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 areas of slow slip have been linked to regions 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 the 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 powdered samples (to simulate fault gouge) while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges, despite only containing 25% phyllosilicates, 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 increased the Nankai gouges become more rate-strengthening, and thus more stable (an increase in ($a-b$) of ∼6 × 10⁻⁵ MPa⁻¹ with increasing pore-pressure). 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.

孔隙流体压力是控制断层力学的一个重要参数，因为它降低了有效正应力，允许断层在较低的剪切应力下滑动。它还被认为会影响断层滑动的性质，特别是在缓慢滑动区域与孔隙流体压力升高区域相关的俯冲带中。尽管孔隙流体压力对断层力学很重要，但它在控制断层稳定性方面的作用由摩擦率参数（$\mathrm{一种}-乙$) 的约束很差，特别是对于来自俯冲带的断层材料。在 2018-19 年冬季，作为综合海洋钻探计划 (IODP) 远征 358 的一部分，对覆盖南海海槽俯冲带（日本西南）的增生复合体进行了钻探。在这里，我们测试了在远征期间回收的增生沉积物的摩擦稳定性：对粉状样品进行一系列速度步进实验（以模拟断层泥），同时系统地改变孔隙流体压力和有效正应力条件。尽管仅含有 25% 的层状硅酸盐，南开凿岩具有很强的速率强化作用，并且速率和状态参数b显示出负值. 我们发现，对于有效法向应力保持恒定且孔隙流体压力增加的实验，南开凿岩变得更加速率强化，因此更加稳定（增加（$\mathrm{一种}-乙$) ~6 × 10 ^-5 MPa ^-1随着孔隙压力的增加)。相反，当孔隙流体压力保持恒定且有效法向应力变化时，对凿井的摩擦稳定性影响很小。在孔隙流体压力升高时凿井摩擦稳定性的增加是由速率和状态参数b的演变引起的，在高孔隙流体压力下该参数变得更负。这些结果对于理解俯冲带中滑动的性质具有重要意义，并表明孔隙流体压力的稳定作用可以促进俯冲界面区域的缓慢滑动或抗震蠕变，否则这些区域可能会经历地震破裂。