Strength evolution of a reactive frictional interface is controlled by the dynamics of contacts and chemical effects

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Abstract

Assessing the healing rate of a fault is relevant to the knowledge of the seismic machinery. However, measuring fault healing at the depths where it occurs still remains inaccessible. We have designed an analog laboratory experiment of a simulated rough fault that undergoes healing and investigate the relative roles of interface chemical reactivity and sliding velocity on the healing rate. Slide-hold-slide experiments are conducted on a bare interface with various materials in contact (glass/glass, salt/glass, and salt/salt) with or without the presence of a reactive fluid and the slider-surface pull-off force is measured. Our results show that the interface strengthens with hold time, whatever the conditions of the experiments. In addition, we quantify the effect of chemical reactivity on the healing rate. Considering the glass/glass case as a reference, we show that the healing rate is increased by a factor of 2 for the salt/glass case; by a factor of 3 for the salt/salt case; and by about a factor of 20 when saturated brine is added on a salt/salt interface. We also measure that the sliding velocity affects the healing rate for salt/salt interfaces at room humidity. A careful optical monitoring of the interface allows a direct observation of the contact growth characteristics associated to each type of materials. Finally, the large differences of healing rate are interpreted through a mechanistic approach, where the various experimental conditions allow separating different healing mechanisms: increase of adhesion of the contacts by welding, contact growth due to creep or due to neck growth driven by surface tension.

Highlights

► Healing controls how an active fault evolves between two earthquakes. ► We reproduced experimentally the healing of a frictional interface. ► The dynamics of asperity contacts, that control friction, is enhanced by chemical effects. ► These effects modify the time scales of fault healing.

Introduction

Immediately after an earthquake, a fault starts healing and strengthening. During the period between two major earthquakes, defined as the interseismic period, the fault may creep, compact, and seal, leading to a progressive increase of the mechanical strength of the fault zone (Li et al., 2003, Vidale and Li, 2003, Gratier and Favreau, 2003). Such fault re-strengthening during interseismic periods can be measured from periodic or continuous records of seismic velocities variations in the damage zone surrounding large earthquakes (Li et al., 2003, Brenguier et al., 2008). It has been proposed that these variations may be related to the closure of partially fluid-filled cracks in the rupture zone and the recovery of microscopic damage; a behavior known as fault healing. The growing interest in fault healing lies in the fact that it controls fault stability and the seismic versus aseismic behavior (Ruina, 1983, Tse and Rice, 1986), and the mode of rupture propagation, crack-like or slip-like (Perrin et al., 1995).

Fluids play a key role in fault zones, as proven by the many structural markers found on exhumed faults (veins, stylolites, mineralogical differentiation) or by their high permeability and porosity measured in damage zones after an earthquake (Lockner et al., 2000). The role of fluids during the seismic cycle and their effects on healing processes (see Hickman and Evans, 1995 for a review) is thus crucial. On the one hand, an increase in fluid pressure may bring the fault closer to failure and control earthquake sequences, as shown, for example, for the 2004 Mid-Niigata Prefecture sequence in Japan (Sibson, 2007). On the other hand, fluid-enhanced chemical effects may control fault re-strengthening, these effects including: lithification (Karner et al., 1997, Renard et al., 2000); crack closure (Vidale and Li, 2003); modification of the nature of the contact, from adhesive to welded, due to mineral precipitation (Fredrich and Evans, 1992, Tenthorey and Cox, 2006); increased contact surface area either due to creep (Nakatani and Scholz, 2004, Yasuhara et al., 2004, Goldsby et al., 2004, Gratier et al., 2009) or to surface tension driven prop growth (Beeler and Hickman, 2004, Hickman and Evans, 1991), and fracture sealing (Boullier et al., 2004). Healing and sealing processes also control the evolution of the permeability of fractures and their ability to transport fluids and transmit pressure in the crust (Matsuki et al., 2001, Polak et al., 2003).

