Earthquake Nucleation Research Articles

Fully Dynamic Spontaneous Rupture Due to Quasi‐Static Pore Pressure and Poroelastic Effects: An Implicit Nonlinear Computational Model of Fluid‐Induced Seismic Events

AbstractFluid perturbations play a pivotal role in triggering earthquakes. However, the role of fluid in the coseismic rupture process remains largely unknown. To this end, we develop a 2‐D fully dynamic spontaneous rupture model for fluid‐induced earthquakes. The effect of fluid in the preseismic quasi‐static regime is modeled as either pore pressure diffusion or fully coupled poroelasticity, using our Jin and Zoback (2017, https://doi.org/10.1002/2017JB014892) computational model. The two approaches lead to radically different predictions on the time of earthquake nucleation. Correspondingly, the evolved fluid pressure or poroelastic stress on the fault, together with the spatially altered density of the fluid‐saturated hosting rock, is passed to the dynamic regime. Under the assumption of an undrained coseismic fluid‐solid system, we discretize the fully dynamic Cauchy equation of motion subjected to an exact fault contact constraint using a split‐node finite element method in space and an implicit Newmark family finite difference method in time. Within each time step, a fully implicit Newton‐Raphson scheme is implemented iteratively for linearizing the fully discrete equations. Within each Newton iteration, a physics‐based and nonstationary preconditioner is designed to accelerate the convergence of the selected generalized minimal residual method iterative linear solver. The effect of fluid is highlighted throughout the discretization and computational procedures. Finally, by conducting a numerical experiment, we illustrate that a fully coupled poroelastic model can lead to significantly different predictions on coseismic rupture behaviors and wavefields compared to a decoupled model. Our computational model can also serve as one of the earliest full‐physics modeling tool for fluid‐induced earthquakes.

Fluid Injection and the Mechanics of Frictional Stability of Shale‐Bearing Faults

AbstractFluid overpressure is one of the primary mechanisms for triggering tectonic fault slip and human‐induced seismicity. This mechanism is appealing because fluid overpressure reduces the effective normal stress, hence favoring fault reactivation. However, upon fault reactivation models of earthquake nucleation suggest that increased fluid pressure should favor stable sliding rather than dynamic failure. Here we describe laboratory experiments on shale fault gouge, conducted in the double direct shear configuration in a true‐triaxial machine. To characterize frictional stability and hydrological properties we performed three types of experiments: (1) stable sliding shear experiments to determine the material failure envelope and permeability, (2) velocity step experiments to determine the rate‐and‐state frictional properties, and (3) creep experiments to study fault slip evolution with increasing pore fluid pressure. The shale gouge shows low frictional strength, μ = 0.28, and permeability, k ~ 10−19 m2 together with a velocity strengthening behavior indicative of aseismic slip. During fault pressurization, we document that upon failure slip velocity remains slow (i.e., v ~ 200 μm/s), not approaching dynamic slip rates. We relate this fault slip behavior to the interplay between the fault weakening induced by fluid pressurization, the strong rate‐strengthening behavior of shales, and the evolution of fault zone structure. Our data show that fault rheology and fault stability is controlled by the coupling between fluid pressure and rate‐and‐state friction parameters.

Recurring Slow Slip Events and Earthquake Nucleation in the Source Region of the M 7 Ibaraki‐Oki Earthquakes Revealed by Earthquake Swarm and Foreshock Activity

AbstractSlow slip events (SSEs) on the plate interface are closely related to the occurrence of earthquakes and often trigger earthquake swarms in subduction zones. Moreover, some SSEs, accompanied by intensive foreshocks, precede large interplate earthquakes. Therefore, detecting and monitoring SSEs is important for assessing the potential of future large earthquakes. However, there are many SSEs not followed by large earthquakes, and it is unclear whether these can be distinguished from SSEs preceding large earthquakes. Here we use the epidemic‐type aftershock sequence (ETAS) model and matched‐filter technique to examine the spatial‐temporal distribution of earthquake swarms and foreshocks at Ibaraki‐Oki in the Japan Trench. We found that 19 swarm sequences repeatedly occurred during the period 1982–2008 at the same location as the foreshock sequences of the 1982 and 2008 M 7 Ibaraki‐Oki earthquakes. Both the foreshock and swarm sequences contain repeating earthquakes and have anomalously high seismicity rates inexplicable by the ETAS model, suggesting the recurrence of SSEs. The foreshock sequences in 1982 and 2008 contain more events inexplicable by the ETAS model than the swarm sequences. The fault slip of repeating earthquakes in the 2008 foreshock sequence was also larger than those of the swarm sequences, and the slip rate showed an abrupt increase 12 hr before the 2008 M 7 event. Our results imply that the SSEs that preceded the M 7 events had larger seismic moments than the other SSEs. These large SSEs might be related to the nucleation phase of the M 7 earthquakes.

