[SGL30-P07] Geological and seismological conditions required for operating coseismic OSL zeroing of quartz gouge
In the experiments, we used quartz powder with a particle size of <150 µm which is extracted from Tsushigawa granite, located in the northern part of Awaji island, Hyogo Prefecture. We irradiated 400 Gy of gamma-ray for the quartz powder before the experiments to simulate natural radiation damage. Friction experiments were conducted under the following conditions in darkroom environment using a rotary-shear, high-velocity friction apparatus in Yamaguchi University; slip rate of 200 µm ⁄ s to 1.3 m ⁄ s, normal stress of 1 to 5 MPa, and displacement of 2 to 10 m. To eliminate the effect of crushing on OSL signal, we measured coarse grains (75–150 µm) extracted from recovered sample. In the experiment conducted under normal stress of 1.0 MPa, OSL signal start to decrease from the slip rate of 0.25 m ⁄ s and becomes zero at slip rate ≧ 0.65 m ⁄ s. Signal zeroing was also observed from the experiments sheared at 1.3 m ⁄ s under water-added condition. At slip rates of 0.25 and 0.40 m ⁄ s, we found “partial resetting” which is characterized by coexistence of both particles with signal and without signal. OSL signal intensities (Lx ⁄ Tx) have a strong correlation between power density, τeV (DiToro et al., 2011, Nature) which is proposed as a parameter of heat generation during high-velocity friction, and measured ⁄ calculated temperature in the fault zone; that is, the measured ⁄ calculated temperature exponentially rises and Lx ⁄ Tx exponentially decreases with increasing the power density. The power densities required for the partial and complete resetting are 0.24 MW ⁄ m2 and 1.0 MW ⁄ m2, respectively. Assuming a coseismic fault slip rate of 0.65 m ⁄ s, depth conditions required for operating partial and complete resetting of OSL age are expected to be 35 and 141 m respectively.
[Acknowledgements] This work was supported by grants from the Nuclear Regulation Authority (FY 2015–2017). We thank Manami Kitamura (AIST) and Takehiro Hirose (JAMSTEC) for the temperature calculation using FEM software COMSOL.