Marone et al. (1995) estimated healing rates or equivalently the strengthening rate from the stress drop variations of small repeating earthquakes as a function of recurrence time intervals. They showed that the stress drop presents a logarithmic growth with time, and that the fault strengthens at a rate in the range 1−3 MPa per decade of time. Such logarithmic time dependence therefore controls the rate of healing of the fault (Beeler et al., 1994).

Similar time dependence appears in laboratory friction experiments, where it is measured, during slide-hold-slide tests, that the pull-off force to slide over an interface increases with the logarithm of hold time (Rabinowicz, 1958, Rabinowicz, 1995). This ageing behavior is almost universal, as it was observed in laboratory experiments for metals (Rabinowicz, 1995), rocks (Dieterich, 1972), ceramics (Dieterich, 1978), polymers (Bureau et al., 2002), and glass (Berthoud et al., 1999). This frictional healing is attributed to the time-dependent growth and strengthening of contact points (Rabinowicz, 1995, Dieterich, 1972, Dieterich and Conrad, 1984). Some of these experiments were performed at high normal stress, relevant to natural conditions, others at low normal stress. In all cases, healing of the interface was observed. Because friction is proportional to the area of real contacts (i.e. asperities), the healing process can be separated, for each contact, into two effects: increase of the surface area itself and increase of the adhesion (i.e. the strength of the contacts). The asperity surface area is determined by the yield stress of the material and the solid flattens at an asperity so that the normal stress does not exceed this yield stress. The increase of adhesion is mainly dependent on the chemistry of the two solids and on the micro-roughness of the contacts.

The healing effect is included in rate-and-state friction laws. These laws describe the evolution of the friction coefficient μ which depends on the velocity of the slider V and a state variable θ that takes into account memory effects (Dieterich, 1979, Ruina, 1983). Accordingly, friction can be described asμ(V,θ)=μ0+aln(VV0)+bln(V0θDc)where μ0 is the friction coefficient for a reference velocity V0, Dc is a characteristic distance, and a and b are material dependent empirical constants. In this equation, the last term on the right hand side represents the memory effect due to hold periods or the previous sliding velocity in the case of velocity-stepping experiments. This is described by the state variable θ that can follow at least two different evolution laws, either to take into account slip-dependent strengthening or the time evolution of the contacts (Ruina, 1983). Only the latter law includes an evolution of the friction when the velocity V is equal to zero. In this case (i.e. V=0), b scales as the frictional healing rate (Beeler et al., 1994) and thus partly controls the stability of sliding via the value of (a–b) (Scholz, 1998). In the present study, we focus on the experimental rate of frictional healing and study how chemical effects may modify it.

Slide-hold-slide experiments have been reported on bare rock surfaces or simulated fault gouges under high pressure and low or high temperature to estimate healing rate and to infer the physical processes at work (Dieterich and Conrad, 1984, Fredrich and Evans, 1992, Beeler et al., 1994, Nakatani and Mochizuki, 1996, Karner et al., 1997, Blanpied et al., 1998, Karner and Marone, 1998, Mair and Marone, 1999, Frye and Marone, 2002, Nakatani and Scholz, 2004, Yasuhara et al., 2005). Experiments conducted on simulated gouges of quartz under hydrothermal conditions show higher healing rates after a temperature-dependent cut-off time attributed to pressure solution activation (Nakatani and Scholz, 2004, Yasuhara et al., 2005). Using a soluble rock analog, several experimental results of deformation of salt grains during shear have also confirmed the logarithmic time dependence of healing and the crucial role of grain chemistry and reactivity (Bos and Spiers, 2000, Niemeijer et al., 2008).

