[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P03] Experimental investigation on effects of diagenesis on frictional properties of incoming sediments from Tohoku subduction zone

*Mizuki Ueda^1,2, Keishi Okazaki^2,1, Asuka Yamaguchi³, Yohei Hamada² (1.Hiroshima University, 2.Japan Agency for Marine-Earth Science and Technology Kochi Institute for Core Sample Research, 3.Atomosphere and Ocean Research Institute, The University of Tokyo)

Keywords:diagenesis, frictional properties , slow earthquake

Plate boundary faults in subduction zones are mainly composed of silica and clay minerals. To reveal the details of various frictional slip behaviors of plate boundary faults (e.g., normal earthquakes, shallow slow earthquake, and tsunami earthquakes), it is important to understand the frictional properties of such sediments. Sediments could change frictional properties by various factors. Especially diagenesis accompanied by subducting, which cause solidification of sediments, may affect their frictional properties and mechanical properties. The temperature and depth conditions at which slow earthquakes and megathrust earthquake at Tohoku subduction zone are corresponding to the depth occurring active diagenesis and dehydration of silica and clay minerals. However, friction experiments considering influence of diagenesis have not been conducted. In this research, incoming sediments sampled during KS-15-3 cruise to Japan Trench were used. From XRD analysis, the sample is mainly composed of clay minerals (smectite, illite and chlorite), terrigenous grains (quartz and feldspar) and biogenic opal. Friction experiments with velocity step were conducted after a long "leaving time" in high temperature, high pressure and hydrothermal condition. The samples were left in the pressure vessel for 10 minutes to 7 days to reproduce the diagenetic reaction and to investigate the effect of diagenesis on frictional properties of incoming sediments. The frictional properties were estimated mainly based on the value of the (a-b), which indicates the velocity dependence of the friction coefficient, from rate-and-state dependent friction law (RSF law). Experimental results showed that the friction coefficient increased with increasing leaving time and temperature, proportional to the logarithm of them. In addition, (a-b) decreased with leaving time, but showed only a slight negative correlation with temperature. In terms of changes in the individual friction parameters a and b, a had no trend for leaving time and temperature, while b showed a negative trend with respect to both leaving time and temperature. This suggests that the decrease of the (a-b) is mainly due to an increase of b potentially by the diagenesis. From the results of this research, we could estimate the conditions of leaving time necessary to show velocity-weakening behavior for each temperature. Although the value of (a-b) depends on the sliding velocity, the calculation results show that after about 15 hours of leaving at the temperature of 61 degrees, it is sufficient to show velocity-weakening behavior at least one velocity step conditions conducted in this research. At higher temperatures, the required leaving time became shorter. The stability of the fault was also estimated by the value of the critical stiffness, Kcr, in the spring-slider model. Kcr is defined as the product of (b-a) and (the effective normal stress ) divided by Dc (the critical slip distance to reach steady state after a velocity step). If (a-b) is positive, Kcr becomes negative, and thus the fault must show a stable sliding. If (a-b) is negative but 0<Kcr<K, the fault is conditionally unstable and slow-slip potentially triggered, and if K<Kcr, an unstable slip, such as stick-slip, can be observed. For all velocity steps where (a-b) was negative in this study, the condition of the fault stability is 0< Kcr <<K, indicating that the fault was conditionally unstable. This is consistent with that all experiment showed only stable sliding while some experiments showed negative (a-b). Our results on friction experiments indicate that the frictional properties of the incoming sediments can gradually change with increasing the progress of diagenesis. These frictional behaviors of the incoming sediments could be the origin of the depth-dependent transition of the fault slip behavior of the plate boundary fault, such as stable sliding, slow earthquakes and fast earthquakes, at the Tohoku subduction zone.