The role of fluid pressure in induced vs. triggered seismicity. Insights from rock deformation experiments on carbonates

Fluid overpressure is one of the primary mechanisms for tectonic fault slip, because fluids lubricate the fault and fluid pressure reduces the effective normal stress that holds the fault in place. However, current models of earthquake nucleation, based on rate- and state- friction laws, imply that stable sliding is favoured by the increase of pore fluid pressure. Despite this controversy, currently, there are only a few studies on the role of fluid pressure under controlled, laboratory conditions. Here, we use laboratory experiments, to show that the rate- and state- friction parameters do change with increasing fluid pressure. We tested carbonate gouges from sub hydrostatic to near lithostatic fluid pressure conditions, and show that the friction rate parameter (a−b) evolves from velocity strengthening to velocity neutral behaviour. Furthermore, the critical slip distance, Dc, decreases from about 90 to 10μm. Our data suggest that fluid overpressure plays an important role in controlling the mode of fault slip. Since fault rheology and fault stability parameters change with fluid pressure, we suggest that a comprehensive characterization of these parameters is fundamental for better assessing the role of fluid pressure in natural and human induced earthquakes

Frictional behavior of talc-calcite mixtures

Faults involving phyllosilicates appear weak when compared to the laboratory-derived strength of most crustal rocks. Among phyllosilicates, talc, with very low friction, is one of the weakest minerals involved in various tectonic settings. As the presence of talc has been recently documented in carbonate faults, we performed laboratory friction experiments to better constrain how various amounts of talc could alter these fault’s frictional properties. We used a biaxial apparatus to systematically shear different mixtures of talc and calcite as powdered gouge at room temperature, normal stresses up to 50 MPa and under different pore fluid saturated conditions, i.e., CaCO3-equilibrated water and silicone oil. We performed slide-hold-slide tests, 1–3000 s, to measure the amount of frictional healing and velocity-stepping tests, 0.1–1000 μm/s, to evaluate frictional stability. We then analyzed microstructures developed during our experiments. Our results show that with the addition of 20% talc the calcite gouge undergoes a 70% reduction in steady state frictional strength, a complete reduction of frictional healing and a transition from velocity-weakening to velocity-strengthening behavior. Microstructural analysis shows that with increasing talc content, deformation mechanisms evolve from distributed cataclastic flow of the granular calcite to localized sliding along talc-rich shear planes, resulting in a fully interconnected network of talc lamellae from 20% talc onward. Our observations indicate that in faults where talc and calcite are present, a low concentration of talc is enough to strongly modify the gouge’s frictional properties and specifically to weaken the fault, reduce its ability to sustain future stress drops, and stabilize slip

The influence of normal stress and sliding velocity on the frictional behaviour of calcite at room temperature. Insights from laboratory experiments and microstructural observations

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

Hypothesis for the mechanics and seismic behaviour of low-angle normal faults: the example of the Altotiberina fault Northern Apennines

Widespread mapping of low-angle normal faults in areas of former continental extension continues to prompt debate as to whether such structures may be seismically active at very low dips (? ? 3 (i.e.? v >0.93).The short-lived attainment of P f >? 3 along small fault portions,in an area characterised by large amounts of CO2,account for the microseismic activity located along the ATF,which occurs on rupture surfaces in the range of 10 ??10 ? 3 km 2.

Physical and Transport Property Variations Within Carbonate-Bearing Fault Zones: Insights From the Monte Maggio Fault (Central Italy)

AbstractThe physical characterization of carbonate‐bearing normal faults is fundamental for resource development and seismic hazard. Here we report laboratory measurements of density, porosity, Vp, Vs, elastic moduli, and permeability for a range of effective confining pressures (0.1–100 MPa), conducted on samples representing different structural domains of a carbonate‐bearing fault. We find a reduction in porosity from the fault breccia (11.7% total and 6.2% connected) to the main fault plane (9% total and 3.5% connected), with both domains showing higher porosity compared to the protolith (6.8% total and 1.1% connected). With increasing confining pressure, P wave velocity evolves from 4.5 to 5.9 km/s in the fault breccia, is constant at 5.9 km/s approaching the fault plane and is low (4.9 km/s) in clay‐rich fault domains. We find that while the fault breccia shows pressure sensitive behavior (a reduction in permeability from 2 × 10−16 to 2 × 10−17 m2), the cemented cataclasite close to the fault plane is characterized by pressure‐independent behavior (permeability 4 × 10−17 m2). Our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone. In situ Vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale. Combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels, M ≤ 1 earthquakes are less favored, in agreement with earthquake‐depth distribution during the L'Aquila 2009 seismic sequence that occurred on carbonates

On the evolution of elastic properties during laboratory stick-slip experiments spanning the transition from slow slip to dynamic rupture