Under room temperature and small stress, the static strength of glass-glass interface was investigated (Berthoud et al., 1999) and it was found that, in addition to the logarithmic strengthening, the rate of healing grows with the temperature of the material, and with the state of shear stress applied during the holding periods. Berthoud et al. (1999) have shown experimentally that the level of healing was almost twice larger if the shear stress was maintained constant compared to tests where the shear stress was decreased to zero during the hold period. This was confirmed by Losert et al. (2000) who measured that healing was negligible when no shear stress was applied. Karner and Marone (1998) observed also that, in the absence of shear stress during the hold period, the healing effect could become negative. Note, however, that one study (Nakatani and Mochizuki, 1996) has seen an opposite effect with the healing rate being 10–20% higher when the shear stress was decreased during the hold periods. Finally, transition after some time in the healing rate was observed and attributed to the activation of different healing mechanisms at short and long hold times (Blanpied et al., 1998, Yasuhara et al., 2005, Niemeijer et al., 2008).

The principal purpose of the present study is to analyze which mechanisms may be responsible for the frictional healing of a rock analog interface in presence or absence of chemical reactions and how it modifies the value of the parameter b in Eq. (1). We propose here an analog experiment where we attempt to separate the effects of the various healing mechanisms, and provide some explanation on the wide range of the values of the b parameter found in experimental studies. Hereafter, we investigate the dependence of the strengthening rate of a frictional interface on the chemical reactivity of the two solids in contact. In order to avoid complexities arising from the use of a granular material (i.e. gouge), we work with rough bare surfaces. The reactivity of the interface is controlled by changing the nature of the surfaces in contact (glass/glass; salt/glass; salt/salt) and the fluid (air, water). A camera integrated at the device allows acquiring images of the asperity contacts. The strengthening is derived from the shear force necessary to break the interface and pull-off the slider. It is measured using a force gage. This experimental set-up is built to be analogous to the two surfaces of a natural fracture during the interseismic period and allows characterizing how a macroscopic variable, the strengthening, is controlled by microscopic processes related to contact dynamics.

Section snippets

Friction experiment apparatus

A 1×1 cm2 surface area slider held under constant normal load, is left in contact with a flat plate (Fig. 1). The slider has a cube shape and is made of either a single crystal of sodium chloride (NaCl) or glass. The 3 cm radius circular flat plate over which the slider is moving is made of either NaCl single crystal or glass. During experiments, the 1 cm2 nominal surface area of the slider bears a constant normal load of 2.65 kg (0.26 MPa normal stress) imposed by dead weights. The loading rate is

Slide-hold-slide experiments: effect of surface reactivity on the ageing rate of the interface

Fig. 2 presents a typical record of frictional force during a slide-hold-slide experiment. Here, we define the degree of healing (Δμ) as the difference between the peak shear-stress upon re-shear and the preceding steady shear stress normalized by the normal load (as in Marone, 1998a, Niemeijer et al., 2008, for example). In the following, we approximate the healing by the ageing of the interface at rest, and we evaluate how the surface chemical reactivity impacts the healing rate. In order to

Discussion

We present experimental observations of the healing and strength recovery of a rough interface in the presence of chemical reactions. Our results first confirm that interface chemical reactivity plays a major role on healing processes by enhancing the rate of asperity growth. Our work furthermore highlights the key role played by reactive fluids in the dissolution–crystallization processes responsible for the strengthening of the interface, which is attributed here mainly to neck growth of

Conclusion

Assessing how an interface may recover strength with time represents a fundamental issue to predict stress evolution in a fault zone during the interseismic period. In the present study, we have monitored the strength evolution of a frictional interface and varied the chemical reactivity of the solids in contact. The strengthening process occurred by the growth of contact asperities with non-linear time dependence, and the increase of adhesion of the welded contacts. The healing effect is

Acknowledgments

We thank Pierre Giroux, Liliane Jenatton, and Benjamin Vial, for their technical help and Benoit Derode, Salif Koné, Danitza Planet, and Anne-Marie Boullier for fruitful discussions. The project was funded by the University Grenoble I and the ANR grant JCJC-0011-01.

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