Masuda et al. (2019) conducted laboratory experiments with quartz and albite and reported that feldspar plays a more dominant role than quartz in limiting the depth extent (thickness) of the seismogenic zone, but the frictional behavior of other minerals in the lower seismogenic zone still need to be characterized. Frictional behavior depends in part on environmental conditions, including temperature and the presence of water (Blanpied et al. 1995; Scholz 1998), and the heterogeneity of fault materials (Barnes et al. 2020) or surface morphology (Kirkpatrick et al, 2020) at the interface between seismic and aseismic zones also affects seismogenic processes. Therefore, it is important to know the physical properties, especially the frictional properties, of materials in the transition zone around this interface to understand seismogenic mechanisms. Recent advances in seismological observation and analysis techniques have allowed seismic velocity and density distributions to be determined in detail, but information on the physical properties of materials in high seismicity regions is still lacking. Only laboratory measurements can provide basic physical information about materials, such as their frictional stability.
Effect of water on the frictional characteristics of materials
The velocity dependence of friction (a – b) of quartz and feldspar, the two main components of the Earth's crust, is mostly positive under dry conditions at temperatures corresponding to those in faulting zones (Masuda et al. 2019; Fig. 2a). Under wet conditions, however, (a – b) is negative at temperatures of 200–400 °C. These results are similar to those observed in experiments using granite gouge, a main component of crustal rocks (Lockner et al. 1986; Blanpied et al. 1995). These experimental findings suggest that seismic behavior is induced in the deep crust in the presence of water. Various studies have shown that the mechanism by which water affects frictional behavior in the deep crust is solution-precipitation-aided cataclastic flow (e.g., Blanpied et al. 1995; Masuda et al. 2019). Here, scanning electron microscope images of anorthite gouge samples after the experiments showed rounded grains only after the experiments conducted under wet conditions (Additional file 1). This result suggests that pressure solution occurred in the experiment under wet conditions.
Compared with quartz, feldspar had negative (a – b) values over a wider temperature range. This result suggests that under wet conditions, the frictional characteristics of feldspar series minerals play an important role in limiting the depth extent of the seismogenic zone. The results of the anorthite experiments conducted in this study (Fig. 2) strongly support the results of the albite experiments conducted by Masuda et al. (2019). Moreover, unlike albite, anorthite showed stick–slip behavior under wet conditions at temperatures of 200 and 400 °C. In geologic materials, stick–slip behavior is associated with brittle deformation such as slip on a fault (e.g., Brace and Byerlee 1966). This observed brittle behavior thus suggests that the frictional characteristics of anorthite can account for fault movements in the brittle–plastic transition region.
Lower boundary of the seismogenic zone
Mechanically weak heterogeneities at the base of the seismogenic zone would localize deformation in the brittle–plastic transition region and cause brittle faulting. The physical properties of anorthite support this concept. The stick–slip behavior observed in anorthite (CaAl2Si2O8) was not observed in albite (NaAlSi3O8) (Masuda et al. 2019). Under ambient conditions, the feldspar mineral structure is based on TO4 tetrahedra (where T = Si4+ or Al3+), within which low-charge cations (Na+ or Ca+) occupy large voids. Commonly, feldspars are thermodynamically stable at pressures up to 3 GPa. Because TO4 tetrahedra show very little compression, they behave as a relatively rigid unit (e.g., Pakhomova et al. 2020). This rigidity may be one reason that feldspar is associated with brittle sliding even at high temperatures. In addition, because Ca+ is heavier than Na+, anorthite, the calcic endmember of the series, may be more rigid and therefore exhibit more brittle behavior (such as stick–slip behavior) than albite. In field observations of an exhumed fault zone exposed along the Hatagawa Fault Zone, northeast Japan, many localized brittle deformation zones, including highly fractured crush zones, are observed. In these zones, numerous fractures may have nucleated by fracturing of highly deformed fine-grained feldspar (Shigematsu et al. 2009).
In this study, we investigated anorthite, one mineral component of the Earth's crust. However, measurements of many component minerals and composite materials under a wide variety of possible conditions, as well as laboratory measurements of materials obtained by deep drilling projects (e.g., Kirkpatrick et al. 2020), are needed to construct a reliable conceptual model of seismic processes in the brittle–plastic transition zone at the seismogenic zone depth limit.