11:55 AM - 12:10 PM
[SSS27-19] Frictional properties of mafic metamorphic gouges: Implication for slow earthquakes along the Nankai Trough
Keywords:metamorphic rock, oceanic crust, frictional behavior, Nankai Trough, amphibole, pore pressure ratio
Based on the observation, mentioned above, we performed hydrothermal ring shear experiments using a mixture of actinolite (Act, ~85%) and chlorite (Chl, ~15%) at effective normal stresses (σneff) of 50-200 MPa, pore fluid pressures (Pf) of 50-200 MPa, T of 22.5-600°C, and sliding velocities (V) of 0.0003-0.1 mm/s. Our results show that the rate- and state-dependent friction parameter (a-b) is affected by both σneff and Pf at T = 200-400°C. At low velocity-step (e.g. 0.0003-0.001 mm/s), (a-b) shows negative at this temperature range, whereas it increases to positive with increasing V.
To extrapolate the results of the mechanical behaviors outside the experimental conditions, we quantify the effects of σneff and Pf on (a-b) in the lowest velocity-step from 0.0003 to 0.001 mm/s, using a multiple regression analysis. By applying the results of these empirical fits to the P-T conditions of the Nankai Trough, we demonstrate that a high λ (Pf / (σneff + Pf), pore pressure ratio) above ~0.92-0.95 is needed for unstable, velocity-weakening behavior on Act + Chl gouge. However, since (a-b) increases with increasing V, unstable slip nucleating in a mixture of Act + Chl, can transition to stable sliding with increasing V, and stop without developing a huge rupture event. Act + Chl gouges therefore might have slipped at low velocity, resulting in slow earthquakes concentrating stress in adjacent undeformed bodies (i.e. asperities) of different assemblages and texture.