[S-SS10] Fault Rheology and Earthquake Physics

[SSS10-P28] Experimental evaluation of friction property of unaltered and altered basalt

*Yohei Donga¹, Minoru Kudose¹, Akito Tsutsumi², Tetsuro Hirono¹ (1.Osaka Metropolitan University, 2.Kyoto University)

Keywords:Nankai earthquake, plate-subduction boundary, basalt, frictional property

The quantitative evaluation of slip-on plate boundary faults during Nankai earthquakes is extremely important because it is directly related to the estimation of the size of the tsunami. Hirono et al. (2016) conducted diversified analyses and laboratory experiments using core samples collected from the Nankai Trough Megasplay fault off the coast of Kumano as the IODP Nankai Trough Seismogenic Zone Experiment, and the slip distance of 30-50 m was calculated through dynamic analysis. However, these results were obtained only from analytical and experimental values of clay samples collected from 271 m below the seafloor, and so this fault model does not reflect the oceanic plate stratigraphy. Therefore, we aim to quantitatively evaluate the peak friction coefficient, the dynamic friction coefficient, and Dc of distances from the start of the experiment to the steady state by our friction experiments using basalt, one of the representative components of the oceanic plate stratigraphy. In addition, considering the alteration of basalt at plate boundaries, we used two types of basalt in our friction experiments; unaltered basalt underlying the oceanic crust of the Cocos Plate and altered basalt exposed in the Mio Mélange of the Hidakagawa Group, Wakayama Prefecture, Japan. First, we ground the basalt samples with a mortar, obtained a specific grain size of 53-150 µm with a sieve, and did not dry samples. We held samples between two 25 mm diameter cylindrical blocks of gabbro, and covered the friction surface with a Teflon ring to prevent leakage of samples. Experiments were then conducted using a rotary-shear intermediate to high-velocity friction apparatus with a constant normal stress of 1.5 MPa, a slip velocity of 1 m/s, slip distances of 10 m, and a sample weight of 2 g. We obtained experimental results as follows. The unaltered basalt shows a peak friction coefficient of 1.00, a dynamic friction coefficient of 0.32, and Dc of 9.70 m. On the other hand, the altered basalt showed a peak friction coefficient of 0.88, a dynamic friction coefficient of 0.23, and Dc of 9.39 m. Comparing the values of the experiments for the two basalts, the altered basalt has the peak friction coefficient of 0.12 lower, the dynamic friction coefficient of 0.09 lower, and the Dc of 0.31 m shorter than those of the unaltered basalt. The powder X-ray diffraction analysis shows that the altered basalt contain more kaolinite and dolomite. This suggests that these minerals with lower strength formed by the alteration at the plate subduction boundary lead to the decrease in the friction coefficient of the basalt. As a future study, we will evaluate more comprehensive frictional properties of the unaltered and altered basalt by experiments at various constant normal stresses and with larger slip distances.