSAR ground deformation in the Delaware Basin have revealed an aseismic slip on semi‐optimally oriented normal faults located close to disposal wells. The slip, occurring over 3–5 years, extends approximately 1 km down‐dip, over 10 km along strike, and reaches 25 cm. We develop and calibrate 2D and pseudo‐3D coupled pore pressure diffusion and rate‐state models with velocity‐strengthening friction tailored to this unique height‐bounded fault geometry. Pressure diffusion is limited to a high‐permeability fault damage zone, and the net influx of fluid is adjusted to match the observed slip. A 1–2 MPa pressure increase initiates slip, with ∼5 MPa additional pressure increase required to produce ∼20 cm slip. Most slip occurs at approximately constant friction. Fault zone permeability must exceed ∼10−13 m2 to match the along‐strike extent of slip. Models of the type developed here can be used to operationally manage injection‐induced aseismic slip. Plain Language Summary: Disposal of wastewater through wells in the Delaware Basin is causing slip on shallow faults. This slip occurs gradually over several years. We develop computer simulations of fluid transport along these faults, calculating pressure changes in the fluid that weaken the fault and cause it to slip. We adjust properties of the fault zone and the rate at which fluid enters the fault zone to achieve consistency with satellite‐based measurements of the amount and spatial extent of slip. Once calibrated, these models can be used to predict the slip response to hypothetical injection scenarios. Key Points: We model the aseismic slip response to injection‐induced fluid infiltration and pressurization of the fault damage zoneMatching the observed slip requires a 5 MPa pressure increase beyond the 1–2 MPa required to initiate slipMatching observed along‐strike extent of slip requires fault zone permeability of 10–13 m2 or greater [ABSTRACT FROM AUTHOR]

On 21 May 2021, an MS 6.4 earthquake occurred in Yangbi, Yunnan province, China. A significant foreshock sequence occurred 3 days before the mainshock, which provides an opportunity to study earthquake nucleation. In this study, we adopt a template matching technique and a double‐difference location method to build a catalog for the foreshock sequence of the 2021 Yangbi MS 6.4 earthquake, which contains 1,086 events with high‐precision locations. The whole foreshock sequence can be grouped into four episodes, each with one MW > 3.7 principal foreshock. The foreshocks within the first episode initiated on a NW‐trending small fault, whereas the subsequent foreshocks within episodes 2–4 jumped onto a nearby major fault to gradually rupture toward NW direction. We identify three MW > 3.4 repeating earthquakes on the NW‐trending small fault in episode 1, implying that aseismic slip occurred in the early stage of the mainshock nucleation. Meanwhile, the rupture dimension estimation and stress disturbance analysis of the later foreshocks in episodes 2–4 reveal a cascading rupture process on the major fault, indicating that stress transfer may dominate the later stage of the mainshock nucleation. Hence, our study suggests that the two end‐member mechanisms of aseismic slip and stress transfer may be compossible in the nucleation process of the 2021 Yangbi MS 6.4 mainshock. Plain Language Summary: The 2021 MS 6.4 Yangbi earthquake was preceded by a significant foreshock sequence that initialized ∼3 days before the mainshock. We first adopt state‐of‐the‐art methods to build a high‐precision and high‐completeness foreshock catalog. The unprecedented foreshock catalog outlines the detailed seismogenic structure accommodating the mainshock. To investigate the relationship between the mainshock and its foreshocks, we further calculate the rupture dimensions of 21 ML ≥ $\ge $ 2.5 foreshocks and estimate the stress change triggered by the two largest foreshocks MW 4.6 and MW 5.2. Our results suggest that the nucleation of the MS 6.4 mainshock may be initiated by an aseismic slip process, then dominated by cascade triggering. Key Points: We detect and locate 9,030 events for the 2021 Yangbi MS 6.4 earthquake sequence including 1,086 foreshocksThe foreshocks consist of four episodes and outline a large through going fault hosting the nucleation of the MS 6.4 mainshockAseismic slip and cascade triggering could be compossible in the nucleation process of the MS 6.4 mainshock [ABSTRACT FROM AUTHOR]