Controls on Mid‐ocean Ridge Normal Fault Seismicity Across Spreading Rates From Rate‐and‐State Friction Models

AbstractRecent seismic and geodetic observations have led to a growing realization that a significant amount of fault slip at plate boundaries occurs aseismically and that the amount of aseismic slip varies across tectonic settings. Seismic moment release rates measured along the fast‐spreading East Pacific Rise suggest that the majority of fault slip occurs aseismically. By contrast, at the slow‐spreading Mid‐Atlantic Ridge seismic slip may represent up to 60% of total fault displacement. In this study, we use rate‐and‐state friction models to quantify the seismic coupling coefficient, defined as the fraction of total fault slip that occurs seismically, on mid‐ocean ridge normal faults and investigate controls on fault behavior that might produce variations in coupling observed at oceanic spreading centers. We find that the seismic coupling coefficient scales with the ratio of the downdip width of the seismogenic area (W) to the critical earthquake nucleation size (h*). At mid‐ocean ridges, W is expected to increase with decreasing spreading rate. Thus, the relationship between seismic coupling and W/h* predicted from our models explains the first‐order variations in seismic coupling coefficient as a function of spreading rate.

Crustal deformation, spatial distribution of earthquakes and along strike segmentation of the Sagaing Fault, Myanmar

The ∼1200 km long north-south oriented Sagaing fault (plate boundary fault between the Burma and Sunda plates), entirely located in Myanmar, passes through some major cities, and has witnessed several major earthquakes since prehistoric time, including the recent Mw 6.8 Thabeikkyin earthquake in 2012. The Sagaing fault accommodates ∼50–55% of dextral motion of the India-Sunda plate motion. Combined, geodetic observations and rheological considerations using topography as a proxy, we propose along strike segmentation of the Sagaing fault from centrally locked segment to creeping segment further north along the Koma fault. Such segmentation facilitates stress transfer (or loading) from creeping segment to locked segment, and influences earthquake nucleation process and along strike spatial extent of great earthquake ruptures. Topographic margins and their relative strength, predicted from the Newtonian channel flow model, also support the along strike segmentation behaviour.

Dynamic and Quasi‐Dynamic Modeling of Injection‐Induced Earthquakes in Poroelastic Media

AbstractEarthquake ruptures in poroelastic media involve a suite of complex phenomena arising from stick‐slip frictional instabilities and thermo‐hydromechanical couplings. In this study we propose a fully implicit, time‐adaptive, and monolithically coupled finite element model to simulate dynamic earthquake sequences in poroviscoelastic media. We consider a Kelvin‐Voigt viscoelastic material and characterize the impact of inertial effects on injection‐induced earthquakes. We present, for the first time, dynamic simulations of ruptures in rate‐and‐state faults in poroelastic media. Our simulations resolve the full earthquake cycle, including the interseismic, spontaneous earthquake nucleation, and dynamic rupture phases. We compare dynamic simulations with quasi‐dynamic ones, in which inertial effects are neglected and the slip singularity is resolved through a radiation damping approximation. Viscous dissipation models the physical process of seismic wave attenuation: As viscous damping increases, the patch size and the maximum fault slip become smaller, hence decreasing the expected earthquake magnitude. From a computational perspective, viscoelasticity helps avoid spurious high‐frequency oscillations during wave propagation. By including inertial effects, the dynamic model accounts for transient fluctuations of pressures and solid stresses during rupture, which are neglected in the quasi‐dynamic approach. Understanding these transient perturbations may shed light on the role of pore pressure in the mechanism of dynamic earthquake triggering. The poroviscoelastic dynamic approach is a good compromise between the inviscid, fully dynamic model, and the quasi‐dynamic one. A small amount of viscous damping allows us more efficient calculations, while preserving the most relevant features of dynamic ruptures, in particular slip velocities, accumulated slip, and seismic moment released.