The physical mechanisms governing slow earthquakes remain unknown, as does the relationship between slow and regular earthquakes. To investigate the mechanism(s) of slow earthquakes and related quasi-dynamic modes of fault slip we performed laboratory experiments on simulated fault gouge in the double direct shear configuration. We reproduced the full spectrum of slip behavior, from slow to fast stick slip, by altering the elastic stiffness of the loading apparatus (k) to match the critical rheologic stiffness of fault gouge (kc). Our experiments show an evolution from stable sliding, when k>kc, to quasi-dynamic transients when k ~ kc, to dynamic instabilities when k<kc. To evaluate the microphysical processes of fault weakening we monitored variations of elastic properties. We find systematic changes in P wave velocity (Vp) for laboratory seismic cycles. During the coseismic stress drop, seismic velocity drops abruptly, consistent with observations on natural faults. In the preparatory phase preceding failure, we find that accelerated fault creep causes a Vp reduction for the complete spectrum of slip behaviors. Our results suggest that the mechanics of slow and fast ruptures share key features and that they can occur on same faults, depending on frictional properties. In agreement with seismic surveys on tectonic faults our data show that their state of stress can be monitored by Vp changes during the seismic cycle. The observed reduction in Vp during the earthquake preparatory phase suggests that if similar mechanisms are confirmed in nature high-resolution monitoring of fault zone properties may be a promising avenue for reliable detection of earthquake precursors

The role of fault rock fabric in the dynamics of laboratory faults

Fault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite-anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak-slip velocities. Subsequent unstable behavior produces slow stick-slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick-slip cycles

Deformation processes, textural evolution and weakening in retrograde serpentinites

Serpentinites play a key role in controlling fault rheology in a wide range of geodynamic settings, from oceanic and continental rift zones to subduction zones. In this paper, we provide a summary of the most common deformation mechanisms and frictional strengths of serpentine minerals and serpentinites. We focus on deformation mechanisms in retrograde serpentinites, which show a progressive evolution from undeformed mesh and bastite pseudomorphic textures to foliated, ribbon-like textures formed by lizardite with strong crystallographic and shape preferred orientations. We also discuss the possible mechanical significance of anastomosing slickenfibre veins containing ultraweak fibrous serpentines or relatively strong splintery antigorite. Our review and new observations indicate that pressure solution and frictional sliding are the most important deformation mechanisms in retrograde serpentinite, and that they are frictionally weak (μ~0.3). The mineralogical and microstructural evolution of retrograde serpentinites during shearing suggests that a further reduction of the friction coefficient to μ of 0.15 or less may occur during deformation, resulting in a sort of continuous feedback weakening mechanism

Complex geometry and kinematics of subsidiary faults within a carbonate-hosted relay ramp

Minor fault geometry and kinematics within relay ramps is strongly related to the stress field perturbations that can be produced when two major fault segments overlap and interact. Here we integrate classical fieldwork and interpretation of a virtual outcrop to investigate the geometry and kinematics of subsidiary faults within a relay ramp along the Tre Monti normal fault in the Central Apennines. Although the Tre Monti fault strikes parallel to the regional extension (NE-SW) it shows predominant dip-slip kinematics, suggesting a NW-SE oriented extension acting at sub-regional scale (1–10 km). Conversely, the slickenlines collected on the front segment of the relay ramp highlight right-lateral kinematics. The subsidiary faults in the relay ramp show a complex geometry (variable attitudes) and slickenlines describe multiple kinematics (left-lateral, dip-slip, right-lateral), independently of their orientation. Our fault slip analysis indicates that a local stress field retrieved from the kinematic inversion of the slickenlines collected on the front segment, and likely promoted by the interaction between the overlapping fault segments that bound the relay zone, can explain most of the geometry and kinematics of the subsidiary faults. Further complexity is added by the temporal interaction with both the regional and sub-regional stress fields

A new biaxial apparatus integrated within a pressure vessel to test the physical properties of brittle rock: the state-of-the-art

A main goal of the European Research Council, Starting Grant, GLASS (InteGrated Laboratories to investigate the mechanics of ASeismic vs Seismic faulting), is to develop a prototype rock deformation biaxial apparatus to examine the physical properties of brittle rocks. Two layers of fault rock are sandwiched between three steel block by a normal load applied using a horizontal oil-dynamics piston. A vertical oil-dynamics piston pushes the internal rock sample of the sandwich in order to slide at constant velocity. With GLASS we are going to build-up a confining pressure around the rock samples under load stress (tri-axial mode) and to measure the fluid flow properties of the rock during the deformation. Working in tri-axial mode with a fluid circulation, the machine is able to measure and to characterize frictional properties of faults on the sample for a wide spectrum of realistic conditions. We have concurrently been working to improve the control and the acquisition system for having a machine very flexible and easy to use for several applications and capable to detect different signals on the rock during frictional sliding in a fluid-rich environment with the goal of comparing these signals to those observed in nature. We began designing the servo controlled machine in October 2010 and have recently installed the apparatus in the HP-HT lab at the INGV in Rome. First tests of this biaxial apparatus confirm the main target of the project