Recent seismic and geodetic observations suggest the involvement of pressurized fluid and aseismic slip during earthquake swarms. However, the interaction between these two factors is not fully understood. In this study, we show geodetic signals induced by aseismic opening and shear dislocation on a fault, accompanied by hypocenter migration during an earthquake swarm in the Hakone volcano, central Japan. The hypocenters were concentrated on a vertical plane, and the focal mechanisms were strike‐slip faults whose nodal planes were consistent with the hypocenter distribution. Precursory seismicity occurred 1 week before the main swarm. We observed a complex pattern of hypocenter migration during the seismic sequence; namely, the lower limit of the seismicity appears to expand downward during the precursory seismicity, followed by an upward hypocenter diffusion. We also observed tilt changes that could be explained by opening and shear dislocation on the fault plane inferred from the planar hypocenter distribution. Furthermore, we identified several groups of repeating earthquakes. The average aseismic slip history inferred from the repeating earthquakes indicated a minor aseismic slip of 0.4 mm during the precursory seismic activity, which accelerated by up to 3 mm during the main swarm. The total cumulative slip was consistent with the geodetic model results. These observations may suggest that fluid intrusion caused the aseismic slip, and the complex migration of the hypocenters reflects the aseismic slip propagation. We hypothesize that aseismic deformation acts as the driving force for the earthquake swarms together with the intrusion of pressurized fluid. Plain Language Summary: An earthquake swarm is seismic activity that deviates from the typical mainshock–aftershock sequence. According to several studies, the triggering of earthquake swarms is related to the migration of highly pressurized fluid. However, aseismic slip on a fault plane is also thought to drive the triggering of earthquake swarms. Based on precise hypocenter locations, a combination of both the driving forces was suggested to be responsible for the occurrence and hypocenter migration of earthquake swarms. However, the interaction between aseismic deformation and fluid injection in earthquake swarms has not been fully clarified. From 11 May to 20 May 2019, an earthquake swarm occurred beneath Lake Ashi in the western part of the Hakone caldera, Japan. In this study, we analyzed geodetic data obtained using tiltmeters and observations of repeating earthquakes and investigated the aseismic deformation associated with the swarm activity. We detected a significant tilt change that was consistent with shear tensile fault movements. We also detected several groups of repeating earthquakes and the migration of hypocenters. The findings suggest that aseismic deformation acts as the driving force for earthquake swarms in addition to the intrusion of highly pressurized fluid. Key Points: A complex hypocenter migration pattern was observed during the earthquake swarmThe complex hypocenter migration, repeating earthquakes, and tilt changes may suggest the occurrence of aseismic slip [ABSTRACT FROM AUTHOR]

The Delaware Basin, Texas is currently a hot‐spot of induced seismicity and ground deformation due to fluid extraction and injection associated with horizontal drilling techniques; however, the driving mechanism behind the seismicity and deformation remains under debate. Using vertical and east‐west horizontal surface deformation measurements derived from Sentinel‐1 interferometric synthetic aperture radar (InSAR), we show that the subsurface responds differently to oil and gas activity in the northern and southeastern portions of the basin. In the north, where there is little seismicity, deformation patterns display long‐wavelengths and equidimensional patterns. In contrast, the southeast region hosts most of the seismicity and displays spatial deformation patterns with narrow linear features that strike parallel to the maximum principal horizontal stress and to trends in seismicity, suggesting movement along normal faults. We model a linear deformation feature using edge dislocations and show that the InSAR observations can be reproduced by slip on normal faults contained within the Delaware Mountain Group (DMG), the formation that hosts local wastewater injection and the majority of earthquakes. Our model consists of three parallel, high‐angle normal faults, with two dipping toward one another in a graben structure. Slip magnitudes reach up to 25 cm and are spatially correlated with injection wells. Measured seismicity can only explain ∼2% of the fault motion predicted by our fault model, suggesting that slip leading to the deformation is predominantly aseismic. We conclude that seismic and aseismic fault motion in the southeastern Delaware Basin is likely driven by wastewater injection near critically‐stressed normal faults within the DMG. Plain Language Summary: In the Delaware Basin, TX, widespread oil and gas operations have been linked to an increase in earthquake frequency and ground deformation. We use satellites to measure the ground deformation and show that the northern and southern portions of the basin respond differently to the pumping and injection of fluids. The southern portion displays narrow linear displacement patterns, whereas the northern region displays wide and equidimensional features. The relationship of the narrow features in the southern portion of the basin to local stress conditions and earthquake locations suggests downward slip on faults. Using analytic models in a small study area, we develop a three‐fault slip model that is consistent with ground displacement measurements, the location and sense of slip of the largest local earthquakes, and wastewater disposal wells. Our findings suggest that wastewater disposal in the Delaware Mountain group is reactivating pre‐existing normal faults, leading to induced earthquakes and non‐seismic slip. Key Points: Linear deformation features in the southeastern portion of the Delaware Basin are attributed to aseismic slip on normal faultsIdentified faults create graben structures that likely belong to a larger graben networkAseismic and seismic fault movement in Texas' Delaware Basin can be linked to wastewater injection in the Delaware Mountain group [ABSTRACT FROM AUTHOR]