Author Correction: Earthquake nucleation and fault slip complexity in the lower crust of central Alaska

In the version of this Article originally published, the ‘Data availability’ section contained an incorrect DOI for data from the FLATS (XV) seismic network ( https://doi.org/10.7914/SN/ZE_2015 ); the correct DOI is: https://doi.org/10.7914/SN/XV_2014 . This has now been corrected in the online versions.

Deformation Response of Seismogenic Faults to the Wenchuan MS 8.0 Earthquake: A Case Study for the Southern Segment of the Longmenshan Fault Zone

The spatiotemporal deformation response of a seismogenic fault to a large earthquake is of great significance to understanding the nucleation and occurrence of the next strong earthquake. The Longmeshan fault, where the 2008 Wenchuan MS 8.0 earthquake and 2013 Lushan MS 7.0 earthquake occurred, provides an opportunity for us to study this important issue. Based on the GPS observations, we exploit the deformation response of the Southern Segment of the Longmenshan Fault (SSLMF) to the Wenchuan earthquake. The results are as follows: (1) during the co-seismic and post-seismic processes of the Wenchuan earthquake, the deformation is dominated by a continuous pattern in the SSLMF, which is different from the rupture pattern in the middle-northern segment of the Longmenshan Fault (LMF). Quantitatively, the compressive strain present between 2008 and 2013 was equal to the strain accumulation of 69 years during the interseismic period in the SSLMF. If the statistics scope is restricted to the eastern region of the Anxian-Guanxian Fault (AGF), which covers the Lushan source area (Abbr.: Eastern Region), the value is about 25 years; (2) After the Wenchuan earthquake, the strain accumulation pattern changes significantly. First, the deformation adjustment (especially the shear deformation) in the region that crosses the Maoxian-Wenchuan Fault (MWF) and Beichuan-Yingxiu Fault (BYF) (Abbr.: Western Region) is significantly greater than that in the Eastern Region. Furthermore, the crustal shortening is significant in the Eastern Region with minor adjustments in shear deformation. Second, the azimuth angles of the principal compressive strain rate in both regions show significant adjustments, which change fast in the first year of the observation period and then turn into the stable state. In general, the deformation responses of the SSLMF reveal that the Wenchuan earthquake promotes the occurrence of the Lushan earthquake. Their differences in the spatiotemporal domain can be attributed to the influence of afterslip, viscous relaxation of the lithosphere, mechanical parameters and block movement.

Nucleation of the 1999 Izmit earthquake by a triggered cascade of foreshocks

Much of what we know about the nucleation of earthquakes comes from the temporal and spatial relationship of foreshocks to the initiation point (hypocentre) of the mainshock. The 1999 Mw 7.6 Izmit, Turkey, earthquake was preceded by a 44-minute-long foreshock sequence, which built in intensity as the time of the mainshock approached. Here we apply a series of high-resolution methods to the sparse seismological observations to determine the spatial and temporal relation of the foreshocks to the mainshock hypocentre. We find that the foreshocks form a contiguous series of ruptures that progressed systematically from west to east towards the mainshock hypocentre, located at the extreme eastern edge of the foreshocks. The Izmit foreshock sequence occurred as a triggered cascade in which one foreshock loads the adjacent fault patch causing it to fail, with the mainshock initiation no different than another foreshock. We find no evidence to support a hypothesized precursory aseismic driving process. If we are to resolve the role of aseismic deformation and possible role of fluid overpressure in the earthquake nucleation process it will likely require measurements made in the near field of the hypocentre. The magnitude 7.6 Izmit earthquake that struck Turkey in 1999 was nucleated by an eastward-migrating cascade of foreshocks, according to high-resolution analyses of seismic data.

