[S-SS10] Fault Rheology and Earthquake Physics

[SSS10-P20] Off-fault Damage Development as Revealed by Fault Surveys

*Kosei Ogita¹, Jun Muto¹, Hiroyuki Nagahama¹, Sando Sawa¹ (1.TOHOKU UNIVERSITY)

Keywords:Pulverized Rocks, Off-fault damage, Itoigawa-Shizuoka Tectonic Line

Off-faults of large-scale strike-slip faults, such as the San Andreas fault (SAL) and the Arima-Takatsuki Tectonic Line (ATTL), are known to contain characteristic fault rocks, called “pulverized rocks” (e.g., Brune, 2001; Dor et al., 2006a; Mitchell et al., 2011; Muto et al., 2015). They are typically characterized by a lack of shear strain and a development of significant crack in minerals. These features indicate that pulverized rocks are formed by a dynamic rupture different from a general fault slip (Muto et al., 2015). On the other hand, the number of samples is still too small to fully understand this comminution process, and conditions and factors related to the formation of pulverized rocks remain unclear, such as the presence of minerals altered by hydrothermal fluids in the ATTL (Shimizu et al., 2021). In this study, we investigated fractured granites in the Off-fault of the Itoigawa-Shizuoka Tectonic Line (ISTL), where new pulverized rocks have been suggested to exist, to better understand the formation process of pulverized rocks. As a result, rocks with the same characteristics as previously reported pulverized rocks were found in the off-fault of the ISTL’s thrust fault. In addition, we conducted a detailed and quantitative record of off-fault damage within a few meters near the fault core using the fractal dimension of the particle size distribution. The presence of the new pulverized rocks in the ISTL discovered in this study is the first example of pulverized rocks found in not only a strike-slip fault but also a thrust fault and indicates that comminution is likely to be a more universal phenomenon that is independent of fault morphology and alteration. Also, fractal dimension measurements show a wide range of high fractal dimensions close to 3 that are indicative of impact comminution. This suggests that high-energy dissipation occurs near the fault core. The damage heterogeneity recorded in the entire off-fault is also observed in the highly pulverized rocks in the vicinity of the fault. This off-fault damage heterogeneity is known to produce variations in the strain rate field and strength of the rock, which in turn affects seismic wave propagation and energy dissipation processes during the next earthquake (Ostermeijier et al., 2022). Therefore, this study provides new information on the off-fault damage of fault pulverized rocks, which is important for understanding the energy dissipation process during earthquakes, especially in the vicinity of faults.