The 700‐km‐long Chaman fault (CF) marks the western edge of the plate boundary between India and Eurasia. Although global plate models predict 2.3–3.6 cm/yr left‐lateral motion between both plates, the fault is known to have hosted few earthquakes in historical times. Recent geodetic measurements attested the presence of aseismic slip locally. To detail the interplay between fast and slow slip along the CF, we build three Interferometric Synthetic‐Aperture Radar time series of ground deformation covering the whole fault length over 5 years (2014–2019). We find that most of the active fault trace slips aseismically and continuously. From south to north, we identify three creeping fault portions: the Nushki, Central, and Qalat segments of lengths between 80 and 130 km. The loading rate is 1.2 ± 0.3 cm/yr for the two southernmost portions, while it is about 0.7 ± 0.2 cm/yr for the Qalat segment. The Central segment and the nearby locked segments have hosted the largest known historical earthquakes on the CF, and three moderate magnitude earthquakes in our observation period. We image these earthquakes for which modeled slip at depth (Mw 5–5.6), time series of surface slip and deformation patterns argue toward large triggered aseismic slip. The June 2018 event displays postseismic moment 3–15 times greater than coseismic moment. Over the two decades covered by geodetic observations, continuous or triggered aseismic slip dominates along most of the fault and co‐locates with earthquakes. We observe that fault geometrical complexities delimit active segments and may be responsible for the kilometer‐scale intertwining between seismic and aseismic events. Plain Language Summary: The Chaman fault (CF) separates Indian and Eurasian tectonic plates moving at about 3 cm/yr with respect to each other. This fault is known to have hosted very few earthquakes (sudden and rapid slip) in historical times, and appears to slip slowly. We measure slip on the CF with radar satellite images covering 2014–2019, and assess the average amount of slip attributed to earthquakes over the past century. We identify three 80–130‐km‐long portions of the 700‐km‐long fault that slip silently, without radiating seismic waves, at rates reaching 1.2 ± 0.3 cm/year along the southern half, and 0.7 ± 0.2 cm/year in the north. The largest historical earthquakes and three earthquakes of moderate size in our observation period occurred on the central part of the CF. We image the surface deformation induced by these three earthquakes and find that they were followed by significant slow slip on the fault, especially the third event in June 2018. Since the 2000s space‐based measurements indicate that the slow mode of slip dominates on the CF plane and co‐locates with earthquakes. Changes of fault orientation may delimit portions of the fault which either slip slowly or break into fast earthquakes. Key Points: Aseismic slip along the Chaman fault (CF) reaches 12 mm/yr with three large distinct aseismic sections, despite marked elastic strain increaseThree earthquakes are imaged along the central CF and two exhibit large post‐seismic afterslipAlong‐strike distribution of slip is compared with historical seismicity and fault geometry [ABSTRACT FROM AUTHOR]

For decades, seismogenic faults of the Zagros orogen that episodically experienced large earthquakes studies in detail; however, the active faults with the aseismic slips have only have been studied as a possible contributor to their seismic hazard assessments. This study identifies the contribution of pressure solution creeps to their seismic hazard assessments. Microscopic measurements of the calcite slickenline fibres on the Sabz-Pushan thrust planes documents a length of 1.28 ± 0.50 mm. Microscopic-scaled stretched calcite length measurements represent the present slip magnitude along with this thrust. This slip magnitude correlates with GPS velocity measurements of the Sabz-Pushan thrust of 1.5 ± 0.2 mm/year. The slip magnitude of the aseismic creep in the Sabz-Pushan thrust is closely associated with postseismic adjustments of the 1824 and 1994 earthquakes. The dynamic fluid flow and fluid circulations of the Mand (Qara Aghj) River along with the Sabz-Pushan and Sepidar thrusts and sub-simple shearing are the main controlling factors in an aseismic slip of the Sabz-Pushan and Sepidar thrusts. These thrusts forming a compressional push-up structure that may be a critical factor in the brittle deformation of the damage zone and ductile flow in-depth on the fault planes. The presence of rotated lens-shaped Type III e-twins within the slickenline fibres on the thrust planes confirms that the depth of aseismic creep occurred at > 400 °C which represents calcite ductile flow of the pressure solution mechanism as a potentially important deformation along with the Zagros thrust system of the Zagros orogen. Analysis of the slickenline fibre orientations indicates that the mean plunge and trend of maximum principal stress (σ1) of the Sabz-Pushan thrust are N31° E–N36° E, sub-horizontal and the mean trend of the minimum principal stress (σ3) is NNW and is sub-vertical. Horizontal maximum principal stress confirms that the Sabz-Pushan characterizes as a dip-slip fault. [ABSTRACT FROM AUTHOR]

