The deformation of quartz and feldspar, major components of granitic rocks, has been well documented (e.g., Tullis 2002; Passchier and Trouw 2005). Quartz deforms by brittle mechanisms up to about 300–350 °C, corresponding to crustal depths of around 10–12 km. At greater depths, crystal-plastic mechanisms (creep mechanisms) and diffusion dominate. Feldspar is different because of the difficulty of dislocation glide and climb (crystal-plastic deformation mechanism). It thus deforms in a brittle manner at temperatures up to 500 °C and depths of 20–30 km (Fossen 2016). However, the responses of both quartz and feldspar to frictional forces on fault planes during faulting are not well understood, especially the effect of the presence of water. Our experiments have shown that the frictional characteristics of quartz and feldspar under deep crustal temperature and pressure conditions differ.
Effect of water on velocity dependence of steady-state friction
The differences of the values of (a – b) that we identified for dry and wet samples of quartz and feldspar demonstrated that the presence of water strongly influence their frictional properties. For both quartz and feldspar, we observed that under wet conditions there were temperature ranges within which (a – b) was negative (corresponding to seismic conditions; Fig. 4), but there was little difference in the frictional coefficient versus displacement curves between the wet and dry samples (Fig. 3). These general trends are similar to those observed in experiments on granite gouge (Blanpied et al. 1991, 1995). Most natural fault zones are fluid-saturated (e.g., Sibson 1992), so we expect that the effect of the presence of water plays an important role in seismogenic processes in the crust. Blanpied et al. (1991) inferred from their experiments on granite gouge that the most likely fluid-related mechanisms are pressure solution of quartz (Rutter and Mainprice 1978) and incongruent pressure solution of feldspar whereby it is dissolved preferentially at stressed interfaces (Beach 1980). Chester and Higgs (1992) and Kanagawa et al. (2000) also suggested that solution transport may be activated by the presence of quartz gouge in a fault.
We suggest that pressure solution occurred in our wet experiments on both quartz and feldspar. SEM images of our samples after the experiments show that rounding of grains occurred during our experiments under wet conditions (Fig. 5), which is supportive of pressure solution taking place. The variations in frictional behavior that we observed can be understood in terms of a change in the dominant mechanisms of slip, such as from cataclastic slip and to solution-precipitation-aided cataclastic flow, as follows. When pressure solution occurred in the wet experiments, dissolved solution was transported to the grain interfaces, where grinding and rounding of the grains of quartz and feldspar occurred, which assisted solution-precipitation-aided cataclastic flow. As a result, frictional resistance was reduced and velocity-weakening behavior was induced. At higher temperatures (> 350 °C), the viscosity of the fluids may have increased, resulting in an increase in resistance to slip. The different behaviors of quartz and feldspar may be explained by the different properties of the dissolved solutions. However, our experimental setup did not allow us to analyze the solutions in the course of our experiments, which we have deferred. Solution analysis is remained for the future studies.
Effect of material properties of quartz and feldspar on the velocity dependence of friction
In almost all of our experiments under wet conditions, the values we obtained for (a – b) were greater for quartz than for feldspar, which indicates that the friction coefficient of quartz becomes larger than that of feldspar with increasing slip velocity. Thus, it is more difficult to initiate slip and cause earthquake nucleation in gouge composed of quartz than in gouge composed of feldspar. Our findings are supported by the work of He et al. (2013), who found that adding a little quartz to the plagioclase–pyroxene mixture they used to simulate fault gouge had a strong stabilizing effect that led to a transition from velocity weakening to velocity strengthening.
Although both quartz and feldspar are ubiquitous in crustal rocks, there have been few experiments on the frictional behavior of feldspar, probably because pure, fine-grained feldspar aggregates are rare in nature (Tullis 2002). As previously described (in our discussion above of the effect of water on the velocity dependence of steady-state friction), we found that in the presence of water the temperature range over which (a – b) for feldspar was negative was wider than was the case for quartz. Moreover, feldspar is more brittle than quartz under the high-temperature and pressure conditions deep in the crust (e.g., Fossen 2016). The brittle–plastic transition zone of the lower crust may be a region of slow deformation. However, because frictional behavior occurs only within the layers where stress is concentrated at the sliding surface, we consider here the friction-plastic transition. Seismic activity takes place where brittle (frictional) materials move quickly on the fault surface, so material that is more amenable to unstable slip plays a more important role in this region. Our results suggest that under some conditions the influence of feldspar in determining the depth extent of the seismogenic zone in the crust is greater than that of quartz.
Implications for the depth extent of the seismogenic zone
To estimate the depth extent of the seismogenic zone in a particular region, regional data are required on the depth–temperature profile, distribution of water, strain rate, and dominant rocks and minerals. The inhomogeneous distribution of earthquakes suggests that the subsurface distribution of the material properties of rocks (i.e., their lithostratigraphy) has a stronger influence on frictional properties at depth than do environmental conditions, such as temperature and strain rate (e.g., Shibazaki et al. 2008).
The availability of data from dense seismic observation networks in and around the Japanese islands allows comparison of precise earthquake hypocenters with subsurface temperature distributions. Hasegawa and Yamamoto (1994) showed that most of the earthquakes in northeastern Japan are confined to the upper 15 km of the crust, which constitutes the brittle seismogenic zone there. Comparison of those hypocenters to the temperature distribution in the crust, estimated from seismic velocity changes inferred on the basis of seismic tomography, shows a clear correspondence of the lower limit of the seismogenic zone with the 350 to 400 °C isotherm (Hasegawa and Yamamoto 1994). This temperature range corresponds better to the transition from brittle deformation to plastic behavior of feldspar than to that of quartz.