It is widely recognized that the significant increase of M > 3.0 earthquakes in Western Canada and the Central United States is related to underground fluid injection. Following injection, fluid ...overpressure lubricates the fault and reduces the effective normal stress that holds the fault in place, promoting slip. Although, this basic physical mechanism for earthquake triggering and fault slip is well understood, there are many open questions related to induced seismicity. Models of earthquake nucleation based on rate- and state-friction predict that fluid overpressure should stabilize fault slip rather than trigger earthquakes. To address this controversy, we conducted laboratory creep experiments to monitor fault slip evolution at constant shear stress while the effective normal stress was systematically reduced via increasing fluid pressure. We sheared layers of carbonate-bearing fault gouge in a double direct shear configuration within a true-triaxial pressure vessel. We show that fault slip evolution is controlled by the stress state acting on the fault and that fluid pressurization can trigger dynamic instability even in cases of rate strengthening friction, which should favor aseismic creep. During fluid pressurization, when shear and effective normal stresses reach the failure condition, accelerated creep occurs in association with fault dilation; further pressurization leads to an exponential acceleration with fault compaction and slip localization. Our work indicates that fault weakening induced by fluid pressurization can overcome rate strengthening friction resulting in fast acceleration and earthquake slip. Our work points to modifications of the standard model for earthquake nucleation to account for the effect of fluid overpressure and to accurately predict the seismic risk associated with fluid injection.
•The conditions for fault reactivation due to fluid overpressure are tested.•First example of creep experiments on fault gouge under fluid overpressure.•Fault weakening by fluid overpressure overcome rate strengthening behavior.•Inform fault deformation with microstructural analysis.•Short term fluid overpressure causes an energy unbalance that trigger dynamic slip.
Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6<μ<0.85. In marked contrast, numerous studies have documented ...significant fault weakening induced by fluid-assisted reaction softening that generally takes place during the long-term evolution of the fault. Reaction softening promotes the replacement of strong minerals with phyllosilicates. Phyllosilicate development within foliated and interconnected fault networks has been documented at different crustal depths, in different tectonic regimes and from a great variety of rock types, nominating fluid-assisted reaction softening as a general weakening mechanism within the seismogenic crust. This weakening originates at the grain-scale and is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones resulting in a friction as low as 0.1<μ<0.3.
Collectively, geological data and results from laboratory experiments provide strong supporting evidence for structural and frictional heterogeneities within crustal faults. In these structures, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor.
•Fault friction drops from 0.6 to 0.2 for interconnected networks of phyllosilicates.•Fluid-assisted reaction softening is a general weakening mechanism.•Structural and frictional heterogeneities of crustal faults are widespread.
Abstract
Analysis of seismicity can illuminate active fault zone structures but also deformation within large volumes of the seismogenic zone. For the M
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6.5 2016–2017 Central Italy seismic ...sequence, seismicity not only localizes along the major structures hosting the mainshocks (on-fault seismicity), but also occurs within volumes of Triassic Evaporites, TE, composed of alternated anhydrites and dolostones. These volumes of distributed microseismicity show a different frequency-magnitude distribution than on-fault seismicity. We interpret that, during the sequence, shear strain-rate increase, and fluid overpressure promoted widespread ductile deformation within TE that light-up with distributed microseismicity. This interpretation is supported by field and laboratory observations showing that TE background ductile deformation is complex and dominated by distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones. Our results indicate that ductile crustal deformation can cause distributed microseismicity, which obeys to different scaling laws than on-fault seismicity occurring on structures characterized by elasto-frictional stick-slip behaviour.
Fluid 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.
Key Points
Laboratory experiments on shale fault gouge designed to test fault slip behavior during fluid pressurization
Shale fault gouge fails by accelerated but slow shear slip not evolving into a dynamic slip instability
Slow shear slip arises from the interplay of fault hydromechanical properties and fault structure evolution
The Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the ...frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes.
Key Points
Sediments subducting at the Hikurangi Trench become weaker and less velocity strengthening with increasing depth
Sediments exhibit a cutoff velocity of 1 μm/s, possibly explaining the presence of shallow slow slip at the Hikurangi Trench
Slow slip at the Hikurangi Trench could result from frictional instability, in addition to factors such as elevated pore fluid pressures
Seismic cycles lead to variations in rock physical properties. Quantifying these changes is of key importance in building reliable crustal deformation models. Here we report laboratory measurements ...of the uniaxial compressive strength (UCS), Young's modulus and Poisson's ratio, both static (Es, νs) and dynamic (Ed, νd) of the seismogenic Triassic Evaporites of the Northern Apennines. Triassic Evaporites are composed of dolostones, anhydrites and gypsum. Gypsum was the weakest lithology, with UCS values ranging from 10 to 26MPa; anhydrite exhibited intermediate values from 52 to 144MPa; and dolostones were the strongest with a maximum UCS of 228MPa.
