The composition and structure of fault gouge affect the magnitude and frequency of seismic activity in the Red River Fault Zone

Seismic records show that seismic activity is strikingly different in the southern and northern Red River Fault Zone (RRFZ). The magnitude and frequency of seismic activity in the southern RRFZ are weak (Less than 3 M) and low, respectively, while the magnitude and frequency of seismic activity in the northern RRFZ are strong (more than 5 M) and high, respectively. However, considerable controversy regarding the reasons for these differences remains. In this paper, to investigate the causes of different seismic activities, we analyze the composition and structure of fault gouges by conducting field investigations, polarized light microscopy, scanning electron microscopy observations, and X-ray diffraction (XRD) experiments. The results indicate that fault gouges in the southern RRFZ with a high illite content have high friction coefficients, and in the nanoscale, illite-rich fault gouges are easily deformed into S–C fabrics or foliated (layers) structure; under shear stress, it can release of stress and strain slowly (creep), so the fault activity shows aseismic creep. While the fault gouges in the northern RRFZ contain mainly montmorillonite, which has a low friction coefficient, and in the nanoscale, montmorillonite-rich fault gouges easily form linear nanogrooves and nanoridges under shear stress, so the fault shows seismic stick–slip, and it can induce earthquakes. Therefore, we propose that the composition and structure of fault gouges are the key factors affecting the different magnitudes and frequencies of seismic activity in the southern and northern RRFZ.