[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-01] The stabilizing effect of pore-fluid pressure along subduction megathrust faults: Evidence from experiments on Nankai Trough sediments

*John D Bedford^1,2, Daniel R Faulkner², Michael J Allen², Takehiro Hirose¹ (1.JAMSTEC, 2.Univ. of Liverpool)

Keywords:Slow slip, Pore-fluid pressure, Nankai Trough, Fault friction

Pore-fluid pressure is an important parameter in controlling fault mechanics as it lowers the effective normal stress allowing fault slip at lower shear stress. It is also thought to influence the nature of fault slip, particularly in subduction zones where the occurrence of slow-slip has been linked to areas of elevated pore-fluid pressure. Despite the importance of pore-fluid pressure on fault mechanics, its role on controlling fault stability, which is determined by the friction rate parameter (a-b), is poorly constrained, particularly for fault materials from subduction zones. In the winter of 2018-19 the accretionary complex overlying Nankai Trough subduction zone (SW Japan) was drilled as part of Integrated Ocean Drilling Program (IODP) Expedition 358. Here we test the frictional stability of the accretionary sediments recovered during the expedition by performing a series of velocity-stepping experiments on simulated fault gouges while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges are strongly rate-strengthening and exhibit negative values for the rate-and-state parameter b. We find that for experiments where the effective normal stress is held constant and the pore-fluid pressure is varied the Nankai gouges become more rate-strengthening, and thus more stable, at higher pore-fluid pressures. In contrast, when the pore-fluid pressure is held constant and the effective normal stress is varied, there is minimal effect on the frictional stability of the gouge. The increase in frictional stability of the gouge at elevated pore-fluid pressure is caused by an evolution in the rate-and-state parameter b, which becomes more negative at high pore-fluid pressure. These results have important implications for understanding the nature of slip in subduction zones and suggest the stabilizing effect of pore-fluid pressure could promote slow-slip or aseismic creep on areas of the subduction interface that might otherwise experience earthquake rupture.