During uniaxial cyclic stressing experiments, we observed complex variations in Es and νs with: large increases are observed in the early cycles (stage 1), followed by essentially constant values (stage 2), before Es decreases and νs increases approaching failure (stage 3). Complementary microseismicity (acoustic emission, AE) data show no significant AE during stage 1, then the stress needed to induce AE remained essentially constant (stages 2 and 3). Integration of mechanical data with microstructural observations suggests a first stage dominated by compaction and strengthening, a second stage characterised by quasi-elastic behaviour associated with the development of randomly oriented microfractures, and a third stage of weakening due to the growth of macrofractures parallel to the direction of the load. Laboratory dynamic elastic moduli are, on average, in agreement with dynamic elastic moduli used in crustal modelling. However static values of Young's modulus are about 50% lower than dynamic ones, and static values of Poisson's ratio are about 40% higher with respect to dynamic values. These observations suggest that the frequency effect on the difference between laboratory and crustal scale dynamic moduli values is rather small and that static values of modulus are more appropriate for crustal deformation modelling than seismically derived values.
► We have measured elastic moduli during increasing-amplitude cyclic stress experiments. ► We observe changes that are interpreted as involving a three stage process. ► Compaction dominates Stage I, crack growth Stage III, and a balance for Stage II. ► We observe large differences between dynamic and static values. ► We suggest that the static values better represent damage zones.
We present seismological evidence for the existence of an actively slipping low‐angle normal fault (15° dip) located in the northern Apennines of Italy. During a temporary seismic experiment, we ...recorded ∼2000 earthquakes with ML ≥ 3.1. The microseismicity defines a 500 to 1000 m thick fault zone that crosscuts the upper crust from 4 km down to 16 km depth. The fault coincides with the geometry and location of the Alto Tiberina Fault (ATF) as derived from geological observations and interpretation of depth‐converted seismic reflection profiles. In the ATF hanging wall the seismicity distribution highlights minor synthetic and antithetic normal faults (4–5 km long) that sole into the detachment. The ATF‐related seismicity shows a nearly constant rate of earthquake production, ∼3 events per day (ML ≤ 2.3), and a higher b value (1.06) with respect to the fault hanging wall (0.85) which is characterized by a higher rate of seismicity. In the ATF zone we also observe the presence of clusters of earthquakes occurring with relatively short time delays and rupturing the same fault patch. To explain movements on the ATF, oriented at high angles (∼75°) to the maximum vertical principal stress, we suggest an interpretative model in which crustal extension along the fault is mostly accommodated by aseismic slip in velocity strengthening areas while microearthquakes occur in velocity weakening patches. We propose that these short‐lived frictional instabilities are triggered by fluid overpressures related to the buildup of CO2‐rich fluids as documented by boreholes in the footwall of the ATF.
The presence of calcite in and near faults, as the dominant material, cement, or vein fill, indicates that the mechanical behaviour of carbonate-dominated material likely plays an important role in ...shallow- and mid-crustal faulting. To better understand the behaviour of calcite, under loading conditions relevant to earthquake nucleation, we sheared powdered gouge of Carrara Marble, >98 per cent CaCO3, at constant normal stresses between 1 and 100 MPa under water-saturated conditions at room temperature. We performed slide-hold-slide tests, 1–3000 s, to measure the amount of static frictional strengthening and creep relaxation, and velocity-stepping tests, 0.1–1000 μm s–1, to evaluate frictional stability. We observe that the rates of frictional strengthening and creep relaxation decrease with increasing normal stress and diverge as shear velocity is increased from 1 to 3000 μm s–1 during slide-hold-slide experiments. We also observe complex frictional stability behaviour that depends on both normal stress and shearing velocity. At normal stresses less than 20 MPa, we observe predominantly velocity-neutral friction behaviour. Above 20 MPa, we observe strong velocity-strengthening frictional behaviour at low velocities, which then evolves towards velocity-weakening friction behaviour at high velocities. Microstructural analyses of recovered samples highlight a variety of deformation mechanisms including grain size reduction and localization, folding of calcite grains and fluid-assisted diffusion mass transfer processes promoting the development of calcite nanograins in the highly deformed portions of the experimental fault. Our combined analyses indicate that calcite fault gouge transitions from brittle to semi-brittle behaviour at high normal stress and slow sliding velocities. This transition has important implications for earthquake nucleation and propagation on faults in carbonate-dominated lithologies.
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