Earthquake nucleation and fault slip complexity in the lower crust of central Alaska

Earthquakes start under conditions that are largely unknown. In laboratory analogue experiments and continuum models, earthquakes transition from slow-slipping, growing nucleation to fast-slipping rupture. In nature, earthquakes generally start abruptly, with no evidence for a nucleation process. Here we report evidence from a strike-slip fault zone in central Alaska of extended earthquake nucleation and of very-low-frequency earthquakes (VLFEs), a phenomenon previously reported only in subduction zone environments. In 2016, a VLFE transitioned into an earthquake of magnitude 3.7 and was preceded by a 12-hour-long accelerating foreshock sequence. Benefiting from 12 seismic stations deployed within 30 km of the epicentre, we identify coincident radiation of distinct high-frequency and low-frequency waves during 22 s of nucleation. The power-law temporal growth of the nucleation signal is quantitatively predicted by a model in which high-frequency waves are radiated from the vicinity of an expanding slow slip front. The observations reveal the continuity and complexity of slip processes near the bottom of the seismogenic zone of a strike-slip fault system in central Alaska. A strike-slip fault zone in central Alaska exhibits a range of earthquake slip processes, including very-low-frequency earthquakes, some of which transition into regular, fast earthquakes.

Unsettled earthquake nucleation

Detailed analyses of the source characteristics of two earthquake sequences lead to seemingly contradictory interpretations: one study concludes that each earthquake triggers subsequent ones, while the other favours a slow-slip trigger.

The geophysics, geology and mechanics of slow fault slip

Modern geodetic and seismologic observations describe the behavior of fault slip over a vast range of spatial and temporal scales. Slip at sub-seismogenic speeds is evident from top to bottom of lithospheric faults and plays an important role throughout the earthquake cycle. Where earthquakes and tremor accompany slow slip, they help illuminate the spatiotemporal evolution of fault slip. Geophysical subsurface imaging and geologic field studies provide information about suitable environments of slow slip. In particular, exhumed fault and shear zones from various depths reveal the importance of multiple deformation processes and fault-zone structures. Most geologic examples feature frictionally weak and velocity-strengthening materials, well-developed mineral fabrics, and abundant veining indicative of near-lithostatic fluid pressure. To produce transient slow slip events and tremor, in addition to the presence of high-pressure fluids a heterogeneous fault-zone structure, composition, and/or metamorphic assemblage may be needed. Laboratory and computational models suggest that velocity-weakening slip patches smaller than a critical dimension needed for earthquake nucleation will also fail in slow slip events. Changes in fluid pressure or slip rate can cause a fault to transition between stable and unstable fault slip behavior. Future interdisciplinary investigations of slow fault slip, directly integrating geophysical, geological and modeling investigations, will further improve our understanding of the dynamics of slow slip and aid in providing more accurate earthquake hazard characterizations.

GPS-Derived Fault Coupling of the Longmenshan Fault Associated with the 2008 Mw Wenchuan 7.9 Earthquake and Its Tectonic Implications

Investigating relationships between temporally- and spatially-related continental earthquakes is important for a better understanding of the crustal deformation, the mechanism of earthquake nucleation and occurrence, and the triggering effect between earthquakes. Here we utilize Global Positioning System (GPS) velocities before and after the 2008 Mw 7.9 Wenchuan earthquake to invert the fault coupling of the Longmenshan Fault (LMSF) and investigate the impact of the 2008 Mw 7.9 Wenchuan earthquake on the 2013 Mw 6.6 Lushan earthquake. The results indicate that, before the 2008 Mw 7.9 Wenchuan earthquake, fault segments were strongly coupled and locked at a depth of ~18 km along the central and northern LMSF. The seismic gap between the two earthquake rupture zones was only locked at a depth < 5 km. The southern LMSF was coupled at a depth of ~10 km. However, regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were not coupled, with an average coupling coefficient ~0.3. After the 2008 Mw 7.9 Wenchuan earthquake, the central and northern LMSF, including part of the seismic gap, were decoupled, with an average coupling coefficient smaller than 0.2. The southern LMSF, however, was coupled to ~20 km depth. Regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were also coupled. Moreover, by interpreting changes of the GPS velocities before and after the 2008 Mw 7.9 Wenchuan earthquake, we find that the upper crust of the eastern Tibet (i.e., the Bayan Har block), which was driven by the postseismic relaxation of the 2008 Mw 7.9 Wenchuan earthquake, thrust at an accelerating pace to the Sichuan block and result in enhanced compression and shear stress on the LMSF. Consequently, downdip coupling of the fault, together with the rapid accumulation of the elastic strain, lead to the occurrence of the 2013 Mw 6.6 Lushan earthquake. Finally, the quantity analysis on the seismic moment accumulated and released along the southern LMSF show that the 2013 Mw 6.6 Lushan earthquake should be defined as a “delayed” aftershock of the 2008 Mw 7.9 Wenchuan earthquake. The seismic risk is low along the seismic gap, but high on the unruptured southwesternmost area of the 2013 Mw 6.6 Lushan earthquake.

