Clay-rich fault gouge in the principal slip zones of faults stores abundant water within pores and between clay interlayers. During the preparation of thin-sections and rock chips for microstructural observations, fault-gouge samples are commonly air-dried at room temperature or in an oven. However, during the drying process, remnant liquid water between gouge grains produces an inter-particle adhesion force (liquid-bridge force), which rearranges the grain-to-grain structure, likely resulting in a disturbance of the original fabric. For this study, we prepared gouge samples from the Itozawa fault, northeastern Japan, using a t-butyl alcohol freeze-drying method that mitigates drying-induced fabric disturbance. We then compared the microstructure of our samples with those prepared using the conventional air-drying method. The freeze-dried samples preserve a smooth fault plane, clearly defined nanoparticles, and well-developed shear-sense indicators, including slickenlines and Riedel shear planes. In contrast, the air-dried samples underwent shrinkage during drying, which distorted the geometry of the fault plane. These air-dried samples lack nanoparticles and display only a weak shear fabric. We conclude that microstructural observations on samples prepared using the t-butyl alcohol freeze-drying method, compared with conventional air-drying, could preserve more evidence for the retrieval of fault information, including the kinematics, slip stability, and dynamic weakening mechanism of a fault.
The principal slip zones of faults comprise fine-grained gouge that commonly contains abundant clay minerals (e.g., Otsuki et al., 2003; Bullock et al., 2014). Microstructural analysis of fault gouge using scanning electron microscopes (SEMs) can constrain the deformation processes associated with past slip events (Boullier et al., 2009). Until the 1970s, incohesive fault rocks were assumed to exhibit random fabrics (e.g., Sibson, 1977); however, more recent field and laboratory investigations have recognized Riedel shear planes in such rocks (e.g., Logan et al., 1979; Chester et al., 1985; Rutter et al., 1986; Tanaka, 1992a). Slickensides with slickenlines and Riedel shear planes preserved in fault rocks can be used to determine the sense of shearing during fault motion (e.g., Logan et al., 1979; Doblas, 1998). The degree of development of such structures in fault gouge is closely related to the stability of fault slip, which is associated with earthquake nucleation (Beeler et al., 1996). As well as fault kinematics, shear-sense indicators can be used to constrain paleostress tensors and tectonic activity around faults (e.g., Petit, 1987; Chorowicz et al., 1999). SEM observations have revealed the micro-to nano-scale morphology and size distribution of gouge grains, which can be used to constrain weakening mechanisms during faulting (e.g., frictional melting, thermal pressurization, powder lubrication and graphization of gouge; Hirono et al., 2006; Han et al., 2007; Boullier et al., 2009; Han et al., 2010, 2014; Kuo et al., 2014b).