Multi-source datasets, including the UAV-Lidar/Photogrammetry, InSAR, and field investigation data, have been used for revealing the complex surface displacement pattern and focal mechanism of the 2021 Maduo Mw 7.3 earthquake. First, a co-seismic surface deformation field was extracted from the Synthetic Aperture Radar (SAR) images captured by ALOS-2 and Sentinel-1 satellites. Second, the SAR pixel offset tracking results were adopted to detect the initial location of the seismogenic fault. Then, the Lidar digital elevation model with high spatial resolution and field investigation were employed to refine and verify the location of the seismogenic fault. It was found that bifurcated strike-slip rupture should account for the 2021 Maduo earthquake. As indicated by the estimated faulting model based on the InSAR data, the maximal fault slip was ~6.2 m, occurring in the southeast of the main seismogenic fault, and five remarkable slip concentrations controlled the surface displacement of the 2021 Maduo earthquake. Furthermore, the co- and post-seismic InSAR deformation, dilatation, shear strain, Coulomb failure stress, and aftershock sequence suggest that the co-seismic rupture of the two main seismogenic faults have triggered the aseismic slip along the Changmahe fault. Lastly, according to the Coulomb failure stress change due to the historical earthquakes and the 2021 Maduo earthquake, the 1937 M 7.8 earthquake predominantly controlled the Coulomb failure stress change along the Kunlun fault, and the Xidatan-Alake Lake and Maqin segments had a higher risk of future earthquake than the other segments.

Abstract Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation.

Strong strain and pore pressure changes are observed after three Mw 4.5+ local and one Mw 7.2 regional earthquake during 2010–2017 in borehole strainmeters near Anza, California. The strain change emerges immediately after the earthquakes and lasts 40–100 days with amplitudes up to 10−7, larger than the coseismic strain offsets. The pore pressure exhibits change immediately after the earthquakes at some boreholes and with a delay of 4–10 days at the others. A joint analysis of the observed postseismic strain and pore pressure change suggests that the postseismic strains could be explained by combined effects of poroelastic deformation due to earthquake‐induced pore pressure change and elastic deformation due to an earthquake‐triggered aseismic slip on a local fault. Our study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, pore pressure changes after the earthquake could be even more important in producing postseismic deformation. Plain Language Summary: Understanding the physical mechanisms producing postseismic underground deformation is important for assessing fault slip budget and seismic hazards. In this study, we seek to clarify the possible roles of aseismic fault slip and underground water in producing postseismic deformation, through a joint analysis of underground deformation and pressure change in the pores of underground water reservoirs observed in the boreholes in southern California following four middle/large‐magnitude earthquakes. We find that both the underground deformation and pore pressure in the underground water reservoir exhibit changes lasting 1–3 months after the earthquakes, with changing amplitudes larger than the coseismic changes. These observations are well explained by a mechanism in which the mainshock earthquakes instantly trigger aseismic slips on the local faults and alter the hydrological conditions in the region; the change of hydrological condition results in a postseismic change of pore pressure in the underground water reservoir and produces poroelastic deformation in the region, while the aseismic fault slips produce elastic deformation. This study indicates that, in addition to possible aseismic fault slips triggered by an earthquake, changes of pore pressure after the earthquake in the underground water reservoir could play an even more important role in producing postseismic deformation. Key Points: We observe the strong months‐long change of strain and pore pressure after four Mw 4.5+ earthquakes in borehole strainmeters at Anza, CaliforniaThe postseismic strains last 40–100 days, and exhibit different trends and larger amplitudes up to 10−7 compared to coseismic strainsPostseismic strain = poroelastic strain by earthquake‐induced pore pressure change + elastic strain by an earthquake‐triggered aseismic slip [ABSTRACT FROM AUTHOR]