Hydromechanical Earthquake Nucleation Model Forecasts Onset, Peak, and Falling Rates of Induced Seismicity in Oklahoma and Kansas

AbstractThe earthquake activity in Oklahoma and Kansas that began in 2008 reflects the most widespread instance of induced seismicity observed to date. We develop a reservoir model to calculate the hydrologic conditions associated with the activity of 902 saltwater disposal wells injecting into the Arbuckle aquifer. Estimates of basement fault stressing conditions inform a rate‐and‐state friction earthquake nucleation model to forecast the seismic response to injection. Our model replicates many salient features of the induced earthquake sequence, including the onset of seismicity, the timing of the peak seismicity rate, and the reduction in seismicity following decreased disposal activity. We present evidence for variable time lags between changes in injection and seismicity rates, consistent with the prediction from rate‐and‐state theory that seismicity rate transients occur over timescales inversely proportional to stressing rate. Given the efficacy of the hydromechanical model, as confirmed through a likelihood statistical test, the results of this study support broader integration of earthquake physics within seismic hazard analysis.

The effect of inertia, viscous damping, temperature and normal stress on chaotic behaviour of the rate and state friction model

A fundamental understanding of frictional sliding at rock surfaces is of practical importance for nucleation and propagation of earthquakes and rock slope stability. We investigate numerically the effect of different physical parameters such as inertia, viscous damping, temperature and normal stress on the chaotic behaviour of the two state variables rate and state friction (2sRSF) model. In general, a slight variation in any of inertia, viscous damping, temperature and effective normal stress reduces the chaotic behaviour of the sliding system. However, the present study has shown the appearance of chaos for the specific values of normal stress before it disappears again as the normal stress varies further. It is also observed that magnitude of system stiffness at which chaotic motion occurs, is less than the corresponding value of critical stiffness determined by using the linear stability analysis. These results explain the practical observation why chaotic nucleation of an earthquake is a rare phenomenon as reported in literature.

Maximum magnitude of injection-induced earthquakes: A criterion to assess the influence of pressure migration along faults

The maximum expected earthquake magnitude is an important parameter in seismic hazard and risk analysis because of its strong influence on ground motion. In the context of injection-induced seismicity, the processes that control how large an earthquake will grow may be influenced by operational factors under engineering control as well as natural tectonic factors. Determining the relative influence of these effects on maximum magnitude will impact the design and implementation of induced seismicity management strategies. In this work, we apply a numerical model that considers the coupled interactions of fluid flow in faulted porous media and quasidynamic elasticity to investigate the earthquake nucleation, rupture, and arrest processes for cases of induced seismicity. We find that under certain conditions, earthquake ruptures are confined to a pressurized region along the fault with a length-scale that is set by injection operations. However, earthquakes are sometimes able to propagate as sustained ruptures outside of the zone that experienced a pressure perturbation. We propose a faulting criterion that depends primarily on the state of stress and the earthquake stress drop to characterize the transition between pressure-constrained and runaway rupture behavior.

Do scaly clays control seismicity on faulted shale rocks?

One of the major challenges regarding the disposal of radioactive waste in geological formations is to ensure isolation of radioactive contamination from the environment and the population. Shales are suitable candidates as geological barriers. However, the presence of tectonic faults within clay formations put the long-term safety of geological repositories into question. In this study, we carry out frictional experiments on intact samples of Opalinus Clay, i.e. the host rock for nuclear waste storage in Switzerland. We report experimental evidence suggesting that scaly clays form at low normal stress (≤20 MPa), at sub-seismic velocities (≤300 μm/s) and is related to pre-existing bedding planes with an ongoing process where frictional sliding is the controlling deformation mechanism. We have found that scaly clays show a velocity-weakening and -strengthening behaviour, low frictional strength, and poor re-strengthening over time, conditions required to allow the potential nucleation and propagation of earthquakes within the scaly clays portion of the formation. The strong similarities between the microstructures of natural and experimental scaly clays suggest important implications for the slip behaviour of shallow faults in shales. If natural and anthropogenic perturbations modify the stress conditions of the fault zone, earthquakes might have the potential to nucleate within zones of scaly clays controlling the seismicity of the clay-rich tectonic system, thus, potentially compromising the long-term safeness of geological repositories situated in shales.