It is widely recognized that fluid injection can trigger aseismic fault slip. However, the processes by which the fluid‐rock interactions facilitate or inhibit slip are poorly understood and some are oversimplified in most models of injection‐induced slip. In this study, we perform a 2D anti‐plane shear investigation of aseismic slip that occurs in response to fluid injection into a permeable fault governed by rate‐and‐state friction. We account for porosity and permeability changes that accompany slip, including dilatancy, and quantify how these processes affect pore pressure diffusion, which couples to aseismic slip. Fault response to injection has two phases. In the first phase, slip is negligible and pore pressure closely follows the standard linear diffusion model. Pressurization eventually triggers aseismic slip close to the injection site. In the second phase, aseismic slip front expands outward and dilatancy causes pore pressure to depart from the linear diffusion model. We quantify how prestress, injection rate, permeability and other fluid transport properties affect the slip front migration rate, finding rates ranging from 10 to 1,000 m/day for typical parameters. The migration rate is strongly influenced by the fault's closeness to failure and injection rate. The total slip on the fault, on the other hand, is primarily determined by the injected volume, with minimal sensitivity to injection rate. Additionally, we show that when dilatancy is neglected, slip front migration rate and total slip can be several times higher. Our modeling demonstrates that porosity and permeability evolution, especially dilatancy, fundamentally alters how faults respond to fluid injection. Plain Language Summary: The underground injection of fluids during wastewater disposal, geothermal operations, and other energy‐production activities has been linked to the occurrence of earthquakes. In addition to earthquakes, fluid injection can also trigger aseismic slip on faults, that is, frictional sliding that occurs so slowly that seismic waves and ground shaking are not produced. Here, we perform computer modeling of fluid injection and aseismic slip, exploring how the injection rate and fluid transport properties influence the aseismic slip response. We speculate that additional complexity in frictional properties and other conditions would cause aseismic slip to be accompanied by numerous, small earthquakes (microseismicity), as is often observed during injection. We quantify the rate at which aseismic slip migrates outward from the injection site and compare predicted migration rates to observed microseismicity patterns. Our model also predicts fluid pressure changes, slip, rock deformation surrounding the fault, and fluid flow paths that might be measurable and used to validate the modeling. Key Points: Modeling of constant rate fluid injection into a fault predicts steadily propagating aseismic slip frontMigration rate of aseismic slip front increases with injection rate and ranges from 10 to 1,000 m/day for typical parametersDilatancy and permeability enhancement alter system response as compared to linear pore pressure diffusion [ABSTRACT FROM AUTHOR]

Understanding the interplay of seismic and aseismic slip is key in seismic hazard evaluation. It is particularly important to know if the same or nearby fault segments can host different slip modes and understand the transition between modes of slip. We investigate this in the trifurcation area of the Anza segment of the San Jacinto fault where deep creep driving seismicity below the geodetic locking depth has been proposed. We focus on periods following local moderate‐sized earthquakes by combining the occurrence of highly correlated families of earthquakes (i.e., closely spaced or partially overlapping events) that we describe here as near‐repeating earthquakes, seismicity, and borehole strainmeter data. We find that all Mw > 4.5 earthquakes between 2010 and 2020 triggered afterslip with coplanar families of near‐repeating earthquakes and off‐fault microseismicity. These observations include newly detected afterslip following the June 10, 2016 Mw 5.2 and April 4, 2020 Mw 4.9 local earthquakes. Afterslip geometries defined by the near‐repeating earthquake families are consistent with strain change observations. We conclude that families of near‐repeating earthquakes, similar to low‐frequency earthquakes within tremor, can be useful indicators of aseismic slip transients and can reveal faulting complexities during aseismic slip. Further, we identify the first evidence of spontaneous aseismic slip in the Anza region from near‐repeating earthquake families on two minor faults in 2015. Taken together, our observations support a model where deep microseismicity is located in a transitional region at the bottom of the seismogenic zone with spatially heterogeneous frictional properties that produce frequent aseismic slip transients near Anza. Plain Language Summary: Slip on faults occurs as a spectrum ranging from continuous sliding, where motion is too slow to radiate seismic waves (aseismic creep), to rapid movement during earthquakes (seismic slip). Within the fault slip spectrum lies the transient release of stress through aseismic slip. We investigate the interplay of these various slip modes near the Anza segment of the San Jacinto fault by combining observations of seismicity and borehole strainmeter data. We test the hypothesis that transient slip drives intervening small strong patches to failure generating near‐repeating earthquakes (i.e., highly correlated events located very close in space). We find that all moderate‐sized (Mw > 4.5) local earthquakes between 2010 and 2020 triggered transient slip events and coplanar families of near‐repeating earthquakes. We use the locations of near‐repeating earthquake families to define transient slip geometries and find that they are largely consistent with strain change observations. Transient slip occurs on main fault strands and minor structures. We also identify the first evidence of spontaneous transient slip in the Anza region from near‐repeating earthquake families and strainmeter data. Key Points: Near‐repeating earthquakes, like low‐frequency earthquakes in tremor, can indicate intermittent periods of aseismic slipLarge afterslip followed all four moderate‐sized earthquakes in Anza from 2010 to 2020; three events have afterslip moments greater than the mainshockAfterslip in the Anza region likely occurs on several fault segments both on and off the mainshock fault [ABSTRACT FROM AUTHOR]