Some Tonal and Rhythmical Sequences in the Vocal Language of Dogs as Significant Earthquake Precursors

A monitoring of multiple physical parameters in a moderate seismic area in Western Piedmont (NW Italy) and the simultaneous observation of the behaviour of numerous species of domestic and wild animals gave in a period of over twenty years the possibility to distinguish the unusual animal behaviours due to local earthquake nucleation from other causes. In particular, the observation of the body and vocal language of dogs (Canis familiaris) in the same area has permitted not only to specify the different meanings of vocal language in connection to their body language, but also to classify the minimum elements into a vocal language that is linked together by tonal and rhythmical sequences of sounds that form a semantic lexicon. The usage of the same tonal and rhythmical vocal sequences in similar or identical situations, which are experienced by different groups of dogs, induces us to verify whether it could be possible to link particular vocal sequences to precise physical anomalies before earthquakes. The individuation of physical anomalies due to an earthquake nucleation or due to a hydro-geological destabilization, is possible thanks to a continuous long-term monitoring of some parameters. Moreover, the complexity of the vocal language of dogs increases if the dogs live in an area with a law population density. Then the correlation between some vocal sequences and some seismic precursors is better if dogs live free in yard or on farms, if they are in good health, and if they can establish a strong social relation of group. When dogs live closed in yards of houses that are far apart, they communicate with each other with an amazing vocal language, full of questions and answers, imitations of sequences, and information about situations that may be harmful to them.

Entropy, Seismicity Monitoring in the Armenian Highlands and Dynamics of the Akhurian Reservoir Filling

The Armenian seismic system, which has a threshold magnitude of 6.2, has been identified based on the method of seismic entropy, makes it possible to monitor nucleation of strong earthquakes in the magnitude range of 6.2 ≤ M < 6.6, as well as to assess the seismic situation in Armenia and neighboring countries. A study of energy and track diagrams has revealed a fan-shaped tectonic pattern of stress concentrations with an apex in the Kurdistan knot. The analysis of entropy accumulation in seismic cycles carried out for the hierarchy of seismic systems of the Armenian Highlands helped in the identification of prognostic criteria. The results have been compared to the dynamics of Akhurian reservoir filling in 1983–2016.

Dynamic fracture and wave propagation in a granular medium: A photoelastic study

In our earlier work on the generation mechanisms of earthquakes and related failures, fracture phenomena were treated in the framework of continuum mechanics, and the existence of the universal critical condition for earthquake nucleation as well as the strong dependence of earthquake-induced structural failure patterns on the frequencies and types of incident seismic waves was pointed out. However, other significant seismic phenomena such as landslides and liquefaction may not be simply explained using the theories established for continuum media. For example, in order to clarify the physics of the formation of the geological flame structure, possibly due to liquefaction and ensuing gravitational instability in water-immersed sediments, the mechanical behavior of particles under dynamic load and the influence of waves, if any, on fracture should be understood for granular media beforehand. Here, as an initial investigation into wave and fracture propagation inside granular media, under dry conditions first of all, experimental technique of dynamic photoelasticity is employed. Penny-shaped particles made of birefringent polycarbonate are prepared and placed on a rigid horizontal plane to form two-dimensional model slopes with certain inclination angles. Dynamic impact is given to the top (approximately) horizontal free surface of the slope, and the transient evolution of stress and fracture is recorded by a high-speed digital video camera. It is shown that depending on the profile of energy imparted by the impact, (i) one-dimensional force-chain-like stress transfer or (ii) widely spread multi-dimensional wave propagation can be found. While the case (i) results in mass flow, i.e. total collapse of the slope, in (ii) waves can induce dynamic separation of only slope faces similar to toppling failure. The experimentally observed wave and fracture phenomena in granular media are compared to those in continuum media and the actual slope failure repeatedly caused in Japan, New Zealand and USA.