Abstract Repeating earthquakes (repeaters) that rupture the same fault area (patch) are interpreted to be caused by repeated accumulation and release of stress on the seismic patch in a creeping area. This relationship between repeaters with fault creep can be exploited for tracking the fault creep (slow slip) based on the repeaters’ activity. In other words, the repeaters can be used as creepmeters embedded on a fault. To do this, it is fundamentally important to select earthquakes that definitely re-rupture in the same area. The selections are usually done based on waveform similarity or hypocenter location. In hypocenter-location based detection, the precision of the relative location compared with the dimension of earthquake sources is critical for confirming the co-location of the source area. On the other hand, waveform-similarity-based detection needs to use appropriate parameters including high enough frequency components to distinguish neighboring sources. Inter-event timing (recurrence interval) and/or the duration of a sequence’s activity are good diagnostic features for finding appropriate detection parameters and eliminating non-repeating events, which are important because an inappropriate selection leads to including triggered sequences that do not re-rupture the same area. Repeaters provide an independent estimation of creep from geodetic data and such estimations are mostly in good agreement when both kinds of data are available. Repeater data are especially useful in the deeper part of strike-slip faults and in near-trench areas of subduction zones where geodetic data’s resolution is usually limited. The repeaters also have an advantage with geodetic data analysis because they are not contaminated by viscoelastic deformation or poroelastic rebound which are prominent postseismic process for large earthquakes and occur outside of faults. On the other hand, the disadvantages of repeater analysis include their uneven spatial distribution and the uncertainty of the estimates of slip amount requiring a scaling relationship between earthquake size and slip. There are considerable variations in the inferred slip amounts from different relationships. Applications of repeater analysis illuminate the spatial distribution of interplate stable slip, after slip, and spontaneous and cyclic slow slip events that represent important components of interplate slip processes in addition to major earthquakes.

Fluid pressure perturbations in subsurface rocks affect the fault stability and can induce both seismicity and aseismic slip. Nonetheless, observations show that the partitioning between aseismic and seismic fault slip during fluid injection may strongly vary among reservoirs. The processes and the main fault properties controlling this partitioning are poorly constrained. Here we examine, through 3D hydromechanical modeling, the influence of fault physical properties on the seismic and aseismic response of a permeable fault governed by a slip-weakening friction law. We perform a series of high-rate, short-duration injection simulations to evaluate the influence of five fault parameters, namely the initial permeability, the dilation angle, the friction drop, the critical slip distance, and the initial proximity of stress to failure. For sake of comparison between tests, all the simulations are stopped for a fixed rupture distance relative to the injection point. We find that while the fault hydraulic behavior is mainly affected by the change in initial permeability and the dilation angle, the mechanical and seismic response of the fault strongly depends on the friction drop and the initial proximity of stress to failure. Additionally, both parameters, and to a lesser extent the initial fault permeability and the critical slip distance, impact the spatiotemporal evolution of seismic events and the partitioning between seismic and aseismic moment. Moreover, this study shows that a modification of such parameters does not lead to a usual seismic moment-injected fluid volume relationship, and provides insights into why the fault hydromechanical properties and background stress should be carefully taken into account to better anticipate the seismic moment from the injected fluid volume.

While the notion that injecting fluids into the subsurface can reactivate faults by reducing frictional resistance is well established, the ensuing evolution of the slip is still poorly understood. What controls whether the induced slip remains stable and confined to the fluid‐affected zone or accelerates into a runaway earthquake? Are there observable indicators of the propensity to earthquakes before they happen? Here, we investigate these questions by modeling a unique fluid‐injection experiment on a natural fault with laboratory‐derived friction laws. We show that a range of fault models with diverging stability with sustained injection reproduce the slip measured during pressurization. Upon depressurization, however, the most unstable scenario departs from the observations, suggesting that the fault is relatively stable. The models could be further distinguished with optimized depressurization tests or spatially distributed monitoring. Our findings indicate that avoiding injection near low‐residual‐friction faults and depressurizing during slip acceleration could help prevent large‐scale earthquakes. Plain Language Summary: Fluid injections into the Earth's crust are common practice in the exploitation of subsurface energy resources such as geothermal energy, shale gas, and conventional hydrocarbons. These injections can perturb nearby fault structures and hence induce earthquakes and transient slow slip. Understanding what controls the stability (i.e., the propensity to generate earthquakes) and spatial extent of the fault response as well as identifying precarious faults is crucial to minimize the seismic hazard associated with these industrial practices. Here, we take a step toward this goal by modeling a unique experiment, in which water was injected into a natural fault and the resulting slip measured directly at depth. We first show that multiple models can explain the observations equally well, while pressure is increased in the experiment. In these models, how stable the fault response is with further injection and how large of a zone is reactivated compared to the fluid‐affected region depends on frictional properties. We then demonstrate that the slow slip response to a decrease in injection pressure further constrains the range of admissible models. Our work suggests that it may be possible to identify potentially hazardous faults with optimally designed injection tests without inducing damaging earthquakes. Key Points: Multiple frictional models with different stability reproduce the slip observed during the pressurization stage of a field experimentThe depressurization phase provides additional constraints on hydromechanical parameters and hence fault stabilityFault stability and the spatial extent of slip relative to the pressurized region depend on residual friction versus initial stress levels [ABSTRACT FROM AUTHOR]

The offshore Castor Underground Gas Storage (UGS) project had to be halted after gas injection triggered three M4 earthquakes, each larger than any ever induced by UGS. The mechanisms that induced seismicity in the crystalline basement at 5–10 km depth after gas injection at 1.7 km depth remain unknown. Here, we propose a combination of mechanisms to explain the observed seismicity. First, the critically stressed Amposta fault, bounding the storage formation, crept by the superposition of well‐known overpressure effects and buoyancy of the relatively light injected gas. This aseismic slip brought an unmapped critically stressed fault in the hydraulically disconnected crystalline basement to failure. We attribute the delay between induced earthquakes to the pressure drop associated to expansion of areas where earthquakes slips cause further instabilities. Earthquakes occur only after these pressure drops have dissipated. Understanding triggering mechanisms is key to forecast induced seismicity and successfully design deep underground operations. Plain Language Summary: Underground fluid injection for energy‐related activities usually induces microseismicity (not felt earthquakes). But felt earthquakes are sometimes induced, which may cause public concern and project cancellation. This was the case of the offshore Castor underground gas storage (UGS), Spain. Gas injection induced numerous seismic events, including three with magnitudes around 4, larger than any other earthquake ever induced by UGS. These earthquakes occurred after the stop of injection in the crystalline basement, which is hydraulically disconnected from, and significantly deeper than the storage formation. To explain this seismicity, we propose a combination of mechanisms. First, the Amposta fault, bounding the site, crept (failed slowly, aseismically) because of pore pressure buildup induced by injection, which reduces the effective compression stabilizing faults, and stress variations caused by gas buoyancy. Aseismic slip of the Amposta fault provoked underground displacements, which reactivated a critically stressed unmapped fault in the crystalline basement. We attribute the delay between earthquakes of the sequence to rock expansion, and stabilizing pore pressure drop, in areas where slip displacement might cause new earthquakes. Earthquakes occur only after the pressure drop dissipates, which takes some time. Assessing fault stability prior to gas injection would have identified the risk of inducing seismicity. Key Points: Gas injection, through both pore pressure buildup and buoyancy, reactivated mainly aseismically the critically stressed Amposta faultShear slip stress transfer due to creep of the Amposta fault reactivated an unmapped critically stressed fault in the crystalline basementFelt earthquakes did not occur until pore pressure reductions had dissipated where shear slip stress transfer led to worsened stability [ABSTRACT FROM AUTHOR]